It may be “mission accomplished” for the Planetary Society’s solar sail experiment, but its privately funded LightSail 2 mission is far from over.
Five weeks after LightSail 2’s launch aboard a SpaceX Falcon Heavy rocket, the nonprofit membership society celebrated the spacecraft’s ability to raise the highest point of its orbit by a little more than a mile (1.7 kilometers), using the force of sunlight pressing against its 18.4-foot-wide, 4.5-micron-thick reflective Mylar sails.
Demonstrating solar sail steerability was the point of the decade-long campaign to build and fly LightSail 2 and its predecessor, LightSail 1. The project’s estimated $7 million cost was covered by contributions from Planetary Society members and other donors.
“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,” Planetary Society chief scientist Bruce Betts, who serves as program manager for LightSail, told journalists today during a teleconference.
The sail maneuvered itself in response to commands beamed up from Earth to take advantage of the push of the sun’s photons, in a style that’s similar to sailboats taking advantage of the wind.
LightSail 2 isn’t the first solar sail to get a push from the sun: That distinction belongs to Japan’s Ikaros spacecraft, which experienced a sunlight-powered course change in 2010. But the Planetary Society’s executive director, Bill Nye (the Science Guy), said LightSail 2 showed that the job could be done using a 3U CubeSat spacecraft that’s about the size of a loaf of bread.
If the technology can be perfected, solar sailing could be suitable for a wide range of fuel-free space applications — for example, keeping spacecraft steady above Earth’s poles or at gravitational balance points in deep space. Sail-borne spacecraft could also be directed from one solar system destination to another, or out of the solar system altogether.
Nye said his favorite solar sailing destination would be the planet next door.
“We’d ferry cargo to Mars and look for signs of life, and change the course of human history. How about that?” he said.
Nye noted that the late astronomer Carl Sagan, one of the Planetary Society’s co-founders, promoted the idea of using a solar sail to send a spacecraft to Comet Halley back in the 1970s. Sagan happened to be one of Nye’s mentors. “I’ve been charmed or thrilled by this idea ever since I heard about it 40 years ago,” Nye said.
LightSail 2 unfurled its tightly packed sail a week ago, after a series of orbital checkouts. Purdue aerospace engineer David Spencer, LightSail 2’s mission manager, said the orbit was shifted in a series of steps. The biggest step increased the spacecraft’s maximum orbit by a little more than 900 meters (half a mile).
Spencer said LightSail 2’s capabilities would undergo further testing during maneuvers that are due to continue through August. But there’s a limit: Every time there’s a rise in the maximum altitude of LightSail 2’s elliptical orbit (known as the apogee) there’s a corresponding drop in the minimum altitude on the other side of the orbit (known as the perigee).
“For simplicity, the plan was never to circularize the orbit, only to raise apogee by thrusting on one side of the orbit, which also drops perigee,” the Planetary Society’s Jason Davis explained in a tweet.
Years of computer simulations. Countless ground tests. They've all led up to now. The Planetary Society's crowdfunded LightSail 2 spacecraft is successfully raising its orbit solely on the power of sunlight.
— Planetary Society (@exploreplanets) July 31, 2019
Eventually, the atmospheric drag at perigee will cancel out LightSail 2’s orbit-shifting capability and pull it back toward Earth. The Planetary Society expects the spacecraft to meet its downfall in less than a year. But before it burns up, Spencer wants to do a final experiment.
“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,” he told GeekWire. “That’s an experiment that, to my knowledge, hasn’t been done before.”
There’s more solar sailing on the horizon: NASA plans to put a solar sail on NEA Scout, an asteroid-observing mission that’s due for launch as a secondary payload on the first Space Launch System heavy-lift rocket in the 2021 time frame. “The NEA Scout engineers have been working with us,” Spencer said.
Meanwhile, the Planetary Society is planning a competition to select its next crowdfunded space mission.
“We are involved in other, we believe, game-changing technologies for planetary exploration,” Nye said. As an example, he pointed to the society-supported PlanetVac mission, which would stir up a sampling of soil from the lunar surface and capture it for chemical analysis.
“This international formal proposal competition is what we’re doing next,” he said.
Update for 5:40 p.m. PT July 31: There’s a technical debate over the results of LightSail 2’s maneuvers, due to the fact that the increase in apogee is accompanied by a decrease in perigee.
I am not seeng the claimed orbit raising in the data. I see the normal orbit decay, but an increase in eccentricity presumably caused by the sail – so yes apogee is going up, but perigee is going down even faster. https://t.co/f9v4IFRhCr
— Jonathan McDowell (@planet4589) July 31, 2019
I'm not quite sure what you mean. Are you taking "raised its orbit" to mean average orbit? They are using the phrase informally; the actual indicators are apogee increase/perigee decrease consistent with solar sailing thrust on one side of the orbit.
— Jason Davis (@jasonrdavis) July 31, 2019
Yes, to me "raise orbit" means increase the orbital period. Other space folks were also confused by the usage to mean increase e only
— Jonathan McDowell (@planet4589) August 1, 2019
This is the key confusion: “solar sailing thrust on one side of the orbit” is not consistent with data. At least, I see in data the solar radiation pressure that is continuous (on average). There is no sail steering, just random tumbling motion. This is most likely explanation
— Tomas Svitek (@Tomas_Stellar) August 1, 2019
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