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An “intruder” asteroid was detected by Australian comet hunter Rob McNaught at an observatory on Siding Spring Mountain on Friday night and will have a close encounter with Earth at about 1am tomorrow.

The object, estimated to be around 35-60 metres at its widest, is similar in size to the dead comet shard that exploded with the force of a large  hydrogen bomb over a then largely uninhabited region of central Siberia on 30 June 1908.

This time there will be no collision, but the object catalogued as 2009 DD45 will come as close as around 63,000 kilometres from the earth’s surface somewhere over the Pacific west of Tahiti.

Given the speed, mass and orbit of the object, this is a wafer-thin encounter, and it is locked into a short period earth-crossing orbit which means it will return and be a future danger to the planet.

2009 DD45 will not be visible to the naked eye but should be easily observed by those who know how to use any modest tracking mounted telescope and an updated ephemeris.

It will move rapidly through the constellation Virgo at an apparent rate of about half a lunar diameter (LD) per minute with a visual magnitude of 10 on current estimates.

The intruder is already inside four LD units or lunar earth distances, meaning earth’s gravitational field will substantially modify its future orbits and this bending effect is causing some uncertainty as to its precise trajectory during tomorrow morning’s closest approach.

For the astronomers working on the Siding Spring Survey of potentially dangerous near earth objects or NEOs the closeness of the approach will allow more accurate predictions of future visits by this intruder, which is big enough to incinerate a large city if it hits Earth or as is more likely, turns into a Tunguska-like fireball on hitting the atmosphere at a closing velocity of tens of kilometres per second.

The fast detection and observation of 2009 DD45 is a major “trophy” for the Siding Spring Survey which is jointly operated by the Australian National University and the University of Arizona and supported by NASA funding.

Like the much larger earth crossing asteroid Apophis, which will pass even closer to Earth on 13 April 2029, and may then suffer enough gravitational deflection to subsequently collide with us on 13 April 2036, 2009 DD45 is also trophy that will remain at large until humans shift it into a safer orbit, or it runs into us.

Peter Fray

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