Launching a solar sail

A solar sail uses photons from the Sun to propel its flight in space. By Cathal O’Connell.

Stowed for launch into space, the LightSail's solar sail folds into a space just 10 x 10 x 20 centimetre in size (left). Once unfurled (right), the sail is 32 square metres in size. The sail is made of Mylar, a strong form of polymer one quarter the thickness of plastic bag. To catch the light the Mylar is coated with a highly reflective aluminium layer. – Anthony Calvert

Provide ships or sails adapted to the heavenly breezes, and there will be some who will brave even that void.”

— Kepler in a letter to Galileo, 1610.

Arthur C. Clarke’s 1964 story Sunjammer describes a race between space yachts sailing the Solar System propelled by sunlight. LightSail is the latest and most compact realisation of this fantasy.

A black box about the size of a loaf of bread, LightSail is a most unlikely looking space yacht. Yet it could potentially cross the heavens faster than any rocket.

Last June, floating free in low-Earth orbit, its four doors opened like petals, a spindly arm emerging from each. Then, like a magician conjuring handkerchiefs from a pocket, the arms unfurled a shiny, metallic sheet that grew and grew, until it flattened into a sail angled towards the Sun. Light yachts sail because particles of light (photons) have momentum. When they bounce off a reflective surface they give it a tiny kick. With enough photons, the tiny kicks add up to a significant thrust. A spacecraft with a large enough sail could eventually reach incredible speeds with zero fuel. Harnessing sunlight could allow us to send faster and cheaper probes to the outer Solar System.

Solar sailing has also captured the attention of amateurs interested in a relatively cheap way of exploring space

Solar sailing projects have been mooted by space agencies since the 1970s (including a plan by NASA to sail to Halley’s Comet in 1986). But it wasn’t until 21 May, 2010 that the first space yacht – the Japan Aerospace Exploration Agency’s IKAROS (Interplanetary Kite-craft Accelerated by Radiation Of the Sun) – was launched into orbit. When its 196-square-metre sail opened to catch the solar breeze, IKAROS became the first craft to use photons as its main form of propulsion. It reached Venus in a little more than six months, completing its flyby on 8 December, 2010. IKAROS is now in a stable seven-year orbit around the Sun between Venus and Mercury.

Solar sailing has also captured the attention of amateurs interested in a relatively cheap way of exploring space, which is how LightSail came into being.

Whereas IKAROS cost about $20 million to build and launch, LightSail is being run on a $1.5 million budget, raised by public donations to a Kickstarter campaign. The project is driven by The Planetary Society, a non-profit co-founded by Carl Sagan in the early 1980s to promote space exploration.

To keep costs low, LightSail is designed as a CubeSat, meaning it’s small and light enough to hitch a ride into space. When a large satellite is launched into orbit, for instance, CubeSats can be tucked into a corner of a launch rocket without inconveniencing the main cargo.

LightSail boxes weigh in at five kilograms and are only 30 centimetres long – you could squeeze two in your carry-on luggage. But once unfurled, LightSail’s microscopically thin aluminium-coated polymer sail covers 32 square metres. Besides the sail and motors, LightSail is packed with a two-way radio, a control system and onboard imaging cameras. “You can package a very large sail into a tiny little spacecraft, and you can do something useful with it,” says Jason Davis from the Planetary Society.

For the June experiment, the LightSail craft hitched a ride on an Atlas V rocket. It was dropped off in a low-Earth orbit and the arms unfurled the sails. “The deployment was perfect, ” says Bill Nye, chief of the Planetary Society. However, at that altitude the drag of the upper atmosphere gradually slowed it down. It fell back to Earth after about a week.

The successful demonstration paves the way for a device to hitch another launch ride in 2016. This time the sails will unfurl at an altitude of 720 kilometres – where conditions should be ripe for fair-weather sailing.

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Cathal O'Connell is a science writer based in Melbourne.
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