Australasian Science Magazine

New technology can warn whether a piece of junk poses a threat to a spacecraft, and if so in which direction the craft should move.

By Stephen Luntz

Satellites are under threat from about 500,000 pieces of space junk, but new Australian technology can now track the orbit of debris as small as 1 cm to within 1 metre.

Far above our heads, even above the ozone layer, the Earth has a pollution problem. Low Earth orbit is becoming increasingly crowded. Not only are satellites ever more common, but they face more and more danger from pieces of space junk.

However, Australian company Electro Optics Systems Holdings Limited (EOS) thinks it has a big part of the answer. For a country that, to a large extent, has turned its back on space research, this could become our major contribution.

The Problem
In addition to 500–600 operating satellites, tens of thousands of pieces of junk are circling the planet. While most are small, the speeds at which they are travelling make them dangerous.

So far one major collision has occurred, in 2009 between Iridium 33 and the deactivated Kosmos 2251. At a speed of 11.7 km/s, the accident unsurprisingly destroyed both satellites.

The great danger with such events is that they will unleash a cascade. In a collision each object is likely to break up into numerous components, many of them large enough to cause future collisions. More than 1700 pieces of debris have been catalogued from the Iridium/Kosmos collision, but that number is incomplete. So far none appears to have triggered their own collisions, but it may be only a matter of time. Moreover, the lack of a cascade from one event does not mean that such things will not occur in future.

“There is a voluntary code of conduct,” says the CEO of EOS Space Systems, Craig Smith. “But even the US has not decided formally to be part of it.”

Spacefaring nations have become more careful about leaving junk in space. “They used to blow off bolts and leave things in orbit,” says Smith. “Third-stage rocket bodies would typically blow up and create new space debris.

“Now they’re supposed to drive them into a safe orbit. This should mitigate the rate of increase, but it can’t stop it, and so far the Europeans are the only ones to have bound themselves to the agreement.”

Moreover, with occasional collisions causing cascades, the problem will get worse. “Even if we never launch another satellite, we’re now past the point of no return,” says Smith. “And of course more launches will happen.”

The lowest parts of Earth orbit are self-cleaning. Although the atmosphere is often defined as reaching up to 100 km above the Earth’s surface, there is actually no sharp line where it stops. All the way up to about 400–500 km there are enough stray particles to exert a small drag on orbiting objects. This drag slows them down and causes them to lose height, bringing them into contact with even more of the atmosphere.

Over a period of a few years, objects below 400 km gradually spiral down until they burn up in the Earth’s atmosphere. A few large objects do not burn up entirely, instead leaving components large enough to make it to Earth. When this fate befell Skylab in 1979 it created panicked headlines.

However, the danger of such an object causing death or destruction is miniscule. The risk is much higher from those at greater altitude.

Objects above 500 km encounter much less resistance, and experience minimal drag. It will take centuries for most of these to fall to Earth, giving them plenty of time to crash into operating satellites.

Since launch costs increase dramatically with height, most satellites are placed at fairly low orbits. However, many functions require geostationary orbits, where a satellite stays above a particular part of the globe. This is only achievable at an altitude of almost 36,000 km. Other satellites, particularly those for navigation purposes, are best placed in medium orbits.

Keeping Track
At the moment the problem is addressed by keeping track of as many objects as possible. When one appears to be on a collision course with an operating satellite, the satellite is given instructions to take evasive action, shifting to a higher or lower orbit.

However, there are a few flaws in this technique. For one thing not all satellites are capable of moving in this manner. More substantially, we simply don’t know the whereabouts of many of the objects that are large enough to cause damage.

There is also a third problem, which EOS believes it can address. Current monitoring does not allow us to track the positions of these pieces of junk as accurately as we would like. “You want to make sure you’re moving a satellite out of the way of an object, not right into its path,” Smith says.

At the moment, space junk is tracked using radar. “Radar is good for looking at large volumes of space,” Smith says. “Where it falls down is in precision measurements. Radar is probably accurate to a few hundred metres.”

EOS, on the other hand, is a leader in laser tracking, which works by getting an approximate orbit from radar results and using this to predict the rough position of an object. A visible light camera attached to a substantial telescope is used to identify the object, after which an infrared laser pulse is fired at the location and the scattered light detected by a camera at the launch site. By dividing the time taken by the speed of light, the length of the path to the object and back can be determined.

Smith says that the shortness of the laser pulses allow the distance of the object to be measured to an accuracy of 1 metre, far more precisely than can be done with the radio frequencies that others use.

With repeated readings these measurements are sufficient to plot an orbit precise enough to warn whether a piece of junk poses a threat to a spacecraft, and if so in which direction the craft should move.

Besides the accuracy in positioning, Smith is also upbeat about the size of objects that EOS will be able to track. He says it will be possible to spot objects down to 1 cm in diameter – of which there are estimated to be 500,000 of human origin currently in orbit. That only 16,000 are currently tracked indicates the scale of the task.

Even if EOS can track every object within its capacity, plenty of dangerous objects will still haunt the skies. An object only 3mm in diameter is considered large enough to damage a spacecraft on its own while smaller objects can create cumulative damage (see box above).

However, perfect safety in space is never possible. Even if every artificial menace was tracked, the possibility of collision with a small meteor cannot be ruled out.

State of Play
EOS operates a satellite laser ranging observatory at Mt Stromlo, which was destroyed and rebuilt after the 2003 fires in Canberra. The station arose from research dating back to the early 1980s.

EOS was founded in 1983 to commercialise satellite laser tracking technology, although military work now makes up about two-thirds of the company’s $35 million per year business. Much of the rest is a result of EOS’ status as one of the largest builders of professional astronomical telescopes – another little-known Australian success story.

Space junk was not the station’s first project. “We are paid by Geoscience Australia to track satellites with reflectors on them,” says Smith. “We know what the orbit should be. Any wobbles tell you about the gravitational field and atmospheric drag. From this we can learn about the way the centre of the Earth moves around. This is important for things like the precession of the poles.”

Such precision tracking is also important for calibrating certain satellite activities, such as sea surface measurements. While Smith describes tracking space junk as “basically the same sort of thing,” he acknowledges that a lot more power is needed to observe a tiny piece of debris than a satellite that has been fitted with a handy mirror specifically designed for bouncing lasers off. While more than 40 facilities around the world track operating satellites, Smith says he’s “not aware of anyone else” capable of tracking small pieces of space junk with lasers. “Everyone else is using radar.”

In July 2010 the federal government announced a $4.04 million grant to upgrade operations so the Mt Stromlo station can automatically track objects passing over the Asia–Pacific region. Smith says this would create a “proof of principle” station capable of demonstrating that small objects can be tracked with high accuracy.

EOS received further encouragement in November when US Vice-President Hilary Clinton’s visit to Australia led to a joint communiqué announcing closer collaboration between Australia and the United States in areas including space surveillance.

Getting laser tracking to work for space junk did not involve a particular technological breakthrough so much as resolving complex systems challenges. “Everything has to work at the same time,” Smith says. “It either works or it doesn’t. Making sure we’re landing pulses on the target is the clever part, as well as the systems software to make it all happen together.”

Smith doesn’t expect lasers to replace radar. Instead he thinks the two technologies will prove complementary, with radar being used to locate the general position of an object and lasers used where radar results raise concerns. Smith believes laser tracking could have given enough warning to avoid the Iridium/Kosmos collision.

If EOS can prove the value of its satellite tracking technology, multiple stations will be needed around the world. “I couldn’t say the number off the top of my head, but probably closer to ten than three,” says Smith.

The business plan involves both government and private satellite operators paying EOS to keep track of anything crossing their satellite’s orbit, offering them plenty of warning when something poses a danger. But this remains some way off as the upgraded station will not be fully operational until 2012.

And as important as tracking is, it cannot completely solve the problem. As Smith notes, “a fleck of paint penetrated five of the space shuttle’s seven layers”.

If space becomes too crowded, no amount of tracking will enable satellites to avoid the debris. Fans of the Reagan’s Administration’s Strategic Defence Initiative may dream of vapourising tricky objects from Earth, but Smith makes it clear that he sees no sign of this proving realistic in the near future. Likewise the idea of robotic garbage cleaners remains very much in the realm of science fiction.

The problem of space junk is a particularly sharp illustration of environmental problems in general. It is easy and cheap to allow bits of debris to float away in space, and the price is paid not by the individual polluter but by everyone with a satellite in orbit.

Nevertheless, a combination of radar and laser tracking may at least keep the problem under control for some time, allowing us to go on enjoying the many benefits that satellites have brought the world. For Australia, moreover, EOS might just offer us a way to regain a place in space.