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Solar sails take shape


  An artist's rendering shows Japan's Ikaros solar sail in flight.

As Japan gears up to send the first working solar sail into deep space in a couple of months, the Planetary Society is moving ahead with its own solar-sail project. You can put your name on both sails … if you act now.

Sunday is the deadline for adding your name to the list for Japan's Ikaros spacecraft, due to piggyback on the May 18 launch of the Venus-bound Akatsuki orbiter aboard a Japanese H-2A rocket. More than 25,000 people have signed up already using the Planetary Society's "Sail Away" Web page - and when those are added to the Japanese list, the tally goes up to 60,000 names.

All those names will be digitally encoded on the same kind of silica glass mini-DVD that was sent to the Red Planet on the Phoenix Mars Lander as well as on each of NASA's Mars rovers, said Bruce Betts, director of projects for the Planetary Society. The California-based society is a space advocacy group co-founded by the late astronomer Carl Sagan.

If you sign up now, you can get a two-fer: The list of names will also be encoded on a mini-DVD accompanying the Planetary Society's LightSail-1. And even if you miss out this weekend, you can still make it onto that later flight.

How much later? When the $1 million-plus LightSail mission was announced last November, its backers hoped to get it off the ground sometime this year. But the Planetary Society's executive director, Louis Friedman, told me today that schedule would have to be stretched.

"We can still be ready by the end of 2010, but the launch vehicle will probably not be available until the first part of 2011," he said.

In fact, LightSail-1's launch vehicle has not yet been selected. The Planetary Society is planning to piggyback on someone else's liftoff - perhaps on a defense-related flight, or a commercial launch, or a mission launched from outside the United States.

"The good news is, we have many options. The bad news is, we have many options," Friedman joked.

One option that's not on the table is Russia's sub-launched Volna rocket. In 2005, a Volna launched the Planetary Society's previous solar-sail prototype, Cosmos 1, but a premature shutdown of the rocket's first stage led to mission failure.

LightSail-1 and Ikaros are both aimed at showing that solar sails can actually propel a spacecraft through the cosmos - something that's never been done before, even though people have tried for more than a decade. Russian solar sail missions failed in 1999 and 2001. The Japanese successfully tested the deployment of a solar sail in 2004, but that suborbital experiment did not address the propulsion question. A NASA experiment known as NanoSail-D went awry in 2008 when its SpaceX Falcon 1 launch vehicle failed to reach orbit.

The idea behind solar sailing is that photons of light bounce off the sail's surface, exerting enough pressure over a wide surface to give a push to the spacecraft. That push is gentle, but over time, the steady acceleration from sunlight could eventually send the sail on an interstellar cruise.

All about Ikaros
The 700-pound (315-kilogram) Ikaros spacecraft's name refers to the classic Greek myth of Icarus, the youth who made wings from feathers and wax but flew too close to the sun. It's also an acronym, standing for "Interplanetary Kite-craft Accelerated by Radiation Of the Sun."

Its objective is not to fly to the sun or to the stars, but merely to show that a sailcraft can be propelled and steered through deep space. Ikaros is launched with its sail rolled up in a flat cylinder. After separation from the launch vehicle, the probe would spin itself at a rate of up to 20 revolutions per minute to unfurl the squarish sail.

When fully deployed, the thin-film sail would stretch 66 feet (20 meters) along its diagonal. Tiny solar cells, dust counters and reflective steering devices are mounted as panels on the sail.

Betts said Ikaros' flight plan is "ideal for solar sailing," because the sail would be unfurled beyond Earth orbit. "It's like sailing in the open ocean as opposed to sailing in the harbor," he explained. "You don't have to keep tacking or changing the orientation of your sail."

Japanese researchers see Ikaros' flight as the first in a series of solar-powered space odysseys, leading up to a mission heading for Jupiter and its retinue of Trojan asteroids sometime in the 2020s. That ambitious journey calls for the use of a solar sail as well as a solar-electric ion engine.

All about LightSail-1
The Planetary Society also sees its LightSail mission as the first of a set. A chart for the mission program suggests that mission planners are working toward a flight in 2015 that would boost LightSail-3 beyond Earth orbit to the L1 gravitational balance point between Earth and the sun. That's about 1 million miles (1.5 million kilometers) away.

Rick Sternbach / Planetary Society
The LightSail-1 craft consists of three CubeSats at the center of four lightweight triangular sails.

LightSail-1 consists of three linked-together microsatellites, or CubeSats, each 4 inches (10 centimeters) on a side. One CubeSat contains the electronics and control module, while the other two contain the folded-up plastic panels for the solar sail. Total weight is projected to be less than 11 pounds (5 kilograms).

When fully unfurled, the sail would have an area of 344 square feet (32 square meters). That works out to about 18.5 feet (5.7 meters) on a side. The craft would look like a giant, silvery kite in orbit.

Although the spacecraft has about as much volume as an express-mail package, Friedman said there's more than enough space for what the Planetary Society wants to do.

"It's so much more interesting than I first realized, even when we started down the path of building these CubeSats," he said. "These 4-inch spacecraft are the wave of the future. ... You can do so much on these spacecraft."

A mini-camera would be built into the package to send back imagery of the sail deployment after its launch on the yet-to-be-named rocket, Friedman said. "We'll be involving people from the public for optical and maybe even ham-radio tracking," he said. The key question would be whether the pressure of sunlight is enough to boost the sail to a higher orbit. 

It all sounds pretty cool ... as long as someone comes up with the right rocket. That's more complicated than you might think, because engineers have calculated that LightSail-1 has to be deployed in an orbit at least 500 miles high (800 kilometers). Otherwise, any propulsive effect from sunlight would be canceled out by atmospheric drag.

"Above 850 kilometers, you're clearly getting the dominant effect of the solar pressure," Betts explained. To achieve that altitude, LightSail-1 would have to piggyback on a launch going beyond low Earth orbit. There aren't that many rockets going that far out with payload capacity to spare, but the folks at the Planetary Society are confident they'll strike a deal with someone in the months ahead.

Friedman said Ikaros' planners were lucky to have a Venus mission they could latch onto. "I'm envious that they can get this ride out to interplanetary space," he said.

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