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(99942) Apophis = 2004 MN4 (First minor planet to have a Torino Scale 4 Earth impact prediction)

Apophis was discovered during an observing run at Kitt Peak on 19 June 2004 by Roy Tucker, David Tholen and Fabrizio Bernardi and given the temporary designation2004 MN4, but was only seen for two days and its orbit was not recognised at that point as one to cause concern.

It was then independently rediscovered on 18 Dec 2004 by Gordon Garrard at Siding Spring in Australia and within four days, as more observations came in it was posted by both the JPL Sentry and NEODys CLOMON2 automated systems as the first ever object predicted with a Torino Scale of 2 (A discovery... of an object making a somewhat close but not highly unusual pass near the Earth, meriting attention by Astronomers).

However, by 24 Dec 2004, as further observations came in, both risk calculators raised their rating to Torino Scale 4 (A close encounter, meriting attention by astronomers. Current calculations give a 1% or greater chance of collision capable of localized destruction). Up until that point no other asteroid had exceeded a Torino Scale 1 rating (A routine discovery in which a pass near the Earth is predicted that poses no unusual level of danger).

As updates were issued by the Minor Planet Center several times a day the predictions were refined and by 27 Dec 2004 had been calculated by JPL to be a 1 in 37 chance of the object hitting Earth just after 21h on Friday 13 April 2029. Many other possible impact predictions, though less likely than the first 13 Apr 2029 event were also found.

Later on 27 Dec, very faint pre-discovery images were found, taken on 15 Mar 2004 from the Steward Observatory, Kitt Peak and published on MPEC 2004-Y70, effectively extending the observed arc from 190 days to 288 days. Revised predictions from JPL's Sentry system and NEODys's CLOMON2 system issued 28 Dec 2004 and taking into account these newly found observations allowed the possible impact solution for 13 Apr 2029 to be discounted and the object was downgraded to a Torino Scale 1 rating, with the most likely chance of it hitting Earth now being given by JPL as a 1 in 43,000 chance on 13 Apr 2053.

See the JPL news release 'Possibility of an Earth Impact in 2029 Ruled Out for Asteroid 2004 MN4' and the 2004 MN4 Earth Impact Risk Summary  page for full details

At Great Shefford Observatory (latitude +51.5N) 2004 MN4 throughout Dec 2004 was a difficult object, being at best just a couple of degrees above the horizon at discovery and just 16 altitude by the end of the month. The image above was taken with the asteroid at  less than 10 altitude and with the full moon brightening the sky. A brief interval of just 18 minutes for imaging was created earlier in the day by 're-arranging' an apple tree with a bow saw. However, astrometry obtained from these images was not of good quality and was not submitted.

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