The sheer expense of manned space travel remains an obstacle for those of us who have been waiting 40 years now for someone to pick up where America left off with the last Apollo mission to the Moon. The USA's retirement of the Space Shuttle this year marks its effective withdrawal from the business of sending humans into space, for several years at least.

The politicians have set a target of 2016 for the development of a new Earth-to-orbit launcher. NASA has said that this is impossible without more money, but the government has refused to move, despite pulling funding last year from the Constellation program, whose goal it was to build just such a vehicle.

Why it isn't possible to just build a rocket to one of the many designs already tried and tested, remains a mystery. The Russians are still using the same basic Soyuz capsule developed in 1964, selling seats on it now at $50 million per ticket to take American astronauts to their own space station.

But just as manned missions have disappointed since Apollo, discoveries made by robotic spacecraft continue to stun and amaze us.

In July our newest asteroid mission reaches the asteroid Vesta. Called Dawn, the robot will begin a ten-month program there, before leaving orbit and travelling on to Ceres.

Though the largest of all the asteroids, that belt of small worldlets circling the Sun between Mars and Jupiter, Ceres weighs in at only one-sixtieth of the Earth’s mass. Dawn will arrive there in early 2015 for five more months of study, becoming the first spacecraft ever to orbit first one and then another solar system body (apart from Earth!).

Three times as far from the Sun as the Earth, asteroids preserve the material of the original solar nebula, that ancient cloud of gas and dust from which the Sun and planets formed, four billion years ago.

We can then compare this to the Earth, whose materials have been completely cooked and churned around by solar heat and plate tectonics.

Asteroids are also important because their gravity is so weak. Much of the effort of space travel at present involves simply getting off the Earth. If 1000 tonnes of fuel gets you a couple of hundred kilometres up into low Earth orbit, only another 400 tonnes would give you the planets.

But at present everything we put into space - rockets, habitats, machines, fuel, oxygen and water - still takes this hideously expensive route.

Ceres’ surface gravity is only one-fortieth of Earth’s, and the raw materials needed to build the things we need in space are there, at the bottom of a very shallow gravity well.

Already mining and industrial complexes exist on Earth that are almost entirely run by robots. All humans do is supervise.

The materials and hardware for the real conquest of the solar system will not be mined or built on Earth, but on small, airless worlds like Vesta and Ceres. Even the Earth’s Moon fits the bill, its nearness to us making up, to some extent, for its deficiency in heavy metals and a surface gravity seven times greater than Ceres’.

Robots will do the work, and then humans will arrive, with nothing more to do than move in.

Even rocket fuel will be only a minor issue. Launches won’t just be cheap. They’ll be free. With no atmospheric drag, payloads from such worlds can be accelerated to orbital speeds at ground level, using no fuel at all, via linear electric motors similar to those used on some of the current generation of ultra-fast trains, powered entirely by solar energy.

This may sound like a pipe-dream, but at some stage a tipping point will be reached, and suddenly the ball will start rolling very quickly indeed.

The trigger may be economic or technological, political, social or even military. But one day it will come. It's no accident that the target date for America's new launch vehicle is the same year as a presidential election.

Perhaps someone will discover an asteroid which is virtually a solid lump of iron, or nickel. One of Dawn's goals is to look for mascons - dense concentrations of heavy metals, close to the surface of the asteroids it will visit.

Perhaps one country poor in resources but advanced in technology will get tired of all the international trade wrangling, take the plunge and set up a mining and manufacturing operation on the Moon. This decision would precipitate a space-race that would eclipse the sixties like a nuclear bomb over a firecracker.

Within a few decades, whoever owns these mines and factories will become rulers of the solar system, independent of those annoyingly high launch costs from Earth, able to travel and live almost anywhere for next to nothing.

Because, once begun, the process will accelerate exponentially, and of all the space hardware they manufacture, among the first things the robot factories build will be more robot factories.