Setting Sail for the Stars

June 28, 2000 -- Now that summer is here, many Americans are planning to take their boats out on the water, to feel the wind in their hair and sail off into the proverbial warm evening sunset. Some NASA scientists, as they gaze into the setting Sun, have a different kind of sailing in mind. They are pondering "solar sails" and how new technological developments and research could take spacecraft flying into deep space.

Some of these scientists gathered in Pasadena, CA, last month to share ideas and present new information about solar sails at the 11th annual Advanced Space Propulsion Research Workshop at the Jet Propulsion Laboratory (JPL).

Above: Nearly 400 years ago astronomer Johannes Kepler observed comet tails blown by a solar breeze and suggested that vessels might likewise navigate through space using appropriately fashioned sails. It is now widely recognized that sunlight does indeed produce a force which moves comet tails and a large, reflective sail could be a practical means of propelling a spacecraft. [more information]

"We are now able to start opening doors for missions we couldn't do with chemical propulsion," said Sarah Gavit, program manager for JPL's Solar Sail Technology Program.

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The idea of solar sails has been around for centuries, said Gavit. Early concepts included huge aluminum-coated Mylar sheets that could be pointed at the sun and "blown" toward deep space, powered by sunlight. However, such relatively heavy sails would take a very long time to go anywhere, so scientists have spent years researching and developing new types of sails and new methods to push them into space faster and more efficiently.

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Why use solar sails at all, one might ask, when many other forms of propulsion are available, such as chemical or electrical propulsion systems? According to Dr. Robert Frisbee of NASA's Jet Propulsion Laboratory, one of the main obstacles to interstellar travel is the weight of propulsion systems and their propellants. New developments in solar sail technology open up the possibility that future spacecraft will be propellant-free.

"Our goal is to perform a flight demonstration of a solar sail," said Gavit. "We're hoping we could do a flight demo in the 2005 timeframe to prepare for the Interstellar Probe, a sail-propelled craft that's slated for launch about 5 years later."

The goal of the Interstellar Probe is to travel beyond the nine known planets.

"This will be humankind’s first planned venture outside our solar system," said Les Johnson, manager of Interstellar Propulsion Research at the Marshall Space Flight Center. "This is a stretch goal that is among the most audacious things we've ever undertaken."

Above: Several NASA spacecraft are searching for the boundary between interstellar space and the heliosphere (a giant bubble blown by the solar wind). The solar-sail propelled Interstellar Probe could overtake all of them if it is launched in 2010. [more information]

Nearly half a kilometer wide, the Interstellar Probe's delicate solar sail would be unfurled in space. Continuous pressure from sunlight would ultimately accelerate the craft to speeds about five times higher than possible with conventional rockets -- without requiring any fuel! Zooming toward the stars at 90 km per second, it could cover the distance from New York to Los Angeles in less than a minute. That’s more than 10 times faster than the Space Shuttle’s on-orbit speed of 8 km per second. An interstellar probe launched in 2010 would pass the Voyager 1 spacecraft, the most distant spacecraft bound for interstellar space, in 2018 going as far in eight years as Voyager will have journeyed in 41 years.

Johnson says transportation is quite possibly the toughest challenge with interstellar missions because they have to go so far, so fast. "The difficulty is that rockets need so much fuel that they can't push their own weight into interstellar space. The best option appears to be space sails, which require no fuel," he said.

A sail that holds particular promise, according to Gavit, is one made of carbon fibers. Timothy R. Knowles, of Energy Science Laboratories, Inc. presented a paper at the conference that discussed these new carbon sails. The fibers are woven together, creating a criss-cross patterned material that is very low-density. The use of carbon makes the sail able to withstand the intense heat from the sun.

"The carbon fiber sail can get closer to the sun allowing it to get more bang for the buck," said Gavit. "Of course, some missions don't require flying so close to the sun, so we are looking at other options, too."

Above: Les Johnson, manager of Interstellar Propulsion Research at NASA’s Marshall Space Flight Center in Huntsville, AL, holds a rigid, lightweight, carbon fiber material that could be used to build a giant space sail.

Other presentations at the conference focused on using lasers or microwave transmitters that would power the solar sails along at a faster rate than sunlight alone. The sails would be powered like those designed to use solar photons, with the lasers or microwave transmitters beaming at the sail for a few days or weeks, providing the sail with the light needed to create thrust.

The Solar Wind, Not!

A common misconception is that solar sails are pushed by the solar wind just as sailboats are propelled by the wind on Earth. This is not so. The solar wind is an extremely tenuous flow of particles streaming away from the Sun. It exerts very little force on anything it hits. The propulsive force for a solar sail arises from the pressure of photons (light) from the Sun or from lasers. Sunlight at 1 Astronomical Unit (1 AU is Earth's distance from the sun = 150 million km or 93 million miles) exerts a force of 9 Newtons per square kilometer (0.78 pounds per square mile) on a solar sail. For more information about the basics of solar sails, see "Intro to Solar Sailing," a web page hosted by the California Institute of Technology.

"Lasers can keep focused on the sail for a longer time," said Frisbee. "Microwave transmitters can be used to blast the sail until it heats to 2000 Kelvin, which would accelerate the sail at 100's of G's to get the sail up to 1/10th the speed of light. Scientists are debating which system -- high-powered lasers or microwave transmitters -- might be the best way to go. Each has its advantages and disadvantages."

Scientists will continue to investigate the possibilities for the use of solar sails on future NASA missions, with the hope that these sails can get probes farther into space than ever before.

"These sails have really only become a reality in the last five years, due to advances in lightweight materials," said Gavit. "There are several near-end and mid-term NASA missions that plan to use sails."

The Marshall Space Flight Center partners with NASA’s Jet Propulsion Laboratory in developing solar sail missions and technology. The Jet Propulsion Laboratory has overall responsibility for NASA’s interstellar missions while the Marshall Center is responsible for developing transportation systems for the missions. Marshall’s effort is part of its Advanced Space Transportation Program, NASA’s core technology program for all space transportation.

Web Links

Highway2Space.com -- news and information about space transportation research from the Marshall Space Flight Center

Recent Stories about Space Transportation:

June 9, 2000: A Little Physics and a Lot of String - Using space tethers for propulsion.

May 31, 2000: Advanced Space Propulsion Conference - Scientists meet to discuss the latest in space transportation.

May 29, 2000: What's the Matter with Antimatter?- It may be the ultimate fuel for space travel, but right now antimatter is fleeting, difficult to work with, and measured in atoms not pounds!

May 11, 2000: NASA charts course to sail to the stars on largest spacecraft ever built- NASA/MSFC press release.

April 11, 2000: Where's the Edge?- NASA's Advanced Space Transportation Program looks at ways to turn science fiction into reality.

Stories from the 1999 Space Propulsion Workshop:

April 6, 1999: Ion Propulsion -- 50 Years in the Making- The concept of ion propulsion, currently being demonstrated on the Deep Space 1 mission, goes back to the very beginning of NASA and beyond.

April 6, 1999: Far Out Space Propulsion Conference Blasts Off - Atoms locked in snow, a teaspoon from the heart of the sun, and the stuff that drives a starship will be on the agenda of an advanced space propulsion conference that opens today in Huntsville.

April 7, 1999: Darwinian Design - Survival of the Fittest Spacecraft

April 7, 1999: Coach-class tickets for space? - Scientists discuss new ideas for high-performance, low-cost space transportation

April 12, 1999: Reaching for the stars - Scientists examine using antimatter and fusion to propel future spacecraft.

April 16, 1999: Riding the Highways of Light - Science mimics science fiction as a Rensselaer Professor builds and tests a working model flying disc. The disc, or "Lightcraft," is an early prototype for Earth-friendly spacecraft of the future.

Setting Sail for the Stars

Scientists met last month to discuss the latest developments in solar sail technology, which could carry a spacecraft beyond the edge of the solar system by 2018.