Issue number 25 : 20/05/2004
A scientific publication by SGF and NEODyS

2004 FH: don't panic!

by Ettore Perozzi

When going through one of the many astronomical calendars available on the internet, side by side with classical celestial phenomena such as eclipses and occultations, an ever increasing number of close encounters of NEOs with the Earth is found. But - don't panic! - this does not mean that an increasing number of NEOs are actually passing close to our planet: that would surely be a source of deep concern. It is good news instead: the increasing number of close encounters on record reflects the improvement in our ability to timely detect smaller and smaller NEOs as they routinely approach our planet. In other words, we are starting to fill the observational gap between "asteroids" and "meteorites". No wonder then that "minimum distance" record-breaking events do often happen. The last "champion" of this kind is 2004 FH, which on March 18, 2004 passed as close as 40,000 km from the surface of the Earth: roughly one tenth the distance of the Moon or seven Earth radia. Just above the geostationary ring of telecommunication satellites. Quite close indeed.

Yet don't panic does not necessarily mean don't worry, thus making it reasonable to try to figure out what would have happened if 2004 FH were on a collision course with our planet. Probably nothing. The estimated size of 2004 FH, is about 30 meters which, in the assumption of a rocky composition, would imply an explosion in the atmosphere without any consequence on the ground. Notwithstanding this, there are a number of issues which deserve more attention. First of all the uncertainty in the size determination and in the composition of 2004 FH. The former is derived from the absolute magnitude, which does not take into account the possibility of an unusual albedo, while the latter is only a tentative guess, based on the most common spectral type among NEOs. Furthermore, a 30 meter sized object is just below the threshold over which an impact can be dangerous. Current estimates indicate that the Tunguska event was generated by a 60 meter impactor exploding before reaching the ground. Some statistics is also meaningful. The population of known NEOs has become three times larger in the last four years (their number is quickly approaching 3000 units), yet it has been computed that the total number of objects with a diameter greater than 100 meters is about 100,000: a figure that rises up to one million when lowering the limiting size to 50 meters.

In conclusion, the case for 2004 FH can be considered as a good example to fully understand the three basic goals of current NEO studies: a) improve the sky surveys for discovery and follow-up; b) increase ground based observations aimed at determining the compositional parameters; c) realizing in-situ exploration by means of dedicated space missions. This would provide the ground truth needed for planning and implementing an efficient deflection strategy. Just in case of panic.