The carbon spinning top
Bennu
Bennu is a dark, carbon-rich asteroid about half a kilometre across, shaped like a spinning top. It is one of the Solar System bodies whose future path we know best, and one of the best studied of all: the OSIRIS-REx spacecraft stayed alongside it for more than two years and in 2023 brought 121.6 grams of its material back to Earth.
- Size
- 565 × 535 × 508 m
- Distance from Sun
- 168 M km
- Orbital period
- 436 days
- Rotation
- 4.3 hours
- Discovered
- 1999
- Samples
- 121.6 g (2023)
Physical characteristics
Bennu measures 565 × 535 × 508 metres. It is a carbonaceous asteroid, darker than coal: it reflects about 4% of the light it receives. Like Itokawa it is a rubble pile, with a density of just 1.19 grams per cubic centimetre, barely more than water: almost half its volume is empty space. Its spinning-top shape, with a bulge at the equator, is the result of rotation, which pushes material outwards.
Up close, the surface turned out to be very different from what was expected: not fine sand but a jumble of boulders, some as tall as buildings. When the spacecraft's arm touched it, on 20 October 2020, it sank half a metre in with almost no resistance, as if into a ball pit. The spacecraft also watched Bennu flinging small stones into space: it is an active asteroid.
Orbit and rotation
Bennu takes 436 days to travel an orbit lying between 0.90 and 1.36 AU from the Sun, 168 million km on average, tilted 6° to the ecliptic. It crosses Earth's orbit, and about every six years it passes relatively close to our planet. It spins once every 4.3 hours, and under the effect of sunlight its day shortens by about one second every century.
Thanks to the spacecraft's data its trajectory is known with unprecedented precision. Through the year 2300 the overall probability of an impact with Earth is 1 in 1,750, or 0.057%; the single riskiest date is 24 September 2182, at 1 in 2,700. Much will depend on the close approach of 2135, when Earth's gravity will alter its orbit in a way that cannot be calculated exactly today.
Discovery and exploration
It was discovered on 11 September 1999 by the LINEAR programme. Its name, that of a heron-like bird from Egyptian mythology, was proposed in 2013 by a nine-year-old boy who won an international contest: the spacecraft's arm with its sampling head reminded him of the bird's neck and head.
NASA's OSIRIS-REx spacecraft arrived at Bennu on 3 December 2018, mapped its surface for nearly two years and collected the sample on 20 October 2020. The capsule landed in Utah on 24 September 2023 with 121.6 grams of dust and pebbles. Analyses published in 2025 found in it 14 of the 20 amino acids used by life on Earth, all five bases of DNA and RNA, and salts left by the evaporation of ancient salty water.
Frequently asked questions
Will Bennu hit Earth?
It is very unlikely. The overall probability through 2300 is 1 in 1,750: that means a 99.94% chance it will not happen. The riskiest date, 24 September 2182, carries odds of 1 in 2,700. Bennu nonetheless ranks among the known asteroids with the highest risk, which is why its orbit is tracked so carefully.
What is in the Bennu samples?
Organic molecules and minerals that form in the presence of water. Analyses identified 14 of the 20 amino acids found in proteins, the five nucleobases of DNA and RNA, ammonia, and salts such as sodium carbonates and phosphates, left by a brine that evaporated about 4.5 billion years ago. They are not traces of life, but the ingredients from which life can start.
Why is Bennu shaped like a spinning top?
Because of its spin. Bennu is a loosely bound rubble pile turning once every 4.3 hours: centrifugal force pushes debris towards the equator, where it piles up in a bulge. The same shape is found in Ryugu and Didymos, and seems typical of small asteroids that rotate quickly.
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