The Near-Earth asteroid (NEA) named 2004 MN4 began to attract the attention of asteroid researchers just before Christmas of 2004. Don Yeomans of JPL summarized the information available on December 22 as follows: 2004 MN4 was discovered on 19 June 2004 by Roy Tucker, David Tholen and Fabrizio Bernardi from Kitt Peak, Arizona, and observed over two nights. On 18 December, the object was rediscovered from Australia by Gordon Garradd of the Siding Spring Survey. Using further observations from around the globe over the next several days, the possibility of impact in 2029 was realized by both the automatic SENTRY system of NASA's NEO Program Office and the similar automatic system at the University of Pisa (NEODyS). The diameter of MN4 was estimated to be about half a kilometer (later revised downward slightly to 400 m). The predicted flyby would take place on April 13, 2029, and the possibility of an impact could not be ruled out.
On December 23 the Pisa system, using improved data, calculated that the nominal orbit solution resulted in a close approach to the Earth at 780,000 km distance. Andrea Milani agreed that, given the current knowledge of the orbit, it was not possible to exclude an impact, although he noted his expectation that further monitoring and further analysis on this object would eliminate its potential hazard. At that time the Pisa system estimated a probability of impact of 1 in 170, which would lead to a Torino Scale value of TS=2, the highest yet found for an NEA. TS=2 means ıan object making a somewhat close but not highly unusual pass near the Earth. While meriting attention by astronomers, there is no cause for public attention or public concern as an actual collision is very unlikely.ı The results from both Pisa and JPL were posted on the Internet but, given that it was just two days before Christmas, there was little interest beyond the NEA astronomers themselves.
On December 24 the addition of further new data increased the calculated probability of an impact rather than lowering it, as most commonly happens. The odds of impact were now calculated at 1 in 60, pushing MN4 to TS=4. The description of TS=4 includes the statement that public officials should be notified if the possible collision is less than a decade in the future, which for this object was not the case.
Journalist Rob Britt of Space.com picked up the story on December 24 and also ran an updated story on Christmas titled Asteroid With Chance of Hitting Earth in 2029 Now Being Watched ıVery Carefullyı. He noted that the impact probability had increased again, to 1 in 45. Britt wrote, ıAstronomers still stress that it is very likely the risk will be reduced to zero with further observations. And even as it stands with present knowledge, the chances are 97.8 percent the rock will miss Earth. The asteroid's risk rating for a possible impact scenario on April 13, 2029 has now been categorized as a 4 on the Torino Scale. The level 4 rating -- never before issued -- is reserved for events meriting concern. Asteroid 2004 MN4 is an unusual case in that follow-up observations have caused the risk assessment to climb -- from Torino level 2 to 4 -- rather than fall. [This asteroid] is bigger than the space rock that carved meteor crater in Arizona, and bigger than one that exploded in the air above Siberia in 1908, flattening thousands of square miles of forest. If an asteroid the size of 2004 MN4 hit the Earth, it would do considerable localized or regional damage. It would not cause damage on a global scale.ı With a diameter of 400 m, MN4 actually had a mass (and energy) more than a hundred times greater than either the Meteor Crater or Tunguska impacts, which are estimated at 10-15 megatons of energy.
On December 26 the tragic Indian Ocean tsunami occurred, and science journalists concentrated their efforts on that story. Meanwhile, astronomers continued to search for pre-discovery observations that would allow further improvement in the orbit calculations for MN4. They were successful, and by mid-day on December 27 Don Yeomans could report an ıall clear.ı With the improved orbit, the closest distance on April 13, 2029, was calculated to be 60,000 km with no chance of impact. (On January 25 this miss distance was further refined to 65,000 km.) At that distance it should be bright enough (about magnitude 4) to be visible without a telescope for viewers in Europe and Asia.
This case of MN4 was unusual in two ways: for a few days it yielded a value of TS=4, far higher than any encountered previously, and for several days the addition of new data served to increase the calculated odds of impact. However, just like a run of good (or bad) throws of the dice, this trend reversed and the calculated impact probability went to zero. This is, of course, what we expect for any particular NEA. The odds are overwhelming that they will miss us. But if we were just playing the odds, we would not be in this business. We are in it because there is a possibility of the statistical fluke, of finding an asteroid that will hit in our lifetimes. From that perspective, we cannot assume that time will resolve all threats -- even though that is the likely outcome for any particular NEA.