Ask Ethan: How empty are the depths of space?

Key Points:

• Density variations on Earth range widely, with a cubic meter of gold weighing about 19 metric tonnes, water about 1 tonne, and air at sea level about 1.2 kilograms, while laboratory vacuums can reach densities less than one-trillionth that of air.
• Space environments exhibit vastly lower densities than Earth’s, with the Moon’s extremely thin atmosphere containing about 80 billion particles per cubic meter, interstellar gas clouds ranging from millions to trillions of particles per cubic meter, and protoplanetary disks reaching densities comparable to Earth's best vacuums.
• Interplanetary space within our Solar System contains 5 to 40 million particles per cubic meter, while true interstellar space beyond the Sun’s influence has densities ranging from a few hundred to a million particles per cubic meter, mostly ionized hydrogen gas.
• The lowest densities are found in intergalactic space and cosmic voids, with average matter densities around 0.25 hydrogen atoms per cubic meter and even the emptiest voids retaining about 0.1 protons per cubic meter, far sparser than any region within galaxies.
• Low Earth orbit, often considered "outer space," is still relatively dense compared to most cosmic environments, containing hundreds of trillions of particles per cubic meter plus significant amounts of micrometeoroids, space debris, and thousands of satellites, making it far from a perfect vacuum.