Apophis (known as 99942 and 2004 MN4) is a near-Earth asteroid discovered on June 19, 2004 by R. A. Tucker, D. J. Tholen and F. Bernardi at the Kitt Peak National Observatory.
ESA’s Herschel observed the asteroid on January 5-6, 2013 during about 2 hours on its approach to Earth at about 14.5 million km.
“As well as the data being scientifically important in their own right, understanding key properties of asteroids will provide vital details for missions that might eventually visit potentially hazardous objects,” said Dr Laurence O’Rourke of the European Space Astronomy Center in Spain.
Herschel provided the first thermal infrared observations of Apophis at different wavelengths, which together with optical measurements helped refine estimates of the asteroid’s properties.
Previous estimates bracketed the asteroid’s average diameter at 270 ± 60 m; the new observations returned a more precise diameter of 325 ± 15 m.
“The 20% increase in diameter, from 270 to 325 m, translates into a 75% increase in our estimates of the asteroid’s volume or mass,” explained Dr Thomas Müller of the Max Planck Institute for Extraterrestrial Physics in Garching, Germany, who led the analysis of the new Herschel data.
By analyzing the heat emitted by Apophis, Herschel also provided a new estimate of the asteroid’s albedo of 0.23. This value means that 23% of the sunlight falling onto the asteroid is reflected; the rest is absorbed and heats up the asteroid. The previous albedo estimate for Apophis was 0.33.
“These numbers are first estimates based on the Herschel measurements alone, and other ongoing ground-based campaigns might produce additional pieces of information which will allow us to improve our results,” Dr Müller said.
“Although Apophis initially caught public interest as a possible Earth impactor, which is now considered highly improbable for the foreseeable future, it is of considerable interest in its own right, and as an example of the class of Near Earth Objects,” said Dr Göran Pilbratt, ESA’s Herschel Project Scientist. “Our unique Herschel measurements play a key role for the physical characterization of Apophis, and will improve the long-term prediction of its orbit.”