Solar Sailing Lessons From LightSail2

Solar Sailing was the subject of a paper released by the Planetary Society last month. Find out what they learned from the highly successful LightSail2 mission.

We’ve been keeping readers updated about the status of LightSail2. That’s the plucky loaf-of -bread-sized spacecraft that propels itself using nothing but sunlight.

It’s a project of the Planetary Society and its goal is to prove that we can power a spacecraft using light energy as its only fuel source. This would get rid of all the challenges posed by using chemical fuels.

Like sailing the seas, solar sailing technology uses a large, thin, reflective sheet to capture the photons from sunlight. Even though photons have no mass, they have momentum, and when they bounce off the solar sail, they give the spacecraft a tiny nudge.

REFLECTIVE SHEET CAPTURES PHOTONS FROM SUNLIGHT

The nudges add up and very, very gradually, the solar sail spacecraft builds up speed. This is a different approach from a rocket launch, where the momentum comes all at once at liftoff.

If we’re patient, solar sailing could theoretically take a spacecraft to speeds of up to 20% of the speed of light. That’s far faster than a chemical rocket can go.

Plus you don’t have to worry about the weight of the rocket fuel. Even better, as long as you have a light source, you never run out of gas.

AS LONG AS YOU HAVE LIGHT YOU NEVER RUN OUT OF GAS

LightSail2 launched on 25 June 2019. It was part of the multi-purpose payload of a SpaceX Falcon heavy rocket. It’s the first spacecraft in Earth orbit powered solely by sunlight.

In our 23 July 2019 update, we reported that LightSail2’s huge, 32-square-meter light sail deployed flawlessly. Our third update told readers that the LightSail 2 spacecraft successfully raised its orbit using light energy alone, reaching an orbit apogee (high point) at that time of 7 kilometers.

LightSail2 has now completed six months in space. Justin Mansell of Purdue University is a mission team member. He has published a research paper on lessons learned from the mission.

LIGHTSAIL2 HAS NOW COMPLETED SIX MONTHS IN SPACE

LightSail2 has now reached an orbit of 720 km. Engineers call that a low earth orbit but it’s higher up than most satellites, or even the International Space Station at 400 km.

There’s still an atmosphere at that altitude but nobody was sure how dense it would be. Now we know that the atmospheric drag even at that altitude overpowers the thrust from solar sailing.

That means that LightSail2’s orbit is now gradually decaying. The plan was that LightSail2 would burn up on re-entry after about a year. The team can’t guarantee that timeline anymore.

LIGHTSAIL2’S ORBIT IS NOW GRADUALLY DECAYING

Whatever happens, LightSail2 will tell the team how long orbital decay really takes when a spacecraft has an area the size of a boxing ring. That’s never happened before.

The drag from the atmosphere won’t be a problem for future solar sailing deployments. They’ll just use higher altitudes or leave orbit and explore the solar system.

For example, NASA plans to send a spacecraft called NEA Scout to a nearby asteroid. It will start off with a rocket launch to the moon and then use solar sailing to reach its destination.

NASA PLANS TO USE SOLAR SAILING TO GO TO ASTEROID

LightSail2’s orbit hasn’t been very consistent. That’s because the Earth isn’t perfectly round, causing more gravity at the equator than at the poles.

Also, since the Earth orbits the sun, the angle of the sunlight changes relative to the high and low points of LightSail2’s orbit over time. So, the orbit is wobbly, like when someone spins a hula hoop around their waist.

Maneuvering the spacecraft has also been challenging. It uses a tool called a momentum wheel to shift itself either parallel to or perpendicular to the sun’s rays.

MANEUVERING SPACECRAFT HAS BEEN CHALLENGING

The wheel has a fixed maximum speed. When it hits that, LightSail2 has to stop solar sailing and use torque rods to stabilize its movements.

At first, the team did this manually from the mission control dashboard. This was tricky if there were lapses in communication or other onboard systems.

They’ve found a way to automate that process now and that got rid of the performance problems. Future missions will have to come up with better ways to use the momentum created by changing the sail position.

AT TIMES, SAIL CASTS A SHADOW OVER THE SOLAR PANELS

Another solar sailing obstacle is the shadow of the sail. At times, LightSail2’s sail casts a shadow over the spacecraft’s solar panels. This drops power levels.

The team can get around that by rationing their power consumption and attitude controls. The designers didn’t manage to take sail shadow fully into account in every single position. LIve and learn.

LightSail2’s approach to solar sailing started off being a bit like the old square-rigged ships on the ocean. The spacecraft could only sail when it was square to the sunlight.

OUT OF 100 MINUTE ORBIT, SAILED FOR HALF AN HOUR

That meant that out of its 100-minute orbit, it only picked up speed for about half an hour. Now the spacecraft successfully deploys a new mode called sun-pointing.

Sun-pointing keeps the sail facing the sun throughout the orbit. That will help with the momentum wheel glitch and also keep the spacecraft oriented properly for charging its batteries.

There’s not much left to do now other than take more pictures and watch the orbital decay until the day comes for re-entry. The data from LightSail2’s final phase will be very helpful for future solar sailing missions.

DATA FROM FINAL PHASE HELPFUL FOR FUTURE MISSIONS

Bill Nye is the president of the Planetary Society. He summed up the project like this, “The LightSail2 mission is a game-changer for spaceflight and advancing space exploration.”

LightSail2 is a test. The goal is to collect information and confirm or falsify ideas.

For that reason, even the technical problems mentioned above make the mission successful. There is no such thing as a failed experiment.

NO SUCH THING AS A FAILED EXPERIMENT

As LightSail program manager, Bruce Betts put it, “We’re thrilled to announce mission success for LightSail2. Our criteria was to demonstrate controlled solar sailing in a CubeSat by changing the spacecraft’s orbit using only the light pressure of the Sun, something that’s never been done before.”

It has now.

We always have more to learn if we dare to know.

Learn more:
Planetary Society
Orbit and Attitude Performance of the LightSail 2 Solar Sail Spacecraft
Solar Sails: A Novel Approach to Interplanetary Travel
NASA Discovery Program – 4 Bids to Explore Solar System
The Gift of the Apollo 11 Story
LightSail2: Come Sail Away
LightSail2 Update: Sails Unfurled
LightSail Update

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