Apophis: mark your calendar for April 2029

If I live, I'll be almost 70 when Apophis flies by. I hope that our orbital calculations are as good as we think they are, I'm looking forward to the show. I recall reading about this in 2004, but I didn't remember that the passage will be so close ...

BBC NEWS | Science/Nature | UK plan to track asteroid threat

.... The 300m-wide (980ft) rock, known as Apophis, will fly past Earth in April 2029 at a distance that is closer than many communications satellites.

Astrium, based in Stevenage, Herts, wants a probe to track the asteroid so its orbit can be better understood.

The concept will compete for a $50,000 (£25,000) Planetary Society prize, but a full mission would cost millions.

The British design calls for a small, remote-sensing spacecraft, dubbed Apex (Apophis Explorer), which could rendezvous with Apophis in January 2014.

It would then spend the next three years tracking the rock, sending data back to Earth about the object's size, shape, spin, composition and temperature.

From this information, orbit modelling would enable a more accurate prediction of the risk of any future collision.

Astrium says that if its concept won the prize, it would donate the money to charity...

... A full mission would be expected to cost about $500m (£250m) dollars to develop, launch and operate.

Apophis caused some consternation in 2004 when initial observations suggested there was an outside chance it might hit Earth in 2029.

Further study by ground-based telescopes indicated there was virtually no possibility of this happening, and the expectation is that the object will whiz past the Earth at a close but comfortable distance of just under 36,000km (22,400 miles).

However, there is always some uncertainty associated with an asteroid's orbit.

One reason is the Yarkovsky effect. This describes what happens when an asteroid radiates energy absorbed from the Sun back into space.

Releasing heat in one direction nudges the object in the opposite direction. The resulting acceleration is tiny, but over the centuries acts like a weak rocket and could make the difference between a hit or a miss in some circumstances.

The close encounter with Earth in 2029 will also perturb Apophis' orbit gravitationally.

A mission like Apex to track and study the rock would help reduce uncertainties and give solid predictions about the rock's course long into the future...

... were such a rock to hit the planet, it could cause devastation on a country-wide scale, leading possibly to the deaths of many millions of people.

An Apophis-like object striking at about 20km/s (45,000mph) would gouge a crater 5km (three miles) wide. Even standing 30km (18 miles) away from the impact site, a thermal blast would ignite your clothes and the ground would shudder with an earthquake measuring more than six on the Richter Scale.

Given sufficient warning, though, a potential impactor could be deflected out of Earth's path, scientists believe.

Some have suggested such a rock might be nudged on to a safe trajectory by hitting it with a small mass. Others have proposed flying a spacecraft next to the object, to use gravity to tug the asteroid clear of the planet....

See also the New York Times, March 2007 for a related discussion focusing on general impact management.