LightSail 2 Nears 2 Weeks of Solar Sailing

The Planetary Society's LightSail 2 spacecraft is continuing to sail on sunlight in Earth orbit. The high point, or apogee of the spacecraft's orbit around the Earth was 729 kilometers on Monday, 5 August—an increase of 3.2 kilometers since sail deployment on 23 July. The spacecraft has also captured a few new images, which are available on our raw image downlink page.

LightSail 2's attitude control system has been actively controlling the orientation of the spacecraft since shortly after sail deployment. As mentioned in our last update, the momentum wheel, which swings the sail broadside and then edge-on to the solar photons each orbit, has been saturating—reaching its speed limit—a couple of times per day. When that happens, LightSail 2 must leave solar sailing mode and enter detumble mode, which spins the wheel down and allows the spacecraft's electromagnetic torque rods to stabilize the vehicle. 

During the first 10 days following sail deployment, the spacecraft was in solar sailing mode about two-thirds of the time. On 3 August, the mission team uploaded a software patch that automatically switches the spacecraft to detumble mode when it is in Earth's shadow.  Over the past couple of days, this update has allowed LightSail 2 to consistently stay in solar sailing mode during the sunlit portion of each orbit.

On 5 August, mission operators uplinked a new set of attitude control parameters to the spacecraft that should result in somewhat slower turns induced by the momentum wheel. This is intended to help keep LightSail 2 from overshooting its targeted orientation when solar sailing.

Justin Mansell, a Ph.D. student at the University of Purdue who works on the LightSail 2 mission, provided the below plot showing the angle between LightSail 2's long axis and the position of the Sun over the course of 3 orbits on 28 July. When the spacecraft's thrust is on, the angle should be zero, meaning the CubeSat axis is aimed directly at the Sun and the sail is perpendicular to the solar photons. When the thrust is off, the angle should be 90 degrees, as the sail turns edge-on to slice through the solar photons. LightSail 2's attitude control system currently has a margin of error of roughly 30 degrees, and the points shown in the plot are five minutes apart, based upon the telemetry data captured in flight.

While the graph shows LightSail 2 tracking the desired orientation fairly well, it also shows why the mission team is trying to optimize the way spacecraft manages its momentum and performs turns. During the first orbit, from 00:00 to 02:00 UTC, the flight data generally tracks the commanded orientation, though the spacecraft tends to overshoot on turns back to the 90-degree Sun angle. By the later orbits, the turns are less accurate as the momentum wheel approaches its saturation limit. The recent updates are aimed at improving this.

Mansell also made a video visualizing that first orbit. You’ll see 2 lines coming off the spacecraft: a red line pointing to the direction of the Sun, and a blue line showing the direction of the local magnetic field.

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 meters, showing the promise of flight by light for small spacecraft—the main goal of the program.

Mansell also simulated how the spacecraft's orbit would change if it were randomly tumbling, and plotted that against recorded flight data. Compared with a simulated tumbling solar sail spacecraft, the actual LightSail 2 rate of orbital decay (as measured by its semi-major axis, or average orbit radius) is markedly less. Apogee is being raised faster than it would be if the spacecraft were tumbling. Similarly, the actual rate of perigee drop is lower for LightSail 2's controlled flight than it would be if it were tumbling.

The team is now analyzing data from the spacecraft following the most recent software updates, in order to track LightSail 2’s solar sailing performance.