] 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.

New Scientist: Solar super-sail could reach Mars in a month