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Duck, everyone


Last October, for the first time in history, an incoming asteroid was spotted and tracked to its final plunge over the Sudan. The resulting fall was then successfully collected. Ted Nield reports on the chances of repeating the trick.

Geoscientist Online 16 July 2009

At 0639 UTC, October 6 last year, Richard A Kowalski at the Catalina Sky Survey, discovered Asteroid 2008 TC3 – a mere 20 hours before it began its final plunge to Earth. The asteroid was tiny – somewhere between two and five metres in diameter – and it exploded 37km above the Nubian Desert, Sudan. In December, a search of the predicted impact area yielded some 280 fragments weighing almost four kilograms – including the Almahata Sitta Fragment (pictured in situ).

The Almahata Sitta Fragment of 2008 TC3, in situ

This was the first time in history that an asteroid had been seen incoming and tracked to its final resting place, and the first time a Near Earth Object (NEO) had been correctly predicted to strike the Earth. The event sent the whole community cock-a-hoop. The Spaceguard Survey had shown it could work, and in spotting such a small object, had vastly exceeded its brief. Meteoriticists were delighted because an object with a known trajectory had been successfully followed to its strewn field.

This helps to map out which types of meteorite originate in which part of the solar system, and that in turn assists scientists’ attempts to understand how the Solar System evolved. The icing on the cake was that the meteorites recovered turned out to be one of the rarest kinds of all – a ureilite. These differentiated achondrites have a mineralogical composition different from any other stony meteorites, contain nanodiamonds, and have no known asteroidal parent body.

Clark Chapman (South West Research Institute, Boulder, Co.) told the MetSoc today that this amazing feat should prove to be an increasingly common occurrence, thanks to the previously unrecognised and increasing capabilities of the instruments currently working for the Spaceguard Survey, and to plans for future NEO surveys. However, he warned, “The meteoritical community must speak up to change current plans” if the full benefits of NEO monitoring for Meteoritical science are to be achieved. “We have to get the telescope people to do the right thing – rather than what they currently plan to do” he said.

Dr Clark Chapman NEOs wth a diameter of four metres (roughly the same as 2008 TC3) strike the Earth annually. Smaller objects of 1.5m diameter arrive at a rate of one per month. An incoming four-metre object is brighter than the lower detection limit of current Spaceguard Survey telescopes for just one to two days. Moreover, the survey is limited to the night sky, favours the northern hemisphere and locations away from the galactic plane and the Moon (whose brightness swamps them). This means that they cover about 35% of the celestial sphere.

Ten years ago this much sky was surveyed once a month – now it is mapped several times per month. This means that current capability gives astronomers an approximately 10% chance of spotting a body like 2008 TC3 – in other words, it might do so once a decade. Monthly impactors are harder still to spot, but as they are also more numerous they might also be found with the same frequency. Larger bodies, like that responsible for the devastating Tunguska event over Siberia 100 years ago, should all be bright enough to provide short-term warning – always assuming, of course, that they come out of the 35% of sky that is being mapped.

Despite this encouraging improvement, Chapman is concerned for the future. The new generation of space-watching telescopes, the Panoramic Survey Telescope And Rapid Response System (Pan-STARRS), will be coming on stream over the next decade. One, in Maui, has already seen first light and is operational. These telescopes will be capable of spotting “once a year” (4-6m diameter) objects as early as 10 days out.

Like Catalina telescopes, the new instruments have relatively wide-angle lenses and are designed principally to perform their central cataloguing role in the NEO monitoring program. Also coming along soon will be the Large Synoptic Survey Telescope (LSST), which should see first light in 2015. “LSST says it will ‘cover the sky’ (i.e., 35% of it)” says Chapman, every five nights; which means that it would be able to detect every monthly incoming meteoroid emerging from that part of the celestial sphere. But for meteoritics, there is a snag.

Despite the capability of the Pan-STARRS instrument, current plans are to report on meteoroids “every few weeks” says Chapman. If this operational protocol is not altered, he warned the MetSoc meeting, most of its potential for tracking final plungers to their impact sites would be lost – as would the opportunity to relay information of incoming objects to a more narrow field instruments equipped with spectroscopes, so that the bodies’ reflected-light could be analysed before leaving space for ever. Connecting particular meteorite classes with asteroid spectra of is one of the main tools for determining where meteorites ultimately come from.

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