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A Kilometer Per Day: LightSail Mission Managers Refine Orbit-Raising Plan

Posted by Jason Davis

04-09-2015 4:04 CDT

Topics: LightSail

LightSail will spiral away from Earth by about one kilometer per day, according to the latest orbital estimates by Georgia Tech's Space Systems Design Laboratory. Mission managers are refining the software algorithms used to swing the spacecraft's 32-square-meter sails in and out sunlight each orbit. Over time, LightSail's 720-kilometer, circular path around the Earth will become egg-shaped as the spacecraft's apoapsis—the highest point of its orbit—gradually increases. 

In order to validate solar sailing as a viable method of propulsion for CubeSats, LightSail must show sunlight has produced an intentional, measurable effect on the spacecraft's orbit. Until recently, the plan was to alter LightSail's 24-degree inclination. Inclination is a spacecraft's orbital angle with respect to the equator. It also corresponds to the north and south latitudes the spacecraft will reach. 

At 720 kilometers above Earth, atmospheric drag will still affect LightSail's orbit, though the force of solar sailing is expected to outweigh drag by a factor of four or five to one. Drag increases whenever the sun ejects large bursts of charged particles, which expands Earth's atmosphere. (These particles are different from the packets of photons responsible for solar sailing.) 

Georgia Tech aerospace engineering graduate student Sean Chait ran simulations to see how much LightSail's inclination could change due to sunlight pressure. The results showed a difference of just a few thousandths of a degree after several weeks of flight. Such a small change would be difficult to measure from the ground, and even more difficult to attribute to solar sailing. 

As an alternative approach, Chait evaluated the effect of orienting the solar sail to increase the spacecraft's orbital energy. His analysis showed that by making two 90-degree turns each orbit—tacking in an out of the sun's photon stream like a sailboat—LightSail's apoapsis should increase by about a kilometer per day. That means by the end of two months, LightSail's maximum altutide should reach 780 kilometers—almost twice as high as the International Space Station.

LightSail will make these back-and-forth turns using its momentum wheel and electromagnetic torque rods. When the spacecraft is moving toward the sun, its sail will be turned edge-on to slice through the solar photon stream. As the spacecraft moves away from the sun, it will turn perpendicular to the photons, allowing the photons to transfer their momentum to the spacecraft and push it along.

"The reason we are doing this while we are moving away from the sun is to increase LightSail's orbital velocity," says Chait. "This increase in velocity is what causes apoapsis to walk out."

In general, a spacecraft raises its apoapsis, or high point, by thrusting at the periapsis, or low point. Starting from LightSail's initial 720-kilometer circular orbit, this sail control strategy will increase the altitude on the side of the Earth opposite LightSail's solar thrusting.

LightSail orbit raising strategy

Jason Davis / The Planetary Society

LightSail orbit raising strategy
By tacking back and forth into the sun's rays like a sailboat, LightSail gradually raises the apoapsis, or high point, of its orbit. The orbit becomes increasingly egg-shaped on the side of the Earth opposite LightSail's solar thrusting.

Chait says that while LightSail's apoapsis altitude will increase, atmospheric drag will cause the periapsis, or lowest point, to slowly decrease. "This drag is what will eventually de-orbit LightSail," Chait says. "But this strategy proves that we are changing LightSail's orbital parameters using solar radiation pressure, due to the fact that atmospheric drag will only decrease the spacecraft's altitude."

Thanks to a highly successful KickStarter campaign, LightSail is funded for four months of orbital operations. It launches in September 2016 with its partner spacecraft, Prox-1, aboard the first operational flight of the SpaceX Falcon Heavy.

See other posts from September 2015


Or read more blog entries about: LightSail


Bob Ware: 09/08/2015 04:52 CDT

This looks like a good strategy but I think the 720 K (447 Mi.) altitude is to low to start. 965.606 (600 Mi.) is a safer to bet to start from for a multitude of solar wind scenarios from weather variations affecting the atmospheric density.

Nick_R: 09/11/2015 04:10 CDT

Sails don't just tack at 90 degree increments - you could go diagonal to get some 'left' impulse when crossing between earth and sun, raising the perigee as well. Additionally, you could continuously adjust the angle to optimize the orbit - like a sail boat.

Nick_R: 09/11/2015 09:20 CDT

Thought about it a bit more - starting at the 3 O'Clock position (at angle theta=0) - start with the sail edge on to the sun as indicated on the diagram (the normal to surface points away from earth. Then rotate the sail normal to negative theta/2 degrees as the craft goes around the earth (equivalent to spin rate of 1/(2*orbital time)). The the normal thrust vector will always be in the direction of travel. A constant spin will match your 3 O'Clock and 9 O'Clock positions.

Marc: 09/14/2015 05:09 CDT

I don't know if you could be one to answer a question I have, but I thought I might as well ask it here. I've read somewhere that solar sails like the lightsail would be able to return safely to earth. I've been wondering how this would work and if someone could explain this to me. Thanks in advance.

Jason Davis: 09/14/2015 01:50 CDT

Bob: Unfortunately, we don't get to choose our orbit -- we get dropped off at 720 km, and that's that. Nick: The limiting factor, as I understand it, is the capability of the ACS. When all was said and done, these 90-degree tacks (which take several minutes to complete each orbit) were shown to be the simplest, most feasible method for raising the spacecraft's apoapsis. Marc: Very few vehicles, such as Soyuz and Dragon, have reentry capabilities. A returning solar sail spacecraft would either need to pack up or discard its sail first, to say nothing of the other logistics involved like heat shields, etc.

kj4saf: 09/30/2015 09:57 CDT

When will the keplerian element set data of the initial orbit estimate be available? What will be the monthly cost of orbital operations?

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