Close encounters
In April 2029 a 300m-wide rock, called Apophis, will fly past the Earth missing us by as little as 36,000 km. How the asteroid’s close encounter with the Earth will affect the rock’s future orbit, and other factors influencing its flight path, are issues the proposed Apophis Explorer space mission could answer.

If commissioned, a UK-designed space probe, called Apex, could reach the asteroid by 2014, just in time to track the rock, send data back to Earth and enable an accurate prediction of any future risk of collision.

The mission
Mission system’s engineer, Marie-Claire Perkinson, at EADS Astrium UK, describes how their proposal was a response to a Planetary Society competition.

‘The approach we took was to have a very stable orbit around the asteroid,’ says Perkinson. ‘Sending a radio signal to a spacecraft and returning it could measure very accurately the distance to the spacecraft and enable us to reconstruct the orbit of the asteroid's center of mass. If we do that over a long period, we could determine very accurately the orbit of the asteroid itself.’

She continues, ‘one of the things we'd be looking at is wide and narrow angle, low and high-resolution mapping of the surface of the asteroid. From that you can also determine the shape of the asteroid.’

The most novel aspect of EADS Astrium’s approach is the thermal measurement of the Yarkovsky Effect.  This phenomenon occurs as the asteroid rotates. The heat from the sun on one side of the asteroid is released back into space. Temperature variation across the surface affects the orbital path.

Unique features
One of the challenges the team faced was maintaining a stable orbit around a small-bodied object with low gravity. ‘For thermal mapping we've employed a novel technique,’ explains Perkinson, ‘using an electric propulsion thruster to constantly thrust towards the asteroid. We're using the spacecraft to create an artificial gravity to allow a stable orbit for the mapping phase.’

EADS Astrium hopes the submissions will stimulate inter-agency discussion. The logical next step would be for agencies in Europe to participate in a common strategy. But with a launch needed in early 2013 to reach the asteroid in time to collect data, the next step will need to happen soon.