Genre: Information/Explanation

The DLR successfuly deployed a 20m2 sail structure in December of 1999 , a ground-based demonstrator that consisted of four booms made of carbon fiber reinforced plastic . In August of 2000 , DLR proposed a dedicated mission to explore near-Earth asteroids ( NEAs ) using a solar sail-driven spacecraft called ENEAS . The mission concept was to rendezvous with an object called 1996FG3 with a payload ( not counting the sail assembly ) of 75kg . Such proposals run into financial difficulties familiar to anyone who recalls NASA's early solar sail work for a Halley's Comet mission that never flew . Tight budgets have kept ambitious sail missions on the ground both within ESA and NASA , but concepts continue to multiply . A new paper by Bernd Dachwald and Wolfgang Seboldt at DLR proposes a mission called ENEAS-SR , a 70m2 sail designed to return a sample from NEA 1996FG3 within ten years of launch , carrying a 300 kg payload including lander and sample return capsule . Why , other than to shake out a new technology , should we go to a near-Earth asteroid ? In addition to pure science , NEAs are significant because they are probably fragments of main belt asteroids , and therefore representative of them . The larger class that contains them is the group called NEOs , those asteroids and comets with orbits that intersect or pass near the Earth's orbit . We need , say Dachwald and Seboldt , to know much more about them . From the paper : It is today widely accepted that NEO impacts have caused at least one mass extinction ( 65 million years ago at the Cretaceous / Tertiary boundary ) , and they are suspected to have caused several global catastrophes before . Even NEOs that do not intersect the orbit of Earth may evolve into Earth-crossers , because their orbits are chaotic , having a relatively short dynamical lifetime of about 107 years . One day , it will become necessary to prevent a specific object from impacting the Earth by nudging it out of its orbit . To be able to do this , the bulk properties of NEOs ( material strengths , composition , structure , moments of inertia , etc. ) should be determined as soon as possible . Solar sails seem ideal for this purpose . Once at NEA 1996FG3 , the sail-driven craft can take advantage of its sail in novel ways . Even close in to such a small object , the solar radiation pressure on the sail would exceed the asteroid's gravitational pull ; the spacecraft would therefore be placed into a hovering position in the hemisphere opposite the Sun ( the sail can be turned to vary the photon pressure upon it , making this possible ) . Although ultimately unstable , these hovering positions could be maintained for extended periods of time , long enough for the release of the lander and its Earth return capsule . Dachwald and Seboldt go on to envision an even more sophisticated triple NEA rendezvous mission with a 75kg payload and speculate on the properties of sails up to 140m2 in size . Missions like these make outstanding sense – they take us to objects like NEOs with highly inclined orbits , the kind of target difficult to reach with conventional spacecraft , and they allow us to develop space experience with lightweight structures for future propulsion systems . The paper is " Multiple near-Earth asteroid rendezvous and sample return using first generation solar sailcraft , " in Acta Astronautica 57 ( 2005 ) , pp. 865-875 . If we find et we can transmit the technology for beamed power propulsion to them , and then both civilizations can build the the interstellar equivelent of the train and the rail road on rails of light . tim