Saturday, December 12, 2009

Arthur Clarke's "Sunjammer" aka "The Wind from the Sun" (novelette, racing): Solar sail power interstellar vehicle!

Illustration accompanying a 9 November 2009 story titled _Setting Sail Into Space, Propelled by Sunshine_ by Dennis Overbye in The New York Times. Story is about a series of forthcoming launches of solar sail powered experimental spacecraft dubbed LightSail, a project of Planetary Society.About a month ago, there was a news item in The New York Times [via Brenda Clough] about a series of forthcoming experiments in solar sail powered vehicles. This story is one of the best pieces of hard sf from Clarke, & among the most exotic racing stories by anyone.

Note: Idea of a solar sail is to use the "pressure" of sunlight - not like solar energy but like particles of light that aren't quite particles though they have momentum - as the motive power. No other energy source is necessary, except for steering.

Story summary.

Seven solar sail powered vehicles of different designs from "four inhabited worlds" will compete in the race from earth to moon. Eventually, of course, it will be fight between US & Russian vehicles!

Beginning point is "twenty-two thousand miles above the [earth's] equator" where the vehicles are anchored to normal ships in a certain formation. At T0, the anchor lines are cut, & the race is on.

At moon, other vehicles will pick up the crew capsules. "the winner will be the one that first drifted past the Moon."

Story is mostly told from the point of view of US participant - John Merton, "Chief Designer of Cosmodyne Corporation", riding the sail ship Diana. He's the only one flying solo; everyone else has a crew of two. He thinks the reduced mass will work in his favor.

They will make two rounds around earth, picking speed during the "powered half of the orbit", before reaching escape velocity. We go through the maneuvers needed when vehicles
  1. pass through earth's shadow ("The shroud lines were slackening, & must be wound in lest they become entangled.")
  2. when moving the unpowered half of orbit (towards Sun) (need to sail "edge-on" to sun, or you're thrown back).
  3. when transitioning to powered part of orbit from unpowered.
We also see a competitive practice where one vehicle tries to cast shadow on another to cut its supply of sunlight, & how the victim escapes.

Participants keep falling off the race till only US & Russia are left: two to collision when passing through earth's shadow, another who could not turn the sail edge-on during unpowered half, another who could not transition from unpowered to powered half, & another where the lines rigging crew capsule to sail gave way.

Neck-to-neck half way to moon a day after reaching escape velocity, the last two participants will also have to abandon their vehicles because of a solar flare (their tiny capsules are unshielded). But Merton had rigged his machine to sail unaided too - so when he jumps, the vehicle is on its way to moon & beyond - eventually attaining solar escape velocity.


  1. Diana: "fifty million square feet of sail, linked to his capsule by almost a hundred miles of rigging... Yet it was little more substantial than a soap bubble; that two square miles of aluminised plastic was only a few millionths of an inch thick."
  2. How much is the pressure of sunlight on Earth? "Hold your hands out to the sun... Over the area of your hands, it comes to only about a millionth of an ounce.

    But out in space, even a pressure as small as that can be important, for it's acting all the time, hour after hour, day after day. Unlike rocket fuel, it's free & unlimited."
  3. Solar sail: "A square mile weighs only a ton, & can collect five pounds of radiation pressure. So it will start moving - & we can let it tow us along, if we attach rigging to it.

    Of course, it's acceleration would be tiny - about a thousandth of a g. That doesn't seem much, but let's see what it means.

    It means that in the first second, we'll move about a fifth of an inch. I suppose a healthy snail could do better. But after a minute, we've covered sixty feet, & will be doing just over a mile an hour. That's not bad, for something driven by pure sunlight! After an hour, we're forty miles from our starting point, & will be moving at eighty miles an hour... You'll be surprised when I tell you what our one-thousandth-of-a-g sailboat will be doing at the end of a day's run: almost two thousand miles an hour! If it starts from orbit - as it has to, of course - it can reach escape velocity in a couple of days. And all without burning a single drop of fuel!"
  4. On inertia: "another rule that was hard to learn: the very moment you had started something happening in space, it was already time to think about stopping it."
  5. Food "crumbs were a greater danger to space vehicles than meteorites; they could drift into the most unlikely places, causing short circuits, blocking vital jets, & getting into instruments that were supposed to be hermetically sealed."
  6. "Those two square miles [of sail] produce a maximum pull of just ten pounds. I can exert more force by my little finger."
  7. "It was hard to keep a good watch on that 60 million square feet of dim plastic out there in the darkness, illuminated only by his narrow spotlight & the rays of the still distant Moon."
  8. By the time of second lap around earth, while "the two square miles of plastic sheet must have been riddled by hundreds of micrometeorites, the pinhead-sized punctures had produced no falling off of thrust."
  9. Diana, after jettisoning: "Two days from now, she would flash past the Moon; but the Moon, like the Earth, could never catch her. Without his mass to slow her down, she would gain two thousand miles an hour in every day of sailing. In a month, she would be traveling faster than any ship that man had ever built.

    As the Sun's rays weakened with distance, so her acceleration would fall. But even at the orbit of Mars, should would be gaining a thousand miles an hour in a day. Long before than, she would be moving too swiftly for the Sun itself to hold her. Faster than a comet had ever streaked in from the stars, she would be heading out into the abyss."

Collected in.

  1. "The Collected Stories of Arthur C Clarke".
  2. Arthur Clarke's "The Wind from the Sun" (coll).
  3. Arthur Clarke's "The Sentinel" (coll).

Fact sheet.

First published: Boy's Life, March 1964 as "Sunjammer".
From author's note, introducing this story in Collected Stories: "The story's original title was 'Sunjammer' but as Poul Anderson had the same idea almost simultaneously, I was obliged to make a quick change of name."
Rating: A.
Related: Stories of Arthur Clarke (only his short fiction/novels).


Larry said...

Ah its been a while since I read this story,must revisit it sometime!

Tinkoo said...

Yesterday I finished Clarke & Pohl's "The Last Theorem". One of its episodes is identical to "Sunjammer" in all essential aspects, even some text appears to be lifted straight off this story; only difference is - the race gets abandoned here at the time one of players starts shadowing the lead character.

Larry said...

Not got that book yet,must find it!
I'm currently reading Richter 10 which isn't strictly Clarke really,but I'll see how it goes.

Tinkoo said...

Good choice - "Richter 10" - first half of it, anyway. May be the best single novel I've seen where Clarke shares his name with someone else on the cover.

Larry said...

Have you read another of his collaborations,called The Trigger about a device that renders all weapons harmless? I forget the co-author's name now.

Tinkoo said...

Yes - here's a bit longish note on "The Trigger". Coauthor is Michael Kube-McDowell.

Had read it when I wasn't very experienced reading the genre - was that less than 3 years ago? Not sure how I'll react on a rereading now.

Larry said...

Thanks for that Tinkoo! I will go look at that review!