After interminable delays and tens of billions of dollars in spending, NASA’s Statue of Liberty–size Space Launch System (SLS) megarocket is at last nearing its inaugural launch. Taking place as early as August 29, the launch will use the SLS’s 8.8 million pounds of thrust (39.1 million newtons) to send an uncrewed Orion spacecraft and accompanying service module into lunar orbit. Dubbed Artemis I, this mission will be the biggest milestone yet in NASA’s Artemis program—a project to send humans to the lunar surface for the first time in more than a half-century.

If successful, the mission could also lead to another, lesser-known milestone—but one for the exploration of near-Earth asteroids (NEAs) rather than the moon. As Artemis I nears the moon, NASA’s NEA Scout space probe, a smaller-than-a-shoebox freeloader piggybacking into space as a secondary “rideshare” payload, will deploy from a dispenser on the adapter ring that connects Orion to the SLS rocket’s second stage. Once free-flying, NEA Scout will prepare to chase down and photograph 2020 GE, a near-Earth asteroid about the size of a school bus—the smallest ever to be studied up close by a spacecraft.

But in another, more profound way, NEA Scout’s groundbreaking voyage will also represent a significant milestone in deep-space propulsion because the science mission will demonstrate the use of solar sails, one of the few methods by which a spacecraft can generate thrust without rocket propellant.

Solar sails have, of course, been built and successfully launched on proof-of-concept missions in both interplanetary space and low-Earth orbit. The Japan Aerospace Exploration Agency first demonstrated controlled solar sailing with its pioneering sailcraft IKAROS, which flew by Venus. And NASA’s NanoSail-D spacecraft orbited with a solar sail, as did the Planetary Society’s LightSail 1 and 2 spacecraft, the latter of which is still circling the Earth.

After separating from Artemis I, the 14-kilogram NEA Scout will first fire its cold-gas thrusters—six small cans of pressurized gas propellant—to stabilize and secure a safe trajectory away from the moon and toward 2020 GE. The spacecraft will then unfurl a thin, aluminum-coated plastic solar sail. About the size of a racquetball court, this sail will catch light rather than wind, accelerating into its deep-space cruise by siphoning momentum from solar radiation.

“NEAs are fragments and ejecta from collisions in the main asteroid belts,” says Julie Castillo-Rogez, NEA Scout’s principal science investigator at NASA’s Jet Propulsion Laboratory (JPL) in Pasadena, Calif. And “2020 GE represents a class of asteroid that we know very little about” because no asteroid smaller than 330 feet (100 meters) across has been explored before.

During its flyby, NEA Scout will use its high-resolution camera to get a closer look at 2020 GE, precisely measuring the asteroid’s size, shape, rotation and surface properties while also assessing any surrounding dust and debris. Because the small spacecraft’s camera has a resolution of less than 10 centimeters per pixel, the science team should be able to pin down 2020 GE’s composition—that is, whether it is a rock-solid object or is instead a loose agglomeration of smaller pebbles and dust like some of its larger asteroid cousins. The asteroid Bennu, which is nearly 30 times larger than 2020 GE, features a seemingly solid, rocky surface that proved to be porous when the OSIRIS-REx spacecraft unexpectedly sank into it while collecting samples.

NEA Scout was originally conceived in 2013 as a reconnaissance mission to survey targets in support of since abandoned plans to send astronauts to visit a NEA. That’s why it’s called “Scout,” says Les Johnson, the mission’s principal technology investigator at NASA’s Marshall Space Flight Center in Huntsville, Ala. The program has since evolved into an unexpected and fruitful science-and-technology collaboration between JPL space scientists and Marshall spaceflight engineers after both teams independently proposed uncannily similar mission and vehicle designs in response to a NASA request.

Castillo-Rogez says the mission should provide important insights into the risks to Earth posed by this class of NEA. “Although large asteroids are of most concern from a ‘planetary defense’ perspective,” she says, “objects like 2020 GE are far more common and can pose a hazard to our planet despite their smaller size.” For example, the Chelyabinsk meteor—a similarly sized NEA named for the Russian city it exploded over on February 15, 2013—created a powerful shock wave that shattered windows across the region and injured more than 1,600 people.

NEA Scout’s other task—demonstrating solar sail propulsion for deep-space operations—arose out of a single question, Johnson recalls: Can a tiny spacecraft produce useful science from deep space at a low cost? “This is a huge challenge,” he says. “For asteroid characterization missions, there’s simply not enough room on a small spacecraft for large propulsion systems and the fuel they require.”

The most fraught part of NEA Scout’s mission will likely be the deployment of its gossamer-thin sail, which must unfold essentially flawlessly in deep space. To do that, the spacecraft will roll out four collapsed metallic booms to which the tightly folded sail is attached, Johnson says. The long, flattened supports will be wound around a spool that will dispense the booms much like a tape measure sends out its metal rule. But whereas a tape measure’s ruler is curved and sometimes becomes floppy as it extends, the booms will spring into a rigid, V-shaped cross section as they emerge.

As the solar sail unfurls, it will expand from its compact package to stretch across a total area of 86 square meters, Johnson says. The sail itself is made from CP-1, a tough, flexible, aluminum-coated polymer film that, he says, is “like Saran Wrap, except it’s much thinner than a human hair, just a few microns thick.”

The lightweight mirror sail will generate thrust by reflecting 90 percent of incoming solar photons—quantum particles of light radiating from the sun. Those visible-wavelength photons, Johnson says, are much like projectiles from a pellet gun “striking the sail and reflecting off, each time transferring a bit of their momentum to the sail.”

Solar radiation pressure is exceedingly weak, however, which is why practical solar sails must be so large. And even when a sunlight-powered sail is supersized, in most circumstances, it will still accelerate in super slow motion. The bad news, Johnson says, is that on Earth, “the solar radiation pressure exerted on the area of two football fields at noon in full sunshine is the force on your palm made by a quarter and a nickel.” And the good news? “Newton’s laws work!” he laughs. Freed from Earth’s gravitational field and unimpeded by atmospheric drag—two forces that otherwise act to resist a solar sail’s equal and opposite reactions to the constant rain of photons—momentum gradually builds up, eventually allowing sailcraft to reach surprisingly high speeds while using scarcely any propellant at all.

NEA Scout will release small amounts of propellant from its cold-gas thrusters to control itself in space, but the sail itself will do most of the work. The spacecraft will maneuver by tipping and tilting its sail to change the angle of incidence for incoming sunlight, altering the amount of thrust and direction of travel, similar to how a sailboat maneuvers by angling its sails with respect to earthly winds. The solar sail will hold any given orientation for days at a time, allowing the momentum from each maneuver to build up.

By September 2023, thanks in part to a gravitational assist from flying by the moon, NEA Scout will have marshaled enough speed to catch up to 2020 GE. Mission navigators will guide the spacecraft to within a kilometer of the asteroid. “NEA Scout will accomplish probably the slowest flyby of an asteroid ever—at a relative speed of 70 feet (20 meters) per second,” Castillo-Rogez says. “This will give us around three hours to gather invaluable science and see it up close.”

Afterward, if intense space radiation has not fried its minimally shielded electronics, NEA Scout could conceivably embark on an extended mission, Castillo-Rogez notes. On such a mission, it could loop back for more flybys of 2020 GE or perhaps visit other NEAs or even voyage to potential Trojan asteroids at Earth’s Lagrange point 5 (L5)—a “gravitational valley” that trails our planet’s orbit.

NEA Scout is an example of the growing power of CubeSats, a class of miniaturized, relatively low-cost satellites that, for around two decades, have been built with simple avionics and power systems on standardized platforms. Crucially, their small size and weight enables them to be launched into orbit on the cheap as secondary payloads. Well more than 1,500 CubeSats have made it into orbit that way. But as NEA Scout now demonstrates, CubeSats are evolving from simple satellites parked in Earth orbit to long-range spacecraft that can capably operate in lunar space and beyond.

For example, NASA’s Lunar Flashlight, another CubeSat space probe that will ride to the moon onboard a later launch, is designed to search for water ice deposits there. The spacecraft will use its near-infrared lasers to beam light into shadowy regions of the lunar south pole, seeking to illuminate and reveal lurking water ice. If found in sufficient abundance, such deposits could be utilized for manufacturing rocket fuel, among other things.

NEA Scout itself will set the stage for two additional NASA solar sailcraft whose missions will help guide the design of subsequent spaceships. Those future sailcraft could, for instance, provide early warning of solar flares, serve as quasi-geostationary communication links over Earth’s poles or conduct more far-ranging and ambitious interplanetary missions.

Planned for liftoff within the next several months from a New Zealand launch site onboard a Rocket Lab booster, the Advanced Composite Solar Sail System (ACS3) will demonstrate lightweight but strong polymer-composite booms that will deploy a solar sail from a CubeSat in Earth orbit, says W. Keats Wilkie, principal investigator of the ACS3 program at NASA’s Langley Research Center in Hampton, Va. ACS3’s sail is slightly smaller than that of NEA Scout, but its booms are also significantly lighter, which should allow it to accelerate more efficiently.

“Our current composite boom technology is best suited for solar sails up to 500 square meters in size, which encompasses the emerging rideshare-scale, CubeSat-class of deep-space payloads,” Wilkie observes. “Small as they are, CubeSats are not toys,” he says. “We don’t need the Battlestar Galactica to do great science out there.”

In 2025 a larger-scale mission, NASA’s Solar Cruiser, should set sail. Spanning more than six tennis courts (nearly 1,700 square meters) in area, the big sail will catch sunlight to propel a 100-kilogram spacecraft toward our home star, blazing a trail for future solar-sail spacecraft to serve as space-weather sentinels.

In time, sailing on sunshine should enable spacecraft to be propelled indefinitely within the inner solar system, to reach and maintain novel polar and out-of-plane orbits that are otherwise inaccessible, and to conduct orbital plane changes to view and even linger at the poles of the sun and planets more efficiently than spacecraft propelled by rockets.

In the nearer term, however, next-gen CubeSat spacecraft such as Lunar Flashlight and NEA Scout are pointing the way toward the greater use of tiny, capable and affordable “SailCubes” based on cheaper, standardized platforms and technology. In the not too distant future, an armada of autonomous surveyor-prospectors could piggyback into orbit and then sail away to investigate asteroids and other targets of scientific—and increasingly commercial—interest.