Sitting duck

ANU ScienceWise Magazine - Autumn 2011

A large communications satellite represents a very significant financial investment typically costing in excess of $500 million to build and launch into orbit. The launch process is inherently risky, so insurance premiums are high but even once it’s parked in space, such a satellite isn’t entirely safe.

Over 50 years of space flight have left the Earth’s near orbital zones absolutely littered with space junk ranging from dead satellites and boosters to nuts, bolts and even an astronaut’s glove. The problem is that this junk is travelling at up to 10,000 metres per second and in multiple directions.

Such huge velocities mean even a small bolt will release the same energy as a stick of dynamite if it strikes another object in space. And a piece the size of a brick will be more like a sidewinder missile.

As a result, any space debris colliding with your expensive communications satellite is likely to result in a multi-million dollar insurance claim. In the last three years two major communications satellites have been lost due to such collisions and with an ever increasing number of launches, the problem only looks like getting worse.

Space debris is notoriously difficult to deal with. It’s small, fast and can orbit for centuries. 20,000 pieces down to 15cm size are currently radar tracked but radar has an inherent limitation on its accuracy placed by the long wavelengths used. Moreover, studies show that there are nearly 500,000 objects smaller than 15cm and still dangerous to space travel, but whose orbits are not known. However, this may soon be about to change.

Electro Optic Systems EOS is a commercial company working collaboratively with the Australian National University on a number of space related projects. EOS created a major stir in the aerospace world during 2000 by demonstrating that they could track space junk using a laser - a feat thought to be very difficult, if not impossible by international aerospace companies at the time.

“Radar tracking is important in that it enables us to stay up to date with the larger pieces of space debris,” says EOS Operations Manager Ian Ritchie, “But using radar wavelengths the best orbital position accuracy you can achieve is of the order of 10-100 metres. This just isn’t good enough to be able to say with certainty where an object will be in a couple of days.”

The EOS system uses a laser coupled to a large and very precise tracking telescope equipped with state of the art auto guiding and laser tracking equipment. “In a given pass we may have only 60 seconds to acquire the target at one horizon, then a further 60 to track it before it’s approaching the opposite horizon, so all our systems have to be the best available,” Ian says.

The smallest piece of debris that EOS have been able to track to date is about 8cm across, as estimated by radar. The range accuracy achievable is of the order of a centimetre using laser tracking. For smaller objects than this, the distortion of the beam profile by the ripples and turbulence in the Earth’s atmosphere spreads the focus too much to get a good return signal. It is possible to simply use a more powerful laser but you can’t fire powerful lasers into the sky at will.

“Our existing tracking system fires an eye-safe pre-pulse and if any reflection comes back from something like an aircraft the main pulse is disabled,” Ian explains, “It’s very unlikely that we would ever hit an aircraft in the first place, but naturally we’d never leave that to chance. And we definitely don’t want to go down the path of more powerful lasers to track smaller debris.”

If you can’t use ever more powerful lasers, one thing you can do is try to improve the beam focus by using adaptive optics to eliminate the distortions generated by the atmosphere. This is where the collaboration with the University comes in. The ANU Research School of Astronomy and Astrophysics are currently developing an advanced adaptive optics suite for the Giant Magellan Telescope and of course the technology works just as well for sending a laser beam up as it does for starlight coming down.

“When this technology reaches maturity, we should be able to track space debris with unprecedented accuracy” Ian says, “and that means we can tell you that in x days, y hours your satellite is going to be in danger.” That should give ample warning to manoeuvre it slightly out of harm’s way. And at half a billion dollars saving each time, that’s the sort of information that satellite operators will doubtless be very happy to pay for.

“Like any successful commercial entity EOS have always adapted our business plan around valid business cases,” Ian says, “But we’ve always looked out for emerging technologies that enable new fields of aerospace capability. We develop technology at the cutting edge and if it doesn’t have immediate recognition in the marketplace, we hold our datapack ready to be retrieved, brought up to date and implemented rapidly. We have to try to be both clever and agile to anticipate the market as a smaller player in the aerospace game, and EOS is always an exciting place to be.”