starship-design: Scientists propose sailing to the stars on solar wind

["L. Parker" <lparker@cacaphony.net>]

Scientists propose sailing to the stars on solar wind

By Richard Stenger
CNN Interactive Staff Writer

(CNN) -- Four hundred years after Johannes Kepler suggested that ships might
someday sail on "heavenly breezes" beyond the Earth, his dream is on the
verge of becoming reality.

Within the decade, NASA scientists think they can demonstrate a
revolutionary propulsion system, an immense, super-lightweight sail that
harnesses the power of solar photons to propel spacecraft faster and farther
than ever.

NASA thinks the technology could push an interstellar probe at five times
the speed of current propulsion systems, meaning it could cover the distance
between New York and Los Angeles in less than a minute.

"This will be humankind's first planned venture outside the solar system,"
said Les Johnson, manager of Interstellar Propulsion Research at NASA's
Marshall Space Flight Center in Huntsville, Alabama.

Conventional spacecraft need too much fuel to push their own weight into
interstellar space, but space sails require no fuel, except for the sun, he
said.

Searching for the heliopause
A probe advancing past the edge of the solar system would give scientists a
remarkable glimpse into the composition of the universe.

"We hope to look at what the composition is beyond our solar system. All of
our measurements are in some way part of our planetary system," said Paulett
Liewer, a physicist at NASA's Jet Propulsion Laboratory in Pasadena,
California, which is working with the Marshall center on interstellar probe
research.

Liewer and others want to search interstellar space for signs of organic
matter and to study the interaction between interstellar space and the solar
system.

The Stardust probe earlier this year gathered samples of a thin interstellar
dust stream drifting in the inner solar system. The preliminary data
indicated the presence of complex carbon molecules. But the solar wind
likely filters out the larger particles, meaning scientists can only guess
how dense interstellar space is, according to Liewer.

"We have no idea what the mass density is. That will be a big surprise," she
said.

The heliopause is the outermost boundary of the solar wind, where the
interstellar medium restricts the outward flow of the solar wind. The space
within the boundary of the heliopause, containing the sun and solar system,
is referred to as the heliosphere. The exact location and shape of the
heliopause is a mystery. Scientists think it is teardrop-shaped or roughly
circular and exists some 90 to 120 astronomical units from the sun.

The resilient Voyager probe passed the planets years ago and continues to
beam back scientific information. But its instruments are not ideal for
collecting the kind of data deep space scientists need, Johnson said.

If NASA launched a probe with a solar sail in 2010, traveling at 150,000 mph
(km/h), it would pass Voyager in 2018, despite the fact the latter would
have had a 41-year head start.

The spacecraft could travel more than 23 billion miles (37 billion km), or
250 astronomical units (AUs), said Johnson. In comparison, Neptune orbits
the sun at about 30 AUs. One AU is 93 million miles (150 million km), the
distance between the Earth and sun.

It could extend far beyond the heliopause in 15 years, a definite plus for
professional research.

"Now if you launch with conventional propulsion, most of the scientists
would be retired or dead" when such a probe reached its target distance,
Johnson said. But event at such a vast range, the probe could beam back
transmissions to Earth in several hours.

Twice as large as the Superdome
Solar sails would require a thinness that rivals cellophane, the strength to
withstand intense solar heat and barrages of micrometeors, and an extremely
large surface area: 440 yards (402 meters) wide, twice the diameter of the
Louisiana Superdome. A highly reflective coating would harness the momentum
of photons streaming from the sun.

Constructing the sail is daunting but doable, NASA scientists said.
Researchers believe they're close to breakthroughs with lightweight
composites, including a carbon fiber material developed by Energy Science
Laboratories in San Diego, California. Its density is the equivalent of a
raisin flattened to 1 square yard (0.8 square meter), according to NASA.

"It looks like it has the right thermal properties, so it can go near the
sun and not overheat," Johnson said.

To generate serious speed, a sail probe must first travel to the vicinity of
the sun, where it juices up on solar photons. On its way out of the system,
it would cast off the sail near Jupiter, where the stream of sun particles
peters out.

"All of the thrusting is in the inner solar system, then it coasts," said
Johnson.

Miles of sail squashed in tiny can
Creating a solar sail is difficult enough, but hoisting it could prove an
even greater feat.

"I am confident that we can get the materials, but packaging and deployment
will be the biggest nut to crack," Johnson said.

One JPL scientist who studies advanced propulsion concepts describes the
challenge ahead: "We have miles of sail, that must be squashed inside an
itty bitty can and placed in a launch vehicle," Robert Frisbee said.

Because of atmospheric drag, a NASA spacecraft would have to travel hundreds
of miles from Earth before it could unfurl a solar sail, Frisbee said.

Space scientists are considering several methods to deploy a giant sail, he
said. One would be to fold it like a sheet of paper, then extend it out with
mechanical booms. Another is using an inflatable structure, like a long
balloon, that inflates with a tiny amount of gas and in so doing pulls out
the sail.

Decades after solar sails ply the celestial seas, they could rely on more
than the sun for power, according to Johnson. Lasers or microwave beams,
perhaps originating from satellites placed strategically around the solar
system, could give spacecraft critical boosts on journeys to the stars.