Tracking a dangerous asteroid

Illustration of a spacecraft heating part of Apophis' surface using mirrors

One way to tackle an errant asteroid is to heat part of its surface. Material will be expelled one way, and the asteroid will move slightly in the opposite direction. Done far enough ahead of any potential collision, it will keep a rocky body from coming near Earth.

  • Asteroid Apophis will make a close approach to Earth in 2029
  • Has the potential to strike our planet later this century
  • More observations are needed to clarify its orbit

ASTRONOMERS HAVE TAKEN THE FIRST new images in over three years of the potentially dangerous near-Earth asteroid Apophis as it emerged into view from behind the Sun.

The object became famous in late 2004, when it appeared to have a 1-in-37 chance of colliding with Earth in 2029, but additional data eventually ruled out that possibility.

However, on April 13, 2029, the asteroid, which is 270 metres in diameter, will come closer to Earth than the geosynchronous communications satellites that orbit Earth at an altitude of about 36,000 kilometres. Apophis will then be briefly visible to the naked eye as a fast-moving starlike object.

This close encounter with Earth will significantly change Apophis’s orbit, which could lead to a collision with Earth later this century. For that reason, astronomers have been eager to obtain new data to further refine the details of the 2029 encounter.

Astronomer David Tholen (University of Hawaii (UH) at Manoa), one of the co-discoverers of Apophis, and graduate students Marco Micheli and Garrett Elliott obtained the new images on January 31 using the UH 2.2-metre telescope on Mauna Kea, Hawaii.

Composite image of Apophis

Apophis (circled) in a composite of five exposures taken on January 31 with the University of Hawaii 2.2-metre telescope. (The blemish in the upper left corner is an artefact caused by a dust speck on the camera.)

At the time, the asteroid was less than 44 degrees from the Sun and about a million times fainter than the faintest star that the average human eye can see without optical aid.

“The superb observing conditions that are possible on Mauna Kea made the observations relatively easy,” said Tholen.

Out of the glare

Astronomers measure the position of an asteroid by comparing it with the known positions of stars that appear in the same image as the asteroid. As a result, any tiny error in the catalogue of star positions—due, for example, to the very slow motions of the stars around the centre of our Milky Way galaxy—can affect the measurement of the position of the asteroid.

“We will need to repeat the observation on several different nights using different stars to average out this source of imprecision before we will be able to significantly improve the orbit of Apophis and therefore the details of the 2029 close approach and future impact possibilities,” noted Tholen.

Apophis’s elliptical orbit around the Sun will take it back into the Sun’s glare in the middle of 2011, inhibiting the acquisition of additional position measurements.

However, in 2012, Apophis will again become observable for approximately nine months. In 2013, the asteroid will pass close enough to Earth for ultra-precise radar signals to be bounced off its surface.

Adapted from information issued by the University of Hawaii at Manoa. Apophis image courtesy D. Tholen, M. Micheli, G. Elliott, UH Institute for Astronomy. Apophis illustration courtesy SpaceWorks Engineering, Inc.

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