There is a lot of junk orbiting the Earth and the problem will worsen unless there are changes in how spacecraft operators operate. But it is not all doom and gloom. The first steps toward a comprehensive solution are already well underway including a
There is a lot of junk orbiting the Earth and the problem will worsen unless there are changes in how spacecraft operators operate. But it is not all doom and gloom. The first steps toward a comprehensive solution are already well underway including a European code of conduct for space debris mitigation.
According to Dr Ruediger Jehn, a space debris specialist working at ESA's Space Operations Centre (ESOC) in Darmstadt, there are several relatively simple measures that will help reduce the amount of debris in space. Some are already being implemented by spacecraft operators at little or no cost.
"These steps," he explains, "are based on common sense and include measures that should be acceptable to any spacecraft operator."
The basic concept is simple: do not make the existing problem worse; reduce or prevent the creation of any new debris; and, in particular, strive to protect the commercially valuable low Earth and geostationary orbits.
The amount of debris created during normal operations can be reduced by not discarding, ejecting or detaching anything that does not have to be discarded, ejected or detached. This includes payload covers, Yo-Yo despinners and instrument covers such as those used to protect the highly sensitive optical windows of sensors during launch. Lastly, minimise break-ups, a major source of small but deadly debris.
Explosions in space
It may be surprising to anyone outside the space community to learn that spacecraft (occasionally) and launch vehicles (frequently) do in fact break up in orbit.
Launch vehicle lower stages generally fall back into the atmosphere and completely burn up, providing a tidy, if fiery, solution but the typical fate of rocket upper stages, which are usually cast off after launch, is to blow up.
Why does this happen?
Spacecraft engineers have traditionally ensured a good margin of launch success by carrying extra fuel onboard, as this comes in handy if the engine has to burn a little longer than planned.
However, that spare fuel mostly remains inside pressurized tanks once the rocket stage is discarded into Earth orbit. Over time, and in the harsh environment of space, the mechanical integrity of the booster's internal components breaks down; lines leak, corrosive fuel seeps into nooks and crevices, micro and not-so-micro meteoroids strike and penetrate. A sudden release of pressure often results, causing an explosion and spewing hard-to-track fragments, large and small, into orbit, adding to the debris field.
Other onboard power sources serve as latent explosion triggers, including batteries, other pressurized systems, fuel cells and hypergolic fuels. "Just stopping launch boosters from exploding is a big first step," says Dr Jehn, "and we are already seeing improvements."
The solution to latent explosions caused by onboard fuel is astoundingly simple: once the upper stage is discarded, simply run the engine until the fuel is depleted. The US Delta launch vehicle upper stage now performs such a burn to depletion.
Another fix is simply to vent any remaining fuel to space. This is called passivation, and both the Ariane upper stage and Japan's H-1 second stage now dump their residual fuel in this manner. Batteries and other onboard energy sources can be similarly passivated, although this is not quite so simple and adds more cost.
These and other measures have been widely adopted by most, but not all, mission operators in the past decade but even so, debris continue to grow as older vehicles, launched 10, 20 or more years ago -- before mitigation requirements were understood -- continue to generate debris.
Parking hulks in graveyard orbits
Spent launch vehicles and expired satellites are themselves debris, even if they do not break up. Thus, another important mitigation step is to manoeuvre these out of the commercially and scientifically valuable low Earth orbit (LEO) and geostationary orbit (GEO) zones upon mission completion. Of course, this requires carrying extra fuel specifically for this, adding to cost, but the effort is well worth it.
According to the 2002 draft Mitigation Guidelines issued by the Inter-Agency Space Debris Coordination Committee (IADC), a grouping that includes ESA and 10 national space agencies, spacecraft in LEO should be deorbited, i.e. allowed to fall into the atmosphere and burn up, within 25 years of mission end while craft in GEO should be boosted to at least 300 km above the geosynchronous orbital ring and parked in a graveyard orbit. Why 300 km? "Solar radiation pressure and other small forces would eventually push any craft back into the GEO ring unless they're high enough," says Dr Jehn.
Both measures require fuel: the former to slow and lower craft from LEO and the latter to raise and park craft from GEO. It is too expensive to bring a spent craft all the way down from GEO to burn up, but graveyard parking is an adequate alternative.
In LEO, the solution is even more straightforward. For example, ESA's ERS satellite orbits at about 800 km altitude. Ideally, if it were slowed and lowered at mission end to 200 km altitude it would naturally deorbit and burn up in about 24 hours; but this would take a lot of fuel.
"For a craft the size of ERS, however, it will deorbit naturally within 25 years if we merely bring it down to 600 km," says Dr Jehn, "so this [altitude] is a fuel-saving compromise."
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