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On Earth, space begins at the Kármán line (100 km above sea level). This is where Earth's atmosphere is said to stop and outer space begins. This is not a firm boundary but is a convention used by scientists and diplomats.
However, the space near Earth is quite crowded by astronomical standards. A list of spaces goes like this:
- Geospace is the region of outer space near Earth. Geospace includes the upper region of the atmosphere and the magnetosphere. The Van Allen radiation belt lies within the geospace. The space inside the magnetosphere is protected from radiation from the Sun. It has a low level of electrically charged particles.
- Interplanetary space is the space around the Sun and planets of the Solar System. It has the solar wind, a continuous stream of charged particles from the Sun. This stream creates a very thin atmosphere (the heliosphere) for billions of miles into space.
Interplanetary space has the magnetic field generated by the Sun. There are also magnetospheres generated by planets such as Jupiter, Saturn, Mercury and the Earth. These magnetic fields can trap particles from the solar wind and other sources, creating belts of magnetic particles such as the Van Allen radiation belt. Planets without magnetic fields, such as Mars, have their atmospheres gradually stripped off by the solar wind.
- Interstellar space is the physical space within a galaxy not occupied by stars or their planetary systems. It continues to the edges of the galaxy, where it fades into the intergalactic void. Most of the mass in this space is made up of single hydrogen atoms, fewer helium atoms and a few heavier atoms formed in stars. Supernovae blow some of their atoms huge diatances.
A number of molecules and tiny 0.1 μm dust particles do exist in interstellar space. About four new types of molecule are discovered each year. Large regions of higher density matter known as molecular clouds allow chemical reactions to occur. This incudes organic polyatomic species. Much of this chemistry is driven by collisions.
- Intergalactic space does have 'cosmic voids' between the large-scale structures of the universe.
Exploring space is very difficult because it contains no air and is so large that even the fastest ships can only explore a tiny part of it. It takes 3 days of traveling to reach the Moon and, depending on speed, it would take a long time to reach the closest star Proxima Centauri.
We gain most of our information about the items in space from different kinds of telescopes. Space probes also explore planets, comets and other space objects that are not too far. In 1998, NASA launched the Deep Space 1, a space probe that explored the planet Mars and the comet Borrelly.
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- "Deep Space 1". National Space Science Data Center. 2012 [last update]. http://nssdc.gsfc.nasa.gov/nmc/spacecraftDisplay.do?id=1998-061A. Retrieved 26 March 2012.