Q+A on solar sails
Nearly 70 years after the launch of the first satellite, we still have more questions than answers about space. But a team of Berkeley researchers, led by electrical engineering and computer sciences professor Kristofer Pister and mechanical engineering Ph.D. student Alexander Alvara, is on a mission to change this. Their idea: the Berkeley Low-cost Interplanetary Solar Sail (BLISS) project, consisting of a fleet of low-cost, autonomous spacecraft, each weighing only 10 grams and propelled by nothing more than the pressure of solar radiation. These miniaturized solar sails could visit thousands of near-Earth asteroids and comets, capturing high-resolution images and collecting samples.
Why solar sails?
AA: Solar sails use a non-consumable propulsion force. They are propelled by sunlight, similar to how a sailboat is propelled by wind. So, unlike other spacecraft, solar sails can travel around the galaxy, or, more specifically, our solar system, without having to carry any fuel or worry about refueling.
KP: The magic is that light, even though it doesn’t have mass, has momentum. When light bounces off a mirror, you get a force due to that change in momentum. And on a square meter sail, that force is tiny. It’s about the weight of a grain of sand, but you get it for free. And you get it for as long as you want, as long as you’re sitting in space with the sunlight striking you.
What are the project’s goals?
KP: Our initial goal for the BLISS project was simple: capture images of all the near-Earth asteroids, starting with the biggest ones. Roughly a thousand near-Earth asteroids are bigger than a kilometer in diameter. And we have pictures, usually fuzzy pictures, of maybe 10 of them. We were excited by the idea that you could potentially take an iPhone camera, orbit around one of these things, take a thousand high-resolution color photographs from a very close distance and then beam that information down.
Why make them small?
AA: A smaller size allows the spacecraft to be more agile. We don’t have to worry about buckling of the sail, which is just one square meter. This is a huge issue with larger solar sails.
KP: Cost is another advantage to going small … if we do everything right, the cost of the solar sails will be a thousand dollars or less. We could then put thousands of these tiny spacecrafts into a little package, the size of a small satellite, and launch them into space.
What were other key design features?
KP: We’re leveraging all the technology, all the miniaturization and low power consumption that goes into the design of cell phones. But there are also many other instruments that MEMS [microelectromechanical systems] has managed to miniaturize. Our little spacecraft has roughly a ½ meter diameter, super-lightweight mirror — maybe the size of a card table — that is connected to the body of the spacecraft by a few carbon fiber filaments. The inchworm [motors] inch their way along those filaments, pulling on the filaments and moving the sail relative to the center of mass of the spacecraft. It turns out that’s what you need to navigate — just like on a sailboat. You pull on the lines and change the attitude of the sail through the wind, and that affects direction.
AA: [For navigation], the majority of the analysis is done using something called the Lost in Space [Identification] Algorithm. The idea is that you map the stars that you can see, then compare them to the pixels of the images that you can get from your on-board cell phone camera. So we can basically use smartphones to help navigate.
What might the concept missions look like?
AA: Kris had mentioned earlier sending the solar sails to explore near-Earth asteroids. One of the other main concept missions is cometary sample retrieval, so getting microdust from comet plumes.
KP: As for the mission durations, they vary a lot. It will take us some number of months to get out of Earth’s orbit, it will take us months or years to get to the asteroid or comet that we’re interested in, and then the reverse of that coming back in. So, certainly months at the short end, and maybe a decade or so at the long end.
Learn more: Small solar sails could be the next ‘giant leap’ for interplanetary space exploration; BLISS: Interplanetary exploration with swarms of low-cost spacecraft (Acta Astronautica)