Space is no longer a vacuum; Earth orbits are full of man-made stuff. Along with working satellites, there are old satellites in orbit that no longer serve a purpose, litter left in space like a park after a three-day weekend, and detritus from collisions of orbital hardware.

There are more than 20,000 pieces of debris larger than a softball orbiting Earth traveling at speeds of up to 17,500 mph. There are 500,000 pieces of debris the size of a marble or larger, and millions of pieces that are too small to track.

A 250 mg object (think ibuprofen tablet) traveling at 17,500 mph has the same energy as 357-magnum bullet. A 5-gram marble has 400 times that much energy.

The probability of two large objects colliding is remote, but when they do, the outcome can be serious. In February 2009 an operational satellite, U.S. Iridium 33, collided with a defunct Russian Kosmos-2251 at 26,170 mph, spraying debris.

In 1978 NASA scientist Donald J. Kessler proposed the idea, now known as “the Kessler Syndrome,” that two colliding objects in space generate more debris that then collides with other objects, creating even more shrapnel and litter, creating a self-sustaining cascading collision of space debris in low Earth orbit.

Orbital dynamics being what they are, each collision creates debris that spreads out as it orbits, gradually encircling Earth until the debris field distributes itself evenly in a shell surrounding the planet.

This would be bad enough if there were no new satellites launched into orbit, but since 1960 there have been more than 100 objects launched into orbit each year by 70 different nations. Unlike here on Earth’s surface, there is no chemical weathering in space, no seafloor to settle onto, no fauna to help break it down and no soil for it to “disappear” into.

With each new satellite launch, the potential magnifies as each piece of junk eventually adds to the threat. According to the United Nations Office for Outer Space Affairs, there are just fewer than 5,000 satellites orbiting Earth, which is nearly 60 percent of all satellites ever launched.

Not counting major events, such as the 2007 destruction of a Chinese satellite to test the country’s missile system, or the Iridium-Kosmos collision, space junk consists of many different kinds of hardware.

There are derelict spacecraft and upper stages of launch vehicles, carriers for multiple payloads, debris intentionally released during spacecraft separation from launch vehicles or during mission operations, debris created as a result of spacecraft or upper-stage explosions or collisions, solid rocket motor effluents, and tiny flecks of paint released by thermal stress or small particle impacts.

Debris left in orbits below 375 miles normally falls back to Earth within several years. At altitudes of 500 miles, it can take decades. Above 625 miles, orbital debris will normally stay in orbit for a century or more.

Without some kind of mediation, space experts at NASA, the European Space Agency and others fear that debris could contaminate low Earth orbit so as to make future launches too risky, too expensive or even impossible.

That would be a shame, or worse, because our day-to-day activities from GPS to cellphone use depend on those satellites.