Asked in AstronomyPhysics
Why is space a vacuum?
February 15, 2011 7:29AM
Space is a vacuum because there is extraordinarily little matter there.
Gravity is the large scale organizer of matter in the universe. It is gravity that causes material objects to attract each other, and gravity "gathers" matter together to make stars and even galaxies themselves. Because gravimetric centers -- black holes, stars, planets, comets or asteroids -- attract other matter, the matter tends to "clump" around other matter. When this matter is gaseous, and it "clumps" around bodies large enough to hold it, you have an atmosphere.
The wide volume of space between planets, or between star systems, or even between the galaxies is therefore fairly free of matter. This is as little as a handful of hydrogen atoms per cubic meter of space, compared to 1024 atoms, a trillion trillion, in a cubic meter of the air we breathe. However, space is readily and regularly traversed by moving particles, from cosmic rays (ionized atomic nuclei) to the incredibly numerous but nearly-massless particles known as neutrinos.
(Due to the energy release from stars, and the interaction of its released particles, some molecules achieve escape velocity and are "lost to space" from planets. This creates an expanded exosphere in their orbits. However, at a distance from the planets, the mass content dwindles to nearly zero.)