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Mote Prime > Science

Popping Next Door: A Mission to Gliese 581

Gliese 581 is a red dwarf star, and host to a system of extrasolar planets. It's also very close, astronomically speaking. So could we "pop next door" and visit the neighbours?

The Star

If you look up into the night sky, in the direction of the constellation of Libra, your gaze will encompass a scattering of dim stars. One of these, first catalogued just over 100 years ago, is the red dwarf known as Gliese 581.

Nothing very special, in cosmic terms, it's one of our nearest neighbours, a mere 20 light years away. Being a red dwarf, it doesn't put out a lot of light, and consequently it doesn't shine as brightly as, say, Vega - further away, but a magnificent bright blue-white pinprick that rides high in the summer sky.

But Gliese 581 is ideally placed to reveal its secret companions, and reveal them it has.

The Companions

Gliese 581 was one of the early crop of stars to be confirmed as having planets of their own. The first was discovered in 2005 by observing the motion of the star itself, and was christened Gliese 581b. The planet itself is too dim to see, but as it orbits, it tugs on the star just as much as the star tugs on it. The small size of the star, and the speed of the orbit, created a signal that was easy to detect. Well, as easy as these things get, anyway. The dedication of the observers should not be underestimated.

Since then, more observations have led to the discovery of several more planets dubbed Gliese 581c, d, e, f and most recently g. G is the most interesting one because although it huddles in close, much closer than Mercury in our solar system, the dimness of its parent does not roast it, but gently warms it to the kind of temperature where liquid water could exist.

This is exciting, since liquid water is a prerequisite for life. And alien life, even if it is just bacteria, would be a momentous discovery.

Let's Go Visit

Naturally, the next question lots of people asked was "How can we send a probe there to look?" Not a silly question. But lots of the answers I heard were silly.

"Impossible!" "Look at how long our rockets take to reach even something as close as Pluto!" "It would take 530 times the mass of a spacecraft in [antimatter] fuel to get to Gliese 581."

All true. But all wrong at the same time.

All these estimates assume that you take your engines and fuel with you. Much better to leave them at home. But what kind of a ship doesn't have an engine? A sailing ship, of course.


First, I'd like to introduce you to Starwisp - a concept invented by the late great Robert L. Forward, engineer, inventor and science fiction author.

A Starwisp probe doesn't look like anything we've ever launched up to now. It's more cobweb than Cassini. It's the world's biggest cobweb though, 100 metres in diameter, and so fragile it could only be built in space. It rotates slowly to keep itself stretched out flat. Overall, it weighs no more than a kilogramme.

Up close, it consists of a mesh of extremely fine wires, of the kind of thickness etched onto today's silicon chips. (Forward though they would be metal. Today's best candidate designs use carbon nanotubes.) They are spaced so that they will reflect microwaves, and at the junctions of some of the wires are tiny silicon chips. These are the payload, and each chip contains a tiny amount of computing power and a few miniaturized sensors. Together, they form a huge redundant array of sensors and the processing required to make sense of what the probe is seeing.

The other half of the probe is the engine, but we leave this at home. It's a solar powered microwave beam satellite. With a beam output of 50 gigawatts, and an array of focussing lenses built with the same technology as the Starwisp itself, the beam acts like the wind on the sail of a ship, and transfers enough momentum to the Starwisp to push it with several gees of acceleration. In a month or two of continuous operation, it can push a Starwisp up to 10% of lightspeed.

Our probe is on its way. And what's great is that we still have the engine! Now have either a 50GW solar power satellite at our disposal, or we can use it to launch another Starwisp to another system. We could have a production line, flinging out probes in all directions.


After the initial acceleration phase, the probe goes to sleep. It has no internal reserves of power, no maneuvering fuel. It cools and heads out into the dark.

It will encounter micrometeorites in its 200 year journey. Most of them will pass through and miss the mesh completely. Those that don't will punch through at 10% of lightspeed, severing connections and destroying processors. The web will become ragged as it ages.

But it is designed for that. The network is redundant, the links reroutable.


And finally, as it draws closer to the Gliese system, from behind comes a dim wash of transmitted microwave power. It is the faint remnant of a 50GW transmission from Earth 20 years previously. Too weak for propulsion, it is just enough to power the probe during the encounter. The Starwisp wakes from its two century old slumber and looks around.

At 10% of the speed of light, the probe would traverse our Solar system in two days. At Gliese 581, the known planets huddle close around their dim sun, and to cross the entire system will take less than an hour. There is no chance of slowing down, so the sensors go all out, analysing the light from the known planets, searching for others, gathering spectra and images for the descendants of the the original team, back home.

Think of that for a moment. A two hundred year mission, culminating in less that an hour of data collection. And at the transmission speeds available, it will take months to return the data.

But it will be worth it.

Fact or Fantasy?

This isn't quite possible using today's technology - we don't have the space manufacturing techniques. But it doesn't need any huge leaps, and could be launched within our lifetime.

Gliese 581g is closer than you think. We should send an ambassador there, even if it is only a cobweb.