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 1. References
 2. Science & Astronomy


WHAT IS SPACE?

By Elizabeth Howell
Contributions from
Chelsea Gohd
published February 17, 2022

Space, the final frontier, what is it?

 * 
 * 
 * 
 * 
 * 
 * 
 * 


What is space? In this image you can see the planetary nebula NGC 6891 glows in
this Hubble Space Telescope image. (Image credit: NASA, ESA, A. Hajian
(University of Waterloo), H. Bond (Pennsylvania State University), and B. Balick
(University of Washington); Processing: Gladys Kober (NASA/Catholic University
of America))
Jump to:
 * Invisible radiation
 * Dark matter and energy
 * Black holes
 * Stars, planets, asteroids and comets
 * Galaxies and quasars

We often refer to our expanding universe with one simple word: space. But where
does space begin and, more importantly, what is it?



Space is an almost perfect vacuum, nearly void of matter and with extremely low
pressure. In space, sound doesn't carry because there aren't molecules close
enough together to transmit sound between them. Not quite empty, bits of gas,
dust and other matter floats around "emptier" areas of the universe, while more
crowded regions can host planets, stars and galaxies.



From our Earth-bound perspective, outer space is most often thought to begin
about 62 miles (100 kilometers) above sea level at what is known as the Kármán
line. This is an imaginary boundary at an altitude where there is no appreciable
air to breathe or scatter light. Passing this altitude, blue starts to give way
to black because oxygen molecules are not in enough abundance to make the sky
blue.




Related: Where DOES Space Begin? Virgin Galactic Flies Right into the Debate

Click here for more Space.com videos...


No one knows exactly how big space is. It's difficult to determine because of
what we can see in our detectors. We measure long distances in space in
"light-years," representing the distance it takes for light to travel in a year
(roughly 5.8 trillion miles (9.3 trillion kilometers)). 



From the light that is visible in our telescopes, we have charted galaxies
reaching almost as far back as the Big Bang, which is thought to have started
our universe about 13.8 billion years ago. This means we can "see" into space at
a distance of almost 13.8 billion light-years. But the universe continues to
expand, making "measuring space," even more challenging. 

Additionally, astronomers are not totally sure if our universe is the only one
that exists. This means that space could be a whole lot bigger than we even
think.


SPACE RADIATION INVISIBLE TO HUMAN EYES

The majority of space is relatively empty, with just stray bits of dust and gas
floating around. This means that when humans send a probe to a distant planet or
asteroid, the craft will not encounter "drag" in the same way that an airplane
does as it sails through space.

In fact, the vacuum environment in space and on the moon, is one reason why the
lunar lander of the Apollo program was designed to have an almost spider-like
appearance, as it was described by the Apollo 9 crew. Because the spacecraft was
designed to work in a zone with no atmosphere, it didn't need to have smooth
edges or an aerodynamic shape.

In addition to the bits of debris that speckle the "emptier" regions of space,
research has shown that these areas are also home to different forms of
radiation. In our own solar system, the solar wind — charged particles that
stream from the sun — emanate throughout the solar system and occasionally cause
auroras near Earth's poles. Cosmic rays also fly through our neighborhood,
stemming from supernovas outside of the solar system.

In fact, the universe as a whole is inundated with what is known as the cosmic
microwave background (CMB), which is essentially the leftover radiation from the
explosion mostly commonly known as the Big Bang. The CMB is the oldest radiation
that our instruments can detect. 

Infographic: Cosmic Microwave Background Explained


DARK MATTER AND ENERGY



There remain two giant mysteries about space: dark matter and dark energy. 

While scientists have provided extensive evidence for the existence of dark
matter and dark energy, they are each still poorly understood as, so far,
scientists cannot directly observe them and can only observe their effects. 

Roughly 80% of all of the mass in the universe is made up of what scientists
have dubbed "dark matter," but it's not known what it actually is or if it is
even matter by our current definition. However, while dark matter doesn't emit
light or energy and cannot, therefore, be directly observed, scientists have
found overwhelming evidence that it makes up the vast majority of the matter in
the cosmos.

Dark energy might have a similar name to dark matter, but it's a whole different
component entirely. 

Thought to make up nearly 75% of the universe, dark energy is a mysterious and
unknown force or entity that scientists think is responsible for the universe's
ongoing expansion.


BLACK HOLES



Matter swirling into the supermassive black hole at the centre of M87. (Image
credit: Event Horizon Telescope)

Smaller black holes can form from the gravitational collapse of a gigantic star,
which forms a singularity from which nothing can escape — not even light, hence
the name of the object. No one is quite sure what lies within a black hole, or
what would happen to a person or object who fell into it – but research is
ongoing.

An example is gravitational waves, or ripples in space-time that come from
interactions between black holes. This was first predicted by Albert Einstein at
the turn of the last century, when he showed that time and space are linked;
time speeds up or slows down when space is distorted.

As of mid-2017, the Laser Interferometer Gravitational-Wave Observatory (LIGO)
Scientific Collaboration has announced three black-hole interactions and mergers
detected through gravitational waves, in just two years.

The team found these three events in about two years, indicating that when LIGO
is implemented at full sensitivity, the observatory may be able to find these
sorts of events frequently, scientists said in May 2017. Should a bunch of these
black hole events be detected, it could help scientists learn how black holes of
a certain size (several tens of sun masses) are born, and later merge into new
black holes.


STARS, PLANETS, ASTEROIDS AND COMETS



The exoplanet wasp-103b and its star. (Image credit: ESA)

Stars (like our own sun) are immense balls of gas that produce their own
radiation. They can range from red supergiants to cooling white dwarfs that are
the leftovers of supernovas, or star explosions that occur when a big one runs
out of gas to burn. These explosions spread elements throughout the universe and
are the reason that elements such as iron exist. Star explosions can also give
rise to incredibly dense objects called neutron stars. If these neutron stars
send out pulses of radiation, they are called pulsar stars.

Planets are objects whose definition came under scrutiny in 2006, when
astronomers were debating whether Pluto could be considered a planet or not. At
the time, the International Astronomical Union (the governing body on Earth for
these decisions) ruled that a planet is a celestial body that orbits the sun, is
massive enough to have a nearly round shape, and has cleared its orbit of
debris. Under this designation, Pluto and similar small objects are considered
"dwarf planets," although not everyone agrees with the designation. After the
New Horizons spacecraft flew by Pluto in 2015, principal investigator Alan Stern
and others again opened up the debate, saying the diversity of terrain on Pluto
makes it more like a planet.



The definition of extrasolar planets, or planets outside the solar system, is
still not firmed up by the IAU, but essentially astronomers understand it to
mean objects that behave like planets in our neighborhood. The first such planet
was found in 1992 (in the constellation Pegasus) and since that time, thousands
of alien planets have been confirmed — with many more suspected. In solar
systems that have planets under formation, these objects are often called
"protoplanets" because they aren't quite the maturity of those planets we have
in our own solar system.

Asteroids are rocks that are not quite big enough to be dwarf planets. We've
even found asteroids with rings around them, such as 10199 Charilko. Their small
size often leads to the conclusion that they were remnants from when the solar
system was formed. Most asteroids are concentrated in a belt between the planets
Mars and Jupiter, but there are also many asteroids that follow behind or ahead
of planets, or can even cross in a planet's path. NASA and several other
entities have asteroid-searching programs in place to scan for potentially
dangerous objects in the sky and monitor their orbits closely. 

In our solar system, comets (sometimes called dirty snowballs) are objects
believed to originate from a vast collection of icy bodies called the Oort
Cloud. As a comet approaches the sun, the heat of our star causes ices to melt
and stream away from the comet. The ancients often associated comets with
destruction or some sort of immense change on Earth, but the discovery of
Halley's Comet and related "periodic" or returning comets showed that they were
ordinary solar system phenomena.


GALAXIES AND QUASARS



An artist's rendering of the double quasar, located in two merging galaxies
about 10 billion light-years away  (Image credit: NASA, ESA, and J. Olmsted
(STScI))

Among the biggest cosmic structures we can see are galaxies, which essentially
are vast collections of stars. Our own galaxy is called the Milky Way, and is
considered a "barred spiral" shape. There are several types of galaxies, ranging
from spiral to elliptical to irregular, and they can change as they come close
to other objects or as stars within them age.

Often galaxies have supermassive black holes embedded in the center of their
galaxies, which are only visible through the radiation that each black hole
emanates as well as through its gravitational interactions with other objects.
If the black hole is particularly active, with a lot of material falling into
it, it produces immense amounts of radiation. This kind of a galactic object is
called a quasar (just one of several types of similar objects.)

Large groups of galaxies can form in clusters that are groups as large as
hundreds or thousands of galaxies bound together gravitationally. Scientists
consider these the largest structures in the universe.

This page was updated in Jan. 2022 by Space.com senior writer Chelsea Gohd.

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Elizabeth Howell
Social Links Navigation
Staff Writer, Spaceflight

Elizabeth Howell (she/her), Ph.D., is a staff writer in the spaceflight channel
since 2022 covering diversity, education and gaming as well. She was
contributing writer for Space.com for 10 years before joining full-time.
Elizabeth's reporting includes multiple exclusives with the White House and
Office of the Vice-President of the United States, an exclusive conversation
with aspiring space tourist (and NSYNC bassist) Lance Bass, speaking several
times with the International Space Station, witnessing five human spaceflight
launches on two continents, flying parabolic, working inside a spacesuit, and
participating in a simulated Mars mission. Her latest book, "Why Am I Taller?",
is co-written with astronaut Dave Williams. Elizabeth holds a Ph.D. and M.Sc. in
Space Studies from the University of North Dakota, a Bachelor of Journalism from
Canada's Carleton University and a Bachelor of History from Canada's Athabasca
University. Elizabeth is also a post-secondary instructor in communications and
science at several institutions since 2015; her experience includes developing
and teaching an astronomy course at Canada's Algonquin College (with Indigenous
content as well) to more than 1,000 students since 2020. Elizabeth first got
interested in space after watching the movie Apollo 13 in 1996, and still wants
to be an astronaut someday. Mastodon: https://qoto.org/@howellspace

With contributions from
 * Chelsea GohdSenior Writer

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