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CENTAURI DREAMS

Imagining and Planning Interstellar Exploration

WIND RIDER: A HIGH PERFORMANCE MAGSAIL

by Paul Gilster on November 19, 2021

Can you imagine the science we could do if we had the capability of sending a
probe to Jupiter with travel time of less than a month? How about Neptune in 18
weeks? Alex Tolley has been running the numbers on a concept called Wind Rider,
which derives from the plasma magnet sail he has analyzed in these pages before
(see, for example, The Plasma Magnet Drive: A Simple, Cheap Drive for the Solar
System and Beyond). The numbers are dramatic, but only testing in space will
tell us whether they are achievable, and whether the highly variable solar wind
can be stably harnessed to drive the craft. A long-time contributor to Centauri
Dreams, Alex is co-author (with Brian McConnell) of A Design for a Reusable
Water-Based Spacecraft Known as the Spacecoach (Springer, 2016), focusing on a
new technology for Solar System expansion.

by Alex Tolley

In 2017 I outlined a proposed magnetic sail propulsion system called the Plasma
Magnet that was presented by Jeff Greason at an interstellar conference [6]. It
caught my attention because of its simplicity and potential high performance
compared to other propulsion approaches. For example, the Breakthrough Starshot
beamed sail required hugely powerful and expensive phased-array lasers to propel
a sail into interstellar space. By contrast, the Plasma Magnet [PM] required
relatively little energy and yet was capable of propelling a much larger mass at
a velocity exceeding any current propulsion system, including advanced solar
sails.

The Plasma Magnet was proposed by Slough [5] and involved an arrangement of
coils to co-opt the solar wind ions to induce a very large magnetosphere that is
propelled by the solar wind. Unlike earlier proposals for magnetic sails that
required a large electric coil kilometers in diameter to create the magnetic
field, the induction of the solar wind ions to create the field meant that the
structure was both low mass and that the size of the resulting magnetic field
increased as the surrounding particle density declined. This allowed for a
constant acceleration as the PM was propelled away from the sun, very different
from solar sails and even magsails with fixed collecting areas.

The PM concept has been developed further with a much sexier name: the Wind
Rider, and missions to use this updated magsail vehicle are being defined.

Wind Rider was presented at the 2021 American Geophysical Union meeting by the
team led by Brent Freeze, showing their concept of the design for a Jupiter
mission they called JOVE. The main upgrade from the earlier PM to the Wind Rider
is the substitution of superconducting coils. This allows the craft to maintain
the magnetic field without requiring constant power to maintain the electric
current, reducing the required power source. Because the superconducting coils
would quickly heat up in the inner system and lose their superconductivity, a
gold foil reflective sun shield is deployed to shield the coils from the sun’s
radiation. This is shown in the image above with the shield facing the sun to
keep the coils in shadow. The shield is also expected to do double duty as a
radio antenna, reducing the net parasitic mass on the vehicle.

The performance of the Wind Rider is very impressive. Calculations show that it
will accelerate very rapidly and reach the velocity of the solar wind, about 400
km/s. This has implications for the flight trajectory of the vehicle and the
mission time.

The first mission proposal is a flyby of Jupiter – Jupiter Observing Velocity
Experiment (JOVE) – much like the New Horizons mission did at Pluto.

Figure 1. The Wind Rider on a flyby of Jupiter. The solar panels are hidden
behind the sun shield facing the sun. The 16U CubeSat chassis is at the
intersection of the 2 coils and sun shield.

The JOVE mission proposal is for an instrumented flyby of Jupiter. The chassis
is a 16U CubeSat. The scientific instrument payload is primarily to measure data
on the magnetic field and ion density around Jupiter. The sail is powered by 4
solar panels that also double as struts to support the sun shield and generate
about 1300 W at 1 AU and fall to about 50W at Jupiter.

Figure 2. Trajectory of the Wind Rider from Earth to Jupiter

The flight trajectory is effectively a beeline directly to Jupiter, starting the
flight almost at opposition. No gravity assists from Earth or Venus are
required, nor a long arcing trajectory to intercept Jupiter. Figure 2 shows the
trajectory, which is almost a straight-line course with the average velocity
close to that of the solar wind.

Although the mission is planned as a flyby, a future mission could allow for
orbital insertion if the craft approaches Jupiter’s rotating magnetosphere to
maximize the impinging field velocity. Although not mentioned by the authors, it
should be noted that Slough has also proposed using a PM as an aerobraking
shield that decelerates the craft as it creates a plasma in the upper atmosphere
of planets.

How does the performance of the Wind Rider compare to other comparable missions?

The JUNO space probe to Jupiter had a maximum velocity of about 73 km/s as
Jupiter’s gravity accelerated the craft towards the planet. The required gravity
assists and long flight path, about 63 AU or over 9 billion km, mean that its
average velocity was about 60 km/s. This is not the fairest comparison as the
JUNO probe had to attain orbital insertion at Jupiter.

A fairer comparison is the fastest probe we have flown – the New Horizons
mission to Pluto — which reached 45 km/s as it left Earth but slowed to 14 km/s
as it flew by Pluto. New Horizons took 1 year to reach Jupiter to get a gravity
assist for its 9 year mission to Pluto, and therefore a maximum average velocity
of 19 km/s between Earth and Jupiter.

Wind Rider can reach Jupiter in less than a month. Figure 2 shows the almost
straight-line trajectory to Jupiter. Launched just before opposition, Wind Rider
reaches Jupiter in just over 3 weeks. Because opposition happens annually, a new
mission could be launched every year.

As the Wind Rider quickly reaches its terminal velocity at the same velocity as
the solar wind, it can reach the outer planets with comparably short times with
the same trajectory and annual launch windows.

The Wind Rider can fly by Saturn in just 6 weeks, and Neptune in 18 weeks.
Compare that to the Voyager 2 probe launched in 1977 that took 4 years and 12
years to fly by the same planets respectively. Pluto could be reached by Wind
Rider in just 6 months.

Because of its high terminal velocity that does not reduce during its mission,
the Wind Rider is also ideally suited for precursor interstellar missions.

The second proposed mission is called Pathfinder, proposed to ultimately reach
the solar gravity focal line around 550 AU from the sun. Flight time is less
than 7 years, making this a viable project for a science and engineering team
and not a multi-generation one based on existing rocket propulsion technology.
As the flight trajectory is a straight line, this makes the craft well suited to
follow the focal line while imaging a target star or exoplanet using the sun’s
diameter as a large aperture telescope to increase the resolving power.

As the Wind Rider reaches the solar wind velocity, it may even be able to ride
the gusts of higher solar wind velocities, perhaps reaching closer to 550 km/s.

While solar sails have been considered the more likely means to reach high
velocities, especially when making sun-diver maneuvers, even advanced sails with
proposed areal densities well below anything available today would reach solar
system escape velocities in the range of 80-120 km/s [3]. If the Wind Rider can
indeed reach the velocity of the solar wind, it would prove a far faster vehicle
than any solar sail being planned, and would not need a boost from large laser
arrays, nor risky sun-diver maneuvers.

I would inject some caution at this point regarding the performance. The
performance is based entirely on theoretical work and a small scale laboratory
experiment. What is needed is a prototype launched into cis-lunar space to test
the performace on actual hardware and confirm the capability of the technology
to operate as theorized.

It should also be noted that despite its theoretical high performance, there is
a potential issue with propelling a probe with a magnetic sail. Compared to a
solar sail or a vehicle with reaction thrusters, the Wind Rider as described so
far has no crosswind capability. It just runs in front of the solar wind like a
dandelion seed in the wind. This means that it would have to be aimed very
accurately at its target, and subject to the vagaries of the strength of the
solar wind that is far less stable than the sun’s photon emissions. Like the
dandelion, if the Wind Rider was very inexpensive, many could be launched in the
expectation that at least one would successfully reach its target.

However, there is a possibility that some crosswind capability is possible. This
is based on modelling by Nishida [4]. This paper was recommended by Dr. Freeze
[7].

The study modeled the effect of the angle of attack of the magnetic field of a
coil against the solar wind. The coil in this case would represent the induced
circular movement of the solar wind induced by the primary Wind Rider/PM coils.

Theoretically, the angle of attack has an impact on the total force pushing past
the magnetic field.

Figure 3 shows the pressure and on the field as the coil is rotated from 0
through 45 and 90 degrees to the solar wind.

The force experienced is maximal at 90 degrees. This is shown visually in figure
3 and graphically in figure 4.

Figure 4. Force on the coil effected by angle of attack. A near 90 degrees angle
of attack increases the force about 50%.

The angle of attack also induces a change in the thrust vector experienced by
the coil, which would act as a crosswind maneuvering capability, allowing for
trajectory adjustments as well as a longer launch window for the Wind Rider.

Figure 5. The angle of attack affects the thrust vector. But note the
countervailing torque on the coil.

If the coil can maintain an angle of attack with respect to teh solar wind, then
the Wind Rider can steer across the solar wind to some extent.

Figure 6. (left) Angle of attack, and steering angle. (right) angle of attack
and the torque on the coil.

Figure 6 shows that the craft could steer up to 12 degrees away from the solar
wind direction. However, maintaining that angle of attack requires a constant
force to oppose the torque restoring the angle of attack to zero or 90 degrees.
The coil therefore acts like a weather vane, always trying to align itself with
the solar wind. To maintain the angle of attack would be difficult. Reaction
wheels like those on the Kepler telescope could only act in a transient manner.
Another possibility suggested is to move the center of gravity of the craft in
some way. Adding booms with coils might be another solution, albeit by adding
mass and complexity, undesirable for this first generation probe. Jeff Greason
has an upcoming paper to be published in 2022 on theoretical navigation with
possible ranges of steering capability.

In summary, the Wind Rider is an upgraded version of the Plasma Magnet
propulsion concept, now applied to a reference design for 2 missions, a fast
flyby of Jupiter, and an interstellar precursor mission that could reach the
solar gravity lens focus. The performance of the design is primarily based on
modelling and as yet there is no experimental evidence to support a finite
lift/drag ratio for the craft.

Having said that, the propulsion principle and hardware necessary are not
expensive, and there seems to be much interest by the AIAA. Maybe this
propulsion method can finally be built, flown and evaluated. If it works as
advertised, it would open up the solar system to exploration by fast, cheap
robotic probes and eventually crewed ships.

References

1. Freeze, B et al Wind Rider Pathfinder Mission to Trappist-1 Solar
Gravitational Lens Focal Region in 8 Years (poster at AGU – Dec 13th, 2021).
https://agu.confex.com/agu/fm21/meetingapp.cgi/Paper/796237

2. Freeze, B et al Jupiter Observing Velocity Experiment (JOVE), Introduction to
Wind Rider Solar Electric Propulsion Demonstrator and Science Objective.
https://baas.aas.org/pub/2021n7i314p05/release/1

3. Vulpetti, Giovanni, et al. (2008) Solar Sails: A Novel Approach to
Interplanetary Travel. New York: Springer, 2008.

4. Nishida, Hiroyuki, et al. “Verification of Momentum Transfer Process on
Magnetic Sail Using MHD Model.” 41st AIAA/ASME/SAE/ASEE Joint Propulsion
Conference & Exhibit, 2005.
https://doi.org/10.2514/6.2005-4463

5. Slough, J. “Plasma Magnet NASA Institute for Advanced Concepts Phase I Final
Report.” 1970.
https://www.semanticscholar.org/paper/The-Plasma-Magnet-NASA-Institute-for-Advanced-Phase-Slough/74e9e914930103f54606bfc335a42b77fd7ae5ef/.
See Figure 12.

6. Tolley, A “The Plasma Magnet Drive: A Simple, Cheap Drive for the Solar
System and Beyond” (2017).
https://www.centauri-dreams.org/2017/12/29/the-plasma-magnet-drive-a-simple-cheap-drive-for-the-solar-system-and-beyond/

7. Generous email communications with Dr. Brent Freeze in preparation of this
article.

{ 7 comments }

TOLIMAN TARGETS CENTAURI A/B PLANETS

by Paul Gilster on November 17, 2021

We talked about the TOLIMAN mission last April, and the renewed interest in
astrometry as the key to ferreting out possible planets around Alpha Centauri A
and B. I was fortunate enough to hear Peter Tuthill (University of Sydney), who
leads the team that has been developing the concept, rough out the idea at
Breakthrough Discuss five years ago; Céline Bœhm (likewise at the University of
Sydney) reported on more recent work at the virtual Breakthrough Discuss session
this past spring. We now have an announcement from scientists involved that the
space telescope mission will proceed.

Eduardo Bendek (JPL) is a member of the TOLIMAN team:

> “Even for the very nearest bright stars in the night sky, finding planets is a
> huge technological challenge. Our TOLIMAN mission will launch a
> custom-designed space telescope that makes extremely fine measurements of the
> position of the star in the sky. If there is a planet orbiting the star, it
> will tug on the star betraying a tiny, but measurable, wobble.”

Work on the mission heated up in April of this year, with scientists from the
University of Sydney working in partnership with Breakthrough Initiatives, the
Jet Propulsion Laboratory and Australia’s Saber Astronautics. The mission could
revolutionize our view of Centauri A and B, according to Tuthill:

> “Astronomers have access to amazing technologies that allow us to find
> thousands of planets circling stars across vast reaches of the galaxy. Yet we
> hardly know anything about our own celestial backyard. It is a modern problem
> to have; we are like net-savvy urbanites whose social media connections are
> global, but we don’t know anyone living on our own block… Getting to know our
> planetary neighbors is hugely important. These next-door planets are the ones
> where we have the best prospects for finding and analyzing atmospheres,
> surface chemistry and possibly even the fingerprints of a biosphere – the
> tentative signals of life.”

Image: The University of Sydney’s Peter Tuthill, project leader for TOLIMAN.
Credit: University of Sydney.

Astrometry tracks the minute changes in the position of a star that are the
result of the gravitational pull of a planet. Detection of tiny angular
displacements of the star allows the planet’s mass and orbit to be recovered,
and unlike the situation with both radial velocity and transit methods, the
astrometric signal increases with the separation of the planet and star.

That takes us out to the orbital distance for an Earth-class planet to be in the
habitable zone, even though the signal is tiny, in the range of micro-arcseconds
for the Alpha Centauri binary. The astrometric signal from an Earth-class planet
orbiting in the habitable zone of Centauri A is 2.5 micro-arcseconds; a similar
planet around Centauri B is roughly half of that.

TOLIMAN uses what the team calls a ‘diffractive pupil’ lens that spreads out the
starlight and allows scientists to eliminate systematic errors and clarify the
underlying signal. The flower-like pattern enhances the detection of star
movement without the need for field stars as references, eliminating the need
for a large aperture (such stars demand a larger collecting area). The pattern
also reduces noise levels in the detector. An online description of the TOLIMAN
technology explains why the nearest stellar system makes an excellent target for
these methods:

> With the fortuitous presence of a bright phase reference only arcseconds away,
> measurements are immediately 2 – 3 orders of magnitude more precise than for a
> randomly chosen bright field star where many-arcminute fields (or larger) are
> required to find background stars for this task. Maintaining the instrument
> imaging distortions stable over a few arcseconds is considerably easier than
> requiring similar stability over arcminutes or degrees. Alpha Cen’s proximity
> to Earth means that the angular deviations on the sky are proportionately
> larger (typically a factor of ~10-100 compared to a population of comparably
> bright stars).

Image: This is from Figure 3 of the online description of TOLIMAN referenced
above. Caption: Left: pupil plane for TOLIMAN diffractive-aperture telescope.
Light is only collected in the 10 elliptical patches (the remainder of the pupil
is opaque in this conceptual illustration, although our flight design will
employ phase steps which do not waste starlight). Middle: The simulated image
observing a point-source star with this pupil. The region surrounding the star
can be seen to be filled with a complex pattern of interference fringes,
comprising our diffractive astrometric grid. Right: A simulated image of the
Alpha Cen binary star as observed by TOLIMAN. Credit: Tuthill et al.

The same description refers to TOLIMAN as a ‘modest astrometric space
telescope,’ and the word ‘modest’ seems to apply in that this is a narrow-field
instrument 30cm in diameter, with what proponents estimate is a fast build time
on the order of 18 months. We might contrast the mission with existing
astrometric missions like the European Space Agency’s space-based GAIA. The
latter can make astrometric measurements in the 10s of micro-arcseconds, which
basically means it is capable of detecting gas giants. TOLIMAN takes us into the
realm of much smaller, rocky worlds. Because it has no need of a large aperture,
it is small, inexpensive and, obviously, tightly focused on a nearby system
rather than surveying a large star field.

Image: Simulated image of the Alpha Centauri system, as could be viewed by the
TOLIMAN telescope. Credit: Peter Tuthill.

TOLIMAN will receive spaceflight mission operations support from Saber
Astronautics, including satellite communications and command. Saber’s
involvement, says Tuthill, is “a critical part of the mission.” The company has
received A$788,000 from an Australian Government International Space Investment:
Expand Capability grant for the telescope’s design and construction, and I
rather like the spirit in CEO Jason Held’s comment on TOLIMAN:

> “TOLIMAN is a mission that Australia should be very proud of – it is an
> exciting, bleeding-edge space telescope supplied by an exceptional
> international collaboration. It will be a joy to fly this bird.”

As to when we can expect the bird to fly, Tuthill speaks of launch by 2023. We
might know by mid-decade whether an Earth-size rocky planet orbits Centauri A or
B. Habitable zone orbits are possible around both stars.

An early description of TOLIMAN is Tuthill et al., “The TOLIMAN Space
Telescope,” Proc. SPIE 10701, Optical and Infrared Interferometry and Imaging
VI, 107011J (9 July 2018). Abstract.

{ 13 comments }

PROBING THE LIKELIHOOD OF PANSPERMIA

by Paul Gilster on November 16, 2021

I’m looking at a paper just accepted at The Astrophysical Journal on the subject
of panspermia, the notion that life may be distributed through the galaxy by
everything from interstellar dust to comets and debris from planetary impacts.
We have no hard data on this — no one knows whether panspermia actually occurs
from one planet to another, much less from one stellar system to another star.
But we can investigate possibilities based on what we know of everything from
the hardiness of organisms to the probabilities of ejecta moving on an
interstellar trajectory.

In “Panspermia in a Milky Way-like Galaxy,” lead author Raphael Gobat
(Pontificia Universidad Católica de Valparaíso, Chile) and colleagues draw
together current approaches to the question and develop a modeling technique
based on our assumptions about galactic habitability and simulations of galaxy
structure.

Panspermia is an ancient concept. Indeed, the word first emerges in the work of
Anaxagoras (born ca. 500–480 BC) and makes its way through Lucian of Samosata
(born around 125 AD), through Kepler’s Somnium, to re-emerge in 19th Century
microbiology. Accidental propagation of life’s building blocks was considered by
Swedish chemist Svante Arrhenius in the early 20th Century. Fred Hoyle and Nalin
Chandra Wickramasinghe developed the idea still further in the 1970s and 80s.

So how do we approach a subject that has remained controversial, likely because
it does not appear necessary in explaining how life emerged on our own Earth? As
the paper notes, modern work falls into three distinct categories, the first
involving whether or not microorganisms can survive ejection from a planetary
surface and re-entry onto another. Remarkably, hypervelocity impacts are not
show-stoppers for the idea, suggesting that a small fraction of spores could
survive impact and transit.

As to timescale and kinds of transfer mechanisms, most work seems to have
focused on mass transfer between planets in the same stellar system, usually
through lithopanspermia, which is the exchange of meteoroids. It’s true,
however, that transit between different stars has been investigated, looking at
radiation pressure on small grains of material. There are even a few studies on
whether or not a stellar system might be intentionally seeded by means of
technology. The term here is directed panspermia, a subject more often treated
in science fiction than academic circles.

Although not entirely. While directed panspermia is off the table for Gobat and
colleagues, we’ll take a look in a month or so at what does appear in the
literature. Some interesting ideas have emerged, but they’re not for today.

What Gobat and co-authors have in mind is to apply a model of galactic
habitability they have developed (citation below) in conjunction with the
simulations of spiral galaxies based on hydrodynamics that are found in the
McMaster Unbiased Galaxy Simulations (MUGS), a set of 16 simulated galaxies
developed within the last decade. On the latter, the paper notes:

> These simulations made use of the cosmological zoom method, which seeks to
> focus computational effort into a region of interest, while maintaining enough
> of the surrounding large-scale structure to produce a realistic assembly
> history. To accomplish this, the simulation was first carried out at low
> resolution using N-body physics only. Dark matter halos were then identified,
> and a sample of interesting objects selected. The particles making up, and
> surrounding, these halos were then traced back to their origin, and the
> simulation carried out again with the region of interest simulated at higher
> resolution.

Simulation and re-simulation allow the MUGS galaxies to reproduce the known
metallicity gradients in observed galaxies and likewise reproduce their
large-scale structure, including disks, halos and bulges. The authors use one of
the simulated galaxies, a spiral galaxy similar to but not identical with the
Milky Way, to investigate the probability and efficiency of panspermia as
dependent on the galactic environment.

Image: This is Figure 1 from the paper. Caption: Mock UV J color images of the
simulated galaxy g15784 (Stinson et al. 2010; Nickerson et al. 2013), for both
edge-on (left) and face-on (right) orientations, using star and gas particles,
and assuming Bruzual & Charlot (2003) stellar population models and a simple
dust attenuation model (Li & Draine 2001) with a gas-to-dust ratio of 0.01 at
solar metallicity. Additionally, we include line emission from star particles
with ages ≤ 50 Myr, following case B recombination (Osterbrock & Ferland 2006)
and metallicity-dependent line ratios (Anders & Fritze-v. Alvensleben 2003). All
panels are 50 kpc across and have a resolution of 100 pc. Two spheroidal
satellites can be seen above and below the galactic plane, respectively. Credit:
Gobat et al.

Panspermia appears to be more likely in the central regions of the galactic
bulge, as we might assume due to the high density of stars there, a factor which
counterbalances their lower habitability in this model. Panspermia is found to
be much less likely as we move out into the central disk. In the model of
habitability as developed by Gopat and Sungwook Hong in 2016, habitability
increases as we depart from galactic center, while the new paper shows that the
likelihood of panspermia works inversely, being more likely toward the bulge.

In a sense, we decouple habitability from panspermia. The paper uses the term
‘particles’ to refer not to individual stars, but to ensembles of stars with a
range of masses but the same metallicity. This reflects, say the authors, the
resolution limits of the simulations, which cannot track individual stars
through time. From the paper, noting the narrow dynamic range of habitability
vs. panspermia [the italics are mine]:

> In dense regions [of the simulated galaxy], many source particles can
> contribute to panspermia, whereas in the outer disk and halo the panspermia
> probability is typically dominated by one or, at most, a few source star
> particles. Unlike natural habitability, whose value varies by only ∼ 5%
> throughout the galaxy, the panspermia probability has a wide dynamic range of
> several orders of magnitudes..

The models used here have a number of limitations, but it’s interesting that
they point to panspermia as being considerably less efficient at seeding planets
than the evolution of life on the planets themselves. At best, the authors find
the probability of panspermia to be no more than 3% of all the star particles in
their simulation. This may be an overly generous figure, and the paper
acknowledges that it cannot be more precisely quantified other than to say that
when it comes to efficiency, local evolution wins going away. Higher resolution
galaxy simulations will offer more realistic insights.

We have a result, as the authors acknowledge, that is more qualitative than
quantitative, a measure of how much we have to learn about galaxies themselves,
and about the Milky Way in particular. The sample galaxy, for example, has a
higher bulge-to-disk ratio than the Milky Way. But more significantly, the
capture fraction of spores by target planets and the likelihood that life
actually does develop on planets considered habitable are subjects with no
concrete data to firm up the conclusions.

We can anticipate that future simulations will take into account a rotating
evolving galaxy as opposed to the single simulation ‘snapshot’ the paper offers.
Nonetheless, this modeling of organic compounds being transferred between stars
points to the orders of magnitude difference in the likelihood of panspermia
between the inner and the outer disk, a useful finding. Given that so few of the
star particles the simulation generates have high panspermia probability, the
process may occur but under conditions that make it much less effective than
prebiotic evolution.

The paper is Gobat et al., “Panspermia in a Milky Way-like Galaxy,” accepted at
the Astrophysical Journal (preprint). The paper on galactic habitability is
Gobat & Hong, “Evolution of galaxy habitability,” Astronomy & Astrophysics Vol.
592, A96 (04 August 2016). Abstract.

{ 24 comments }

TESS: AN UNUSUAL CIRCUMBINARY DISCOVERY

by Paul Gilster on November 12, 2021

Circumbinary planets are those that orbit two stars, a small but growing
category of worlds — we’ve detected some 14 thus far, thanks to Kepler’s good
work, and that of the Transiting Exoplanet Survey Satellite (TESS). The latest
entry, TIC 172900988, illustrates the particular challenge such planets
represent. Transit photometry is a standard method for finding planets,
detecting the now familiar drop in starlight as the planet moves between us and
the surface of the host star. Kepler found thousands of exoplanets this way. But
when two stars are involved, things get complicated.

Image: The newly discovered planet, TIC 172900988b, is roughly the radius of
Jupiter, and several times more massive, but it orbits its two stars in less
than one year. This world is hot and unlike anything in our Solar System.
Credit: PSI/Pamela L. Gay.

Three transits are required to determine the orbital path of a planet. For us to
make a detection, a circumbinary planet will have to transit both stars, but the
timing of the transits can vary. The planet may transit the first star, then the
second, before returning to transit the first. Nader Haghighipour (Planetary
Science Institute) points out that the orbital period of a circumbinary planet
will always be much longer than the orbital period of the binary star, and that
means detecting three transits will be problematic for a telescope like TESS,
which observes each portion of sky for only 27 days.

The paper on the discovery of TIC 172900988b lays out these problems:

> Finding transiting planets orbiting around binary stars is much more difficult
> than around single stars. The transits are shallower (due to the constant
> ‘third-light’ dilution from the binary companion), noisier (due to starspots
> and stellar activity from two stars), and can be blended with the stellar
> eclipses. This difficulty is greatly compounded when the observations cover a
> single conjunction and, even if multiple transits are detected as in the
> system presented here, they are neither periodic, nor have the same depth and
> duration… The transit times and shapes depend on the orientation and motion of
> the binary stars and of the CBP [circumbinary planet] at the observed times.
> The complexity of such transits is both a curse and a blessing…

A blessing, the authors argue, because such a detection yields information
“richer than what can be obtained from a single transit of a single-star
planet,” offering better estimation of the planet’s orbital period.

Image: A newly discovered planet was observed in the system TIC 172900988. In
TESS data, it passed in front of the primary star (right) and 5 days later
(shown) passed in front of the second star (left). These stars are just over 30%
larger than the Sun, and differ very little in size. Credit: PSI/Pamela L. Gay.

Haghighipour is part of a team of astronomers with circumbinary planet
experience; he also contributed to a 2020 paper in The Astronomical Journal that
produced a technique for discovering circumbinary planets using only two
transits, one across each star during the same conjunction. It was this method,
ideally suited for TESS, that led the same team to make the just announced
discovery of TIC 172900988b. This is the first TESS circumbinary planet to be
found using these methods.

TIC 172900988b takes 200 days to complete a full orbit of the binary system. The
planet is a gas giant of Jupiter size, the most massive transiting circumbinary
planet found thus far. The team, led by Veselin B. Kostov (SETI Institute),
observed it transit the primary star, followed five days later by a transit of
the secondary, as the binary eclipsed itself over a 20-day orbit.

The Kepler mission discovered its circumbinary worlds by finding pairs of
transits during a single conjunction, making it clear that the phenomenon is
common. In fact, Jean Schneider and Michel Chevreton (both at the Paris
Observatory) analyzed this likely observational signal as far back as 1990 in a
paper for Astronomy and Astrophysics. Now TESS has a circumbinary discovery of
its own, despite its much shorter dwell time on the stars in its field. Adds
Kostov:

> “The occurrence of multiple closely-spaced transits during one orbit is a
> unique observation signature of transiting circumbinary planets. This is a
> geometrical phenomenon that provides a new planet detection method. The
> discovery of TIC 172900988b is the first demonstration that the method works.”

Image: This is Figure 5 from the paper. Caption: The photometric data shown in
Figure 4 phase-folded on a linear period of P = 19.65802 days. The left panel
shows the primary eclipse and the right panel shows the secondary eclipse. The
different data sets are vertically offset in the lower panels for clarity. The
phase change of the secondary eclipse relative to the primary—indicative of the
apsidal motion of the binary—is clearly seen in the right panels. Credit: Kostov
et al.

We learn in the paper’s analysis of the detection that no further data from TESS
will become available on this planet, making future study the province of other
instruments. From the paper:

> We note that TESS will observe the target again in Sectors 44 through 47 (2021
> October to 2022 January). Unfortunately, it will miss the predicted transits
> for the corresponding conjunctions by several weeks. Thus follow-up
> observations from other instruments are key for strongly constraining the
> orbit and mass of the CBP [circumbinary planet]. In particular, observing the
> predicted 2022 February-March conjunction of the CBP is critical for solving
> the currently-ambiguous orbit of the planet. As a relatively bright target (V
> = 10.141 mag), the system is accessible for high resolution spectroscopy, e.g.
> Rossiter-McLaughlin effect, transit spectroscopy. TIC 172900988 demonstrates
> the discovery potential of TESS for circumbinary planets with orbital periods
> greatly exceeding the duration of the observing window.

The paper is Kostov et al., “TIC 172900988: A Transiting Circumbinary Planet
Detected in One Sector of TESS Data,” The Astronomical Journal 162, No. 6 (10
November 2021), 234 (abstract / preprint).

{ 8 comments }

SPARCS: ZEROING IN ON M-DWARF FLARES

by Paul Gilster on November 10, 2021

Although we’ve been talking this week about big telescopes, from extremely large
designs like the Thirty Meter Telescope and the European Extremely Large
Telescope to the space-based HabEx/LUVOIR descendant prioritized by Astro2020,
small instruments continue to do interesting work around the edges. I just
noticed a tiny one called the Star-Planet Activity Research CubeSat (SPARCS)
that fills a gap in our study of M-dwarfs, those small stars whose flares are so
problematic for habitability.

Under development at Arizona State University, the space-based SPARCS is just
halfway into its development phase, but let’s take a look at it in light of
ongoing work on M-dwarf planets, because it bodes well for turning theories
about flare activity into data that can firm up our understanding. The problem
is that while theoretical studies delve into ultraviolet flaring on these stars,
the longest intensive UV monitoring on an M-dwarf done thus far has been a
thirty hour effort with the Hubble instrument.

We need more, which is why the SPARCS idea emerged. A team of researchers led by
ASU’s Evgenya Shkolnik has produced an overview of the NASA-funded mission’s
science drivers and its intention of deepening our understanding of star-planet
interactions. “Know thy star, know thy planet,. . . especially in the
ultraviolet (UV),” comments the team in their abstract, which also points to the
necessity of data collection for these intensely studied stars, ubiquitous in
the galaxy and known to host interesting planets like Proxima Centauri b.

Image: An example of M-dwarf flaring. DG CVn, a binary consisting of two red
dwarf stars shown here in an artist’s rendering, unleashed a series of powerful
flares seen by NASA’s Swift. At its peak, the initial flare was brighter in
X-rays than the combined light from both stars at all wavelengths under typical
conditions. Credit: NASA’s Goddard Space Flight Center/S. Wiessinger.

Can such a world be habitable? Recent observations have shown that flare events
produce a more severe flux increase in the ultraviolet than the optical; a flare
peaking on the order of 0.01x the star’s quiescent flux in the optical, write
the authors, can at UV wavelengths brighten by a factor of 14000. UV
‘superflare’ events — as much as 10,000 times more energetic than the flares
produced by our G-class Sun — can produce 200x flux increases that are expected
to occur on a daily basis on young, active M-dwarfs.

Thus habitability can be compromised, with UV radiation damaging planetary
atmospheres, eroding ozone and producing lethal levels of radiation at the
surface. An Earth-like planet in the habitable zone can likewise be subject to
methane depletion under the kind of flaring Proxima Centauri has been known to
produce. Thus the composition of an M-dwarf planet’s atmosphere is subject to
interactions with its star that may prevent life from ever arising, or
drastically affect its development.

SPARCS is a CubeSat observatory carrying a 9-cm telescope and the associated
gear to perform photometric monitoring of M-dwarf flare activity in the near
(258−308 nm) and far ultraviolet (153−171 nm). The target: 20 M-dwarfs in a
range of ages from 10 million to 5 billion years old, examined during a mission
lifetime of one year. Planned for launch in 2023 into a heliosynchronous orbit
that offers “decent thermal stability and optimized continuity in target
monitoring,” SPARCS will track flare color, energies, occurrence rate and
duration on active as well as inactive M-dwarfs.

The authors believe the observatory will also improve our atmospheric models for
M-dwarf planets, useful information as we look toward future biosignature
investigations, and helpful as we fill an obvious gap in our data on this class
of star. The software onboard is interesting in itself:

> The payload software is able to run monitoring campaigns at constant detector
> exposure time and gain, but due to the expected high amplitudes of M dwarf UV
> flares, observations throughout the nominal mission will be conducted using a
> feature of the software that autonomously adjusts detector exposure times and
> gains to mitigate the occurrence of pixel saturation during observations of
> flaring events. SPARCS will be the first space-based stellar astrophysics
> observatory that adopts such an onboard autonomous exposure control.

So we have a small space telescope that will be able to monitor its targets in
both near- and far-ultraviolet wavelengths simultaneously, managed by a
dedicated onboard payload processor that allows the observatory to adjust for
pixel saturation during flare events. This “autonomous dynamic exposure control
algorithm” is a story in itself, adding depth to a mission to investigate the
most extremely variable stars in the Hertzsprung–Russell diagram. SPARCS should
help us learn whether these long-lived stars can allow planetary habitability as
they age into a less dramatic maturity.

The paper is Ramiaramanantsoa et al., “Time-Resolved Photometry of the
High-Energy Radiation of M Dwarfs with the Star-Planet Activity Research CubeSat
(SPARCS),” accepted for publication in Astronomische Nachrichten (preprint).

{ 4 comments }

THE EXOPLANET PIPELINE

by Paul Gilster on November 9, 2021

Looking into Astro2020’s recommendations for ground-based astronomy, I was
heartened with the emphasis on ELTs (Extremely Large Telescopes), as found
within the US-ELT project to develop the Thirty Meter Telescope and the Giant
Magellan Telescope, both now under construction. Such instruments represent our
best chance for studying exoplanets from the ground, even rocky worlds that
could hold life. An Astro2020 with different priorities could have spelled the
end of both these ELT efforts in the US even as the European Extremely Large
Telescope, with its 40-meter mirror, moves ahead, with first light at Cerro
Armazones (Chile) projected for 2027.

So the ELTs persist in both US and European plans for the future, a context
within which to consider how planet detection continues to evolve. So much of
what we know about exoplanets has come from radial velocity methods. These in
turn rely critically on spectrographs like HARPS (High Accuracy Radial Velocity
Planet Searcher), which is installed at the European Southern Observatory’s 3.6m
telescope at La Silla in Chile, and its successor ESPRESSO (Echelle Spectrograph
for Rocky Exoplanet and Stable Spectroscopic Observations). We can add the NEID
spectrometer on the WIYN 3.5m telescope at Kitt Peak to the mix, now operational
and in the hunt for ever tinier Doppler shifts in the light of host stars.

We’re measuring the tug a planet puts on its star by looking radially — how is
the star pulled toward us, then away, as the planet moves along its orbit? Given
that the Earth produces a movement of a mere 9 centimeters per second on the
Sun, it’s heartening to see that astronomers are closing on that range right
now. NEID has demonstrated a precision of better than 25 centimeters per second
in the tests that led up to its commissioning, giving us another tool for
exoplanet detection and confirmation.

But this is a story that also reminds us of the vast amount of data being
generated in such observations, and the methods needed to get this information
distributed and analyzed. On an average night, NEID will collect about 150
gigabytes of data that is sent to Caltech, and from there via a data management
network called Globus to the Texas Advanced Computing Center (TACC) for analysis
and processing. TACC, in turn, extracts metadata and returns the data to Caltech
for further analysis. The results are made available by the NASA Exoplanet
Science Institute via its NEID Archive.

Image: The NEID instrument is shown mounted on the 3.5-meter WIYN telescope at
the Kitt Peak National Observatory. Credit: NSF’s National Optical-Infrared
Astronomy Research Laboratory/KPNO/NSF/AURA.

What a contrast with the now ancient image of the astronomer on a mountaintop
coming away with photographic plates that would be analyzed with instruments
like the blink comparator Clyde Tombaugh used to discover Pluto in 1930. The
data now come in avalanche form, with breakthrough work occurring not only on
mountaintops but in the building of data pipelines like these that can be
generalized for analysis on supercomputers. The vast caches of data contain the
seeds of future discovery.

Joe Stubbs leads the Cloud & Interactive Computing group at TACC:

> “NEID is the first of hopefully many collaborations with the NASA Jet
> Propulsion Laboratory (JPL) and other institutions where automated data
> analysis pipelines run with no human-in-the-loop. Tapis Pipelines, a new
> project that has grown out of this collaboration, generalizes the concepts
> developed for NEID so that other projects can automate distributed data
> analysis on TACC’s supercomputers in a secure and reliable way with minimal
> human supervision.”

NEID also makes a unique contribution to exoplanet detection by being given over
to the analysis of activity on our own star. Radial velocity is vulnerable to
confusion over starspots — created by convection on the surface of exoplanet
host stars and mistaken for planetary signatures. The plan is to use NEID during
daylight hours with a smaller solar telescope developed for the purpose to track
this activity. Eric Ford (Penn State) is an astrophysicist at the university
where NEID was designed and built:

> “Thanks to the NEID solar telescope, funded by the Heising-Simons Foundation,
> NEID won’t sit idle during the day. Instead, it will carry out a second
> mission, collecting a unique dataset that will enhance the ability of machine
> learning algorithms to recognize the signals of low-mass planets during the
> nighttime.”

Image: A new instrument called NEID is helping astronomers scan the skies for
alien planets. TACC supports NEID with supercomputers and expertise to automate
the data analysis of distant starlight, which holds evidence of new planets
waiting to be discovered. WIYN telescope at the Kitt Peak National Observatory.
Credit: Mark Hanna/NOAO/AURA/NSF.

Modern astronomy in a nutshell. We’re talking about data pipelines operational
without human intervention, and machine-learning algorithms that are being tuned
to pull exoplanet signals out of the noise of starlight. In such ways does a
just commissioned spectrograph contribute to exoplanetary science through an
ever-flowing data network now indispensable to such work. Supercomputing
expertise is part of the package that will one day extract potential
biosignatures from newly discovered rocky worlds. Bring on the ELTs.

{ 12 comments }

TWO TAKES ON THE EXTRATERRESTRIAL IMPERATIVE

by Paul Gilster on November 5, 2021

Topping the list of priorities for the Decadal Survey on Astronomy and
Astrophysics 2020 (Astro2020), just released by the National Academy of
Sciences, Engineering and Medicine, is the search for extraterrestrial life.
Entitled Pathways to Discovery in Astronomy and Astrophysics for the 2020s, the
report can be downloaded as a free PDF here. At 614 pages, this is not light
reading, but it does represent an overview in which to place continuing work on
exoplanet discovery and characterization.

In the language of the report:

> “Life on Earth may be the result of a common process, or it may require such
> an unusual set of circumstances that we are the only living beings within our
> part of the galaxy, or even in the universe. Either answer is profound. The
> coming decades will set humanity down a path to determine whether we are
> alone.”

A ~6 meter diameter space telescope capable of spotting exoplanets 10 billion
times fainter than their host stars, thought to be feasible by the 2040s, leads
the observatory priorities. As forwarded to me by Centauri Dreams regular John
Walker, the survey recommends an instrument covering infrared, optical and
ultraviolet wavelengths with high-contrast imaging and spectroscopy. Its goal:
Searching for biosignatures in the habitable zone. Cost is estimated at an
optimistic $11 billion.

I say ‘optimistic’ because of the cost overruns we’ve seen in past missions,
particularly JWST. But perhaps we’re learning how to rein in such problems,
according to Joel Bregman (University of Michigan), chair of the AAS Committee
on Astronomy and Public Policy. Says Bregman:

> “The Astro2020 report recommends a ‘technology development first’ approach in
> the construction of large missions and projects, both in space and on the
> ground. This will have a profound effect in the timely development of projects
> and should help avoid budgets getting out of control.”

Time will tell. It should be noted that a number of powerful telescopes, both
ground- and space-based, have been built following the recommendations of
earlier decadal surveys, of which this is the seventh.

SUBORBITAL BUILDING BLOCKS

We’re a long way from the envisioned instrument in terms of both technology and
time, but the building blocks are emerging and the characterization of habitable
planets is ongoing. What a difference between a flagship level space telescope
like the one described by Astro2020 and the small, suborbital instrument slated
for launch from the White Sands Missile Range in New Mexico on Nov. 8. SISTINE
(Suborbital Imaging Spectrograph for Transition region Irradiance from Nearby
Exoplanet host stars) is the second of a series of missions homing in on how the
light of a star affects biosignatures on its planets.

False positives will likely bedevil biosignature searches as our technology
improves. Principal investigator Kevin France (University of Colorado Boulder)
points particularly to ultraviolet levels and their role in breaking down carbon
dioxide, which frees oxygen atoms to form molecular oxygen, made of two oxygen
atoms, or ozone, made of three. These oxygen levels can easily be mistaken for
possible biosignatures. Says France: “If we think we understand a planet’s
atmosphere but don’t understand the star it orbits, we’re probably going to get
things wrong.”

Image: A sounding rocket launches from the White Sands Missile Range, New
Mexico. Credit: NASA/White Sands Missile Range.

It’s a good point considering that early targets for atmospheric biosignatures
will be M-dwarf stars. Now consider the early Earth, laden with perhaps 200
times more carbon dioxide than today, its atmosphere likewise augmented with
methane and sulfur from volcanic activity in the era not long after its
formation. It took molecular oxygen a billion and a half years to emerge as
nothing more than a waste product produced during photosynthesis, eventually
leading to the Great Oxygenation Event.

Oxygen becomes a biomarker on Earth, but it’s an entirely different question
around other stars. M-dwarf stars like Proxima Centauri generate extreme levels
of ultraviolet light, making France’s point that simple photochemistry can
produce oxygen in the absence of living organisms. Bearing in mind that M-dwarfs
make up as many as 80 percent of the stars in the galaxy, we may find ourselves
with a number of putative biosignatures that turn out to be a reflection of
these abiotic reactions. Aboard the spacecraft is a telescope and a spectrograph
that will home in on ultraviolet light from 100 to 160 nanometers, which
includes the range known to produce false positive biomarkers. The UV output in
this range varies with the mass of the star; thus the need to sample widely.

SISTINE-2’s target is Procyon A. The craft will have a brief window of about
five minutes from its estimated altitude of 280 kilometers to observe the star,
with the instrument returning by parachute for recovery.

An F-class star larger and hotter than the Sun, Procyon A has no known planets,
but what is at stake here is accurate determination of its ultraviolet spectrum.
A reference spectrum for F-stars growing out of these observations of Procyon A
and incorporating existing data on other F-class stars at X-ray, extreme
ultraviolet and visible light is the goal. France says the next SISTINE target
will be Alpha Centauri A and B.

Image: A size comparison of main sequence Morgan–Keenan classifications. Main
sequence stars are those that fuse hydrogen into helium in their cores. The
Morgan–Keenan system shown here classifies stars based on their spectral
characteristics. Our Sun is a G-type star. SISTINE-2’s target is Procyon A, an
F-type star. Credit: NASA GSFC.

Launch is to be aboard a Black Brant IX sounding rocket. And although it sounds
like a small mission, SISTINE-2 will be working at wavelengths the Hubble Space
Telescope cannot observe. Likewise, the James Webb Space Telescope will work at
visible to mid-infrared wavelengths, making the SISTINE observations useful for
frequencies that Webb cannot see. The mission also experiments with new optical
coatings and what NASA describes as ‘novel UV detector plates’ for better
reflection of extreme UV.

Image: SISTINE’s third mission, to be launched in 2022, will target Alpha
Centauri A and B. Here we see the system in optical (main) and X-ray (inset)
light. Only the two largest stars, Alpha Cen A and B, are visible. These two
stars will be the targets of SISTINE’s third flight. Credit: Zdenek
Bardon/NASA/CXC/Univ. of Colorado/T. Ayres et al.

{ 37 comments }

WHITE DWARF CLUES TO UNUSUAL PLANETARY COMPOSITION

by Paul Gilster on November 3, 2021

The surge of interest in white dwarfs continues. We’ve known for some time that
these remnants of stars like the Sun, having been through the red giant phase
and finally collapsing into a core about the size of the Earth, can reveal a
great deal about objects that have fallen into them. That would be rocky
material from planetary objects that once orbited the star, just as the planets
of our Solar System orbit the Sun in our halcyon, pre-red-giant era.

The study of atmospheric pollution in white dwarfs rests on the fact that white
dwarfs that have cooled below 25,000 K have atmospheres of pure hydrogen or
helium. Heavier elements sink rapidly to the stellar core at these temperatures,
so the only source of elements higher than helium — metals in astronomy parlance
— is through accretion of orbiting materials that cross the Roche limit and fall
into the atmosphere.

These contaminants of stellar atmospheres are now the subject of a new
investigation led by astronomer Siyi Xu (NSF NOIRLab), partnering with Keith
Putirka (California State University, Fresno). Putirka is a geologist, and thus
a good fit for this study. Working with Xu, an astronomer, he examined 23 white
dwarfs whose atmospheres are found to be polluted by such materials. The duo
took advantage of existing measurements of calcium, silicon, magnesium and iron
from the Keck Observatory’s HIRES instrument (High-Resolution Echelle
Spectrometer) in Hawai‘i, along with data from the Hubble Space Telescope, whose
Cosmic Origins Spectrograph came into play.

Their focus is on the abundance of elements that make up the major part of rock
on an Earth-like planet, especially silicon, which would imply the composition
of rocks that would have existed on white dwarf planets before their
disintegration and accretion. The variety of rock types that emerge is wider
than found in the rocky planets of our inner Solar System. Some of them are
unusual enough that the authors create new terms to describe them. Thus “quartz
pyroxenites” and “periclase dunites.” None have analogs in our own system.

The finding has implications for planetary development, as Putirka explains:

> “Some of the rock types that we see from the white dwarf data would dissolve
> more water than rocks on Earth and might impact how oceans are developed. Some
> rock types might melt at much lower temperatures and produce thicker crust
> than Earth rocks, and some rock types might be weaker, which might facilitate
> the development of plate tectonics.”

The paper goes into greater detail:

> …while PWDs [polluted white dwarfs] might record single planets that have been
> destroyed and assimilated piecemeal, the pollution sources might also
> represent former asteroid belts, in which case the individual objects of these
> belts would necessarily be more mineralogically extreme. If current petrologic
> models may be extrapolated, though, PWDs with quartz-rich mantles…might create
> thicker crusts, while the periclase-saturated mantles could plausibly yield,
> on a wet planet like Earth, crusts made of serpentinite, which may greatly
> affect the kinds of life that might evolve on the resulting soils. These
> mineralogical contrasts should also control plate tectonics, although the
> requisite experiments on rock strength have yet to be carried out.

Image: Rocky debris, the pieces of a former rocky planet that has broken up,
spiral inward toward a white dwarf in this illustration. Studying the
atmospheres of white dwarfs that have been polluted by such debris, a NOIRLab
astronomer and a geologist have identified exotic rock types that do not exist
in our Solar System. The results suggest that nearby rocky exoplanets must be
even stranger and more diverse than previously thought. Credit:
NOIRLab/NSF/AURA/J. da Silva.

High levels of magnesium and low levels of silicon are found in the sample white
dwarfs, suggesting to the authors that the source debris came from a planetary
interior, the mantle rather than the crust. That contradicts some earlier papers
reporting signs of crustal rocks as the original polluters, but Xu and Patirka
believe that such rock occurs as no more than a small fraction of core and
mantle components.

Adds Putirka:

> “We believe that if crustal rock exists, we are unable to see it, probably
> because it occurs in too small a fraction compared to the mass of other
> planetary components, like the core and mantle, to be measured.”

The paper is Putirka & Xu, “Polluted white dwarfs reveal exotic mantle rock
types on exoplanets in our solar neighborhood,” Nature Communications 12, 6168
(2 November 2021). Full text.

{ 7 comments }

GOING AFTER SAGITTARIUS A*

by Paul Gilster on November 2, 2021

Only time will tell whether humanity has a future beyond the Solar System, but
if we do have prospects among the stars — and I fervently hope that we do — it’s
interesting to speculate on what future historians will consider the beginning
of the interstellar era. Teasing out origins is tricky. You could label the
first crossing of the heliopause by a functioning probe (Voyager 1) as a
beginning, but neither the Voyagers nor the Pioneers (nor, for that matter, New
Horizons) were built as interstellar missions.

I’m going to play the ‘future history’ game by offering my own candidate. I
think the image of the black hole in the galaxy M87 marks the beginning of an
era, one in which our culture begins to look more and more at the universe
beyond the Solar System. I say that not because of what we found at M87,
remarkable as it was, but because of the instrument used. The creation of a
telescope that, through interferometry, can create an aperture the size of our
planet speaks volumes about what a small species can accomplish. An entire
planet is looking into the cosmos.

So will some future historian look back on the M87 detection as the beginning of
the ‘interstellar era’? No one can know, but from the standpoint of symbolism —
and that’s what this defining of eras is all about — the creation of a telescope
like this is a civilizational accomplishment. I think its cultural significance
will only grow with time.

Image: Composite image showing how the M87 system looked, across the entire
electromagnetic spectrum, during the Event Horizon Telescope’s April 2017
campaign to take the iconic first image of a black hole. Requiring 19 different
facilities on the Earth and in space, this image reveals the enormous scales
spanned by the black hole and its forward-pointing jet, launched just outside
the event horizon and spanning the entire galaxy. Credit: the EHT
Multi-Wavelength Science Working Group; the EHT Collaboration; ALMA
(ESO/NAOJ/NRAO); the EVN; the EAVN Collaboration; VLBA (NRAO); the GMVA; the
Hubble Space Telescope, the Neil Gehrels Swift Observatory; the Chandra X-ray
Observatory; the Nuclear Spectroscopic Telescope Array; the Fermi-LAT
Collaboration; the H.E.S.S. collaboration; the MAGIC collaboration; the VERITAS
collaboration; NASA and ESA. Composition by J.C. Algaba.

INTO THE MILKY WAY’S HEART

The Event Horizon Telescope (EHT) is not a single physical installation but a
collection of telescopes around the world that use Very Long Baseline
Interferometry to produce a virtual observatory with, as mentioned above, an
aperture the size of our planet. Heino Falcke’s book Light in the Darkness
(HarperOne, 2021) tells this story from the inside, and it’s as exhilarating an
account of scientific research as any I’ve read.

M87 seemed in some ways an ideal target, with a black hole thought to mass well
over 6 billion times more than the Sun. In terms of sheer size, M87 dwarfed
estimates of the Milky Way’s supermassive black hole (Sgr A*), which weighs in
at 4.3 million solar masses, but it’s also 2,000 times farther away. Even so, it
was the better target, for M87 was well off the galactic plane, whereas
astronomers hoping to study the Milky Way’s black hole have to contend with
shrouds of gas and dust and the fact that, while average quasars consume one sun
per year, Sgr A* pulls in 106 times less.

But the investigation of Sgr A* continues as new technologies come into play,
with the James Webb Space Telescope now awaiting launch in December and already
on the scene in French Guiana. Early in JWST’s observing regime, Sgr A* is to be
probed at infrared wavelengths, adding the new space-based observatory to the
existing Event Horizon Telescope. Farhad Yusef-Zadeh, principal investigator on
the Webb Sgr A* program, points out that JWST will allow data capture at two
different wavelengths simultaneously and continuously, further enhancing the
EHT’s powers.

Among other reasons, a compelling driver for looking hard at Sgr A* is the fact
that it produces flares in the dust and gas surrounding it. Yusef-Zadeh
(Northwestern University) notes that the Milky Way’s supermassive black hole is
the only one yet observed with this kind of flare activity, which makes it more
difficult to image the black hole but also adds considerably to the scientific
interest of the investigation. The flares are thought to be the result of
particles accelerating around the object, but details of the mechanism of light
emission here are not well understood.

Image: An enormous swirling vortex of hot gas glows with infrared light, marking
the approximate location of the supermassive black hole at the heart of our
Milky Way galaxy. This multiwavelength composite image includes near-infrared
light captured by NASA’s Hubble Space Telescope, and was the sharpest infrared
image ever made of the galactic center region when it was released in 2009.
While the black hole itself does not emit light and so cannot be detected by a
telescope, the EHT team is working to capture it by getting a clear image of the
hot glowing gas and dust directly surrounding it. Credit: NASA, ESA, SSC, CXC,
STS.

Thus we combine radio data from the Event Horizon Telescope with JWST’s infrared
data. How different wavelengths can tease out more information is evident in the
image above. Here we have a composite showing Hubble near-infrared observations
in yellow, and deeper infrared observations from the Spitzer Space Telescope in
red, while light detected by the Chandra X-Ray Observatory appears in blue and
violet. Flare detection and better imagery of the region as enabled by adding
JWST to the EHT mix, which will include X-ray and other observatories, should
make for the most detailed look at Sgr A* that has ever been attempted.

What light we detect associated with a black hole is from the accreting material
surrounding it, with the event horizon being its inner edge — this is what we
saw in the famous M87 image. The early JWST observations, expected in its first
year of operation, are to be supplemented by further work to build up our
knowledge of the flare activity and enhance our understanding of how Sgr A*
differs from other supermassive black holes.

Image: Heated gas swirls around the region of the Milky Way galaxy’s
supermassive black hole, illuminated in near-infrared light captured by NASA’s
Hubble Space Telescope. Released in 2009 to celebrate the International Year of
Astronomy, this was the sharpest infrared image ever made of the galactic center
region. NASA’s upcoming James Webb Space Telescope, scheduled to launch in
December 2021, will continue this research, pairing Hubble-strength resolution
with even more infrared-detecting capability. Of particular interest for
astronomers will be Webb’s observations of flares in the area, which have not
been observed around any other supermassive black hole and the cause of which is
unknown. The flares have complicated the Event Horizon Telescope (EHT)
collaboration’s quest to capture an image of the area immediately surrounding
the black hole, and Webb’s infrared data is expected to help greatly in
producing a clean image. Credit: NASA, ESA, STScI, Q. Daniel Wang (UMass).

Whether we’re entering an interstellar era or not, we’re going to be learning a
lot more about the heart of the Milky Way, assuming we can get JWST aloft. How
many hopes and plans ride on that Ariane 5!

{ 13 comments }

TALKING TO THE LION

by Paul Gilster on October 29, 2021

Extraterrestrial civilizations, if they exist, would pose a unique challenge in
comprehension. With nothing in common other than what we know of physics and
mathematics, we might conceivably exchange information. But could we communicate
our cultural values and principles to them, or hope to understand theirs? It was
Ludwig Wittgenstein who said “If a lion could speak, we couldn’t understand
him.” True?

One perspective on this is to look not into space but into time. Traditional
SETI is a search through space and only indirectly, through speed of light
factors, a search through time. But new forms of SETI that look for
technosignatures — and this includes searching our own Solar System for signs of
technology like an ancient probe, as Jim Benford has championed — open up the
chronological perspective in a grand way.

Now we are looking for conceivably ancient signs of a civilization that may have
perished long before our Sun first shone. A Dyson shell, gathering most of the
light from its star, could be an artifact of a civilization that died billions
of years ago.

Image: Philosopher Ludwig Wittgenstein (1889-1951), whose Tractatus
Logico-Philosophicus was written during military duty in the First World War. It
has been confounding readers like me ever since.

Absent aliens to study, ponder ourselves as we look into our own past. I’ve
spent most of my life enchanted with the study of the medieval and ancient
world, where works of art, history and philosophy still speak to our common
humanity today. But how long will we connect with that past if, as some predict,
we will within a century or two pursue genetic modifications to our physiology
and biological interfaces with computer intelligence? It’s an open question
because these trends are accelerating.

What, in short, will humans in a few hundred years have in common with us? The
same question will surface if we go off-planet in large numbers. Something like
an O’Neill cylinder housing a few thousand people, for example, would create a
civilization of its own, and if we ever launch ‘worldships’ toward other stars,
it will be reasonable to consider that their populations will dance to an
evolutionary tune of their own.

The crew that boards a generation ship may be human as we know the term, but
will it still be five thousand years later, upon reaching another stellar
system? Will an interstellar colony create a new branch of humanity each time we
move outward?

Along with this speculation comes the inevitable issue of artificial
intelligence, because it could be that biological evolution has only so many
cards to play. I’ve often commented on the need to go beyond the conventional
mindset of missions as being limited to the lifetime of their builders. The
current work called Interstellar Probe at Johns Hopkins, in the capable hands of
Voyager veteran Ralph McNutt, posits data return continuing for a century or
more after launch. So we’re nudging in the direction of multi-generational
ventures as a part of the great enterprise of exploration.

But what do interstellar distances mean to an artilect, a technological creation
that operates by artificial intelligence that eclipses our own capabilities? For
one thing, these entities would be immune to travel fatigue because they are all
but immortal. These days we ponder the relative advantages of crewed vs. robotic
missions to places like Mars or Titan. Going interstellar, unless we come up
with breakthrough propulsion technologies, favors computerized intelligence and
non-biological crews. Martin Rees has pointed out that the growth of machine
intelligence should happen much faster away from Earth as systems continually
refine and upgrade themselves.

It was a Rees essay that reminded me of the Wittgenstein quote I used above. And
it leads me back to SETI. If technological civilizations other than our own
exist, it’s reasonable to assume they would follow the same path. Discussing the
Drake Equation in his recent article Why extraterrestrial intelligence is more
likely to be artificial than biological, Lord Rees points out there may be few
biological beings to talk to:

> Perhaps a starting point would be to enhance ourselves with genetic
> modification in combination with technology—creating cyborgs with partly
> organic and partly inorganic parts. This could be a transition to fully
> artificial intelligences.
>
> AI may even be able to evolve, creating better and better versions of itself
> on a faster-than-Darwinian timescale for billions of years. Organic
> human-level intelligence would then be just a brief interlude in our “human
> history” before the machines take over. So if alien intelligence had evolved
> similarly, we’d be most unlikely to “catch” it in the brief sliver of time
> when it was still embodied in biological form. If we were to detect
> extraterrestrial life, it would be far more likely to be electronic than flesh
> and blood—and it may not even reside on planets.

Image: Credit: Breakthrough Listen / Danielle Futselaar.

I don’t think we’ve really absorbed this thought, even though it seems to be
staring us in the face. The Drake Equation’s factor regarding the lifetime of a
civilization is usually interpreted in terms of cultures directed by biological
beings. An inorganic, machine-based civilization that was spawned by biological
forebears could refine the factors that limit human civilization out of
existence. It could last for billions of years.

It’s an interesting question indeed how we biological beings would communicate
with a civilization that has perhaps existed since the days when the Solar
System was nothing more than a molecular cloud. We often use human logic to talk
about what an extraterrestrial civilization would want, what its motives would
be, and tell ourselves the fable that ‘they’ would certainly act rationally as
we understand rationality.

But we have no idea whatsoever how a machine intelligence honed over thousands
of millenia would perceive reality. As Rees points out, “we can’t assess whether
the current radio silence that Seti are experiencing signifies the absence of
advanced alien civilisations, or simply their preference.” Assuming they are
there in the first place.

And that’s still a huge’ if.’ For along with our other unknowns, we have no
knowledge whatsoever about abiogenesis on other worlds. To get to machine
intelligence, you need biological intelligence to evolve to the point where it
can build the machines. And if life is widespread — I suspect that it is — that
says nothing about whether or not it is likely to result in a technological
civilization. We may be dealing with a universe teeming with lichen and pond
scum, perhaps enlivened with the occasional tree.

A SETI reception would be an astonishing development, and I believe that if we
ever receive a signal, likely as a byproduct of some extraterrestrial activity,
we will be unlikely to decode it or even begin to understand its meaning and
motivation. Certainly that seems true if Rees is right and the likely sources
are machines. A SETI ‘hit’ is likely to remain mysterious, enigmatic, and
unresolved. But let’s not stop looking.

{ 56 comments }
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Charter

In Centauri Dreams, Paul Gilster looks at peer-reviewed research on deep space
exploration, with an eye toward interstellar possibilities. For the last twelve
years, this site coordinated its efforts with the Tau Zero Foundation. It now
serves as an independent forum for deep space news and ideas. In the logo above,
the leftmost star is Alpha Centauri, a triple system closer than any other star,
and a primary target for early interstellar probes. To its right is Beta
Centauri (not a part of the Alpha Centauri system), with Beta, Gamma, Delta and
Epsilon Crucis, stars in the Southern Cross, visible at the far right (image:
Marco Lorenzi).

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