Saturday, April 18, 2009

Solar sails

Thomas Mallon has a piece in this month's Atlantic about solar sails. The last I heard about any form of solar sailing (other than Count Dooku's silly ship) was in an Arthur C. Clarke short story about a competitive sport in which people use solar winds to propel themselves past the moon. Mallon reports that people are now taking this technology seriously and are proposing using it to send a payload to another solar system. Unfortunately, Mallon's piece focuses largely on international politics and funding (it is a political magazine, after all), and not on the technology, about which I was particularly curious.

Apparently, I had some misunderstandings about what solar sails do. I had thought that they collected solar wind, which is a collection of sub-light particles streaming from the sun. In fact, they are propelled by light itself. I don't really get how that works, since I understood photons to have no mass, thus being unable to exert any force, but this is hardly my field of specialty. But assuming that it works, I don't get why it would be a good vehicle for interstellar travel. As Mallon writes,
Once a sailing vehicle leaves the solar system, it will run out of gas—that is, sunlight—and space-based lasers, put into orbit around the planets, would have to keep beaming the necessary juice over vast distances.
Solar sail proponent Louis Friedman suggests a mission that could carry a 1-ton payload to Alpha Centauri in under 20 years. Isn't this kinda impractical? Beaming lasers into deep space for 20 years? Aren't there other forms of propulsion that could do roughly the same job?


Anonymous said...

Bob Forward has a couple of books that feature interstellar solar sails.

Also, _The Mote in God's Eye_ starts with one.

Anonymous said...

Also, the advantage is that you don't need to accelerate the fuel you're using, and you don't need to accelerate the fuel you're using to accelerate the fuel you'll use in five years, and so on. The ship itself is just payload + sail.

Seth Masket said...

Okay, good point about the fuel, but how does the ship come home? Does it just rely on Alpha Centauri's radiation and come home at a really slow pace? Or does it not bother, just beaming us information about what it finds?

Anonymous said...

The easiest is just that it doesn't. In Forward's books, there are human crew but the trip is explicitly billed as one-way with the ship engineered to be able to sustain a normal lifespan. Not a suicide mission, just permanent exile.

You can use a solar sail to accelerate the other way too -- the way you do it is cut loose part of the sail (or another sail) and use it to bounce the light onto your drive sail the other way. The mirror sail goes the opposite way from you real fast, but you can alter the shape to maintain focus on the drive sail.

Seth Masket said...

So, a ship leaving Alpha Centauri will be powered by a laser beamed from Earth and reflected off another sail? This just really sounds like a stretch to me.

Anonymous said...

That's how they slow down in the Forward books; there's no return trip AFAIK. But slowing down = accelerating the other way.

That would hardly be the biggest stretch in the relevant books.