A US-based non-profit organisation led by science educator Bill Nye is poised to launch a crowd-funded space mission that, if successful, will be the first ever to use a solar sail to manoeuvre an Earth-orbiting satellite.
Solar sails have long been a darling of science fiction. But it is only in the past few years that scientists have taken serious steps towards making them feasible.
Driven by large, lightweight mirrors, they work by bouncing away incoming photons — just like a normal mirror lets you see your image when you gaze into it.
But rather than being designed for reflection, these are made to take advantage of the fact that photons, although massless, have momentum. That means that when they bounce off a suitable surface like myriad tiny billiard balls, they push on it, very gently.
The push from a single photon is minute. But if you shine enough light on a big enough surface, the effect adds up. It’s not quite like wind pushing on a yacht sail, but it’s not as different as it might sound.
“The thing that’s interesting is that it works so much like a sailboat,” Nye says.
There have been a handful of prior attempts to demonstrate the feasibility of solar sails, ranging from a project called Cosmos 1, which failed on launch in 2005, to a 2010 Japanese mission called Interplanetary Kite-craft Accelerated by Radiation of the Sun (IKAROS), which was designed to test the ability of a light sail to function in interplanetary space.
But the new project, called LightSail 2, will be the first to attempt to use a solar sail to manoeuvre in Earth orbit.
Built by The Planetary Society, a nonprofit organisation dedicated to the advancement of space science and exploration, it is scheduled to be launched the night of 24 June, Florida time, as part of the payload of a Falcon Heavy rocket, the largest rocket currently built by the private company Space Exploration Technologies (SpaceX).
“This $7 million project has been 100% funded by individuals — people who are passionate about space flight and willing to put their own dollars behind [it],” says The Planetary Society’s Jennifer Vaughn.
In total, she says, more than 40,000 people contributed, chipping in anywhere from $5 to more than $1 million.
The spacecraft itself, explains Bruce Betts, the organisation’s chief scientist and LightSail program manager, is a “three-unit cube-sat”. “It’s around the size of a loaf of bread,” he says.
It weighs a mere five kilograms, keeping down launch costs and allowing it to share space with other projects. {%recommended 7954%}
Once in orbit, it will extrude four cobalt-alloy booms that will deploy 32 square metres of shiny Mylar film — the sail itself.
“That’s about the size of a boxing ring,” Betts says, adding that “one of the big challenges was to figure out how to cram all that stuff into a tiny spacecraft”.
To navigate, says David Spencer, the mission’s project manager, the spacecraft will use a solar-powered momentum wheel to alter the angle of the entire vehicle, including the light sail.
This will allow it to steer by changing the angle at which sunlight strikes the sail. When the spacecraft needs a push, it will rotate so the sail is facing the sun. When such a push would shove it in the wrong direction, it will rotate so the sail is edge on to the sun.
Even when the sail is turned fully toward the sun, the pressure on it will be very small — akin to the weight of a housefly sitting on your forearm, Betts says.
But, Spencer adds, “it’s always there, and you’re able to use it”.
The launch vehicle will deliver it to a circular orbit at an altitude of about 720 kilometres, but the sail, Spencer says, should allow it to increase that by about 500 metres each day.
“If we get a measurable increase per day, that’s a win,” he says.
Not that the mission is without risks. Spencer thinks that the biggest one is failure in the sun sensors and magnetometers used to help the spacecraft know the direction in which it’s pointing.
“Those things have to work properly for the spacecraft to control itself,” he says.
Errant cosmic rays or other radiation could also produce problems with software and damage hardware.
Meanwhile the LightSail 2 mission is also of great interest to NASA scientists, who in 2020 hope to launch their own light sail-propelled mission, called NEA Scout, to explore a small near-Earth asteroid (NEA) called 1991VG.
That spacecraft weighs 13 kilograms, uses the launch space of six cube-sats, and has an 82-square-metre light sail.
Even that large a surface area, however, provides only a very low acceleration — about .07 millimetres of velocity per second per second, explains Les Johnson of NASA’s Marshall Spaceflight Centre in Alabama, US, and principal investigator on the mission.
But that’s enough for it to speed up by four to five kilometres per second during the two years it will take it to rendezvous with the asteroid, then slowly fly by it.
The mission will only cost $28 million — a fraction of the cost of conventional asteroid-rendezvous missions.
“What’s really cool about a sail,” he adds, “is that if everything is still working, we can do a loop and come back. It’ll take a few months, but if we need more data, we’ll return.”
His team and the LightSail 2 team, he adds, have long been in communication.
“We are all solar sailors, wanting to achieve the same goals, so we talk to each other,” he says.
What’s interesting from a technological perspective, he adds, is that neither NASA’s light sail nor that of the Planetary Society use exotic new materials.
“These booms and sail materials have been around for a long time,” he explains.
“What enabled this is the miniaturisation of spacecraft. When everything started shrinking, sails that we could build suddenly started getting useful acceleration. That’s what enabled the whole field to take off.”
Meanwhile, LightSail 2’s manoeuvres should be visible from large parts of both America and Australia — basically anyone within 42 degrees of the equator, Betts says.
How bright it will be will depend on the angle of its light sail at any given moment, but if you catch it at the right time, Betts says, “it’s going to be incredibly bright — much brighter than the brightest star”.
Nye adds that there will be a place on The Planetary Society’s website where you can enter your latitude and longitude and get directions where to look.
“It’s very likely that people who don’t live too far from the Equator will get more than a glimpse of it, more than once,” he says.
