By Clara Moskowitz
SPACE.com Senior Writer
posted: 04 May 2010
The Galaxy 15 commercial satellite that recently lost contact with the ground has joined the ranks of a boatload of other debris adrift in space. It's now termed a "zombiesat" by engineers who have a better sense of humor than you might have imagined. So what's next for this 4,171-pound (1,892-kg) zombiesat?
This defunct satellite will probably drift to one of two "gravity wells" that catch most out-of-control spacecraft, experts say. Galaxy 15 could threaten nearby satellites because its communications package is stuck on and it may start interfering with its neighbors by siphoning off their signals. It's the first time such an event has ever occurred, and it sent Orbital Sciences, the satellite's builder, on a dash to figure out how to stop the satellite-run-amok.
Galaxy 15, like many communications satellites, was circling Earth about 22,369 miles (36,000 km) high in what's called geosynchronous orbit, meaning that it orbited at the same speed the Earth rotates, so that it sat perched above the same part of Earth all the time. "There are two points in geosynchronous orbit called geopotential wells," explained Nicholas Johnson, chief scientist for Orbital Debris at NASA's Johnson Space Center in Houston. "These are perturbations in Earth's gravity field. Typically when satellites lose control they will drift toward the nearest geopotential well and just oscillate around it." The two spots, also called libration points, are located at longitudes of 105 degrees west and 75 degrees east. There are already between 150 and 200 objects oscillating around these points, Johnson said.
Still a large place
In that sense, the new zombiesat doesn't significantly increase the space debris problem or pose a serious risk of colliding with an operational satellite.
"Space is still a very large place," Johnson told SPACE.com. "There are a lot of objects that are drifting back and forth. Galaxy 15 really just kind of joins a relatively large number of objects – it's not a significant new hazard from a global standpoint. But if your satellite happens to be near where Galaxy 15 is drifting then it's of more concern." Eventually, everything in low-Earth orbit will eventually fall back down toward Earth because of atmospheric drag. The small amount of atmospheric particles in space create friction with spacecraft, causing their orbits to decay. The time it takes for an object's orbit to decay depends on its altitude.
For example, the International Space Station orbits at about 250 miles (400 kilometers) above Earth, while the Hubble Space Telescope circles much higher, at 353 miles (569 km). "When things fall off the International Space Station, they typically fall back within a couple months, but where Hubble is, it typically takes several years to fall back to Earth," Johnson said. "At 800 km you're talking many decades or even hundreds of years."
To prevent the buildup of dead spacecraft in heavily trafficked areas of geosynchronous orbit, guidelines recommend that when a satellite reaches the end of its life it is boosted to a higher orbit out of the way. This "graveyard orbit" is about 186 miles (300 km) above where most satellites orbit. "The whole idea is to get to an altitude so they don't drift back into the operational region for a very, very long time – over 100 years," Johnson said.
It's actually easier to boost a spacecraft up just this much higher than to maneuver a craft down to where it would immediately fall back to Earth and burn up in the atmosphere, he said.
Getting rid of space junk
To actually go and collect defunct spacecraft to remove the collision risk altogether is currently beyond our ability. "Unfortunately we haven't found a concept which appears to be both technically feasible and affordable," Johnson said. The best way to remove spent rocket stages and other large objects from orbit is to simply send up another spacecraft to rendezvous and dock with it and drag it back down to earth. This method would be extremely expensive and time-consuming, and isn't viable for the vast number of objects already in space. Some more exotic measures involving tethers and other props have been proposed, Johnson said, but aren't yet feasible.
For getting rid of very small pieces of space junk, there are two favorite ideas, he said. One involves shooting lasers at the objects to push them into lower-altitude orbits so they fall back down to Earth more quickly. "That has technical, economic, as well as policy issues," Johnson said. Another concept is to fly up a structure with a large area but low mass so that when particles strike the surface they will penetrate and lose some of their orbital energy, causing them to fall back to Earth more quickly. This option would also need many technical issues ironed out. "If it was easy we'd already be doing it," Johnson said of tackling the debris problem. "But it's prudent to be working the issue now before it becomes a serious impediment to space operations."