In the accompanying diagram, the most likely position of 430-metre-long asteroid 2004 MN4 is shown at the end of the blue line near the Earth on April 13th, 2029. However, since the asteroid's position in space is not perfectly known at that time, the white dots at right angles to the blue line are possible alternate positions of the asteroid. Neither the nominal position of the asteroid, nor any of its possible alternative positions, touches the Earth, indicating that an Earth impact in 2029 is ruled out. Image credit: Paul Chodas / NASA / JPL.

Over the past week, several independent efforts were made to search for pre-discovery observations of 2004 MN4. These efforts proved successful today when Jeff Larsen and Anne Descour of the Spacewatch Observatory near Tucson, Arizona, were able to detect and measure very faint images of asteroid 2004 MN4 on archival images dating to March 15th, 2004. These observations extended the observed time interval for this asteroid by three months allowing an improvement in its orbit so that an Earth impact on April 13th, 2029 can now be ruled out.

As is often the case, the possibility of future Earth impacts for some near-Earth objects cannot be entirely ruled out until the uncertainties associated with their trajectories are reduced as a result of either future position observations, or in this case, heretofore unrecognized, pre-discovery observations. When these additional observations were used to update the orbit of 2004 MN4, the uncertainties associated with this object's future positions in space were reduced to such an extent that none of the object's possible trajectories can impact the Earth (or Moon) in 2029.

The passage of the asteroid by the Earth in 2029 alters its subsequent trajectory and expands the asteroid's position uncertainty region (i.e., the line of white dots increases in extent) so the asteroid's subsequent motion is less certain than it was prior to the 2029 close Earth approach. However, our current risk analysis for 2004 MN4 indicates that no subsequent Earth encounters in the 21st century are of any concern.

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