More Data Yields Better Information

Remember that near-Earth asteroid that whizzed by us last week and caused a flurry of excitement? There was a lot of speculation about how much damage 2009 DD₄₅ would have caused if it had hit our planet. Several of us wrote about that in a special New York Times blog discussion and there were numerous press reports about “‘Roid Rage” and so forth.

Well, with the passage of time and continuing observations of this tiny interloper, astronomers have been able to tighten up the size estimates on 2009 DD₄₅. In a short note to subscribers of the NEO News listserv, scientist David Morrison reported:

Rick Binzel and colleagues observed DD₄₅ in the infrared from Mauna Kea and reported as follows to the Central Bureau for Astronomical Telegrams: R. P. Binzel, M. Birlan, and F. E. DeMeo, Paris Observatory, made 0.8- to 2.5-micron spectroscopic measurements on Mar. 2.6 UT using the NASA Infrared Telescope Facility 3-m reflector on Mauna Kea. Absorption bands revealed at 1 and 2 microns show the characteristics of the S-type class of asteroids. Using the average albedo value of 0.36 for small NEAs in this class and based on its observed brightness, the diameter is estimated to be 19 +/- 4 m. This is near the lower size limit and, together with the stony composition, suggests that this object would likely have disintegrated too high to do any ground damage. Knowing the size also allows us to revise the estimate of how often a NEA of this size passes this close to Earth, to roughly once per year.

So, essentially what happened is that observers figured out what this object is made of — a stony mix of silicates and other rocky materials. We know how much light such materials can reflect (what their brightness, or albedo, is), and based on the observed brightnesses, astronomers were able to figure out a size of somewhere between 15-23 meters (roughly  50 to 75 feet) in diameter. That’s not large enough to make it safely through the atmosphere — this object would have burned up had it been on a collision course with Earth.

This illustrates the value of continuing observations of asteroids, or any celestial object for that matter. The more you observe, the more you learn and the more you ultimately know about an object.  No doubt continued observations of DD₄₅ will refine its orbital parameters, its spin, and other factors about this piece of rock that orbits the Sun and occasionally comes close to Earth.