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Space junk is a serious problem, particularly in some orbits where debris is increasing at alarming rates.

While there are some 900 active satellites orbiting the Earth, there are 19,000 bits of junk larger than 10 cm across. This stuff is big enough to be tracked and catalogued on the ground so that operational satellites can move away if it becomes a threat.

But it’s the smaller stuff that represents a more insidious threat since it cannot be seen and therefore can’t be avoided. Most experts agree that there’s at least an order of magnitude more of this small stuff than large bits up there.

So what to do? Various organisations have suggested ways of minimising junk, such as reducing the amount of deliberately jettisoned junk such as lens caps, and by deorbiting defunct satellites or moving them into safe orbits using space tugs.

But these measures will only help reduce the amount of big junk. The smaller stuff is much harder to clean up.

There is a natural process that can help. Below 900km, the Earth’s atmosphere generates a small but significant amount drag, which deorbits small junk in 25 years or less. So here the orbits are naturally flushed clean. But above 900km, the life time of junk stretches into centuries.

Today, Gurudas Ganguli at the US Naval Research Laboratory and a few pals describe a novel way of getting it down.

Their idea is to increase the drag on the stuff above 900 km so that their orbits decay more rapidly. That sounds perfectly sensible but their method is likely to be controversial.

Their scheme is to release some 20 tons of tungsten dust at an altitude of 1100km, creating a thin shell of particles that will entirely envelop the Earth. These tungsten particles will be just 30 micrometres across but still capable of packing a punch, tungsten being 1.7 times denser than lead.

Ganguli and co say that the dust’s interaction with the atmosphere will cause its orbit to decay slowly. But within 10 years or so, it should drop below the critical 900 km level. After that, it will deorbit more quickly.

However, the crucial point is that the tungsten particles will naturally collide with any debris it encounters, taking this junk with it. The dust and the debris will then burn up in the Earth’s atmosphere over the next 25 years or so.

So over period of 35 years, the orbits up to 1100km will be scrubbed clean. Ganguli and co call it a “dust snow plow”.

There’s an obvious question here: what of larger objects that get caught up in the dust storm, operational satellites, for example?

Ganguli and co say the risk is manageable. First, these satellites could be designed to move above the cloud. But even if they don’t move, Gangulia and co claim these spacecraft will not be significantly damaged by the dust. “Dust grains of the size proposed by NRL will certainly not penetrate thermal blankets, spacecraft structure, or sensor baffles,” they say.

They add that more sensitive equipment, such as the optics of Earth observing sensors or space telescopes, usually point straight up or straight down and so should be protected from dust flying in from the side.

One concern is solar panels which are likely to be sand blasted by the cloud. But Ganguli and co say that panels for the next generation of spacecraft could be strengthened to cope with this kind of problem.

There’s also the question of the tungsten cloud’s dynamics. Ganguli and co imagine it forming a shell about 30 km thick. This shell would then deorbit steadily. But there’s another possible scenario: that the tungsten band simply widens to form a cloud several hundred kilometres thick!

The NRL will need to do more work on this problem.

Then there is one group of people whose concerns Ganguli and co fail to address entirely in this paper: astronomers. While a cloud of tungsten particles would have little affect at visible frequencies, astronomers will want to know what kind of effect this cloud will have at other wavelengths.

Is it possible that a cloud of metal particles encircling the Earth could significantly degrade our view of the Universe at certain frequencies, perhaps even acting like a giant spherical mirror? More work is needed here too.

But before dismissing the proposal out of hand, the alternative has to considered.

In 2007, the destruction of a defunct communications satellite at 900km by a Chinese anti-satellite weapon created, in an instant, 2400 pieces of large debris and countless smaller ones. The collision between the Iridium 33 and Kosmos 2251 satellite in 2009 created a similar amount of debris.

It’s likely that we’ll see more events of this kind in future and the possibility of a catastrophic cascade of collisions from the debris they produce.

So Ganguli and co are presenting the space-faring world with a choice: the controlled exposure of all satellites to a low level of small collisions or the uncontrolled exposure of a few satellites to catastrophic collisions.

Viewed in that way, NRL’s orbiting dust storm may not be the perfect answer but it could well be the lesser of two evils.

Ref: arxiv.org/abs/1104.1401 : A Concept For Elimination Of Small Orbital Debris

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