Prepared by Richard P. Binzel , Secretary WGNEO,

on behalf of the Working Group on NEOs of IAU Division III

STATEMENT DATE: 30 December 2004


Precise determination of the orbit (and future location) of an asteroid or comet typically requires measurements of its position and motion in space over a period of many months to years. Naturally for a newly discovered asteroid or comet, the time baseline for these position and motion measurements is limited by the recentness of the discovery. Consequently, orbit calculations based on these limited data can yield only preliminary results. In some cases, these preliminary results may indicate one or more future close approaches with the Earth for which the possibility of a collision is not completely ruled out. Because the results of these orbit calculations are only preliminary, they are subject to revision as new data become available.

The International Astronomical Union Working Group on Near Earth Objects (WGNEO) provides, as a service to the international astronomical community, expert review of reports of newly discovered objects for which preliminary orbit calculations indicate some possibility of a collision on a future date. Individuals and organizations are free to make their discoveries, observations, and calculation results public at any time. If they so choose, the discoverer or the scientist(s) producing the orbit calculations may voluntarily submit their report to the WGNEO for Technical Review. The purpose of the WGNEO Technical Review is to provide independent expert verification of the preliminary orbit solution based on all currently available data.

This IAU WGNEO announcement pertains to a near-Earth object designated as 2004 MN4. This object was discovered on 19 June 2004 by Roy Tucker, David Tholen and Fabrizio Bernardi of the University of Hawaii Asteroid Survey (UHAS), from Kitt Peak, Arizona, and observed over two nights. Most likely this object is a small asteroid about 400m in diameter. On 18 December, the object was rediscovered from Australia by Gordon Garradd of the Siding Spring Survey. Further observations from around the globe over the next several days allowed the IAU Minor Planet Center to confirm the connection to the June discovery. Orbit calculations based on the observed path over the period 19 June through 27 December were performed and cross-verified by two separate systems, CLOMON2 in Pisa/Valladolid and Sentry in Pasadena, where this cross verification satisfied the requirements for a Technical Review. Beginning with observational data available on 23 December, these solutions indicated a non-negligible impact probability with the Earth corresponding to a value of 2 on the 10 point Torino Scale. Continual updates to these solutions were performed as new data were obtained, with these ongoing preliminary results giving a chance of as much as about 1 in 40 for a collision with Earth on 13 April 2029. This collision probability corresponded to a 4 on the 10 point Torino Scale, which describes Level 4 as:

     "A close encounter, meriting attention by astronomers.  
      Current calculations give a 1% or greater chance of 
      collision capable of regional devastation.   Most likely, 
      new telescopic observations will lead to re-assignment 
      to Level 0 [No Hazard].  Attention by the public and by 
      public officials is merited if the encounter is less than 
      a decade away." 
These calculations also indicated a value of about +1.0 on the Palermo Technical Impact Hazard Scale, meaning that the potential 2029 impact was about ten times more likely than a random impact of a similar-sized object before 2029.

As appropriate for the Torino Scale 4 rating, astronomers continued their routine positional measurements of 2004 MN4 so as to extend its orbital arc over a longer baseline to allow further refinement to its orbit and to the Earth close approach situation in 2029. At the same time, astronomers also methodically searched through archives to locate "prediscovery" observations made prior to June 2004. These archive search efforts proved successful on 27 December when Jeff Larsen and Anne Descour of the Spacewatch Observatory near Tucson, Arizona, were able to detect and measure very faint images of 2004 MN4 on archival images dating to 15 March 2004. These observations extended the observed orbital arc for this object by an additional three months. The resulting improvement to the orbit solution allowed any chance of an Earth impact on 13 April 2029 to be ruled out, reducing the hazard index for this encounter to 0 [No Hazard] on the Torino Scale. The current orbital solution also indicates that no future Earth close approaches by 2004 MN4 during the 21st century rank higher than 1 ["Normal"] out of 10 on the Torino Scale. These 21st century encounters also merit a score of -2.0 or less on the Palermo Technical Impact Hazard Scale, meaning that a random impact of a similar-sized object is about 100 times more likely than a 2004 MN4 impact.

The IAU WGNEO commends all parties, beginning with both amateur and professional observers, for routinely making publicly available in real-time all observations and cross-verified collision probabilities through Minor Planet Electronic Circulars issued by the IAU Minor Planet Center and through postings on the NEODyS and NASA Near-Earth Object Program Office web pages. At all times, these parties also appropriately gave greatest emphasis to the likelihood that further observations would rule out any chance of collision.