Where Satellites Go When They Die

Where Satellites Go When They Die
According to regulations from the Federal Communications Commission, any satellite in geosynchronous orbit—i.e., at an altitude of just under 36,000 kilometers—must be moved farther away from the Earth at the end of its useful life. A complicated formula determines how high the satellite must go to reach a suitable "disposal orbit," but most end up about 300 km above where they started.

Up to a dozen geosynchronous satellites go out of service every year, and there are now several hundred derelicts in the disposal orbit. They may drift slightly from that altitude under the influence of the sun and moon, but they won't interfere with the operational satellites below.

Unless they explode, of course. Any unused energy source material on an abandoned satellite can pose a significant danger, and many derelicts have blown up over the years. An exploding satellite in a disposal orbit can spray space debris down toward the operational satellites. To prevent such explosions, defunct satellites are supposed to "passivate" by discharging batteries, releasing compressed gases, or dumping propellant.

Of course, a satellite needs a bit of propellant to reach a disposal orbit, a fact that limits the life span of some satellites. Even if all of the electronics and moving parts on the DirecTV satellite are still functioning after 12 years, it will still need to be decommissioned before it runs out of fuel.

For satellites that operate at lower altitudes (including those used for surveillance and GPS), a trip up to disposal orbits may not be necessary. Orbits tend to decay over time, at a rate determined by the initial altitude. Anything above 2,000 km will take millennia to return to Earth, while satellites further down might take centuries or decades. A satellite that starts out in a low orbit can be maneuvered downward to ensure that it falls from space in a reasonable amount of time. NASA recommends that low-altitude derelicts be brought close enough to the Earth to return within 25 years. The satellite's operator could also decide that it would be more energy efficient to push it up to a disposal orbit above 2,000 km.

When a satellite falls out of orbit, between 10 percent and 40 percent of its mass survives re-entry. Hundreds of objects from orbit do hit the surface of the Earth at random locations every year, but no one has ever been reported injured or killed by falling space debris.

How dangerous is the debris that stays in space? Space agencies around the world keep track of any objects that are more than a few inches in diameter. (There are about 13,000 of these.) Even a small bit of debris could cause significant damage to a working satellite, although collisions are rare and only a handful have ever been observed. The most recent occurred on Jan. 17, when a part from a U.S. rocket launched in 1974 collided with a piece of a Chinese spacecraft from 2000. Three new chunks of debris were created by the crash.

Graveyard Orbit 300 km above GEO

In order to eliminate collision risk, GEO satellites should be moved out of the geostationary ring at the end of their mission. It is recommended that their orbit should be raised by about 300 km, which is considered a safe distance to avoid future interference with active GEO spacecraft.

The change in velocity that is required to raise the orbit altitude by 300 km is 11 meter/sec, and the required propellant corresponds to that necessary for three months of station keeping. This means spacecraft operators have to stop operations three months before the spacecraft runs out of fuel and give up considerable revenue in order to reorbit their spacecraft. However, today, this is the only possibility for preserving the unique resource of the geostationary ring.

Space Junk May Crash Earth's Communication Networks
WASHINGTON/LONDON: Junk of abandoned rockets, shattered satellites and missile shrapnel in space may cause collision between satellites, destroying
communication facilities on earth, the US defence department has warned.

According to scientists, the debris scattered in the earth's orbit is reaching a "tipping point" and pose a threat to the $250 billion space services industry.

A single collision between two satellites or large pieces of "space junk" can send thousands of pieces of debris spinning into orbit, triggering an "uncontrolled chain reaction".

Services such as global positioning systems, telephone networks, television signals and weather forecasts are at risk of crashing to a halt.

The "chain reaction" can leave some orbits so cluttered with debris that they become unusable for commercial or military satellites. Large pieces of debris threaten the lives of astronauts in space shuttles or at the International Space Station, the Pentagon said in its review.

The report, which was sent to Congress in March and not publicly released, said space is "increasingly congested and contested" and warned the situation is set to worsen, The Telegraph reported.

According to Bharath Gopalaswamy, an Indian rocket scientist researching space debris at the Stockholm International Peace Research Institute, there are now more than 370,000 pieces of junk compared with 1,100 satellites in low-Earth orbit (LEO), between 490 and 620 miles above the planet.

The space junk, dubbed "an orbiting rubbish dump", also comprises nuts, bolts, gloves and other debris from space missions.

"This is almost the tipping point," Dr Gopalaswamy said. "No satellite can be reliably shielded against this kind of destructive force."

In February last year, a crash between a defunct Russian Cosmos satellite and an Iridium Communications Inc. satellite left about 1,500 pieces of junk whizzing around the earth at 4.8 miles a second.

A Chinese missile test destroyed a satellite in January 2007, leaving 150,000 pieces of debris in the atmosphere, Dr Gopalaswamy said.

The Chinese missile test and the Russian satellite crash were key factors in pushing the United States to help the United Nations issue guidelines urging companies and countries not to clutter orbits with junk, the Pentagon's Space Posture Review said.

The United Nations Office for Outer Space Affairs (UNOOSA) issued Space Debris Mitigation Guidelines last year, urging the removal of spacecraft and launch vehicles from the earth's orbit after the end of their missions.

Space needs "policies and laws to protect the public interest", saud Mazlan Othman, director of UNOOSA.

"We should have all the instruments to make sure that lifestyles are not disrupted because of misconduct in space when people switch the television to watch the World Cup next month in Johannesburg," he added.

The GEO Graveyard May Not Be Permanent
Since the 1970s, a number of geostationary satellites have been placed in the so called "graveyard orbit," an orbit just above the GEO altitude, roughly 100 to 300 km. The sole purpose of this "burial" location is to remove expired satellites from the highly-congested GEO ring about the equator.

Although most GEO satellite operators have not taken advantage of removing their old spacecraft, there are over 100 already there. This number will continue to grow, because some 20 GEO birds expire each year, and some of these will be sent to the graveyard. Thus, the total number of graveyard residents will increase indefinitely.

Operators that use the "end-of-life" maneuver to rid GEO of future debris feel relieved that they do not have to be concerned about their old satellites interfering with operating birds. After all, as the number of dead satellites in GEO increases, the probability of collisions increases.

Once collisions start occurring the frequency of such events will surely increase dramatically. So, the graveyard orbit seems like a good long-term solution to eventual uncontrolled debris generation in GEO.

But, hold on one second.

There is a well-known theory that the tidal effects of the moon on the Earth are slowly changing the Earth-moon relationship. The moon creates friction due to tidal motion of the oceans. This friction causes a loss of the Earth's rotational energy.

The net effect is a slowing of the Earth's spin rate, i.e., a lengthening of the day. However, the period of the GEO orbit is synchronized with the Earth's rotation rate in order to assure satellites placed in this orbit will remain fixed over one longitude. If the Earth's rotation rate changes, then GEO satellites will no longer appear to be stationary.

This implies GEO satellite will have to be placed in orbits that match any new rotation rate. As the rate decreases, the altitude for synchronized motion will increase, i.e., GEOs will have to fly at ever higher orbits in order to stay stationary over the Earth.

A 10-minute increase in a day will require that GEOs be placed in orbits that are now reserved for expired satellites. This could mean bad news for satellite operators, because the probability of collisions with space debris will increase dramatically.

Hold on!

Although this may sound alarming, let's take a look at the rate of increase in the day. Estimates indicate that the day will increase just a few minutes over the next 20 million years. Given the typical satellite life span of less than 20 years, there appears to be no immediate concern regarding maintenance of station or increases in collision threats at GEO.

However, GEO satellite operators need to be aware of many subtle influences on this all-important orbit. Such influences and a complete treatment of geostationary orbits and operations are covered in Launchspace's new course for 2011: "Advanced Geostationary Orbit Mechanics and Operations." This will soon also be available on demand at your facility.