FTV: R.I.P. Lightsail 2
On November 1 , 2022, an Earth orbiting probe ended its three and a half year mission with a firey plunge through the atmosphere. Amazingly, the mission was slated to last a year, so the sponsoring organization, The Planetary Society, got a lot more bang for their buck. Launched on June 25, 2019, the Lightsail 2 mission was the culmination of a series of events that actually can be traced back to 1976. It was the brain-child of Planetary Society co-founders Carl Sagan (1934-1996), Bruce Murray (1931-2013), and Louis Friedman (1941). Current PS CEO Bill Nye was a student in Sagan’s class at Cornell in the spring of 1977 when Professor Sagan spoke about a plan the trio had pitched to NASA. Their idea was to send a lightsail powered craft with a large solar sail (nearly a kilometer long on each side) on a ten year mission to catch up to Halley’s Comet to collect data and pictures up close. NASA passed on the concept but the rejection began another series of events that are still reverberating in the field of space exploration nearly a half century later.
They were disappointed their concept was not embraced by NASA, but the rejection planted a seed: Why not form a non-governmental nonprofit organization to advocate for projects relating to astronomy, planetary science, and space exploration? Founded in 1980, The Society’s goals have grown to include the search for both near-Earth objects and extraterrestrial life. The Planetary Society’s mission statement reads: “Empowering the world’s citizens to advance space science and exploration.” They are actively engaged in advocating within NASA to fund space exploration and actively lobby Congress to support NASA’s mission. In an age before the term ‘crowdfunding’ had even been coined, The Society had garnered enough support by 2009 to move the Lightsail program forward.
So, what exactly is a light (or solar) sail? The concept can be distilled into a simple phrase, ‘light for flight’. Solar sails (also known as lightsails, light sails, and photo sails) are a form of spacecraft propulsion that uses radiation pressure exerted by sunlight on large surfaces to move a craft through space. It is analogous to sailing a boat where the pressure of the wind on a large surface area (the sail) propels a sailboat over the water. It is a low-cost method of sending exploratory craft on long duration missions in and beyond our solar system. In the frictionless vacuum of space, even the small, constant push of solar radiation on a solar sail can generate a great deal of speed without the weight or cost of burning chemical fuel. The concept has been around for a while, but it took some missteps before The Society was able to send out their first test craft for a true shakedown cruise.
The Society’s first attempt to launch a solar sail took place in 2005. To orbit the Cosmos 1 crowdfunded solar sail, The Society made arrangements to launch this first effort aboard a Russian Volna booster. A rocket malfunction prevented Cosmos 1 from reaching orbit and it eventually found a home at the bottom of the Barents Sea. NASA began their own solar sail program with the launch of a test probe called NanoSail-D on August 3, 2008. An issue with one of the first SpaceX Falcon 1 launch vehicles also prevented NS-D, like Cosmos 1, from reaching orbit. NASA’s second attempt was launched in 2010 but the satellite containing the probe failed to deploy. In January 2011, the NanoSail-D package was unexpectedly ejected from the satellite and it deployed its sails a few days later. It managed to circle the Earth for 240 days gathering data about atmospheric drag before its orbit decayed and the probe burned up on re-entry.
The Japanese are credited with orbiting the first spacecraft to successfully use solar sails in interplanetary space. Their IKAROS craft (Interplanetary Kite-Craft Accelerated by Radiation Of the Sun) took flight on May 20, 2010 and flew along with the Japanese Space Agency’s (JAXA) Akatsuka probe to Venus. After the primary goals were met, JAXA continued gathering data with the probe until contact was finally lost in 2015.
After the failure of the 2005 Cosmos 1 vehicle, roughly 50,000 donors contributed enough for The Planetary Society to make another attempt to orbit a solar sail in 2015. The program, now known as LightSail, launched the LightSail 1 test flight on June 1, 2015. Though LightSail 1 reentered the Earth’s atmosphere only 14 days later, it provided a wealth of data the Society would use in the development of the LightSail 2 vehicle. As a further demonstration of light sail technology, LightSail 2 was designed to show that a small spacecraft could carry, deploy, and utilize relatively large solar sails for propulsion.
LightSail 2 was carried into orbit packaged as a ‘CubeSat’. Picture a container roughly the size of a loaf of bread containing a Mylar solar sail. When fully extended, the solar sail covered an area of some 344 square feet (roughly 18.5 feet by 18.5 feet). According to Jason Davis (as reported in The Planetary Report, Vol 43, Number 1, March 2023 Equinox Edition), “Using a momentum wheel and three electromagnetic torque rods, the spacecraft oriented itself each orbit to get a slight push from sunlight. At LightSail 2’s starting altitude of 450 miles, Earth’s atmosphere is still thick enough to create drag and slow down a spacecraft. Using solar sailing, LightSail 2 slowed its decay rate and even overpowered drag on some occasions, showing that the technology is ready for wider use. LightSail 2 may be gone, but the future of solar sailing is bright.”
The technology demonstrated by LightSail 2 is now being analyzed as scientists and engineers examine more ways to utilize solar sails. For example, a spacecraft put in orbit to circle the poles of a planet would require constant acceleration to maintain its position. Such a ‘pole sitter’ probe would use the unlimited thrust provided by a solar sail to conduct long term studies of polar processes taking place on Earth, the Moon, and other planets.
Solar sailing would also allow a spacecraft to be parked between the Earth and Sun. From this vantage point, the craft could monitor the Sun for solar storms like the one that caused the spectacular displays of northern lights in late March 23. These ejections of high-energy particles from the Sun can disrupt power grids, cause communication blackouts, and harm astronauts in space. Parking a vehicle to observe solar activity while being propelled by solar sails would allow for protective measures to be taken if they sound the alarm when a solar storm is first detected.
The use of solar sails won’t be limited to spacecraft exploring our own solar system. Solar sailing can propel spacecraft to distant destinations more quickly than conventional chemical rocket fuel propulsion. Missions to our outer planets and the halo of ancient objects orbiting the Sun well beyond Pluto (called the Oort Cloud), will become more cost effective. A new type of space telescope could be orbited to the outer reaches of the solar system where the Sun’s gravitational field can be used like a magnifying lens to image and study distant exoplanets in high resolution. The ‘jewel in the crown’ of solar sailing would be sending a craft to our closest stellar neighbor, Proxima Centauri. An organization known as Breakthrough Starshot has proposed a mission that could send a small probe to this star system in as little as 20 years. Their version of solar sailing would use laser pulses to push the solar sail probe up to 20 percent the speed of light. Unlike conventional rockets that would take hundreds of years to make such a trip, a solar sail probe could be visiting Proxima Centauri in our lifetime.
In the meantime, NASA is taking more interest in solar sailing. When the Artimus 1 rocket was launched in November 2022, it carried a CubSat containing their own NEA Scout probe (short for Near-Earth Asteroid Scout). The plan was to visit Asteroid 2020 GE, thus becoming the smallest asteroid to be visited by a spacecraft. NEA-Scout’s solar sail was two and a half times larger than the one deployed by LightSail 2. It would have departed the Earth/Moon system to perform a slow flyby of the asteroid. Unfortunately, NEA-Scout never ‘phoned home’ and the mission was lost before it really began.
NASA had another more ambitious solar sail mission on the drawing board called Solar Cruiser. Like the vehicle mentioned earlier, this craft would have utilized a solar sail the size of six tennis courts to park between the Earth and Sun on solar storm watch. Soon after the loss of the NEA-Scout probe, NASA announced it was not longer pursuing Solar Cruiser. The only future testing NASA has scheduled is slated for late in 2023 when it launches ACS3. The Advanced Composite Solar Sail System will be used to test future solar sail technologies. They will use carbon fiber booms (which are 75 percent lighter than metal booms and less prone to buckling) to support a solar sail of some 860 square feet (nearly 30 feet on a side). The mission’s principal investigator, W. Keats Wilkie wants to see the technology of solar sailing ‘mature’ so scientists will begin to see the value for future exploration. Wilkie says, “Once we start flying these, we’ll get people who say, “Hey, this isn’t science fiction any more.’”
The French aerospace company Gamma recently launched the first of two probes, dubbed Alpha and Beta, to test a boomless technology first employed by the JAXA IKAROS craft. While little information has been shared since the Alpha test, their concept is to use a spinning core to deploy the solar sails without the need for rigid booms. Gamma hopes to provide low cost solar sailing platforms for scientific explorations to Venus, the outer planets, and even the Oort Cloud beyond Pluto. Co-founder Andrew Nutter would like to see their probes hitch a ride on energetic rocket launches like the ones needed to travel to the Moon, for instance: “It allows us to launch as a rideshare on many different missions without needing a private launch, reducing launch cost.”
One disadvantage of using solar sails for deep space propulsion is the angle they need to maintain to keep sunlight providing thrust. If their trajectory requires the angle between the solar sail and the Sun to be 35 percent, for example, thrust is lost. A version of a solar sail called a ‘diffractive sail’ might overcome some of these limitations. According to Davis, “These sails use tiny gratings that diffract incoming light through the sail like a prism rather than reflecting it. The diffracted light has a force component that pushes the sail in a direction perpendicular to the incoming solar photons, allowing the sail to capture the full force of the Sun’s rays. Unlike the Mylar sails used on LightSail 2, diffractive sails would be nearly transparent and the light passing through them would give them a rainbow-like appearance.
Amber Dubill is the principal investigator for the John Hopkins University Applied Physics Laboratory. She sees the potential of diffractive solar sails: “We think we can overcome a lot of the challenges that are keeping traditional solar sailing from becoming widely implemented much more across the board.” Another area being studied is the electric sail, or E-Sail. A small, spinning core with 20 thin, positively charged wires extending up to 12 miles out from the craft would generate even more speed than a traditional sail. As the positively charged photos from the Sun encounter these wires, they ‘plus’ and ‘plus’ forces would repel each other. A trip to the edge of the Sun’s influence, called the Heliopause, could be accomplished in half the time as a mission using traditional solar sails.
Though the LightSail 2 mission lasted much longer than The Society had planned, in the end, it was brought back to Earth by atmospheric drag. The force of the thickening air molecules pushing against the sail eventually tore it apart. Data from its demise will not only help with future solar sail designs, it may also aid in cleaning up some of the 25,000 objects in Earth orbit larger than four inches across. Scientists point out that should there be alien civilizations looking for planets harboring advanced life forms, they need only observe the swarm of working and derelict satellites orbiting the Earth to find us. The damage this debris field can pose for the International Space Station and satellite networks can not be ignored. If LightSail 2’s legacy includes helping us declutter our near space by intentionally deorbiting space junk, then that will make its mission a double success.
A company called Vestigo Aerospace is currently studying the concept aided by personnel who also worked on the LightSail program. They would like to equip all satellites with drag sails. When a satellite reaches the end of its mission life, the drag sail would be deployed to hasten their reentry into the Earth’s atmosphere. With this much junk orbiting the Earth, we are well beyond the time when such ‘end of mission’ planning should have been required for all new spacecraft. Interest in this project by companies like Vestigo tells us it is better late than never to begin cleaning up the Earth’s near space.
As Davis states at the end of his excellent history of solar sailing, “The co-founders of The Planetary Society believed that sailing on sunlight could revolutionize space travel. As the LightSail 2 mission ends, the baton is being passed to the next generation of solar sailors. Who knows what distant shores they will visit as they explore our cosmic ocean?”
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