Wednesday, January 20, 1999 Published at 15:46 GMT
Looking for junk in space
Argos is the largest US Air Force research and development satellite ever to fly
By BBC News Online Science Editor Dr David Whitehouse
A US Air Force satellite with the mission to study man's growing pollution of the heavens has been prevented from launching by strong high level winds.
The next attempt for the launch at Vandenberg Air Force base will be made on Friday at 1037 GMT.
Onboard the Argos technology demonstration satellite is an instrument designed by University of Chicago scientists to monitor the amount of man-made debris orbiting the Earth.
There is more of this than you would think and in some Earth orbits it could become a problem.
Space debris can come from many sources, discarded satellites, covers and pieces ejected from satellites and even flecks of paint.
It is now a general policy that every effort must be taken to minimise polluting outer space but it has not always been that way.
Millions of objects
Since the space age began we have accumulated around the Earth an estimated 9,000 objects greater than 10 cm in size, 100,000 objects greater than 1 cm and tens of millions less than 1 cm in size.
Usually they are travelling at 7-8 km a second. At that speed even a speck of paint packs a heavy punch. After returning from an early mission the Space Shuttle was found to have a fleck of paint embedded in its window.
The Space Shuttle now takes evasive action if it comes within a few kilometres of a piece of space debris. This happens about once a year or so.
The first servicing mission to the Hubble Space Telescope counted 260 impacts, some the size of a coin, on the telescope's exterior.
Despite this, there has been only one, possibly two, instances of a satellite being damaged by man-made space debris.
Space debris is tracked by the United States Space Surveillance Network that uses radar that can detect an object the size of a pea at 400 miles.
Observations of space debris suggest that at altitudes between 250 and 400 miles (400 and 640 km) there is less of it than was feared. However between 500 and 600 miles (800 and 960 km) there is more space debris than predicted, no one knows why this is so.
The equipment on the Argos satellite will be carried through the most troublesome region of space debris.
Despite the worry about the space debris problem, the outlook for the partly-built International Space Station is good. The ISS will be the most heavily shielded structure ever put into space. It will be designed to withstand an impact with a one centimetre object.
It will be also be in roughly the same orbit as the Russian space station Mir. Although Mir has suffered a large number of tiny impacts over the years, it has sustained no damage.
And if the mission controllers see anything larger coming then they will have the option to move the ISS out of the way.
Ultimately, however, nothing can be done about space debris, except to not make any more.