In 1418, European sailing vessels left their ports to explore the Atlantic Ocean, initiating a great Age of Discovery.
In 2018, a small space probe will unfurl a sail and begin a journey to a distant asteroid. It’s the first NASA spacecraft that will venture beyond Earth’s orbit propelled entirely by sunlight. This technology could enable inexpensive exploration of the solar system and, eventually, interstellar space.
The $16 million probe, called the Near-Earth Asteroid Scout, is one of the 13 science payloads that NASA announced Tuesday. They will hitch a ride on the inaugural flight of the Space Launch System—the megarocket designed to replace the space shuttle and, one day, send the Orion spacecraft to Mars.
It will take 2.5 years for the NEA Scout to reach its destination, a smallish asteroid named 1991 VG. But it won’t be a leisurely cruise. The continuous thrust provided by sunlight hitting the solar sail will accelerate the probe to an impressive 63,975 mph (28.6 km/s) relative to the sun.
Given enough time, a spacecraft equipped with a solar sail can eventually accelerate to higher speeds than a similarly sized spacecraft propelled by a conventional chemical rocket.
“A sail wins the race in terms of final velocity because it's the tortoise and the hare,” says Les Johnson, the Technical Advisor for NASA’s Advanced Concepts Office at the Marshall Space Flight Center. A chemical rocket provides tremendous initial thrust, but eventually burns up its fuel. “Since the sail doesn't use any fuel, we can keep thrusting as long as the sun is shining.”
The light stuff
Solar sails are made of ultrathin, highly reflective material. When a photon from the sun hits the mirror-like surface, it bounces off the sail and transfers its momentum to the spacecraft—the same way that a cue ball transfers its momentum when it smacks into another ball in a game of pool.
The solar sail concept has been around since 1924, when Soviet rocket pioneers Konstantin Tsiolkovsky and Friedrick Tsander speculated about spacecraft "using tremendous mirrors of very thin sheets" and harnessing “the pressure of sunlight to attain cosmic velocities.” Forty years later, science fiction author Arthur C. Clarke popularized the idea in his influential short story about a solar sail racing tournament, Sunjammer.
NASA began investing in solar sail technology in the late 1990s. In 2010, it successfully launched a small, sail-propelled satellite into Earth’s orbit, where it remained for 240 days before reentering the atmosphere.
That same year, the Japanese space agency demonstrated the feasibility of solar sails for interplanetary travel. A test craft hitched a ride aboard the Venus probe Akatsuki. The solar sail, dubbed the Interplanetary Kite-craft Accelerated by Radiation Of the Sun (IKAROS), was released into space by the probe when it was 4.3 million miles away from Earth. Six months later, IKAROS made history when it successfully flew by Venus.
Solar sails have become feasible thanks to the revolution in electronics.
That’s because solar sail design is hostage to Newton’s Second Law of Motion: Force = Mass x Acceleration. The force from sunlight is constant, so, in order to achieve high acceleration, you need to have low mass.
“Back 25 or 30 years ago, electronics were not so lightweight,” says Johnson. “You couldn't imagine building a small enough spacecraft that didn't require a ginormous sail. With the advent of smart phones and the miniaturization of components, we're now able to make really lightweight, small spacecraft, which makes the size of the sail more reasonable.”
In particular, Johnson points to the development of CubeSats—boxy mini-satellites designed to use off-she-shelf technology. The NEA Scout will be a CubeSat roughly the size of a large shoebox, propelled by a solar sail measuring 925 square feet (86 square meters).
Despite its modest size, the probe is packed with enough instruments to conduct an extensive survey of asteroid 1991 VG, taking pictures and measuring its chemical composition, size, and motion.
NASA sees such reconnaissance as an essential first step for future crewed missions to asteroids. If an astronaut is going to explore the surface of a space rock, NASA wants to be sure that it’s rotating in a slow, predictable way, as opposed to rapidly tumbling in multiple directions. Likewise, the space agency needs to know ahead of time whether the asteroid is a solid object or a pile of rubble held together by gravity.
All the light moves
During its mission, the NEA Scout will perform at least one slow, close flyby—reducing speed to less than 22 mph (10 meters per second) and passing about half a mile above the asteroid’s surface.
That highlights another advantage of solar sails: They’re very maneuverable, sometimes outperforming conventional methods of propulsion.
The key to steering a sail—whether it’s in the Atlantic Ocean or in space—is to create an asymmetric thrust. There are various ways do this, using the celestial equivalents of masts and rigging. IKAROS had an electro-optic coating that went dark when voltage was applied, absorbing light instead of reflecting it. That made it possible to “tune” one part of the sail so that it got half as much solar push than the other side, causing the spacecraft to tip and tilt.
The NEA Scout will take a different approach, using a sliding mechanism that moves the CubeSat back and forth relative to the booms where the sail is deployed.
“If you imagine a Coke can and that's our spacecraft, and you put a piece of paper on top of it, flat on top, that's the sail,” says Johnson. “Then, you can imagine just physically sliding the piece of paper to the left and the right. That's what we're going to be doing.” Tilting the sail also makes it possible to adjust the speed.
The agility of solar sail spacecraft—coupled with the constant thrust from an inexhaustible supply of fuel—opens the door to some intriguing possibilities.
Let’s say you want to send a probe above the ecliptic plane of the solar system to study the north pole of the sun. In order to achieve the drastic change in direction and velocity—without using precious propellant—engineers would rely on a slingshot maneuver. “Right now, we’d have to send a spacecraft out to Jupiter for a gravity assist to get it out of the ecliptic plane and have a higher angle of orbit around the sun,” says Johnson. “With a sail, you can just kind of crank it up.”
Another potential application, closer to home, is a “pole sitting” satellite. At present, if you want a satellite to remain in a fixed position relative to a certain location on the ground—which is highly desirable for communications technology—your only option is to send it into geostationary orbit, 22,236 miles above the Earth and directly above the equator.
But with a sail, “you can go above the Earth's North or South Pole and orbit the sun at the same rate the Earth is orbiting the sun,” says Johnson. “To keep the Earth’s gravity from pulling you in, you tip the sail so that it’s thrusting upward all the time. That way, you appear motionless above the North or South Pole.”
Photons—which we see as sunlight—aren’t the only spacecraft fuel generated by the sun. NASA researchers have recently received more funding to investigate an advanced concept for a superfast sail propelled by charged particles in the solar wind.
It’s called an electric sail, or e-sail. The idea, first proposed by Pekka Janhunen, a researcher at the Finnish Meteorological Institute, envisions a spacecraft encircled by 20 hair-thin wires that are each 12 miles (20 kilometers) long.
The wires generate a positively charged electrical field extending dozens of meters into space. Protons in the solar wind, traveling at speeds as high as 466 miles per second (750 kilometers per second), are repelled by this electric field, thrusting the spacecraft forward as they are pushed away. The solar wind’s negatively charged particles are discharged by means of an “electron gun,” so that the e-sail maintains a positive electric field.
The e-sail would have plenty of fuel. While the sunlight that propels a solar sail significantly diminishes once a spacecraft reaches the asteroid belt, the solar wind is still blowing strong. Over time, an e-sail can accelerate to speeds on the order of 62-93 miles per second (100-150 km/s).
That means space probes could reach Jupiter in just two years, or Pluto in five. E-sails could enable an entirely new opportunity for exploration by providing express travel beyond the solar system, into interstellar space.
By way of comparison, it took the Voyager I spacecraft 35 years to reach the boundary of the solar system. A solar sail could make the same trip in 20 years, while an e-sail would arrive in just 10.
“I have to admit, about two and a half years ago, when my boss first came to me and said, 'we want you to look at this,' I laughed a little bit," says Bruce Wiegmann, a systems engineer at NASA's Advanced Concepts Office. "Then we looked at it and said, ‘this is pretty interesting.' We went from nonbelievers to believers."
In fact, Wiegmann believes that a prototype could be launched in five years. In the meantime, some key issues need to be addressed. Although an e-sail doesn't need fuel, it requires a power source for the electron gun that expels electrons. How much power would an e-sail need? That depends on the number of electrons that the e-sail collects. NASA researchers are studying the question with charged wire in a plasma chamber that simulates the solar wind.
Another challenge is preventing the long, thin wires from bending as they are pummeled by the solar wind. The solution: rotating the spacecraft at a speed that will produce enough centrifugal force to keep the wires taut.
Next stop, Alpha Centauri
Unsurprisingly, he has big dreams for the distant future. He envisions sending a solar sail all the way to another solar system.
“We could build a big laser,” he says. “As the sail moves away from the sun and the sunlight gets dimmer, you could then shine the laser light on it to keep pushing it. The laser remains here in solar orbit, so it's continuing to push the sail faster and faster as it leaves the solar system.”
Of course, there are some technical details to work out. For starters, the sail would need to be the size of Texas. And the orbiting laser would require an energy output comparable to the amount produced by the whole world today.
It sounds daunting, but in a later century, it might be doable. And the plan has the virtue of being steeped in actual physics.
The first space vessel made by humans and sent to another solar system could arrive just like its ocean faring predecessors did during the Age of Discovery: sails unfurled and guided by the stars.
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