Polluting the Heavens
May 15, 2014 | by Elizabeth Wason
This is an article from the Spring 2014 issue of LSA Magazine. To read more stories like this, click here.
Even before a satellite makes itself useful in space, it leaves a trail of litter in its wake. A satellite hitches a ride into orbit aboard a launch vehicle, which jettisons its spent engines, one by one. Pieces of the dismantled rocket continue to orbit Earth, creating a floating trail of discarded bolts and tanks in outer space.
Some of those rocket engines may not yet be entirely empty of fuel. When leftover chemicals mix or pressure builds up in an old tank, the resulting burst multiplies one piece of space flotsam into hundreds or thousands of spiraling shards, creating even more orbital junk.
The satellite payload, meanwhile, separates from the launch vehicle and circles Earth in a prescribed orbit. Throughout its functioning lifetime, a satellite might help you get directions using GPS, check the weather, or watch an international sporting event on TV. After about 10 years, the satellite dies. Often, it is decommissioned and abandoned, left to circle Earth unattended as a “zombie satellite.” Zombies clog useful orbits, and their crisscrossing trajectories may intersect with other space objects at just the wrong moment.
Crashing Space Junk
Collisions happen. In 2009, an accidental, unpredicted crash between a defunct Russian satellite and a live U.S. communications satellite sent splinters of debris every which way. That one moment of impact congested the surrounding orbits with more than 2,000 trackable pieces of space debris, along with countless smaller pieces.
By far the biggest upsurge in space debris was intentional. In 2007, a Chinese anti-satellite weapon test-blasted a dead weather satellite into smithereens. A high-velocity cloud of debris formed immediately, comprising some 3,000 visible fragments and countless more, each of which careened into its own separate orbit.
Any orbiting object that does not perform a useful function — discarded rocket pieces, zombie satellites, shards from satellite collisions and dismemberments — becomes space debris. What once seemed like a vast, empty, infinite belt of outer space has turned out to be an increasingly scarce natural resource, especially in terms of elbow room for orbiting satellites.
And when the space gets scarce in outer space, things can get really bad really fast.
Running Out of Space
A cascade of satellite collisions produces a chain reaction known as the Kessler syndrome, which causes an exponential increase in the number of space objects orbiting Earth. In this scenario, the resulting population of space junk won’t stop multiplying, even if humans quit launching new satellites.
Have we passed the point of no return? We’re not to that point yet, says LSA Astronomy Professor Patrick Seitzer, but we do need to take measures to control the amount of debris in space. He notes that the highest densities of space debris occur in two orbits — low Earth orbit (LEO; where important satellites like the International Space Station and the Hubble Space Telescope hover) and geosynchronous orbit (GEO; the orbit of choice for communications and weather satellites).
Seitzer studies debris at GEO, which contains a special orbit above the equator called the geostationary orbit. Satellites in geostationary orbit circle Earth in the same amount of time that the planet rotates, in a 24-hour cycle, which makes the orbit especially useful for applications like satellite TV. Geostationary satellites float directly above a single point on Earth at all times, so a satellite dish mounted on your house can receive TV broadcasts by staying pointed at one spot in the sky.
At an altitude of nearly 36,000 km (almost 22,500 miles), GEO is the most distant orbit with a notable number of artificial satellites. Only the brightest objects at GEO are visible without the use of ultra-powerful telescopes under the darkest of skies, which is why Seitzer conducts his research in Chile. At the Cerro Tololo Inter-American Observatory, Seitzer uses U-M’s Curtis-Schmidt telescope in a NASA-supported project called MODEST (for Michigan Orbital DEbris Survey Telescope) to track space debris as small as 20 cm (about 8 inches) at GEO. He can view even smaller objects at Las Campanas Observatory, using the twin Magellan telescopes owned jointly by U-M. Seitzer studies the distribution of space objects in GEO and whether their numbers are increasing, with the aim of preventing collisions in orbit.
How to Clean Your Orbit
Tracking existing pieces of space debris is one way to keep live satellites and orbiting astronauts safe from impact; active removal of space debris is another. Special spacecraft can retrieve zombie satellites from space, deliberately burn them in Earth’s atmosphere, or kick them into unoccupied “graveyard orbits.” But when it costs $3,000 to send something the size of a soda can into orbit, space junk cleanup missions are a last resort.
Prevention may be the easiest method of reducing space debris. Engineers can design launch vehicles to minimize jettisoned debris in the first place, and engineers on the ground can prevent accidental explosions by remotely draining and venting spent fuel tanks.
In any case, one thing is clear: As humans surround Earth with a floating landfill, we will need to come up with more innovative, and more desperate, ways of clearing out that same space to keep our satellites working. If not, all the critical things that satellites make possible — communications, satellite TV, GPS navigation, weather reports, military systems — could get lost in contemporary constellations of tumbling space junk.
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Photo credit: Bettmann/Corbis.