Apophis: why Nasa takes a special interest in the asteroid's latest pass






While there is no cause for alarm, similarly there is no room for complacency either. Apophis remains on the list of Potentially Hazardous Asteroids compiled by the International Astronomical Union's Minor Planet Center.



Having crossed outside Earth's orbit, Apophis will appear briefly in the night-time sky. Wednesday 9 January will afford astronomers the rare opportunity to bring a battery of telescopes to bear: from optical telescopes to radio telescopes to the European Space Agency's Infrared Space Observatory Herschel. Two of the biggest unknowns that remain to be established are the asteroid's mass and the way it is spinning. Both of these affect the asteroid's orbit and without them, precise calculations cannot be made.

Another unknown is the way sunlight affects the asteroid's orbit, either through heating the asteroid or the pressure of sunlight itself. Russia has announced tentative plans to land a tracking beacon on Apophis sometime after 2020, so that its orbit can be much more precisely followed from Earth.

Wednesday's pass is only really close by astronomical standards, taking place at around 14.5 million kilometres above Earth's surface. The moon's orbit is 385,000 km. The 2029 close pass is another matter entirely, however.


So, although Apophis poses no immediate danger, we are almost certain to hear a lot more about it over the coming years and decades. Apart from all the science we can learn, its orbit's proximity to Earth's makes it a potential target for future robotic and even manned missions.
