Spring Books

Nature 452, 696-697 (10 April 2008) | doi:10.1038/452696a; Published online 9 April 2008

Catching a ride on sunshine

Stuart Clark1

BOOK REVIEWEDSolar Sails: A Novel Approach to Interplanetary Travel

by Giovanni Vulpetti, Les Johnson and & Gregory L. Matloff

Springer: 2008. 250 pp. £16.50

Conceptually simple and romantic, solar sailing is an enchanting technological solution for space exploration. When a large reflective sail is unfurled in space, photons of sunlight collide with the sail fabric, imparting pressure and causing the sail to move. Such photons are not the electrically charged particles that constantly flow from the Sun to create the solar winds, they are the actual sunlight itself. The angle of the sail to the Sun and its direction of travel determine whether a propelled craft speeds up or slows down, just as a yacht changes course on the sea.

Solar Sails: A Novel Approach to Interplanetary Travel is the latest book to explore this topic, one that has been tackled only a handful of times in the past 20 years. Aimed at undergraduates, the book convincingly captures the history of ideas about solar sails, their current state of play and their future promise.

Catching a ride on sunshine

Moving according to the constant interplay of gravity and the pressure of sunlight, spacecraft pushed by solar sails are highly manoeuvrable. They can skate along unusual interplanetary trajectories that traditional point-and-shoot rocket-propelled craft would find difficult, if not impossible, to navigate. In the flexibility stakes, the only current competition is from the newly tested but expensive ion-drive engine that powers the SMART-1 Moon mapper built by the European Space Agency (ESA) and NASA's Deep Space One asteroid probe. These propulsion modules run by expelling charged particles, or ions, and can operate using less fuel than standard chemical engines; however, they are technologically trickier and thus expensive to build.

The idea that sunlight exerts pressure has been around for more than a century, since physicist James Clerk Maxwell proposed it in the 1860s. In the 1970s, metre-long solar sail fins — rather like the fins on a 1950s American car — were attached to the Mariner 10 Mercury space probe to adjust its alignment. Today, some satellites are steered with small sail vanes, a technology patented by the aerospace company EADS Astrium. The extra force of sunlight is a hindrance when fine control of movement is required, as with the next generation of formation-flying spacecraft in ESA's proposed Darwin interferometry mission to search for life on extrasolar planets. Such vessels must instead be designed to minimize displacements or, at least, to all suffer the forces equally.

Despite the opportunities, solar sails have yet to be used for propulsion in space. The pressure of sunlight is so slight that a vast sail area would be needed to carry a worthwhile payload of instruments through space. Deploying such a sheet presents an equally vast challenge, and has remained the solar sailor's Achilles' heel.

With useful sails being many tens to hundreds of metres long, these mighty structures must be packed into the equivalent of a suitcase for launching and then faultlessly unfurled once in space. If the sail snags, tears or fails to deploy, the mission is over. This risk deters many potential users; according to one project scientist at the ESA: "Why jeopardize your science by relying on an untested technology?"

Scientists and space agencies have, until recently, been resistant to solar sailing. This negative attitude was reinforced by the failure of the Planetary Society's Cosmos-1 sail, launched atop a converted Russian intercontinental ballistic missile on 21 June 2005 from a submarine in the Barents Sea north of Russia. The upper-stage rocket motor failed, dooming the mission to failure before the sail mechanism could even be tested. Although the test was inconclusive, the perceived lack of success reflected badly on the solar-sail initiative itself.

Now the tide is beginning to turn. Ground-based tests in Europe and the United States have successfully deployed sails of about 20 square metres thanks to improvements in sail-opening mechanisms. The German Aerospace Centre has used plastic booms reinforced with carbon fibre, and NASA has used inflatable booms that harden when exposed to the coldness of space. Even more impressively, the Japanese space agency JAXA has carried out two successful sub-orbital deployment tests. Made of reflective films 7.5 micrometres thick and some 10 metres in diameter, the sails were flown to an altitude of 122 kilometres, where one opened up like a clover-leaf, the other like a fan. JAXA followed this up two years later in 2006, with a successful 20-metre-wide sail deployment from a balloon at an altitude of 35 kilometres.

Some space missions can be performed efficiently only with solar sails. Placing a satellite in a polar orbit around the Sun using a rocket requires a large expenditure of energy, and hence fuel. A craft propelled by a solar sail would take only five years to fly there from Earth but would require a huge sail area of 25,600 square metres (160 metres by 160 metres: larger than five American football pitches side by side) to provide the necessary thrust. Because sunlight holds the sail in space, it can be angled so it hovers like a kite over the poles of a planet, making solar-sail craft ideal anchors for communications and remote-sensing satellites.

Of course, there are limitations. Solar sails lose their power and manoeuvrability when they are far from the Sun, out beyond Jupiter. They are also unable to assume low orbits around planets with atmospheres because the sails are susceptible to drag.

Suitable for aerospace students and keen enthusiasts alike, this book may one day inspire some of them to build a solar-sail-powered vessel. Although there is still a long way to go, this useful volume will help speed up that day.

  1. Stuart Clark is a visiting fellow at the Centre for Astrophysics Research, University of Hertfordshire, Hatfield, AL10 9AB, UK. He is author of The Sun Kings.