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Last Updated: Thursday, 5 May, 2005, 16:47 GMT 17:47 UK
CO2 prolongs life of 'space junk'
By Carolyn Fry

Artist's impression of space debris, ESA
There are about 100,000 objects the size of a marble in orbit currently
The potential of the huge quantities of carbon dioxide (CO2) pumped out by human activities to warm the Earth's surface is a well understood concept.

But hundreds of kilometres above Earth, where satellites circle and survey the planet, the gas is having the opposite effect.

Scientists at Southampton University, UK, have discovered that cooling caused by increased CO2 in the thermosphere is lowering atmospheric density, which could prolong the time a satellite stays in orbit.

It's the equivalent of exploding several sticks of dynamite in your spacecraft
Dr Hugh Lewis, Southampton University
While this might sound like good news, the lower density is also likely to extend the orbital life of all kinds of space debris.

Over time, this will significantly increase the number of destructive collisions in space, which could prove extremely costly for spacecraft manufacturers.

"We created two models, one for the standard density and one for the decreasing density, to predict what might happen in the future," explained Graham Swinerd, co-author of a paper presented at the 4th European Conference on Space Debris at the European Space Operations Centre, in Germany.

"One of the most significant findings was that the number of collisions increased significantly due to the decrease in atmospheric density, with the two models diverging at around 2060 to 2070.

"The lower density causes all objects to stay in orbit for longer. So, if you have a collision that produces debris, the pieces go off in their various orbits and produce more collisions. It's like a chain reaction," he told the BBC News website.

Collision course

The atmosphere up to about 2,000km is extremely rarefied. Even so, carbon dioxide molecules will still bump into oxygen atoms.

The ISS has some protection against impacts
On impact, the CO2 molecules emit a photon in the infrared wavelength - as heat - which then radiates away into space. The cooling reduces the density - and this in turn reduces the drag on spacecraft and space debris, lengthening the time it takes for them to fall back to Earth.

Direct monitoring shows CO2 concentrations have been rising since the 1950s, and levels are likely to continue to climb in the near future - so the scientists fully expect density in the thermosphere to continue to fall.

The first verified case of a collision between two objects occurred in 1996 - the French satellite Cerise was hit by a fragment from an Ariane rocket.

The reduced density model predicts there may be as many as 50 collisions by the end of this century.

There are currently some 9,000 objects over 10cm in diameter in orbit above the Earth's surface. These comprise active satellites and man-made objects such as parts of launching stages of rockets.

About 10,000 catalogued items
7% - operational spacecraft
22% - redundant spacecraft
17% - old rocket bodies
13% - mission-related objects
41% - miscellaneous fragments
Source: Esa
"During the '70s to the '90s, there've been quite a few explosions of rocket upper stages, and each explosion produces a large number of smaller items," explained Southampton colleague Dr Hugh Lewis.

"But whenever you launch a satellite, you've got associated objects, such as lens covers and explosive bolts that are released, and these contribute to the space-debris environment.

"The velocities of objects in low-Earth orbit - that's up to about 2,000km altitude - are of the order of 7km/s. If you have two satellites travelling in opposing directions, you will have relative velocities of anywhere up to 14-15km/s.

"Because the velocities are so high, the kinetic energy is very high. It's the equivalent of exploding several sticks of dynamite in your spacecraft."

The smallest satellites such as Cerise cost a few tens of millions of dollars, while larger ones like the European Space Agency's 8-tonne Envisat can cost as much as two to three billion.

Only in the largest spacecraft of them all, the human-inhabited International Space Station, has shielding been a major driver in design.

Hubble Space Telescope (HST) solar array (Esa)
Damage in a Hubble telescope solar array caused by a space impact
If manufacturers are to avoid the huge costs of losing satellites through collisions, they may have to introduce mitigating policies to reduce the amount of debris, as well as incorporating protective shields into their designs.

"Ultimately, if there is a decrease in density, we will probably still operate these craft but they will have to be shielded and that will increase the already significant cost of space exploitation," warned Dr Swinerd.

David Wade is a space underwriter from Lloyd's Insurance. He commented: "This won't change the space field overnight. Communication satellites tend to be in much higher orbits than 2,000km; they sit at 36,000km.

"This is not really going to affect the satellites that beam TV pictures around the world; they are fine. However, this does suggest we should keep an eye on debris and try to come up with plans to limit the debris increase."

One giant (scrap)heap for mankind?
15 Apr 04 |  Magazine
Nasa expands search for debris
04 Feb 03 |  Americas
Earth's new 'moon' is space junk
12 Sep 02 |  Science/Nature
Space debris lights up sky
03 Dec 01 |  England

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