Why explore space? Well, the goal is usually to find out about the objects within space, particularly our neighbouring worlds.

Some of us are driven by curiosity: what’s out there, and why aren't Mars and Venus more like the Earth? Is there any microbial life on Mars, or was there once, which has now become extinct? Is there a life-bearing ocean below the icy surface of Jupiter’s moon Europa?

We won’t find definite answers to any of these questions without going to these places. Ideally we need to land and bring back samples, though sending unmanned spacecraft to study these worlds from close orbit and from their surfaces are useful first steps.

To explore like this requires collaboration between scientists of many nations. Personally, I am involved in a British instrument to be flown on a European Space Agency mission to Mercury in 2013, and our closest partners are from Finland, Spain, Germany and Japan.

Most of these nations were at war with each other little more than sixty years ago; nowadays, that such conflict should happen again seems unthinkable, and one of the reasons why is joint ventures in space - or maybe they're a symptom of how far we've come. Whichever way you look at it, international space projects are surely a 'good thing'.

'Buzz' Aldrin stands on the surface of the moon [Image: NASA]

Astronaut Robert L Gibson, Space Shuttle STS-71 mission commander, shakes the hand of cosmonaut Vladimir N Dezhurov, Mir-18 commander during a rendezvous in space in 1995 [Image: NASA]

For others, a significant drive to explore space comes from the lure of commercial gain. Surely, one day, the asteroids will be mined for their content of metals and organic compounds? Some current and recent Japanese and US missions represent the first step down this road.

The 500 metre-long near-Earth asteroid Itokawa, seen in 2005 by the Japanese probe Hayabusa. [Image: ISAS/JAXA]

NASA’s Dawb spacecraft, to be launched to the asteroid Vesta in June 2007 [Image: Background painting, "A cocoon nebula, perhaps the primoridal solar nebula" by William K. Hartmann Courtesy of UCLA]

Others would say that we can’t rely on the Earth for ever. Even if we don’t ruin the climate or cause too much pollution, there are natural threats that will inevitably make life on Earth unpleasant - or even impossible - from time to time.

Impacts by asteroids in the 100 metre to 1 kilometre size range, fall in the 'unpleasant' category. A 100 metre asteroid hits the Earth about every thousand years, and will usually devastate only a small area (unless it hits a shallow sea and causes a tsunami).

Bigger asteroid strikes are rarer (about once every 200,000 years for a 1 kilometre asteroid), but could devastate whole continents. No wonder missions to asteroids also have at least half an eye on determining their composition and strength so that we can learn how to deflect any that we find on a collision-course with the Earth.

Artist’s impression of the European Space Agency Don Quijote mission, conceived as a trial of our ability to intercept and deflect an asteroid posing a threat to the Earth [Image: ESA]

The worst natural disasters, such as impact by a 10 kilometre asteroid (every few million years) or supervolcano eruptions (about every 100,000 years) could make it impossible for a technological civilization to survive. Probably the biggest asteroid to hit the Earth in the past billion years was a ten kilometre asteroid that struck what is now the coast of Mexico, 65 million years ago. This killed off not just the dinosaurs, but made life so intolerable that more than 80% of all land and marine species of animals and many kinds of plants disappeared forever. Such a global catastrophe resulted from the months- to years-long pollution and darkening of the atmosphere because of the debris thrown out of the crater.

Artist’ impression of the asteroid impact that killed-off the dinosaurs. [Image: NASA/Don Davies]

You don't need an asteroid impact to make the sky go black. Very large volcanic eruptions (super-eruptions) can do it too, and, worryingly, these tend to happen more often. We can expect a super-eruption bad enough to cause global ecosystem collapse about once every hundred thousand years. Crops would fail for several years in succession. Some humans would survive this, but billions would die. Do you think civilization would continue, or would the survivors find themselves back in a new Stone Age? Personally, I don't think it is worth the gamble, which is why, for me, the main imperative for space exploration is so that humanity no longer has all its eggs in one basket.

In the very long run, about 5 billion years from now, the Sun will swell into a Red Giant and fry the Earth anyway. Sooner or later (sooner, if we are wise) we have to become a space-faring species, with self-sufficient colonies on the Moon, Mars, asteroids, and maybe even permanently in space.

If we want to survive nearby cosmic catastrophes such as supernova explosions and Gamma-Ray Bursts, we will need to spread throughout the Galaxy. A 'colonisation front' hopping from star to star and expanding much slower than the speed of light, would take only about ten million years to reach across the entire Galaxy.

But if it is so simple, why does no alien species seem to have done it during the ten thousand million years so far available?