August 25, 2019

LightSail team declares success in solar sail experiment


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This image was taken by a camera aboard the LightSail 2 satellite during the solar sail deployment sequence at 1849 GMT (2:49 p.m. EDT) on Tuesday, July 23. Credit: The Planetary Society

The gentle push of sunlight is slowly changing the orbit of the Planetary Society’s crowd-funded LightSail 2 satellite after it unfurled a thin solar sail the size of a boxing ring last week, officials confirmed Wednesday.

LightSail 2 is the capstone of a decade-long, $7 million effort to advance the science of solar sailing, a technique that could allow small probes to travel across the solar system, or to other stars, at faster speeds and lower cost.

“On behalf of the tens of thousands of people around the world who came together to help the dream of solar sailing move forward, we’re thrilled to declare mission success for LightSail 2,” said Bruce Betts, the LightSail program manager at the Planetary Society, a non-profit space advocacy group headquartered in Pasadena, California.

LightSail 2 was one of 24 satellites launched June 25 by a SpaceX Falcon Heavy rocket. At the time of launch, LightSail 2 was cocooned inside a larger spacecraft named Prox 1, which released the solar sail craft a week into the mission.

Based on a CubeSat platform, LightSail 2 was about the size of a loaf of bread when folded up for launch. A few days after separating from Prox 1, LightSail deployed solar panels to begin recharging the craft’s lithium-ion batteries, then officials sent the command to open the sail July 23, somewhat later than originally planned to allow extra time for engineers to fine-tune the CubeSat’s attitude control system.

“Our criterion for mission success was to demonstrate controlled solar sailing in a CubeSat by changing the spacecraft’s orbit using only light pressure of the sun,” Betts said. “This is something that’s never been done before in a small spacecraft.”

In a press teleconference Wednesday, Betts said the high point, or apogee, or LightSail 2’s orbit had increased by a little more than a mile — about 1.7 kilometers — over the preceding four days. LightSail 2 does not carry any other means of propulsion, so engineers have attributed orbit change to solar sailing.

LightSail 2’s ultra-thin sail was stowed into the spacecraft’s toaster oven-sized body during launch, but once unfurled, it covers an area comparable to that of a boxing ring, or about 344 square feet (32 square meters). The pressure from solar photons, or units of light, imparts a tiny measure of acceleration on the sail, but it will be enough to change LightSail 2’s orbit.

“This is a very exciting day for us, and for me personally,” said Bill Nye, CEO of the Planetary Society. “This idea that you could fly the spacecraft, that you could get propulsion in space, from nothing but photons is really counterintuitive. It’s surprising. And for me, it’s very romantic that you’ll be sailing on sunbeams.”

This chart shows LightSail 2’s orbit apogee and perigee since launch. From 26 July to 30 July, the spacecraft raised its orbital high point, or apogee, by about 2 kilometers. Credit: The Planetary Society / Data provided by NXTRAC

Telemetry radioed to the ground by LightSail 2 shows the spacecraft is turning its sail broadside to the as it goes around the Earth. The CubeSat’s orientation allows for pressure from solar photons to push on the sail with a force no more than the weight of a paperclip, according to the Planetary Society.

“Our acceleration divided by mass, or thrust to mass, is the highest of any sail launch so far,” Nye said. “And in order to do this, we have to tack in Earth orbit, which means we have to twist the spacecraft 90 degrees every 50 minutes.”

The LightSail 2 experiment is the second solar sailing CubeSat developed by the Planetary Society, and the group’s third attempt overall at a solar sail experiment in low Earth orbit.

A predecessor mission named LightSail 1 lifted off aboard a United Launch Alliance Atlas 5 rocket in May 2015.

The LightSail 1 CubeSat successfully expanded its solar sail, but the spacecraft’s altitude was too low to demonstrate sailing on sunlight. At LightSail 1’s altitude, aerodynamic drag influenced the craft’s orbit more than the minuscule effect from sunlight.

An earlier solar sail experiment led by the Planetary Society, named Cosmos 1, failed to reach orbit after a launch from a Russian submarine.

The Planetary Society says the LightSail program, comprising both CubeSat missions, cost around $7 million from 2009 through March 2019. Planetary Society members, private citizens, foundations and corporate donors helped pay for the program.

NASA provided the launch of the LightSail 1 mission in 2015 on an Atlas 5 rocket, and the U.S. Air Force Research Laboratory provided LightSail 2 with a ride into orbit on SpaceX’s Falcon Heavy.

LightSail 2’s demonstration comes after the Japanese IKAROS mission became the first spacecraft to rely solely on solar sailing for propulsion. IKAROS launched with Japan’s Akatsuki mission toward Venus in 2010, and opened its 2,110-square-foot (196-square-meter) sail in interplanetary space.

But the IKAROS spacecraft was much larger than LightSail 2. The solar sailing success announced Wednesday proves a relatively low-cost CubeSat-class nanosatellite — within the budget of small space agencies, universities and numerous private companies — can use light pressure to reach far-flung destinations.

A NASA-funded CubeSat mission named NEA Scout will employ a solar sail to travel to a near-Earth asteroid after flying into deep space on the first mission of NASA’s Space Launch System heavy-lift rocket.

“This is demonstrating solar sale propulsion in these CubeSats, in small spacecraft,” Betts said. “That means it’s a possible interplanetary propulsion technique for possible piggyback missions of CubeSats, small missions in the future. And in the fairly near future, NASA’s NEA Scout will do this with a spacecraft twice as large. But it’s really demonstrating it can be done, and so hopefully opening up a whole new field of spacecraft and spacecraft propulsion between the planets.”

Future solar sails could be powered by laser light arrays to accelerate toward other stars, achieving speeds impossible with conventional rocket engines.

Artist’s concept of LightSail 2. Credit: The Planetary Society

Officials expect LightSail 2 to remain in orbit for about a year before atmospheric drag pulls it back to Earth, when it will burn up during re-entry.

“We’re going to be, over the next several weeks, continuing to raise the orbit apogee, and we think that we can do that for about a month, probably through the end of August,” said Dave Spencer, LightSail 2’s project manager.

“As we’re doing that, our perigee, or the close point in the orbit, is going to move slightly lower over time,” Spencer said. “And as it 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 atmospheric drag through the use of solar pressure.”

LightSail 2’s attitude control system does not have the ability to point the solar sail to circularize its orbit. Future missions will have that capability.

“There’s one experiment that I really am looking forward to, Spencer said. “If the spacecraft is still functional, once we get down to the point of re-entry, I’d like to see if we can actually control the re-entry point somewhat by changing the orientation of the solar sail. That’s an experiment that to my knowledge, hasn’t been done before, and that’s called targeted re-entry. That would be a really fascinating experiment.”

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Follow Stephen Clark on Twitter: @StephenClark1.


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