What next for the human exploration of space? One idea is to send the next generation of astronauts to explore a near Earth asteroid.

Let’s set aside, for a moment, the question of whether human exploration of space is viable and look at the supposed benefits of visiting a passing rock.

First, asteroids are of enormous scientific interest, being remnants of the primordial Solar System. Second, they need to be well-characterised so that we can head one off should it ever come our way. And finally, they may provide the raw materials and resources for future missions which can use them as stepping stones to Mars and beyond.

But what makes near Earth asteroids particularly inviting from an engineering point of view is their small velocity relative to Earth. A small delta-V, as rocket scientists call this, means less fuel and more payload. And that translates into longer missions with a better scientific return.

That raises an obvious question: which asteroid do we aim for?

Today, Martin Elvis at the Harvard-Smithsonian Center for Astrophysics in Cambridge and a few buddies examine the possibilities. It turns out that of the 6699 near-Earth asteroids we know, only half a dozen have a delta-V worth considering and are big enough to land on (unless we want to land on an asteroid that is smaller than spacecraft visiting it).

Of course, there are many other near-Earth asteroids that we haven’t discovered, probably an order of magnitude more.

But finding them is particular problem. Their very proximity in an orbit similar to Earth’s means that they spend much of their time on the other side of the Sun and in any case are mainly visible only from Earth’s day-side. That makes them almost impossible to see and track from the ground.

So not only do we have a embarrassing of poverty of choice when it comes to deciding which to visit, there is not much prospect of increasing it in the near future.

And if that weren’t bad enough, human spaceflight is about to come to a sudden end in America. In a few weeks, NASA won’t be able to visit the even International Space Station little more than 200 miles above the surface. And yet it has tentative plans to visit a near Earth Asteroid in 2025 or so.

Even now, this looks ambitious. Robots anyone?

Ref: arxiv.org/abs/1105.4152: Ultra-Low Delta-v Objects and the Human Exploration of Asteroids