Richard A Lovett
A small non-profit organisation has achieved a space-travel feat dreamed about for more than 40 years: proving that it is possible to manoeuvre a spacecraft in Earth orbit using only the power of sunbeams. Seven weeks ago, The Planetary Society, which has 50,000 members in 109 countries, launched a tiny, five-kilogram spacecraft into orbit, aboard a SpaceX Heavy Falcon rocket that also carried two dozen spacecraft for the US Air Force. From there, the spacecraft, called LightSail 2, was delivered to an orbit about 720 kilometres above the Earth.
After preliminary tests, it deployed a boxing-ring-sized sheet of reflective Mylar film, which, since then, it has been using to “sail” on the pressure of sunlight.
As far back as 1977, astronomer Carl Sagan – The Planetary Society’s founder – was promoting light sails as a way of propelling spacecraft.
They work because the photons that comprise light, even though they are massless, possess momentum, causing them to exert a gentle pressure on whatever object the light falls on. That pressure can be doubled by reflecting the light with a mirror, which is why LightSail 2 uses a silvery film for its sail.
Sagan’s dream was to use a big lightsail to propel a big spacecraft.
“Back in the 1970s,” says The Planetary Society’s CEO, science educator Bill Nye, “the proposal was to have a solar sail a kilometre on a side that would catch up with comet Halley [after it last passed close to the Sun in 1986].”
The idea was quickly abandoned due to budget concerns and the sense that it was impractical. Revived interest, however, came from the development of miniaturised spacecraft known as CubeSats, which are constructed in blocks measuring 10 × 10 × 11.35 centimetres. (LightSail 2 is built in three of these blocks).
Not only are these inexpensive to launch, their low mass makes them perfect for light sailing. Instead of needing a 100-hectare lightsail, like Sagan’s comet-chaser proposal, LightSail 2 only needs a 32-square-metre sail.
The spacecraft’s small size also makes it easy to manoeuvre – important, because its operations require the orientation of its lightsail to be changed rapidly as it circles the Earth, like a sailboat tacking in changing winds.
The results were dramatic, says Bruce Betts, the project’s chief scientist and program manager. In the eight days since the light sail was deployed, the spacecraft has raised its orbit’s apogee (the point at which it is most distant from the Earth) by 1.7 kilometres.
“Our biggest change was a little over 900 metres, three days ago,” project manager David Spencer, an associate professor of aeronautics and astronautics at Purdue University, Indiana, told a press conference.
LightSail 2 isn’t designed to do anything other than test the ability to use lightsails to manoeuvre. “This has been a demonstration mission all along,” says Jennifer Vaughan, The Planetary Society’s COO.
But its success has important applications. NASA has a six-CubeSat near-Earth asteroid (NEA) mission, called NEA Scout, scheduled for launch sometime in 2020, which will also use light-sail propulsion.
“We’ve got an agreement to share technologies and findings,” Spencer says. “We really look forward to them carrying solar sailing technology to the next level.”
NEA Scout’s principal investigator, Les Johnson of NASA’s Marshall Spaceflight Centre, Huntsville, Alabama, agrees.
“We are all solar sailors, wanting to achieve the same goals,” he told Cosmos shortly before LightSail 2 launched.
And that might just be the beginning. Because lightsails don’t require fuel – and can therefore never run out of it – they are ideal for long-term missions, especially those in which spacecraft might have to do a lot of low-acceleration manoeuvring.
For example, Nye says, they could be used to position a spacecraft between the Earth and the Sun, where it could provide as much as four to six hours of advance warning of dangerous bursts of solar radiation heading our way. Or, such a spacecraft could look outward, seeking out previously unknown asteroids that might pose a risk to the Earth.
“Solar sails are uniquely suited to positioning themselves, station keeping, in orbits closer to the Sun than the Earth’s orbit,” Nye says.
They can also be used to hold spacecraft in orbits around the Earth that are not otherwise stable, such as one that puts a satellite permanently above one of the Earth’s poles. Or, they can be used for long missions to multiple asteroids, the Outer Solar System, or even to other stars.
With a normal spacecraft, Nye says, the fuel eventually runs out. “This fuel never runs out.”
Meanwhile, LightSail 2’s mission isn’t over. “We’re still working to improve sail control,” Spencer says.
The short-term goal, he says, will be to continue working to raise the spacecraft’s apogee. But the manner in which that is being done has the side effect of lowering the spacecraft’s perigee (its closest approach to the Earth).
And, even though it’s 700 kilometres above the Earth’s surface, there’s enough of our planet’s tenuous exosphere up there to exert drag on such a lightweight spacecraft. “As perigee moves lower, the atmosphere is going to cause more drag, to the point where it’s going to be impossible for us to overcome that drag,” Spencer says.
It would be possible, he adds, to employ different sailing techniques to raise the spacecraft’s perigee, along with its apogee, but that wasn’t the goal of the mission. “It’s certainly possible from a physics standpoint to do that, but that’s not in our near-term plan,” he says.
Instead, LightSail 2 will then use its sail to study the drag of the far-outer atmosphere, until the spacecraft eventually falls back to Earth and burns up.
Meanwhile, Nye says that in addition to its scientific objectives, the mission has achieved to other important goals.
First, it was conducted at about “one-twentieth” the cost that would have been incurred by a “regular” space agency like NASA.
Sure, more money would have been nicer – if for no other reason than to allow the control team to talk to the spacecraft throughout its orbit, rather than just when it was above one of their few tracking stations. “But nevertheless, we were able to pull it off,” Nye says.
But more importantly, he says only days after the anniversary of the first Apollo Moon landing, “it shows, once again, that space exploration brings out the best in us.”
