1. Leave 'em out there.
2. Send them hurtling into the atmosphere of the nearest planet, where they will die a fiery, spectacular death.
The first option is a popular one. Earth's orbit is clogged with more than 500,000 piece of space junk the size of a marble or larger, according to NASA. These bits of trash travel at speeds up to 17,500 miles per hour and include abandoned launch vehicle stages, space station trash, stuff that astronauts lost (gloves, tool bags, a spatula), as well as abandoned satellites and spacecraft. Most space debris eventually falls out of orbit and burns up as it re-enters Earth's atmosphere, but plenty more is still out there. Vanguard I, a solar-powered U.S. satellite, has been hurtling around our planet since it was launched in 1958.
All this orbiting trash poses a threat to the spacecraft that are still operational. In 1999, a French satellite was damaged after being struck by debris from a rocket that had exploded 10 years earlier. In the spring, a window on the International Space Station was chipped by a fleck of paint — really, just a fleck.
“The greatest risk to space missions comes from non-trackable debris,” Nicholas Johnson, then NASA's chief scientist for orbital debris, said in 2013.
For that reason, space agencies have been collaborating to find better ways of dealing with defunct spacecraft. One option is to send them up to an orbit so high above the Earth that they are out of the way of most active spacecraft. This requires that aging satellites save a bit of juice at the end of their lives for a burn that will boost them into their high-altitude final resting place. Nearly 200 miles above crowded low Earth orbit, they can spin through space for eternity.
Other spacecraft are programmed to slow down at the end of their mission. Within 25 years, they will start falling toward Earth, burning up from the heat generated by friction as they hit the atmosphere.
If engineers can't be certain that the spacecraft will burn up before they reach the ground, they are required to program the machines to fall into what's known as the “spacecraft cemetery.” This open expanse of the South Pacific Ocean is uninhabited, so old cargo craft and resupply ships can be dropped there without fear of harming anyone. The largest resident of this watery graveyard is the 143-ton Russian Mir space station, which deorbited in spectacular fashion in 2001.
People are no better at picking up after themselves when we visit other solar system bodies. More than 70 vehicles are scattered across the moon, according to the Atlantic. Mars is the final resting place of over a dozen robots, from the 40-year-old Viking 2 lander to the smashed remains of Schiaparelli, the ill-fated European and Russian lander that crashed on impact with Mars's cold, hard surface in the fall.
A few lucky spacecraft get to go out in a blaze of glory, by plunging heroically toward the nearest solar system body in pursuit of one last bit of science. This kind of Hollywood ending awaits Cassini, the NASA spacecraft that has been orbiting Saturn since 2004. Now in the final phase of a historic mission that included the first-ever outer solar system landing (Cassini's companion spacecraft, Huygens, landed on Saturn's moon Titan in 2005), Cassini is dancing through orbits that take it through Saturn's many rings. On April 22, it will begin the “Grand Finale,” dives between the planet and the inner edge of the rings. It'll all come to an end on Sept. 15, when Cassini will take one last dive toward Saturn and burn up in a fireball as it plunges into the gas giant's depths.
The “Grand Finale” will give scientists a view of Saturn as they've never seen it before, allowing researchers to map the planet's gravity and magnetic fields, sample the icy rings, and take ultra-close photographs of the rings and clouds. But that's not the main reason for Cassini to sacrifice itself. Two of Saturn's moons, Enceladus and Titan, are thought to contain potentially habitable environments.
“In order to avoid the unlikely possibility of Cassini someday colliding with one of these moons and contaminating them with any hardy Earth microbes that might have survived on the spacecraft, NASA has chosen to safely dispose of the spacecraft in the atmosphere of Saturn,” the space agency says on its website.
You gotta admit: It's a pretty cool reason for a spacecraft to die.