FYI,

"Solar Super-Sail Could Reach Mars in a Month"
New Scientist
http://www.newscientist.com/channel/mech-tech/mg18524846.500

: A Lick of paint could help a spacecraft powered by a solar sail get
: from Earth to Mars in just one month, seven times faster than the
: craft that took the rovers Spirit and Opportunity to the Red Planet.

: Gregory Benford of the University of California, Irvine, and his
: brother James, who runs aerospace research firm Microwave Sciences
: in Lafayette, California, envisage beaming microwave energy up from
: Earth to boil off volatile molecules from a specially formulated
: paint applied to the sail. The recoil of the molecules as they
: streamed off the sail would give it a significant kick that would
: help the craft on its way. "It's a different way of thinking about
: propulsion," Gregory Benford says. "We leave the engine on the
: ground."

: Solar sails are in essence nothing more than giant mirrors. Photons
: of light from the sun bounce off the surface, giving the sail a
: gentle push. It was while developing a solar sail five years ago
: that the brothers stumbled upon their idea for enhancing the effect.

: The pair were testing a very thin carbon-mesh sail by firing
: microwaves at it. To their surprise, the sail experienced a force
: several times stronger than they expected. They eventually worked
: out that the heat from the microwave beam was causing carbon
: monoxide gas to escape from the sail's surface, and that the recoil
: from the emerging gas molecules was giving the sail an extra push.

: In a forthcoming issue of the journal Acta Astronautica, the
: Benfords explain how a sail covered with a paint designed to emit
: gas when it is heated might propel a spacecraft to Mars in just a
: month. A rocket would take the craft to low-Earth orbit,
: 300 kilometres up. After the craft unfurls a solar sail 100 metres
: across, a transmitter on Earth would fire microwaves at it to heat
: it up. The Benfords calculate a one-hour burst of microwaves could
: accelerate the craft to 60 kilometres per second, faster than any
: interplanetary spacecraft to date.

: The feat would require a 60-megawatt microwave beam with a similar
: diameter to the sail. It would also have to be capable of tracking
: the craft as it accelerated away. But this power level could not be
: delivered by any existing microwave transmission system. The deep-
: space communications network that NASA uses to communicate with
: Mars rovers and the Cassini probe now orbiting Saturn can only
: manage half a megawatt. The Benfords say the power could be ramped
: up in future and hope to persuade NASA to consider doing this as
: part of a future upgrade to the network.

: A further challenge is how to formulate the evaporating paint. The
: ideal material would lock up large amounts of a light gas like
: hydrogen and only release it at very high temperature, when the
: high speed of the gas molecules would maximise the recoil. Ideally
: all the paint would boil away, leaving a micrometre-thin sail to
: continue the voyage to Mars.

Mark Reiff