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Copyright 2003, all rights reserved

A spacefaring sailing ship, chicks grow fast, light takes time

Q: The idea of using sunlight to blow spacecraft across the Solar System will not work, suggests a new analysis by Thomas Gold. How do solar sails work and is Gold right? — Lanney, Sandia Park, New Mexico

Nearly half a kilometer wide, the fragile solar sail unfurls in space, as envisioned by NASA. [NASA/MSFC]

A: A solar sail is a 5-micron thick reflecting film (as thin as a spider silk strand) held perpendicular to the Sun’s rays. The gigantic sheet works like a wind sail, except bits of light push against it instead of bits of air. A photon smacks the sail and reflects off with lower energy. The sail moves away from the bounced photon like a struck punching bag. The same force blows comet tails away from the Sun.

A rain of solar photons pelting a sail as big as 200 football fields (a square sail 1 kilometer on a side) doesn’t generate much force. If the sail is as far from the Sun as Earth is, the force is about 9 Newtons or 2 pounds. A sailboat on Earth would be dead in the water given such wind. However, space is a different story. No drag from air or water. Give that sail time and the little push adds up: five times greater than possible with conventional rockets, says NASA.

That’s the standard theory of solar sailing.

Thomas Gold, physics professor emeritus at Cornell University in New York disagrees. He says that the sail is a perfect mirror and, as such, will reflect the photon with no loss of energy. No loss of energy on the photon’s part means no gain for the sail. The sail won’t move.

Is Gold right?

We’ll know soon. Cosmos I, a solar-sail spacecraft is scheduled to launch by the end of October. This is a privately-funded project of the Planetary Society, an organization founded by the late astronomer, Carl Sagan. The Society’s goals are small, like the Wright brother’s first flight. They’ll use a cheap Russian rocket (Volna), blast off from a Russian submarine, get the sail above the Space Shuttle altitude of 500 miles (800 km), launch the sail into space, and sail for a couple of orbits. That’s enough to answer the question: does it work? Similar to Orville Wright’s 12- second flight.

Physicists who disagree with Gold cite past experience with light pressure effects:

"Gold is wrong," says Deborah Jackson, quantum-computing technologies, Jet Propulsion Laboratory. She says that the experiment is not to test solar-sail theory. Instead, "what the Cosmos mission tests is our ability to engineer and deploy these gossamer sail materials in space."

If all goes well, we’ll soon watch Cosmos I sail into space like sailing ships of yore. Stay tuned...

Further Surfing:

NASA/JPL: Solar sail technology development

The Planetary Society: Cosmos I mission

Thomas Gold: The solar sail and the mirror

Chicks grow fast

Q: How fast do baby chickens grow (how much do they grow in a month)? — Tennille, Adelaide, Australia

A peregrine falcon’s chicks leave the nest when about 5 to 6 weeks old. [US Fish and Wildlife Service]

A: Broiler chickens grow at an incredible rate: 1.66 ounces (47 g) per day, which is 3 pounds (1.4 kg) per month. Scaling that growth rate to a human, a child would weigh 2,835 pounds (1.4 tons!, 1286 kg) by age two. Chickens have a normal life span of seven years but broilers are slaughtered in six to seven weeks. Twenty years ago, it took 84 days for a broiler chicken to reach market size. Now it takes 42.

Light takes time

Q: The nearest star from Earth is Proxima Centauri, which is 4 light years from Earth. So, is it true the stars that we see in the sky at any instant are not the stars exactly at that instant but more than 4 years old, which we see now due to the sluggishness of light? (P.S. Can we see asteroids with the naked eye?) — "Triangular"

The blue streak is an uncatalogued mile-wide bit of rocky debris orbiting the Sun only light -minutes away: an asteroid. Most of the stars in the image are about 25,000 light years away. Hubble accidentally caught the asteroid’s passage as it flickered across Hubble’s view. [Hubble Heritage Team (AURA/STScI/NASA]

A: Yes, that’s true (not counting the Sun, which is Earth’s nearest star, of course). The nearest known star to our Sun is Proxima Centauri, 4.224 light years away. Light travels at 186,411 miles (300,000 km) per second in a vacuum — not instantaneously. Four light years is the distance that light can travel at light speed. So, light will take about four years to reach Earth from Proxima Centauri and we see the star as it existed four years ago.

That’s why looking at the stars is a form of time travel — into the past. We can see quasars, for example, as they were 4 billion years ago. A quasar is the extraordinary bright center of a distant galaxy, whose energy is powered probably by the in-fall of matter into a supermassive black hole.

To answer your "p.s.", we can see an asteroid with our naked eye but only one. Only Vesta is bright enough to see of the hundreds of thousands of asteroids spinning about the Sun between the orbits of Mars and Jupiter. Asteroids shine by reflected sunlight, like our Moon.

(Answered Sep. 5, 2003)