LightSail - Flying to Other Worlds on Sunlight

It's been a few weeks since LightSail 2 unfurled its Mylar solar sails during a day-in-the-life test at Cal Poly San Luis Obispo. What has the little CubeSat been up to? Here's a roundup of updates that were made in response to the test results, and a look at the project's next steps.

Did LightSail stop short?

The biggest anomaly during the day-in-the-life test was the booms stopping just a few motor counts shy of full deployment, as indicated by telemetry coming over the air to Cal Poly's ground station.

 Telemetry also showed that the motor was still active, struggling to push the booms to their imaginary finish lines. This was a potentially dangerous situation—in a worst-case scenario, one might expect smoke to start rolling from the jammed motor—so the team commanded the motor to power down.

LightSail didn't respond, so Ecliptic Enterprises Corporation engineers Riki Munakata and Alex Diaz quickly unhooked one of the four triangular sail sections, opened an access panel in the deployment table and manually powered off the spacecraft.

It turns out the motor count required for a full deployment varies slightly from deployment to deployment. This is mostly based on environmental factors, including how the sail and booms are repackaged after a test. Additionally, all four sail quadrants pull on their adjacent quadrants, but because we aren't testing in zero-G, friction and gravity prevent the sail from working itself into a more relaxed state.

So despite the motor count, the sail was probably tight enough. The tension on the sails at the end of the deployment range—combined with boom flexure and deployment table friction—was enough to overpower the motor.

The mitigation? To be on the safe side, the team will lower future target deployment counts. We can always incrementally walk the booms out farther.

I can't hear you over this solar sail motor!

Regardless of why the motor stopped, LightSail didn't respond to commands in its stuck state. The likely reason? Electromagnetic interference around the spacecraft's aft compartment from the motor itself.  LightSail's antenna is located right beneath the motor and boom spindle:

 LightSail is able to send out telemetry chirps while the motor is running, but it apparently can't hear anything. This isn't usually a problem, since sail deployment is the biggest milestone of the mission, and a point in time where there's not much to do except sit back and watch.

 Rather than solve the more difficult (and costly) problem of the interference itself, the easiest fix is adding a contingency timer to the flight software that powers off the motor after a few minutes, regardless of whether the target deployment count has been met.

Power problems

During the first phase of the day-in-the-life test, where LightSail sits in the clean room for antenna and solar panel deployment, there was some odd battery behavior.

LightSail has two battery boards holding four cells each—eight cells total. During DITL testing, not all of the cells were initially supplying power to the spacecraft. This was unexpected, but not necessarily a problem; it mostly resulted in a change in understanding about the spacecraft's power system.

However, this behavior also led to the discovery of an underlying hardware problem: one of the eight battery cells' protection circuit has failed.

When LightSail's solar panels receive sunlight, they charge the battery cells. When a battery is fully charged, its protection circuit stops more power from flowing into that particular cell. An overcharged cell can distend, or in the worst case, catch fire.

The safest fix here is to replace the battery board with the affected protection circuit. Ecliptic has already ordered the new components and expects to have them installed in the next week or so.

It's been a busy two months of testing for The Planetary Society's LightSail 2 spacecraft.

Engineers spun the CubeSat like a record and shined flashlights in its electronic eyes in order to make sure everything goes according to plan when the solar sail heads into space. More trials are on the horizon, including a trip to a special magnetic cage at Utah State University's Space Dynamics Laboratory.

Most of those tests were relatively straightforward: Does the momentum wheel spin at the commanded rate? Do the gyroscopes record the correct orientation of the spacecraft? Are the sun sensors able to track a light source?

But now, an even bigger challenge awaits. In order for LightSail to raise its orbit by a kilometer each day, the spacecraft must "tack" its solar sails broadside against the sun's rays for half of each orbit. During the other half, LightSail must be rotated 90 degrees to face the solar photons edge-on. 

LightSail orients itself using an attitude control system, which consists of magnetometers, torque rods and a momentum wheel.

The magnetometers read the direction and strength of Earth's natural magnetic field as the spacecraft zips around the planet. The torque rods are three, golden cylinders (one each for the X, Y and Z axes) that electrically charge to generate a magnetic field, torquing against Earth's field to rotate the spacecraft. And the momentum wheel turns the spacecraft on a single axis (many spacecraft have three of these—one per axis). 

So, in order to solar sail, we first stabilize the spacecraft using the torque rods. Then, the momentum wheel, which has more muscle, gives us the 90-degree swings into and out of the sun's rays each orbit.

But how do we make sure that's all going to work?

In early May, LightSail will travel to Utah State University's Space Dynamics Laboratory. There, the CubeSat will be placed into a Helmholtz cage (named after a German physicist).

It's difficult to calibrate a magnetometer on Earth because we are constantly being bombarded by magnetic interference from electronic devices. LightSail's own electronics, contribute to these distortions. A Helmholtz cage creates its own magnetic field, isolating its contents from outside interference. That means the team can simulate the same magnetic field profiles LightSail will see in orbit.

Ecliptic's Alex Diaz, as well as Barbara Plante of Boreal Space, will oversee the Utah trials. There are three main tests: First, Diaz and Plante will see if LightSail's magnetometers correctly read the fields generated by the Helmholtz cage. Next, they'll put LightSail on an air bearing—basically, a floating platform suspended by jets of air. The Helmholtz cage will generate a magnetic field, and LightSail will use its torque rods to swivel the way it will in space. Finally, Diaz and Plante will test LightSail's momentum wheel, commanding the spacecraft to make its 90-degree tacking maneuvers.

If all goes well, the tests shouldn't last more than a few days, paving the way for a day-in-the-life test back at Cal Poly San Luis Obispo, where the solar sails will be deployed for the last time before launch.