This year a spacecraft will set sail to the planet Venus - literally.

The idea of solar sails as a propulsion for spacecraft has been around for a long time. Russian spaceflight pioneer Kostantin Tsiolkovsky and Latvian engineer Fridrickh Tsander wrote about it in the 1920s.

Sunlight exerts a feeble pressure which we normally do not notice on Earth. But in space even those small forces are important and must be considered for satellite orbits. What's more if you take a reflective surface you have some control over the direction in which the force acts depending on how you orient the surface with regard to the Sun.

Taking a thin foil not too different from the aluminum wrapper you might have in your kitchen and deploy it over a large enough area you get enough acceleration to propel a spacecraft. This is the principle of solar sails.

The advantage of a solar sail is that it doesn't need any fuel. The Sun keeps shining all the time and the continuous acceleration acting on a solar sail allows for orbits which are difficult or impossible to achieve with other means.

Although a small solar sail was used for Mariner 10 flying to Mercury to assist in attitude maneuvers and very recently solar sails have been employed on some geostationary satellites to balance the torque of asymmetric solar power arrays no solar sail has been used so far as a means of propulsion.

In the 1970s NASA did a study of a solar sail mission to rendezvous with comet Halley. But the technology of the day required a spacecraft of 850 kg with a solar sail 800m x 800m large and the plan was dropped. In the ensuing years more plans for solar sail missions popped up by various space agencies but they vanished almost as quickly as they appeared altough a few tests to deploy sail-like structures in space were done.

Thanks to technological advances micro and nano satellites are now possible with a mass of just a few kg which can be launched cheeply as secondary payloads. This allowed a private organization living off donations to get into the act. In 2005 the Planetary Society attempted the first solar sail flight but the Volna rocket failed to reach space.

Using an improved design and a different carrier rocket the Planetary Society will try again this year. LightSail-1 has less than 4.5 kg and a sail with 5.5 m to the side. It is scheduled for deployment as a secondary payload into an orbit of at least 800 km altitude.

Although adequate for a prove-of-concept this is a poor orbit for solar sails. Atmospheric drag will still be noticable on such a light structure and a high concentration of space debris as well as the strong gravitational force of the Earth make this a place to avoid. The situation can be compared to that of a large sailing vessel trying to navigate in the narrow confines of a busy harbour using only sails.       

For a meaningful mission solar sails have to get away from the vicinity of the Earth. This is what the Planetary Society will try next. The plan is to fly a solar sail craft toward the Lagranian L1 point. This is a place about 1.5 million km from the Earth where an equilibrium exists between the gravitational and centrifugal forces of the Earth and the Sun. A number of spacecraft studying the Sun fly in large loops around this point.

Due to the continuous acceleration by sunlight a solar sail can establish an artificial L1 point closer to the Sun. Even a small solar sail can already double the L1 distance to the Earth. This is attractive for an early warning system of solar magnetic storms which might affect satellites in Earth orbit as it doubles the time for such warnings compared to spacecraft at the natural L1 point.

But the Planetary Society isn't the only one trying to fly a solar sail in space this year. Of all the various space agencies the Japan Aerospace Exploration Agency (JAXA) alone has kept working on solar sails in a consistent way. Later this year Japan plans to launch the Venus Climate Orbiter AKATSUKI toward the planet Venus. It will be joined by the "Interplanetary Kite-craft Accelerated by Radiation Of the Sun" (IKAROS) as secondary payload.

IKAROS (not to be confused by the Icarus project I wrote about last month!) is a 14m x 14m  square solar sail. Although originally on a trajectory toward Venus its purpose is not to investigate our sister planet but rather to prove the concept of solar sailing in interplanetary space. It is the precursor to a much more ambitious mission.

To this end some sections of the IKAROS solar sail are covered by thin film solar cells for generating electricity. This will be needed by a second mission to be flown late in the decade. That spacecraft will have a 50m sail and the electricity produced by the solar cells will be used by an ion-propulsion engine. This hybrid propulsion system will be essential for the planned mission objective: Jupiter and the Trojan asteroids.

So 2010 seems to be the year we finally see spacecraft sailing across the vastness of space!

If you are interested in more details about those exciting missions (like how do you actually deploy a solar sail in space?) check the following sites:

The JAXA page of the ICAROS sail.

The Planetary Society page for LightSail-1.