Bennu - the asteroid
NASA sampled

Bennu belongs to the Apollo group of near-Earth asteroids - objects whose orbits cross Earth's path around the Sun. Discovered in 1999 by the LINEAR survey in New Mexico, it was formally designated 101955 Bennu after a public naming competition in 2013. The name comes from the Bennu bird of ancient Egyptian mythology, a heron-like figure associated with the Sun god Ra and the concept of renewal.

Spectroscopically, Bennu is a B-type asteroid within the broader C-type (carbonaceous) class. Its surface reflects only about 4% of incoming sunlight - darker than charcoal. The diameter of roughly 490 metres places it well above the 140-metre threshold for a potentially hazardous asteroid (PHA, meaning it is 140 metres or wider with an orbit that passes within 0.05 AU of Earth's orbit, where 1 AU is an astronomical unit, approximately 150 million km).

Bennu is a rubble pile: not a single solid body but a loose aggregate of rocks and dust held together by its own weak gravity. Its density - around 1,190 kg per cubic metre - is lower than solid rock and consistent with a structure that is roughly 40% empty space. The asteroid also has a distinctive spinning-top shape, bulging at the equator, shared by several other small rubble-pile asteroids.

NASA's OSIRIS-REx (Origins, Spectral Interpretation, Resource Identification, Security - Regolith Explorer) launched in September 2016 and arrived at Bennu in October 2018. For nearly two years the spacecraft mapped the surface in detail from orbit, initially at altitudes of just a few kilometres - the smallest orbit ever flown around a solar system body at the time.

The sample collection took place on 20 October 2020. The spacecraft briefly touched down on the Nightingale crater region, firing a burst of nitrogen gas to kick up material and capture it in a collection head. The mechanism collected far more than planned - so much that the sample container lid did not seal cleanly, requiring an early lid closure to preserve what had been collected.

The sample capsule separated from the spacecraft and landed in the Utah desert on 24 September 2023. Scientists recovered approximately 121 grams of material - significantly exceeding the 60-gram mission minimum. It was the first asteroid sample returned to Earth by a US mission, following Japan's Hayabusa and Hayabusa2 missions to asteroids Itokawa and Ryugu respectively.

Before OSIRIS-REx arrived, radar observations suggested Bennu's surface would be relatively smooth - perhaps covered in fine, sandy regolith. The mission found the opposite. The surface was strewn with boulders, some the size of buildings, making sample site selection a considerable challenge. The largest single boulder, nicknamed "Mount Doom" by the science team, stands roughly 58 metres tall.

During proximity operations, the spacecraft observed particle ejection events: small rocks, up to several centimetres across, being ejected from the surface and entering temporary orbit around the asteroid before falling back or escaping. The causes remain under investigation - candidates include thermal fracturing, trapped gas release, and meteorite impacts - but the phenomenon was entirely unexpected.

Laboratory analysis of the returned sample confirmed the presence of hydrated silicate minerals and carbon-bearing organic molecules. These materials are thought to have formed in the presence of liquid water, consistent with Bennu originating from a larger, wetter parent body that broke apart long ago. The organics are not biological in origin, but they are exactly the kind of prebiotic chemistry that researchers believe was delivered to the early Earth by carbonaceous asteroids.

Bennu's next close approach to Earth falls in September 2135, when it will pass within roughly 7.5 lunar distances (LD; 1 LD = 384,400 km, the average Earth-Moon distance) - close enough that Earth's gravitational influence will alter its subsequent trajectory. Precisely how that trajectory changes depends on where within the uncertainty envelope Bennu actually passes during the 2135 encounter.

If Bennu passes through a specific narrow zone - a gravitational "keyhole" - the altered orbit places it on a collision course with Earth on 24 September 2182. The probability of threading that keyhole, combined with subsequent orbital dynamics, gives a roughly 1 in 2,700 chance of impact on that date. This has made Bennu the longest-running probabilistic impact scenario in NASA's CNEOS (Center for Near Earth Object Studies) catalogue.

The figure needs context. A 1 in 2,700 probability means a 99.96% chance of no impact on that date. More importantly, that number will change as the orbital solution improves. OSIRIS-REx data is refining the Yarkovsky effect - a subtle force caused by thermal emission from the asteroid's surface that gradually shifts its orbit over decades. As precision improves, the keyhole question becomes answerable. Historical precedent strongly favours resolution to zero risk, as occurred with Apophis and dozens of other objects.