Using Herschel data, mission scientists have deduced that Apophis is 1,066 feet (325 meters) wide. That's 20 percent larger than the previous estimate of 885 feet (270 meters).
"The 20 percent increase in diameter, from 270 to 325 m, translates into a 75 percent increase in our estimates of the asteroid's volume or mass," said Thomas Müller of the Max Planck Institute for Extraterrestrial Physics in Garching, Germany, and lead scientist of the study.
Herschel was also able to map the heat emitted from Apophis' surface, providing astronomers with valuable data on the object's albedo (or reflectivity). It turns out that the asteroid reflects 23 percent of sunlight that hits it (it therefore has an albedo of 0.23). 77 percent of the sunlight is absorbed by the space rock, causing heating. Previous albedo estimates were higher - 0.33 - meaning Apophis is darker than previously thought.
This factor is critical. Knowing how much light is absorbed and reflected can significantly alter orbital models of asteroid orbits. Sunlight imparts a tiny, yet measurable force on asteroids and heat emitted from the asteroid can do the same (called the Yarkovsky effect). As an asteroid rotates, it is heated on the surface facing the sun and cools on the surface facing away. This heating/cooling cycle can cause changes in the trajectory of the asteroid over long periods of time.
"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," added Müller.
"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 Göran Pilbratt, ESA's Herschel Project Scientist, in a statement.
"Our unique Herschel measurements play a key role for the physical characterization of Apophis, and will improve the long-term prediction of its orbit."
Live SLOOH Flyby Broadcast
In an effort to observe the asteroid, a public event has been planned for tonight at 7pm EST (midnight, Jan. 10 GMT) through the SLOOH Space Camera. Although the faint dot of Apophis would be an unlikely target for backyard telescopes, Slooh will livestream views from professional observatories in Italy and the Canery Islands via the Virtual Telescope Project so you can see the space rock for yourself. Make sure you tune in!
Images: Top: Apophis temperature model using data from Hershel. Temperature scale is in Kelvin. Credit: ESA/Herschel/MACH-11/T.Müller MPE (Germany). Middle: Herschel's three-color view of asteroid Apophis. Credit: ESA/Herschel/PACS/MACH-11/MPE/B.Altieri (ESAC) and C. Kiss (Konkoly Observatory)