LightSail 2, Pushed By Sunlight, Raises Its Orbit By 3 Kilometres In Just Two Weeks

LightSail 2 over Baja, California. (Image: Planetary Society)

Two weeks after entering solar sailing mode, the Planetary Society’s LightSail 2 spacecraft has managed to raise its orbit by nearly 3km, in an important test of this promising new means of propulsion.

LightSail 2 unfurled its 32-square-metre solar sail on July 23, around one month after it was deposited into low Earth orbit by a SpaceX Falcon Heavy rocket. At the time, the spacecraft’s apogee, or high point of its orbit, was just shy of 726 kilometres. But now, a mere two weeks after entering into solar sailing mode, LightSail 2's apogee is now at 729 kilometres, a gain of nearly 3.2 kilometres, according to the Planetary Society.

It’s still early days, but things are going smoothly.

“Mission operations is proceeding very well,” Dave Spencer, LightSail 2 project manager, told Gizmodo earlier today. “The spacecraft is healthy, and we are communicating with the spacecraft several times each day.”

The point of this mission is to test the viability of solar sailing, in which photons from the Sun bounce off a large solar sail, giving the spacecraft a small but continuous boost. Eventually, this form of propulsion could be used to move small satellites in orbit, or even enable interstellar travel.

To maintain optimal performance, mission controllers need to orient the position of the sail relative to the Sun. To that end, the spacecraft performs two 90-degree turns each Earth orbit, which it does with a momentum wheel—basically a flywheel that moves along a single axis. When in solar sailing mode, the sails are positioned to be broadside, or perpendicular, to the Sun to maximise the number of photons hitting the sail.

To prevent LightSail 2 from tumbling, and thus losing control of the sail orientation, the sail is oriented to move edge-on relative to the incoming solar photons. It takes about 90 minutes for LightSail 2 to make a complete orbit of Earth.

“For the first 10 days after sail deployment, we were in solar sailing mode about two-thirds of the time,” explained Spencer. “The momentum wheel was reaching its saturation limit—its maximum rotation rate—a couple of times per day, at which point we would slow down the wheel and use magnetic torque rods to remove angular velocity from the system. This is called ‘detumble’ mode, and it’s used to reduce the angular rates about each axis of the spacecraft. Early on, we were in detumble mode about one-third of the time, including time during the sunlit part of the orbit.”

On August 3, the Planetary Society team uploaded a software patch that enables the spacecraft to autonomously switch into its detumble configuration when it’s inside Earth’s shadow. The means LightSail 2 can now stay in solar sailing mode throughout the sunlit portion of each orbit to “maximise the time spent solar sailing,” explained Spencer.

“We also did some fine-tuning of the sail control algorithm to refine the turn rate of the spacecraft, and reduce the tendency of the spacecraft to overshoot the targeted orientation at the end of a turn,” said Spencer.

Indeed, a graph compiled by Justin Mansell, a Ph.D. student at the University of Purdue who’s involved with the LightSail 2 mission, illustrates this issue rather nicely.

Sail orientation of LightSail 2 during 3 orbits on July 28, 2019. (Image: Planetary Society)

Looking the graph, the red lines represent the commanded sail orientation (i.e. the idealised sail configuration), in which 0 degrees represents a perfectly perpendicular orientation to the Sun, while 90 degrees represents the parallel, edge-on orientation.

The plot lines are actual data taken on July 28 during three orbits, and show the sail orientation overshooting its target when approaching 90 degrees. LightSail 2 is currently working with a margin of error around 30 degrees, which is ok, but it could be better — hence the updated algorithm, uploaded to LightSail 2 on August 5.

“Both of these updates have been effective in improving the sail control performance,” Spencer told Gizmodo. “Based upon spacecraft telemetry, over the past two days our 90-degree turns have been much more crisply done, and the momentum wheel has not reached its saturation limit. We look forward to seeing the result of this improved sail control in the orbit evolution over the coming days,” he said.

A video produced by Mansell shows LightSail 2's orientation with respect to the Sun during a single orbit on July 28, 2019, which happened prior to the recent software updates. The red line shows the direction of the Sun, while the blue line shows the direction of the local magnetic field. Jason Davis from the Planetary Society describes the video:

In the first half of the video, LightSail 2 attempts to fly edge-on into the solar photons, maintaining a 90-degree angle with the Sun, as indicated by “feather” mode. The gaps between data points are not interpolated so as to not misrepresent the data, which makes it look like the sail is jumping around more than it actually is. In the second half of the video, LightSail 2 is in “thrust” mode, trying to keep its long axis to the Sun at about zero so the sail can get a push from sunlight.

As the video shows, there are times where the sail behaves beautifully. And when that happens, the mission team sees excellent orbital performance. On LightSail 2's best day so far, the spacecraft raised its apogee by about 900 metres, showing the promise of flight by light for small spacecraft — the main goal of the program.

Interestingly, the Planetary Society team also conducted simulations to see how the performance of a randomly tumbling spacecraft would compare to actual recorded flight data. Not surprisingly, the controlled orientations resulted in faster rates of lift compared to a tumbling spacecraft.

Spencer told Gizmodo that all LightSail 2 mission performance, including the orientation control, will eventually be documented in conference papers and journal articles compiled by the Planetary Society mission team. A primary goal of the mission, he said, will be to provide this data to the solar sailing community, which includes NASA’s Near Earth Asteroid Scout (NEA Scout) mission — a proposed solar sail mission to an asteroid.

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