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LightSail 2
An image returned from the LightSail 2 spacecraft during deployment of its solar sail July 23. (credit: The Planetary Society)

Solar sailing, at long last

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For decades, The Planetary Society and its founders had sought to demonstrate the viability of solar sailing. The technology offers the opportunity to explore the solar system without the limitations of a conventional propulsion system, able to travel using the power of sunlight alone.

“Today is the day we declare mission success,” said Nye. “We are going to a higher orbital altitude without rocket fuel, just from the push of sunlight.”

That demonstration turned out to be far more difficult than expected. Carl Sagan advocated for a solar sail mission in the 1970s to go to Comet Halley, but slashed NASA budgets cancelled that effort—and led to the formation of the organization itself, to advocate for planetary exploration at a time when such missions appeared to be in jeopardy.

In the early 2000s, The Planetary Society worked on a mission called Cosmos 1 to demonstrate solar sail technology in Earth orbit, combining American concepts and funding with Russian hardware and launch. Cosmos 1 launched in June 2005, but the Volna rocket malfunctioned shorrly after liftoff, and Cosmos 1 never reached orbit.

The organization used a little bit of funding left over from Cosmos 1 to reboot their solar sailing plans. That led to the start of the LightSail program, leveraging advances in small satellite technologies to enable a solar sail mission to be done with a much smaller, and less expensive, spacecraft.

The first LightSail spacecraft, a 3U cubesat, launched in 2015 and successfully deployed a solar sail. The spacecraft, though was in a low orbit where atmospheric drag dominated any thrust the sail could generate. To demonstrate propulsion, The Planetary Society sought to do a second mission launched into a higher orbit.

That came with the long-awaited (and long-delayed) Falcon Heavy launch of the Space Test Program 2 mission for the US Air Force June 25. Its collection of payloads included Prox-1, a small satellite built by Georgia Tech that contained The Planetary Society’s LightSail 2 cubesat. A week after launch, Prox-1 successfully deployed LightSail 2 and, after a few weeks of on-orbit testing, LightSail 2 deployed its 32-square-meter solar sail July 23.

Eight days later, the organization said that LightSail 2 was a success. “Today is the day we declare mission success,” said Bill Nye, CEO of The Planetary Society, in a teleconference. “We are going to a higher orbital altitude without rocket fuel, just from the push of sunlight.”

Strictly speaking, the average orbital altitude is not increasing. Instead, thrust from the solar sail is increasing the apogee of the spacecraft’s orbit: 1.7 kilometers in the four days after LightSail 2 deployed its sail. The spacecraft’s perigee, though has been dropping, which project officials say is due to atmospheric drag on the sail. That change in orbital parameters, project officials said, can only be explained if the sail is providing thrust on the spacecraft.

“It’s possible that we could try to raise perigee as well,” said David Spencer, the LightSail 2 project manager and an associate professor at Purdue University. “The objective of the mission is to just show a definitive change in our orbit apogee, through solar sailing. And so, that’s been our focus, and we want to continue to do that well.”

The solar sailing phase of the mission should continue through late August, Spencer said, with the spacecraft’s apogee continuing to rise and perigee dip until atmospheric drag overwhelms the thrust from the sail. Exactly when that will happen, and by how much orbit the orbit changes, isn’t clear because of uncertainties in atmospheric density at the altitudes LightSail 2 is flying, about 705 and 725 kilometers.

“We don't really know at what point atmospheric drag is going to overcome our ability to continue orbit raising,” he said. “We’ll keep doing this as long as we can.”

“It’s a possible interplanetary propulsion technique for possible piggyback missions of cubesats, small missions in the future,” Betts said of solar sails.

Once the sailing phase of the mission ends, the orbit will gradually decay, reentering in about a year. Engineers plan to do additional tests on the spacecraft during that time, manipulating the sail to see how different orientations affect the drag on the spacecraft. Spencer said that, at the end of the mission, they may attempt to use the sail for a “targeted reentry” where they try to use the sail to steer the spacecraft to reenter over a specific location. “That’s an experiment that, to my knowledge, hasn’t been done before,” he said.

The success of LightSail 2 is a vindication of the decades-long effort by The Planetary Society to demonstrate solar sailing, even though they were not the first to do it. The Japanese spacecraft Interplanetary Kite-craft Accelerated by Radiation Of the Sun, or IKAROS, launched in 2010 as a secondary payload on the Akatsuki mission to Venus. It deployed a much larger sail, 196 square meters in area, which produced thrust as the spacecraft flew in interplanetary space toward Venus.

Nye said that LightSail 2 had a better thrust-to-weight ratio than the much heavier IKAROS. “We are the highest performing solar sail,” he said, with a thrust-to-weight ratio “the highest of any sail launch so far.”

The use of the cubesat form factor is key to the success of LightSail 2 mission, and its ability to support future missions. “This is demonstrating solar sail propulsion in these cubesats,” said Bruce Betts, the LightSail program manager at The Planetary Society. “That means it’s a possible interplanetary propulsion technique for possible piggyback missions of cubesats, small missions in the future.”

One such mission is NEA Scout, a NASA-funded 6U cubesat mission that is among the cubesats that will launch on the first Space Launch System mission no earlier than late 2020. NEA Scout will use a solar sail to propel itself from cislunar space to make a flyby of a near Earth asteroid. Spencer said that the society has a Space Act Agreement with NASA to exchange data on solar sail technology, and members of the NEA Scout have attended LightSail 2 meetings.

Betts said the solar sail technology could be useful for any number of mission concepts, from asteroid flybys like NEA Scout to “pole sitter” missions for space weather monitoring. “Basically, you want to take advantage of the fact that you don’t require fuel. So, you can have very long, very flexible missions,” he said.

The society has no plans for a LightSail 3, Nye said. Instead, the organization is planning an international competition to solicit ideas for new projects that support its broader efforts to support exploration of the solar system, in addition to supporting ongoing technology development efforts.

He left the door open, though, to do another solar sail mission if such a concept emerged from that competition. “If a formal solar sail proposal emerges that is reasonable and worthy, then we would pursue that,” he said.

“For me,” Nye said, “it’s very romantic that you’ll be sailing on sunbeams.”s

Society officials were very satisfied with the outcome of the LightSail 2 mission and the overall program, which cost $7 million, according to Jennifer Vaughn, COO of the organization. That funding came from donations from nearly 50,000 people in 109 countries. “They are truly the heroes of the story,” she said of the project’s supporters. “LightSail 2 is their spacecraft, and I’m so very grateful for the decade of support for LightSail.”

“This is a very exciting day for us and for me personally,” said Nye, who recalled first hearing about solar sails in a class he took at Cornell University taught by Sagan in 1977. “I’ve been charmed or thrilled by this idea ever since I first heard about it over 40 years ago.”

“This idea that you could fly the spacecraft, that you could get propulsion in space from nothing but photons, is really counterintuitive,” he said. “It’s surprising. And, for me, it’s very romantic that you’ll be sailing on sunbeams.”

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