Modern astronomy is giving us unprecedented views of the asteroids, comets, and other small bodies that litter our cosmic home. These planetary leftovers offer clues to our creation—and potential destruction.
In 2015 comet C/2014 Q2 Lovejoy—seen here in a two-photo mosaic—neared the sun for the first time in millennia. Lovejoy likely hails from the Oort cloud, a distant shell of icy objects thought to surround the solar system. It’s one of the roughly 4, 000 known comets among the billions estimated to exist in our cosmic backyard.
Hear more about how scientists are using the universe's small objects to answer its big questions on our podcast, Overheard at National Geographic.
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Lauretta, a University of Arizona planetary scientist, is transfixed by a monitor showing three simulated views of a rubbly, top-shaped object floating in a sea of stars. That’s the asteroid known as 101955 Bennu. He’s watching it while perched on an upholstered metal stool inside an unassuming building in Littleton, Colorado. With its cinder-block hallways, pop-out ceiling panels, and the occasional wasp problem, the building could be mistaken for a run-of-the-mill office suite. But the spacecraft decals on the walls and the labels above each cubicle—Electrical Power; Telecom; Guidance, Navigation & Control—reveal its true function: mission control at Lockheed Martin Space.
It’s 1:49 p.m. mountain time on October 20, 2020, and the screen shows Bennu sitting within a green hoop that represents the orbit of a NASA spacecraft with a mouthful of a name: the Origins, Spectral Interpretation, Resource Identification, and Security-Regolith Explorer—OSIRIS-REx for short. In less than three hours, this robotic emissary will attempt to descend and touch Bennu for the first time, hopefully trapping a sample of extraterrestrial dust and pebbles for return to Earth. (How did NASA land on and grab stuff from an astroid?)
A solar array for NASA’s Lucy spacecraft unfurls as it is tested at a Lockheed Martin facility in Colorado. Set to launch in October, Lucy will need two of these arrays to generate power during its 12-year mission to explore Jupiter’s Trojan asteroids. These ancient swarms, which orbit the sun alongside the giant planet, may hold clues to the solar system’s original layout.
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Launched in 2016, OSIRIS-REx had to orbit the sun twice to catch up with Bennu, which is more than 200 million miles away on this fateful October day. At roughly a third of a mile wide, Bennu is the smallest celestial body a spacecraft has ever orbited. Its surface is so rugged, Lauretta’s team spent a year mapping it to find a safe place to descend. All this buildup should make today’s main event a tense moment, but at this late stage of the billion-dollar mission, Lauretta seems at peace.
Why go through all this stress and effort for a few pounds of dust and rubble? For starters, the asteroid’s building blocks formed during the solar system’s earliest days, more than 4.5 billion years ago. These rocks, which show hints they contain carbon, represent a pristine archive of how the planets formed and perhaps where Earth got the starter materials for life. “Scientifically, it’s literally pay dirt, ” Lauretta says.
But just as Bennu carries the stuff of creation, it also has the power to destroy. Bennu comes close enough to Earth that astronomers believe there is a small but serious chance—one in 2, 700—that it could collide with us between 2175 and 2199. The samples OSIRIS-REx brings back could be key to designing the right defense against an impact that could release more than two million times the energy of the ammonium nitrate blast that rocked Beirut a year ago—enough to devastate a state or province, possibly even a continent.
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On February 22, 2019, the Japanese spacecraft Hayabusa2 was hovering within a hundred feet of the surface of the near-Earth asteroid Ryugu. Moments before, Hayabusa2 had fired a projectile into Ryugu’s surface—creating this image’s central blemish—and captured some of the resulting debris. Hayabusa2 returned those samples to Earth in December 2020, completing humankind’s second mission to bring pristine bits of an asteroid home for study.
On December 6, 2020, a capsule released by the Japanese spacecraft Hayabusa2 landed among the silvery saltbushes and terra-cotta soil of the Australian outback. The container held debris the spacecraft had collected from the near-Earth asteroid Ryugu in 2019. The ancient material now resides at the Extraterrestrial Sample Curation Center in Sagamihara, Japan. Scientists hope it will help unlock secrets about early planet formation and perhaps even the origins of life on Earth. Although Hayabusa2’s cargo has returned, the spacecraft is now on an extended mission that will take it to another asteroid in 2031.
On a grander scale, Bennu and OSIRIS-REx symbolize two parallel revolutions in modern astronomy that are upending old conceptions of the solar system. Today’s telescopes can see more small, faint objects than ever before, allowing astronomers to survey the skies and fill in the cosmic population that surrounds the eight planets. Twenty years ago, humans knew of roughly a hundred thousand celestial bodies in the solar system. By early 2021, we’d cataloged slightly more than a million objects orbiting the sun.(Find out more about the historic OSIRIS-REx mission.)
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At the same time, space agencies around the world have developed the tools and technologies to visit and explore these worlds—and even bring pieces of them back to Earth for closer study.
The picture of the solar system we all learned in school seems to have a logical architecture. But astronomers and planetary scientists have suspected for decades that something was amiss, since by the looks of it, it’s extremely difficult to explain how Uranus and Neptune could have formed where they orbit today. Our cosmic home appears to be missing some of the most common types of planets that orbit alien stars. And as of 2021, Earth is the only known harbor for life.
As wide as the Empire State Building is tall, the asteroid Bennu is the smallest body ever orbited by a spacecraft. On October 20, 2020, Bennu became the third asteroid to be sampled by spacecraft when NASA’s OSIRIS-REx plunged its arm into the surface (below) and collected some of its dust and pebbles. A capsule carrying the sample should land on Earth in 2023.
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Small bodies such as Bennu were long dismissed as mere leftovers in the process that created the planets. But now researchers know how important these bodies are in the search to answer such massive questions. Like Bennu, many are time capsules, essentially unchanged since the birth of our sun. Others could similarly pose a threat to life on Earth. By tracking, visiting, and sampling these primordial worlds, we’re finally getting a chance to see where we came from—and to hopefully stop these objects from destroying who we’ve become.(Tumbling, leftover rubble reveals the chaotic birth of our solar system)
Humankind's interest in small bodies—astronomer-speak for every natural object orbiting the sun that isn’t a planet, dwarf planet, or moon—has been with us as long as there have been people looking up. For millennia, cultures around the world have spotted the comets and meteors visible in the night sky and treated them as important omens. There was only so much people could do to learn more, though, because small bodies reflect very little sunlight and therefore are hard to find in the blackness of space.
By the dawn of the 20th century, astronomers had found roughly 500 asteroids orbiting the sun, starting with the 1801 discovery of Ceres. The pace of discovery really began to crank up in the 1980s and ’90s as telescopes improved. In 1992 astronomers spotted the first world—aside from Pluto and one of its moons—that is beyond Neptune’s orbit, confirming theories of the solar system’s outer zone now called the Kuiper belt. Today astronomers know this far-flung region is filled with thousands—perhaps hundreds of thousands—of icy bodies.
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Near-infrared images, captured by the Gemini South telescope in Chile, reveal planetary leftovers around other stars. Each disk of icy, rocky debris surrounds a young star (blocked out here). Many disks have inner “holes” likely carved out by newly formed planets. These disks resemble our solar system’s Kuiper belt, which lies beyond the orbit of Neptune.
But if you had to pinpoint when the small-body frenzy began, a reasonable choice would be March 11, 1998. That’s when the U.S.-based Minor Planet Center, the world’s official repository of all asteroid and comet orbits, issued an ominous-sounding press release: An asteroid discovered the previous December would come within 26, 000 miles of Earth’s surface in 2028, with a small chance it would hit the planet.
The story quickly made headlines around the world, and the news hit a public increasingly aware of how much damage an asteroid could deal. A few years earlier, geologists had identified the crater left by the asteroid that struck Earth 66 million years ago, killing off all the dinosaurs except birds. Was the incoming space rock the next Big One?
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Astronomers raced to double-check their calculations. By the next day, Don Yeomans and Paul Chodas with NASA’s Jet Propulsion Laboratory had figured out that the asteroid would sail harmlessly by
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