Harrow and Hillingdon Geological Society
Comets and Satellites
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How comets threaten satellites
Anton Kearsley
It has always been assumed that comets threaten satellites but it is not proven. Anton Kearsley works in the Science Facilities Department of the Natural History Museum, using scanning electron microscope (SEM) and microprobes to look at anything he is asked to, including meteorites, most of which are pretty definitely pieces of asteroids, ie ordinary chondrites, with carbonaceous chondrites from a different type of asteroid or a comet.
An electron beam is fired at the sample and sensors detect secondary electrons , back-scattered electrons and characteristic x-rays to define the arrangement of individual mineral grains and their chemical composition. Chondrules are blobs of material that were formerly molten. Olivine is very common and next in abundance is pyroxene. Universal abundance of chemical elements is H, He, O, C, Ne, N, Mg Si, Fe S followed closely by Al, Ca and Na.
What threatens spacecraft in orbit?
Essentially the threats arise from 2 sources:
• Orbital debris put into space by human activity, eg rocket and satellite debris;
• Micrometeoroids, mainly from comets but also from asteroids.
Do spacecraft really suffer damage?
Mariner IV in the 1970s was hit 17 times in 15 minutes. The Olympus telecommunications satellite was hit 4 times on 12 August 1993. Landsat was hit on 13 August 1995. The last 2 coincide with the Perseid meteor shower, as does the Newton X-ray telescope hit on 12 August 2002. The Chandra X-ray telescope was hit in November 2003 and the microsatellite Cerise was hit by orbital debris.
Orbital debris does seem to be accumulating. An orbital debris impact has the energy equivalent of being hit by a rifle bullet, while a space dust impact is equivalent to being hit by a pistol bullet. We do not know how many small comet grains are likely to damage satellites.
Long-period comets like Hale-Bopp in 1997 with a period of 4,200 years are not the problem. Much more worrying are the Jupiter family comets. 2 of these have been sampled, Wild2 by the Stardust spacecraft and Hartley2 by the Deep Impact spacecraft, which when it passed the comet encountered 10cm snowballs of ice and dust, with dust around the whole of the comet track.
What are comets made of? We know there is a lot of ice but what of the dust?
Stardust was a $2 million mission, which brought back about 1μg of dust and photographed the nucleus of Wild2. Over 100 impact tracks on the dust collector have been examined in detail. They contain Ca, Al and Mg, a very high temperature assemblage very similar to carbonaceous chondrites and lots of olivine, pyroxene nickel-iron and iron sulphide typical of chondrites. The minerals of Wild2, olivine, pyroxene and sulphides comprise 95% of everything found.
Is there any evidence this material has hit spacecraft?
A 1993 service mission to the Hubble Space Telescope by the shuttle Endeavour removed the wide-field camera and one solar array, which had been in orbit for about 2½ years. The solar cells had numerous impacts and these were examined to look for the composition of anything left behind by the impacts. The results showed they were dominantly micrometeoroid impacts comprising Mg, Si and Fe.
In 2002, the shuttle Columbia retrieved the solar array from the telescope in its last successful mission before the 2003 tragedy. It had many impact craters visible to the naked eye. Examination by SEM found examples of pyroxene and Al2O3 from solid rocket fuel. Bigger impacts were mainly SiO2. 38 impacts were identified as being due to space debris, 45 to micrometeoroid impact and 28 unresolved.
In 2009, the shuttle Atlantis retrieved the wide-field planetary camera 2 and its 2.2 x 0.8m radiator, holes in which were visible from the shuttle and could be dated (relatively). A laser survey of impacts was undertaken at the Goddard Space Flight Center and the Natural History Museum, the European Space Agency, National Aeronautics and Space Administration and the Ion Beam Centre at Surrey University had a contract from March to September 2012 to find out what was making the big holes.
Specimens were 1cm wide metal with holes and 157 samples had been examined by the end of September (it is now 200). Scanning produced a digital depth profile and the composition of debris. Mg-Si-Fe and FeS are typical of micrometeoroid impacts. Many of the holes were full of melted paint bubbles but there was nearly always Mg & Fe. The largest impact feature was over 1cm across. One hole had a 50μ piece of olivine (Mg-Fe-Si), definitely a micrometeoroid.
The Tandetron Laboratory at the Ion Beam Centre at the University of Surrey fires a very high energy proton beam at specimens, which excites x-rays and reveals micrometeoroids from those uncertain on the SEM. The results show 97 micrometeoroids, 21 probable micrometeoroids, 2 either micrometeoroid or orbital debris, 7 unresolved, 0 orbital debris and 5 contaminated. Almost 90% of the impactors have now been recognised. Micrometeoroid impacts cut across the whole size range of impacts, while orbital debris a large number of very small impacts and a few large ones, though these are now doubtful as no orbital debris has been found in this size range.
Conclusions
Orbital debris and comets both threaten satellites. The numbers of small micrometeoroids pose a hazard in low-earth and geostationary orbits – while they are small, generally less than 1mm, they impact at very high velocity. In low-earth orbit, relatively large pieces of orbital debris pose a real and unproven hazard.