101955 Bennu

Based on Wikipedia: 101955 Bennu

A Rock That Might Hit Us

Somewhere out there, spinning through the darkness between Earth and Mars, is a pile of rubble the size of the Empire State Building that has a small but real chance of slamming into our planet in the year 2182.

Its name is Bennu.

The odds are about one in 1,750. Not high enough to lose sleep over, but high enough that scientists have spent billions of dollars studying this particular space rock in extraordinary detail. In 2023, a spacecraft returned actual pieces of Bennu to Earth—the first time America has ever brought home samples from an asteroid. Those fragments are now being analyzed in laboratories around the world, and what they're revealing is reshaping our understanding of how life might have begun.

But let's start at the beginning. What exactly is Bennu, and why does it matter so much?

Portrait of an Asteroid

Picture a diamond-shaped spinning top, dark as charcoal, about 500 meters across. That's roughly five football fields from end to end. If you could stand on its surface—which you couldn't, since the gravity is almost nonexistent—you'd find yourself surrounded by a chaotic landscape of boulders, some as large as houses. The whole thing completes one rotation every four hours and seventeen minutes, fast enough that loose material tends to drift toward the equator, creating a distinctive ridge that circles the asteroid like a belt.

Bennu is astonishingly dark. It reflects only about 4.6 percent of the sunlight that hits it, making it blacker than fresh asphalt. This darkness comes from its composition: Bennu is made primarily of carbon-rich materials, the same stuff that forms the basis of all life on Earth.

And here's something remarkable: this asteroid is mostly empty space. Scientists have calculated that Bennu's density is only slightly greater than water, which means it's not a solid chunk of rock at all. Instead, it's what researchers call a "rubble pile"—a loose collection of boulders and pebbles held together by nothing more than their own feeble gravity. If you could somehow grab Bennu and squeeze it, about 40 percent of its volume would simply collapse into itself, filling in all the gaps and voids.

This fragility has profound implications. If we ever needed to deflect Bennu—say, because those one-in-1,750 odds started looking uncomfortably close—we couldn't just slam something into it at high speed. That might shatter it into thousands of pieces, potentially making the problem worse. The rubble pile structure demands a gentler approach.

Water in the Void

When the OSIRIS-REx spacecraft—that stands for Origins, Spectral Interpretation, Resource Identification, and Security-Regolith Explorer, a tortured acronym if ever there was one—first approached Bennu in late 2018, scientists detected something extraordinary while the craft was still thousands of kilometers away.

Water.

Not liquid water, of course. Bennu's surface temperature swings between scorching and freezing as it rotates in and out of sunlight, and there's no atmosphere to speak of. But the minerals that make up Bennu are shot through with water molecules locked into their crystal structures. These are called hydrated minerals, and they're like tiny time capsules preserving water from the earliest days of our solar system.

Dante Lauretta, the mission's principal investigator at the University of Arizona, has been emphatic about this finding. "Bennu appears to be a very water-rich target," he's said, "and water is the most interesting and perhaps the most lucrative commodity that you would mine from an asteroid."

That word "lucrative" might seem odd when discussing a rock floating half a billion kilometers from the nearest convenience store. But Lauretta is thinking ahead to a future where humanity expands beyond Earth. Water is extraordinarily heavy, and launching it from Earth's surface costs a fortune in rocket fuel. If future astronauts could extract water from asteroids like Bennu—splitting it into hydrogen and oxygen for rocket propellant, or simply drinking it—the economics of space exploration would transform completely.

Current estimates suggest Bennu contains at least 700 million kilograms of water in one form alone, with the total water content potentially reaching six percent of the asteroid's mass. That's enough to fill hundreds of Olympic swimming pools, trapped in an object smaller than most shopping malls.

The Touch

On October 20, 2020, OSIRIS-REx performed one of the most delicate maneuvers in the history of space exploration. After two years of carefully mapping Bennu's surface, the spacecraft descended toward a small crater near the asteroid's north pole—a site the team had named Nightingale.

The approach had to be extraordinarily precise. Nightingale is a hazardous spot, surrounded by boulders that could have damaged the spacecraft. But it was chosen because its surface showed signs of being relatively fresh, less weathered by billions of years of cosmic radiation and micrometeorite impacts.

When OSIRIS-REx touched down, it extended a robotic arm called TAGSAM—the Touch-And-Go Sample Acquisition Mechanism—and fired a burst of nitrogen gas into the surface. The gas stirred up a cloud of particles, which were captured in a collection chamber.

What happened next surprised everyone.

The spacecraft sank. Not dramatically, not dangerously, but enough to alarm the mission controllers. Bennu's surface turned out to be far less solid than anyone expected. The particles making up the exterior were so loosely packed that if OSIRIS-REx hadn't fired its thrusters to back away immediately, it might have continued to sink into the asteroid like a foot into beach sand.

This discovery revealed something important about Bennu's nature. The surface isn't really a surface at all—it's more like a very diffuse pile of gravel with nothing holding it together except the whisper-thin pull of gravity. Step on it, and you'd sink. Dig into it, and material would flow around you like water.

An Asteroid That Acts Like a Comet

In December 2018, OSIRIS-REx captured something utterly unexpected: Bennu was shooting rocks into space.

Not dust—actual particles, some as large as ten centimeters across, ejecting from the surface in sporadic plumes. Over the following months, the spacecraft observed these ejection events repeatedly. Bennu, it turned out, is what scientists call an "active asteroid."

This blurs a distinction that astronomers once thought was clear. Asteroids, the traditional thinking went, were inert chunks of rock and metal. Comets were icy bodies that sprouted tails when they approached the sun, as frozen gases vaporized and carried dust with them into space. Bennu doesn't fit neatly into either category.

Several theories have been proposed to explain the particle ejections. The sun's heat may be cracking rocks through thermal stress, causing fragments to break off and fly away in Bennu's negligible gravity. Alternatively, pockets of water ice or other volatile materials beneath the surface might occasionally vaporize and burst outward. Tiny meteorites constantly peppering the surface could also be kicking up debris.

Whatever the cause, Bennu's activity connects it to a broader category of objects that challenge our traditional taxonomy. Some researchers have suggested that if the International Astronomical Union ever formally recognizes Bennu as a dual-status object—both asteroid and comet—it would receive the comet designation P/1999 RQ36, referring to its provisional name and the LINEAR survey that discovered it.

Where Did It Come From?

Every atom in Bennu was forged in a dying star.

This is true of everything in our solar system, of course, but it's worth pausing to consider what that means. The carbon, the iron, the oxygen, the traces of heavy elements—all of it was synthesized in the nuclear furnaces of ancient suns that exploded billions of years ago, scattering their enriched remains across the cosmos. That material eventually coalesced into our sun, our planets, and countless smaller bodies like Bennu.

But Bennu itself is relatively young, at least in its current form. Scientists believe it originated in the inner asteroid belt, between Mars and Jupiter, as a fragment of a much larger parent body—perhaps a planetoid a hundred kilometers across. That parent body was big enough to generate internal heat and pressure, which transformed the raw carbonaceous material into more complex minerals. Some of those minerals incorporated water, which is why Bennu is so rich in hydrated compounds today.

At some point—simulations suggest it happened sometime in the past one to 2.5 million years—Bennu was knocked out of the main asteroid belt. The leading candidates for its original home are two asteroid families named Polana and Eulalia, both located in the inner belt. There's about a 70 percent chance Bennu came from Polana and a 30 percent chance it came from Eulalia.

Once dislodged, Bennu's orbit slowly evolved through a phenomenon called the Yarkovsky effect. Here's how it works: as an asteroid rotates in sunlight, one side gets heated while the other cools. When the heated side rotates into darkness, it radiates that warmth back into space. This creates a tiny but persistent thrust, like a very slow rocket engine. Over millions of years, this gentle push can dramatically alter an asteroid's orbit.

For Bennu, the Yarkovsky effect is shifting its orbital path by about 284 meters per year. That might not sound like much, but over geological time, it's enough to send an asteroid on a completely new trajectory through the solar system—eventually bringing it close to Earth.

A Spinning Top Speeding Up

The Yarkovsky effect does more than just move asteroids around. A related phenomenon—with the tongue-twisting name of the Yarkovsky-O'Keefe-Radzievskii-Paddack effect, or YORP—can actually change how fast an asteroid spins.

Bennu is speeding up. Every hundred years, its rotation period decreases by about one second. That might seem trivial, but over millions of years, it adds up. Eventually, Bennu could spin so fast that centrifugal force would overcome its weak gravity, flinging material off the surface and potentially tearing the whole rubble pile apart.

This is probably how Bennu got its distinctive spinning-top shape in the first place. As it rotated faster and faster, loose material migrated toward the equator, building up the ridge that now circles its middle. It's a shape shared by many small asteroids, suggesting this is a common fate for rubble piles that get caught in the YORP spin-up process.

The Impact Question

Let's return to that one-in-1,750 chance.

Bennu has been tracked since its discovery on September 11, 1999, when the Lincoln Near-Earth Asteroid Research project—LINEAR—spotted it during a routine survey. In the years since, radar telescopes at Arecibo Observatory in Puerto Rico and the Goldstone Deep Space Network in California have bounced radio waves off Bennu repeatedly, building up an extraordinarily precise picture of its orbit.

The danger comes from a close approach scheduled for September 25, 2135. On that date, Bennu will pass within about 200,000 kilometers of Earth—closer than the Moon. The encounter won't cause an impact, but Earth's gravity will bend Bennu's path in ways that are difficult to predict precisely. Depending on exactly how close it passes and at what angle, Bennu could be deflected onto a trajectory that brings it back for a collision later in the 22nd or 23rd century.

The date of greatest concern is September 24, 2182. If Bennu hits Earth on that day, the consequences would be catastrophic—but not civilization-ending. Scientists estimate the impact energy would be equivalent to about 1,200 megatons of TNT. For comparison, the most powerful nuclear weapon ever tested, the Soviet Union's Tsar Bomba, released about 54 megatons. The Tunguska event of 1908, when an object exploded over Siberia and flattened 2,000 square kilometers of forest, has been estimated at 3 to 30 megatons.

An impact would be devastating to any region it struck, potentially destroying a major city or triggering tsunamis if it hit an ocean. But it wouldn't cause mass extinctions or threaten human civilization as a whole. The asteroid that killed the dinosaurs was at least ten times larger and released millions of times more energy.

Still, one in 1,750 is not zero. And we have 160 years to prepare—plenty of time to develop deflection technologies if we choose to use them.

What the Samples Revealed

On September 24, 2023—almost exactly on schedule—a small capsule fell from the sky over the Utah desert. Inside was approximately 250 grams of Bennu, the largest sample ever returned from an asteroid by a NASA mission.

The analysis is still ongoing, but early results have been tantalizing. The samples contain abundant organic compounds, the carbon-based molecules that form the basis of life as we know it. They're rich in hydrated minerals, confirming what OSIRIS-REx observed remotely. And they include veins of carbonate minerals—white streaks running through darker rock—that formed when hot water flowed through Bennu's parent body billions of years ago.

These carbonate veins are remarkable. They range from 3 to 15 centimeters wide and can stretch over a meter in length, far larger than anything seen in meteorites that have fallen to Earth. Their presence tells us that Bennu's parent body had active hydrothermal systems, something like the hot springs and geysers we find on Earth today. Such environments are considered prime candidates for the origin of life.

The samples also contain ammonium compounds—molecules incorporating nitrogen, another element essential to life. Together with the water and organics, these findings support a hypothesis that has been gaining strength for decades: that asteroids like Bennu may have delivered the raw ingredients for life to early Earth, seeding our planet with the building blocks that eventually assembled into living cells.

A Name from Mythology

Bennu takes its name from an ancient Egyptian deity, a mythological bird associated with the sun, creation, and rebirth. The name was proposed by a third-grade student from North Carolina named Michael Puzio, who won a 2012 contest that drew more than 8,000 entries from students around the world.

Puzio noticed that the OSIRIS-REx spacecraft, with its extended sampling arm, resembled the heron-like form in which Bennu is traditionally depicted. The connection was fitting: the Egyptian Bennu was said to have appeared at the dawn of creation, rising from the chaos of the primordial waters. The asteroid Bennu carries material from the dawn of our solar system, preserved across 4.5 billion years of cosmic history.

Following the mythological theme, all geological features on Bennu are now named after birds or bird-like creatures from world mythology. The sample collection site, Nightingale, takes its name from the songbird. Other sites considered included Kingfisher, Osprey, and Sandpiper. Craters, ridges, and boulders across the asteroid bear similar avian names, creating a poetic connection between this dark wanderer and the flying creatures of human imagination.

The Bigger Picture

Bennu is not unique. Thousands of asteroids cross Earth's orbit, and many of them share Bennu's carbonaceous composition and rubble-pile structure. What makes Bennu special is simply that we've studied it in such extraordinary detail—mapping every boulder on its surface, measuring its rotation down to fractions of a second, returning actual pieces of it to Earth for analysis.

That knowledge has practical applications. If we ever need to deflect an asteroid heading for Earth, Bennu has taught us that rubble piles require careful handling. We now understand how the Yarkovsky effect slowly reshapes asteroid orbits, knowledge that will help us predict hazards centuries in advance. We've learned that the line between asteroids and comets is blurrier than we thought, with implications for understanding how water and organic compounds move through the solar system.

But beyond the practical, there's something profound about holding pieces of Bennu in our hands. This material has been drifting through space since before Earth existed. It carries water and carbon and nitrogen that were ancient when the first dinosaurs appeared, when the first fish crawled onto land, when the first cells divided in primordial oceans. In a very real sense, we are made of the same stuff.

The atoms in your body were once part of stars that exploded billions of years ago. Some of them may have spent eons floating through the void in objects like Bennu, waiting to fall to Earth and become part of you. Studying this asteroid is, in a small way, studying ourselves—learning how the universe assembles matter into complexity, and eventually into minds capable of looking back at the cosmos and wondering where they came from.

As for whether Bennu will eventually strike our planet—that remains to be seen. The odds are low, but they're not zero. We have time to watch, and time to prepare. And in the meantime, we can marvel at what a small, dark, ancient piece of cosmic debris has taught us about our own origins and our place in the universe.