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.

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