Solar Powered Space Travel

A long-time goal of space enthusiasts is about to reach fruition — the first solar sail is about to take flight.

The really neat thing about it, to me, is that it's privately sponsored. I remember discussing this at dinner in 1982 with Rob Staehle, the JPL engineer who was planning the project back then as an extra-curricular activity (more recently, he was the pre-project manager for the Pluto Express mission and he is now the Deputy Project Manager of the Europa mission.) 

It's great to see it finally happening.

To the degree that many people are aware of the concept of solar sails, they mistakenly believe — taking the nautical analogy — that they are blown by the solar wind. But solar sails, or light sails (the more generic term, because they could be powered with lasers as well as the sun) actually get their thrust from radiation pressure. The solar wind is composed of heavy, highly-energetic particles that would blow right through a sail, destroying rather than propelling it. The sail is instead impinged by photons, the components of light.

The article linked above says that the sail absorbs them, and gains their momentum, but if this occurs, it's actually less efficient. Ideally, the photons actually reflect off the sail, imparting twice the momentum that they would if they were absorbed. Thus, a well-designed sail has a mirrored surface, or at least a surface that acts as a mirror for the frequencies of light for which it's designed.

Also, since the lighter the vehicle, the greater the acceleration for a given force, it's made as thin as possible while still maintaining structural integrity. Finally, since force is pressure times area, the bigger the sail, the more thrust can be attained.

Because the solar radiation pressure is so small, even for a large sail, the total force might only amount to a few pounds. But if that's the only force acting (other than gravity), it can still add up, and with continuous acceleration, get you to an outer planet faster than chemical propulsion.

One question often asked is: if the radiation pressure always acts outward from the sun, how can a sailing spacecraft come back into the solar system. Answer: like conventional sailing ships, it tacks (though the analogy is imperfect — being in a vacuum, unlike the water for a ship, there is no medium in which it travels, and it thus has no use for a keel).

Imagine that the sail is at an angle with respect to the sun. Some of the thrust is directed tangentially along its orbit. Add to orbital velocity, and the energy increases, and the sail heads out to the outer system. Change the angle to subtract from it, and the sail will slow, and fall back in toward the sun. Angle it out of the orbital plane, and you can slowly perform a plane change.

If we really did want to drop nuclear waste into the sun, a sail is probably the only affordable way to do it, with the additional advantage that as the star is approached, the thrust increases as the square of the distance (twice as close means four times the thrust). Unfortunately, because they're such delicate things, the sail might burn up before it had decreased its velocity sufficiently to drop all the way. So a final booster rocket might still be needed.

Here's an extremely little-known fact. Solar sails played a significant role in the conceptualization and development of nanotechnology. Back in the 1970s, a young student at MIT, enamored with space, was trying to figure out how to develop the minimum thickness for a sail. He came up with a concept for laying out an ultra-thin layer of aluminum on a wax, using a technique called vacuum-vapor deposition, in which the metal would be heated to a vapor, and sprayed on a substrate in a vacuum chamber.

Afterwards, the wax would be melted away, leaving the thin aluminum foil. He reasoned that he could get a sail that was only a few atoms thick--strong and reflective enough to be a good sail (as long as it was handled properly) while providing maximum performance.

One thing led to another, and he eventually came up with other techniques for building things at atomic-level scale, and gave some serious thought to the implications of such manufacturing. He wrote a book on the subject in the mid-1980s, and eventually, in 1991, received the first doctorate in the field, having played a major role in inventing it, from MIT. He later expanded his doctoral thesis in another book.

His name, of course, was K. Eric Drexler.

The Truth Trickles Out

But only from the "Moonie" newspaper. The Palestinians desecrated the Church of the Nativity (requiring reconsecration) and brutalized their hostages.

Don't expect to get the story from the mainstream press. After all, it was all the Israelis' fault...

Recharging Your Space Batteries

If you don't already have plans for Memorial Weekend, and you have the time/money to get to Denver and are interested in space, you might want to consider attending the International Space Development Conference, sponsored by the National Space Society.

I've been to many of these, and you'll find programming to suit every taste, from whiz-bang technologies, to recent results in space science, space law and discussions of asteroid mining, colonization and settlement. It's probably the largest gathering of space enthusiasts you'll find during the year.

Don't miss it if you want to find out the latest in our progress to spread life into the universe.

Accessing Space

I was going to write up a report of my trip to the Space Access Conference, but my friend Leonard David has beaten me to it.

Clark Lindsey at Hobby Space has a good review as well.


On the subject of piloting space transports, reader Carter Leffen gratified me and my other correspondents with:

It's great to finally get a hard science writer with a good sense of humor- and honor. Wish you were running NASA, instead of current chowder-headed blowhards... Their problem? No imaginaton, and no engineering sense. Privatization of space is best way to go... Heinlein would be proud of you! Also Kudos to Jody Granier, and those like her. Perhaps through their efforts, our grandchildren may be pioneers on Mars... Keep-up the good work. Great website, Transterrestrial Musings- have your website locked in "favorites" on AOL.

But Mike Horn, former BART Commuter, complains:

Well, you lost me (and I lost it). BART was/is the WORST example one could use to prove a point about anything. As a survivor of the 1989 earthquake (at the Montgomery Street station), I had the pleasure to watch every BART employee abandon the station. EVERY ONE. What did they know about BART's tunnel construction that I didn't?

Other exploits of BART's famed employee professionalism included a train I was on going INTO a smoke filled underbay tunnel as a rescue train — with the one little mistake of failing to evacuate existing passengers — bringing an additional rush-hour trainload of passengers into an already life threatening situation with little or no room in the train to take other unfortunates out of harm's way.

Lastly, who could forget the famed BART transit workers professionalism and humanity of bringing me to work one fine morning and THEN GOING out on strike over lunch time. Whew, I feel better now. Where IS my lithium?

If you're giving me a choice between a BART employee or a computer taking me into space — I'll take the (non-union, non-government) computer every time!!! (as long as I write the code)

Well, I wasn't exactly proposing that BART employees be the pilots. And unfortunately, not everyone can write the code...

I also received a couple of negative emails, which were lengthy, and to which I similarly responded at length here.

Keep that feedback coming! It's the only way this column will improve.

Rand Simberg is a recovering aerospace engineer and a consultant in space commercialization, space tourism and Internet security. He offers occasionally biting commentary about infinity and beyond at his Web log, Transterrestrial Musings.

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