UK space scientists and engineers have designed a mission to investigate a potentially hazardous asteroid.

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.

"The real prize for us would be if the European or US space agencies thought there was merit in our proposal and asked us to carry the feasibility study forward," said Dr Mike Healy, the company's space science director.

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

Early warning

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).

APOPHIS - SPACE ROCK

Orbits the Sun every 324 days, crossing Earth's orbit twice

Not spherical, diameter of 300m; a mass of 27m tonnes

Name: Ancient Egyptian god Apep ('The Destroyer')

Discovery: In 2004 by a University of Hawaii team

Talk of a possible strike on another visit in 2036 has also been dampened by astronomers who have kept a careful watch on the rock's progress through space.

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.

Political support

At the moment, scientists stress Apophis is not thought to be a serious danger to Earth.

But 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.

The dinosaurs are thought to have suffered in a space impact

Rocks thrown up into the air by the impact would rain down - many of them huge lumps a metre wide.

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.

The issue of asteroid or comet strikes is a topical one as researchers continue to gather more information about their frequency during Earth history.

At least one of the planet's mass extinction events - which included the demise of the dinosaurs 65 million years ago - has been attributed to the impact of a large space object.

Liberal Democrat MP Lembit Opik has campaigned for the research area to be given more funding. His grandfather, renowned Estonian astronomer Ernst Opik, did much to raise science's understanding of Earth-crossing comets and asteroids.

The politician told the BBC News website: "The question isn't whether Earth is hit by an asteroid - it is when.

"Good luck to Astrium; they are showing that if we have the political will, we certainly have the technical know-how to do something about threatening objects."

The US-based Planetary Society has organised its competition in co-operation with the European Space Agency (Esa), the US space agency (Nasa), the Association of Space Explorers (ASE), the American Institute of Aeronautics and Astronautics (AIAA), and the Universities Space Research Association (USRA).

The winning entry will be submitted to space agencies to see if they want to carry the ideas through.