Chinks in ISS armour deliver data on space junk impacts
Speeding along in orbit at more than seven kilometres per second, the International Space Station has its surfaces carefully shielded against potentially catastrophic collisions with micrometeoroids or man-made debris. Except, that is, for a trio of unprotected panels until recently attached to an external payload platform of ESA’s Columbus module, specifically intended to sustain impacts from tiny specks of space dust.
Each pointing in a different direction – forward in the direction of orbit, upwards and sideways out to space - the three 10 by 10 cm aluminium foil panels formed the sensor units of an instrument called the DEBris In-orbit Evaluator-2 (DEBIE-2), returning telemetry on impact events to a separate data processing unit.
These units were sited on the European Technology Exposure Facility (EuTEF) which was mounted on the Columbus exterior until returned to Earth on the Space Shuttle this week. Columbus is an ideal position for impact monitoring since the module forms part of the leading edge of the International Space Station (ISS).
There are around 13 000 catalogued pieces of orbital debris larger than 10 cm across. These are big enough to be tracked by terrestrial radar so the ISS, the Shuttle and other satellites can manoeuvre out of their path. In addition there are many millions more items too small to be monitored from the ground. Traditionally most of what is known about these orbital populations has come from simply examining space hardware returned to Earth and counting the number and size of impact craters they have sustained while in orbit.
However the DEBIE system actively measures impact energies and velocities as strikes occur. Possessing only a limited detection area – three times 100 cm2 – the instrument is designed around impacts from particles around a micrometre (a thousandth of a millimetre) in size, the type of debris about which least is known.
Around the same size as smoke particles, they may be tiny but their effect is still dramatic: impacting at hypersonic velocities, they briefly heat the aluminium foil locally to thousands of degrees Kelvin, hotter than the surface of the Sun. This forms a charged plasma cloud whose charge is measured by positive and negative plasma detectors adjacent to the foil. Meanwhile a piezoelectric device converts the force of impact into an electric charge, providing additional information on particle momentum. And if a particle strikes with foil-piercing force then another plasma detector behind the foil performs measurements.
Such small particles pose little danger to the ISS. DEBIE-2’s results are important however for understanding the general characteristics and behaviour of space debris, to feed into future spacecraft designs.
"One of the surprises we have found so far is that impact events come in clusters and are not randomly distributed," explains Gerhard Drolshagen of ESA’s Space Environment & Effects Section, DEBIE-2 Principal Investigator. "These peaks can be concentrated within the space of perhaps a minute to 80 seconds at a time, indicating the existence of clouds of dust along the ISS’ orbit."
These particles might be either natural or artificial in origin. Asteroids and comets leave trails of fine dust behind them while solid rocket boosters often used to transfer satellites into higher orbits spray out fine droplets of aluminium oxide.
Second in the DEBIE series
The first ever standardised impact detector, DEBIE was based on an initial concept by a UK company and then developed under ESA contract by companies in Finland.
DEBIE-1 operated in a 600 km orbit on board ESA’s Proba technology demonstrator satellite for a total of five years from its launch in 2001. DEBIE-2, was launched aboard the same Shuttle that delivered Columbus to the ISS in February 2008, then installed during a subsequent spacewalk.
"We had some operational problems to start off with but updated software installed in December improved our reliability," says Alessandra Menicucci DEBIE-2 Co-Investigator. "Since then we have been gathering data 70-80% of the time."
DEBIE-2 has now been returned to Earth after 1.5 years of exposure to open space in low Earth orbit, along with the other eight instruments within EuTEF, as part of the Shuttle mission STS-128 / 17A on 10 September. The instrument could potentially then be refurbished for another space mission in future– the team would like to make measurements from the Moon or around the L2 Lagrangian Point in space behind Earth where a number of science missions are planned in the next decade.
"We’re also considering a bigger detector capable of getting information on bigger particles in the future," concludes Mr Drolshagen. "Around 0.5 mm is where you start to be really concerned; that size could cut a wire or damage the soft part of an astronaut’s spacesuit."
Gerhard Drolshagen of ESA’s Space Environment & Effects Section, DEBIE-2 Principal Investigator.