In a story written by the science fiction author Arthur C Clarke in 1964, the hero asks a group of students to hold their hands out to the sun. “What do you feel?” he asks. “Heat, of course. But there’s pressure as well — though you’ve never noticed it, because it’s so tiny. Over the area of your hands, it only comes to about a millionth of an ounce.”
The scientist is right: sunlight has no mass, but it has momentum and it exerts pressure — a property that will soon be harnessed when Nasa unfurls the largest solar sail it has ever launched.
Last month a rocket blasted off from New Zealand carrying an unmanned spacecraft called an Advanced Composite Solar Sail System (ACS3) into orbit around the Earth. In the next few weeks, the microwave-sized craft is due to deploy a kite-like sail. All going well, photons, or particles of starlight, will bounce off the sail and push the spaceship along — much as a sailing ship is carried across the sea by the wind.
The sail has an area of 80 sq m, about the size of six car parking spots, and weighs only 400 grams.
The sunlight that reflects off it will exert a force equivalent to the weight of four staples — each weighing 0.02 grams — held in the palm of your hand.
Given that the body of the spacecraft weighs 16kg, this may not sound like very much. But as Clarke’s sci-fi hero explained, out in the vacuum of space, “even a pressure as small as that can be important — for it’s acting all the time, hour after hour, day after day”.
Acceleration will be much slower than that of a rocket engine, but a solar sail will never run out of fuel and it can continuously pick up speed. According to Nasa, that may make the technology the key to covering the vast distances needed to explore “deep space”.
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Nasa has calculated that a square solar sail 500m wide could cover the 15 billion miles travelled in four decades by Voyager 1, our most distant spacecraft, in only eight years.
“The Sun will continue burning for billions of years, so we have a limitless source of propulsion,” said Alan Rhodes, the lead systems engineer for the ACS3 mission at Nasa’s Ames Research Center in California. “We will demonstrate a system that uses this abundant resource to take those next giant steps in exploration and science.”
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James Clerk Maxwell, the Scottish physicist, realised in the middle of the 19th century that light exerts pressure. The novelist Jules Verne, writing shortly afterwards, suggested that it could be used to move a spaceship. It was not until 2010, however, that the Japanese space agency launched the first successful solar sail.
According to Greg Dean, lead assembly integration and test engineer for ACS3, the physics involved is relatively simple, but because the sails must be large and extremely light, they are difficult to build.
The sail for ACS3 is made from a polymer material sandwiched between layers of aluminium and chromium and is thinner than a human hair. It had to be tightly folded up to be carried into orbit. “And if you look at it wrong, it’s going to rip,” said Dean.
The delicate sail is being constructed at Nasa
NASA
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The next step in the mission will involve extending the four masts, or booms, that will support the sail. Made from a polymer and carbon fibre material, each boom is about seven metres long but weighs just 165 grams. At the moment they are rolled up as flat spirals inside the spaceship body.
As they extend, they’ll assume a rigid tube shape, and the sail itself will be unfurled. Just getting this step right will count as making the mission a success, said Dean. The goal will then be to harness the pressure of the sun to shift the spacecraft’s orbit by about 2 km in 24 hours.
This could be a first small step towards far longer voyages. “With a perfect sail, you can pretty much travel infinitely,” said Keats Wilkie, the mission’s principal investigator at NASA Langley.
Dr. Thomas Hughes is a UK-based scientist and science communicator who makes complex topics accessible to readers. His articles explore breakthroughs in various scientific disciplines, from space exploration to cutting-edge research.
