Asteroid Apophis passes safely past Earth

Posted by Emma Woollacott

The massive asteroid Apophis shot past Earth last night, on a trajectory that will bring it even closer during its next flyby in 2029.

Observations from ESA's infrared Herschel Space Observatory show it's even bigger than previously believed. The new estimate is that it's about 1,066 feet wide, nearly 20 percent larger than thought.

"The 20 percent increase in diameter translates into a 75 percent increase in our estimates of the asteroid's volume or mass," says Thomas Müller of the Max Planck Institute for Extraterrestrial Physics.

Apophis has had a particularly high profile, thanks to a 2004 study that found it would pass within 22,364 miles of Earth in April 2029, with a 2.7 percent chance of hitting Earth. geostationary satellites, at around 36 000 km, it's since been established that there's no threat from that particular flyby.

However, it's due to make another in 2036, which could pose more of a danger; it's not known just how much, as Apophis's orbit will be affected substantially by the 2029 flyby. There's a silver lining to every cloud, though, and astronomers and engineers have been having great fun working out how to deflect it if necessary, with suggestions ranging from lasers to paintballing.

Tonight's flyby will see it getting within about 14.5 million miles from Earth, says ESA, which got a good look at Apophis last weekend.

"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," says Laurence O'Rourke of the European Space Astronomy Centre in Spain.

"Apophis is only the second near-Earth asteroid observed by Herschel, and these were the fastest tracked observations by the space telescope – the asteroid moved at a rate of 205 arcseconds per hour as seen from Herschel’s viewpoint."

 By analysing the heat emitted by Apophis, Herschel now also has a new estimate of the asteroid’s albedo: 0.23. This means that 23 percent of the sunlight falling onto the asteroid is reflected; the rest is absorbed and heats it up.

Knowing this indicates how Apophis' orbit might be altered due to subtle heating by the sun - or indeed by lasers or paintballs.

"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," says Göran Pilbratt, ESA’s Herschel project scientist.

"Our unique Herschel measurements play a key role for the physical characterisation of Apophis, and will improve the long-term prediction of its orbit."