At 06:21 GMTShort for Greenwich Mean Time this morning, with a thunderous roar, the mighty Falcon 9 rocket took off from Vandenberg Space Force Base in California. Carried by its thrust was a spacecraft called DartShort for Double Asteroid Redirection Test., bound on a journey of 6 million miles. Its mission, to be completed in 10 months' time: to smash into the surface of an asteroid to change its orbit.

The aim is to tackle a threat which could be catastrophic - that of an asteroid crashing into Earth. This is what scientists believe happened 66 million years ago, throwing enough debrisThe remains of something that has been destroyed. It comes from the French word "debriser" (to break up). When the Chinese destroyed one of their own weather satellites, it produced an estimated 150,000 pieces of debris. into the atmosphere to obscure the sun, and resulting in the extinction of the dinosaurs.

Earth is bombarded with debris from space, most of which disintegrates. The chances of an asteroid hitting us are thought to be small.

It has been estimated that an asteroid 10km across would wipe out the human race. At £240m, Dart - a joint project between NASAThe National Aeronautics and Space Administration, responsible for the US space programme. and the European Space AgencyFounded in 1975, it is based in Paris and has a membership of 22 countries, including Britain.  - looks like a good investment.

Dart is headed for a pair of asteroids, neither of which poses a threat to Earth. The smaller one, Dimorphos, is 160m long. The larger one, DidymosA Greek word meaning twin., is five times bigger.

Both asteroids orbit the Sun. Dimorphos orbits Didymos. The two will be at their closest point to the Earth - 6.7 miles away - at the end of September 2022. Dart will be there to meet them.

Dart's target is Dimorphos, which the 610kg spacecraft is supposed to hit at 15,000 mph. Scientists believe this will nudge it closer to Didymos and shorten its orbit around the larger asteroid by about 10 minutes.

Though the shift would be very small, "Really that's all that's needed in the event that an asteroid is discovered well ahead of time," according to a NASA spokesperson.

Assessing the results is another challenge. Dart is carrying a small satelliteIt is called LICIACube, which is short for Light Italian CubeSat for Imaging of Asteroids. which is designed to separate before impact and send pictures back to Earth. These will be analysed together with observations from telescopes on Earth and those from another spacecraft called Hera, which will follow in 2024.

Didymos and Dimorphos are so small and so far away that even the most powerful telescopes can only see them as a single point of light. But Dimorphos blocks some of the light from Didymos whenever it passes in front of it, which allows scientists to time its orbit.

While the project is undoubtedly ingenious, some people have reservations. One expert, Natalie Trevino, compares it to damming a river for humans' benefit without considering the environmental consequences.

Yes. The effect of a large asteroid hitting the Earth would be appalling. It is only right to take measures to stop it happening - and this project suggests the degree of interference needed would be tiny.  

No. Humans have meddled enough with their own planet, and we are now seeing the awful consequences of that. We should focus on climate change rather than embarking on projects in space.