Over the next few weeks, you will have a chance to see an exciting new type of satellite in our skies – a Solar Sail.
Solar Sails work much like sails do on Earth. If a wind is blowing they will enable a craft to move. If you orient the sail in different ways you can control the speed and direction of this motion, even allowing you to move against the wind if you so desire. It’s free and so long as a wind exists, you can technically keep moving forever.
Now, you may be asking if there is such a thing as a wind in space? Isn’t space not meant to be devoid of air? Well, it turns out that light itself has a pressure. It’s very small but it’s quite measurable. Furthermore, with the right materials you can build a sail that can use this wind to move around in space.
Now, here’s the exciting bit. Most solar system exploration to date has been performed with rockets: spacecraft that are powered by their own internal fuel tanks, as it were. When those fuel tanks run out, the spacecraft have few options to go any faster. The further you want to go, the bigger the fuel load must be, the bigger the rocket must be and the more expensive the mission. Rockets are great for near-Earth discovery, but further away from us this becomes a big problem. But solar sails are different. They are very light, they do not require a fuel tank and once unfurled they will continue to accelerate indefinitely. A solar sail might start its journey travelling very slowly indeed but if the acceleration is continuous it can eventually reach speeds unheard of by existing spacecraft. Solar sails may some day be used in future missions beyond our solar system.
At a more mundane level, solar sails may be a good solution for the growing “space junk” problem. Over the last 40 years, successive space missions have left thousands of pieces of debris in low orbit: old satellites, boosters, tools, etc. These items pose a major and growing problem for future forays into space. They are orbiting the Earth at considerable speed where they could crash into important equipment and even manned craft such as the International Space Station. Solar sails would enable old satellites to be de-orbited, where they could burn up safely in the atmosphere.
The Solar Sail currently in orbit is called NanoSail-D. It was launched in November and after some technical difficulties it unfurled just a few days ago on the 20th of January, covering an area of just 9 square metres. It is the first solar sail ever to orbit the Earth. Over the next two months, the craft will be used to test its ability to slow itself down in orbit, eventually re-entering the Earth’s atmosphere.
NanoSail-D will be visible from Ireland as it crosses over some 650 km above us. Tonight it will cross over at 5.27pm and 7.09pm. On Thursday it will cross over at 5.51pm and 7.35pm, and on Friday it will cross over at 6.15pm. Please see the Heavens Above website for more information about visible passes over your area.