Planetary Society Has a Year To Iron Wrinkles out of Solar Sail Mission
WASHINGTON — The Planetary Society has roughly a year to absorb lessons from its successful but far from flawless solar sail deployment test before the 2016 launch of a second, more ambitious LightSail mission.
“I’m so excited because this mission is a success,” Planetary Society Chief Executive Bill Nye said during a June 10 press conference. “The sail really did deploy.”
Solar sails require no propellant, using the weak but continuous thrust provided by sunlight to gradually accelerate to high speeds, potentially shortening travel time for solar system exploration missions.
Successfully deploying the solar sail, not actually using it for propulsion, was the main objective of the privately funded LightSail mission this time out.
A partial image LightSail sent down June 8 confirmed that the 32-square-meter sail had successfully deployed the previous day from the LightSail spacecraft, a 30-centimeter-long cubesat. But the image also showed ripples in the 32-square-meter mylar sail.
Doug Stetson, LightSail project manager, told reporters the team was considering extending the spacecraft’s booms more to tighten the craft’s sail. Ripples can be seen in the color image LightSail completed transmitting June 9.
However, the head of the California company that designed LightSail told SpaceNews that the ripples are normal and that taught solar sails are not necessary or even desirable.
“[T]he sail looks like it was designed to be,” Stellar Exploration President Tomas Svitek said June 11 via email. “Unlike some intuitive expectations of tight sailboat aerofoils, there is absolutely no reason for solar sail material to be completely flat or wrinkle free. In fact, you want to be very careful about placing any tension or strain on material as it will tear up much faster. That is much bigger threat to the long-term missions than any loss of efficiency due to wrinkles or small loss of surface area.”
When the LightSail project started in 2009, San Luis Obispo, California-based Stellar Exploration was in charge of building the craft. But in 2014 Ecliptic Enterprises Corp. took over the integration and testing of LightSail, working out of Cal Poly San Luis Obispo. Borealis Space also worked on LightSail as a subcontractor to Ecliptic.
The Planetary Society expects to have spent some $5 million on LightSail by the time next year’s follow-on mission concludes. The group raised the money through crowdfunding and other private donations.
A successful solar sail deployment has been a long time coming for the Planetary Society, a Pasadena, California-based nonprofit that promotes solar system exploration. In 2005, a Russian-built solar sail demonstrator dubbed Cosmos 1, launched atop a converted submarine-launched ballistic missile but didn’t reach orbit. A 2001 suborbital flight test of Cosmos 1 technology, also launched on a Volna, failed when the spacecraft didn’t separate from the Russian rocket.
For the LightSail mission, the Planetary Society flew domestic, hitching a ride on the United Launch Alliance Atlas 5 rocket that launched the U.S. Air Force’s X-37B spaceplane May 20 from Cape Canaveral, Florida.
After two days in orbit, LightSail went silent when a software glitch crashed the onboard computers. Ground stations reestablished contact with LightSail May 31 after a cosmic ray hit the spacecraft’s circuit boards and initiated a reboot (The Planetary Society says it knows what caused this problem and will update the spacecraft’s software for next year’s mission.)
The team proceeded with the mission and commanded the craft to open its hinged side panels, literally clearing the way for for the four-segement solar sail to deploy. But an issue involving the craft’s batteries, which had entered a safe mode-like state, caused another shutdown June 4.
Officials now think that the LightSail’s batteries, which were thought to have malfunctioned, actually worked as intended. Rex Ridenoure, president of Pasadena, California-based Ecliptic Enterprises Corp., said that once the solar panels opened it took a few days for the batteries to adjust to the new power levels. “It was not an anomaly,” he said. “It was as designed.”
Controllers restored contact with LightSail for the second time June 6, and decided to push ahead with a sail deployment after seeing that the batteries were back online. The first two attempts to command a sail deploy were unsuccessful for reasons that remain unknown. But on the third try, the deployment motors spun up and began to unfurl the solar sail.
With its sail deployed, it was only a matter of days before atmospheric drag at LightSail’s 350- to 650-kilometer eliptical orbit pushed the mission toward a fiery finale. Stetson predicted the tiny spacecraft would reenter the atmosphere June 13-14.
The Planetary Society’s next LightSail spacecraft, which is planned to launch aboard a SpaceX Falcon Heavy in late summer 2016, will be placed into a 720-kilometer orbit — high enough to allow the craft to make full use its solar sail for propulsion before eventually falling back to Earth.
The primary objective for next year’s mission is to actually use the solar sail to change the inclination of the craft’s orbit. “What we’ll be doing in 2016 is deploying the sail and learning how to fly it,” Stetson said. Officials say the spacecraft will be virtually identical to this year’s version and that the mission will go on for three to six months.