Studying the Junk Yard Around Earth
Space debris is causing growing concern as a threat to the future utilisation of near- Earth space for manned and unmanned activities. The potential danger was brought home on 24 July 1996 by the first documented space collision involving the French Cerise microsatellite and a piece of an Ariane rocket launched in 1986.
The latest studies of this human junkyard in space will be presented at the Second European Conference on Space Debris which is being held in Darmstadt, Germany from 17 to 19 March. The purpose of the conference is to provide a forum for presentations of results from research into this growing menace.
Over 200 experts from around the world are expected to attend, with UK space scientists playing a leading role in the proceedings. Among those presenting papers to the conference will be representatives from the Defence Research Agency, the University of Kent, the Royal Greenwich Observatory, Southampton University, the Open University and Queen Mary & Westfield College, London.
"We have had a mandate since the early 1990's to be proactive in this area," said Richard Tremayne-Smith, the British National Space Centre (BNSC) representative on the conference programme committee. "Last year Britain formally joined the Inter-agency Debris Coordination group (IADC) in its own right. Since the Cerise satellite was hit, everyone is taking space debris a lot more seriously. It is important to make sure the message gets through to industry."
Scientists from the University of Kent have been studying space debris through detailed examination of damage to hardware which has been returned to Earth after long-term exposure to the space environment. Their work has included close scrutiny of the surfaces of the LDEF and EURECA satellites and a solar panel from the Hubble Space Telescope. In addition, experiments using a particle accelerator called a Light Gas Gun facility provide first hand evidence of what happens when debris of different velocities, sizes and densities strikes a spacecraft at different angles.
Apart from modelling the behaviour of artificial space debris, the Kent team also studies the effects of natural meteor showers. These small dust particles, often called meteoroids, are travelling at around 70 km per second, much faster than orbital debris, when they arrive in the vicinity of Earth. Of particular concern to satellite owners is the Leonid meteor shower, a stream of particles shed by comet Temple-Tuttle as it travels around the Sun. This shower is expected to peak in 1998 and 1999, when it may pose a serious threat to Earth-orbiting spacecraft, manned or unmanned. A paper attempting to predict the outcome of these events and possible strategies for safe deployment of spacecraft will be presented by Professor Tony McDonnell.
Space tethers are seen as a promising source of future power generation, radio communications and propellant-free propulsion. However, according to a paper by Dr Neil McBride and Dr Emma Taylor, a tether system launched in March 1994 may have broken as the result of a meteoroid or debris impact. They conclude that the typical lifetime to be expected for a 20 km-long tether would be in the range of a few days to a few weeks.
The team from the Defence Research Agency uses information obtained from the space tracking centre at Fylingdales in Yorkshire to model the artificial space debris environment and investigate ways of shielding spacecraft from these projectiles. Of particular concern is the possible future worsening of the situation by the launch of satellite constellations for mobile phones. One of these systems alone is planning to place 840 satellites into low-Earth orbit. According to a study presented by Roger Walker, the deployment of these systems is likely to result in the overall collision rate more than doubling over the next 50 years and a 30% long term increase in the debris population larger than 1 cm in size.
Other papers presented by DRA at the conference suggest ways of reducing the debris hazard. One suggestion is to deorbit satellites at the end of their lives by means of advanced propulsion systems such as arcjets and ion thrusters. The arcjet uses electrical power to greatly accelerate gases produced during a combustion process. Ion thrusters produce lower thrust levels by using electrical power to accelerate charged particles (ions) to extremely high velocities.
Another alternative is to improve satellite design, in particular protective shielding. At present, hardly any satellites have any shielding against impacts. DRA's Hedley Stokes is applying the principle of biological evolution to this problem through genetic algorithm theory. The model simultaneously identifies the optimum physical shielding required to protect a satellite while also taking into account the best internal arrangement of components. A prototype of the computer model has already been developed with a 3-dimensional model envisaged as the next step.
According to Hedley Stokes, the debris problem is likely to get worse over the long term, particularly in low-Earth orbit, if present trends continue. "Geostationary orbit (35,900 km above the Earth) isn't too much of a problem at the moment," he said. "Velocities and impact energy are lower, so it's a bit easier to shield against and the population is nowhere near as high, so the collision hazard is less."
There are currently some 8,500 man-made objects orbiting the Earth which are sufficiently large to be tracked by ground-based tracking systems. 94% of this is space debris, a mixture of fragments from explosions in space, discarded rocket upper stages, dead satellites and miscellaneous items including a screw driver dropped by an astronaut. In addition, there may be more than 100,000 pieces of space litter as small as 1 cm across.
The conference is being hosted by the European Space Agency at its European Space Operations Centre (ESOC). The British, French, German and Italian space agencies (BNSC, CNES, DARA and ASI) and the International Academy of Astronautics (IAA) are co-sponsoring this event.
CONTACTSDuring the conference, UK representatives may be contacted through the ESOC public relations office. Tel: +49 6151-902696. Fax: +49 6151-902961 or 6151-902752.
For the University of Kent space debris group, contact: Dr Simon Green. Tel: +44 (0)1227-764000 ext. 3780; or Dr John Zarnecki. Tel: +44 (0)1227-764000 ext. 3237.
After the conference:
Richard Tremayne-Smith, BNSC, 151 Buckingham Palace Road, London SW1W 9SS. Tel: +44 (0)171-215-0821. Fax: +44 (0)171-215-0807.
Dr Jacqueline Mitton, RAS Public Relations Officer
Peter Bond, Space Science Advisor