Bennu Asteroid Shape Explained: Scientists Discovered Why This Space Rock Looks Like a Spinning Top

Every 436 days, a near-Earth object (NEO) called asteroid Bennu completes its orbit around the Sun. The 500-meter space rock known as one of the most dangerous asteroids in the Solar System is famous for its unusual shape which resembles a spinning top.

What is Asteroid Bennu?

On September 11, 1999, the asteroid was discovered by the Lincoln Near-Earth Asteroid Research (LINEAR) using a one-meter telescope near Socorro, New Mexico. Originally designated as 1999 RQ36, it was named Bennu in 2013 by a third-grade student who won a contest to name the asteroid. Today, it has an official designation of Asteroid 101955.

This asteroid is within 0.05 astronomical units (AU) of Earth, equivalent to 20 times the distance from Earth to the Moon. Bennu has moved into near-Earth region due to its gravitational interactions with giant planets as well as the gentle push of heat from the Sun. It gets in close approach to our planet every six years.

In 1999, astronomers gathered data about Bennu using radar imaging and ranging from radio telescopes and through spectroscopy. When it approached Earth again from 2005 to 2007, different space-based and ground-based telescopes observed this asteroid. Another set of observations was carried out from 2011 to 2012 which helped the experts get a better glimpse of its rotation period and shape.

NASA planned an ambitious asteroid-sampling mission called the Origins, Spectral Interpretation, Resource Identification, Security-Regolith Explorer (OSIRIS-REx) with the goal of studying one of the 7,000 near-Earth space rocks known at that time. When designing this mission, the engineers looked for a rock that travels between 0.8 and 1.6 AU from the Sun. The rock to be chosen should not also spin too fast to allow the spacecraft to collect samples safely. Lastly, the experts are interested in visiting a carbon-rich type of asteroid that has not changed much since the beginning of the Solar System. Five asteroids fit in all these criteria, but Bennu stood out among all of them.

On September 8, 2016, the OSIRIS-REx was launched to study Bennu up close. The spacecraft arrived on its target destination on December 31, 2018, and started collecting samples from its surface on October 20, 2020.

What Does the Mission Say About Bennu Asteroid Shape?

Although the OSIRIS-REx spacecraft is expected to return to Earth on September 24, 2023 with the sample return capsule, its MapCam provided images about its signature shape and hydration levels. The spinning-top shape of this asteroid indicates that it possesses a pronounced equatorial ridge. For many years, scientists have assumed that this unusual shape resulted from thermal forces called the YORP effect, but the experts from the OSIRIS-REx mission cast doubt about it.

The asteroid contains large impact craters on its equator, and its size suggests that these craters belong to Bennu's oldest surface features. To confirm this claim, computer simulations were used by scientists to model the impact that shattered the asteroid's parent body. The simulations show that Bennu either formed directly as top-shapes or achieved its spinning-top shape early after being formed in the main asteroid belt. The presence of huge equatorial craters rules out the theory that this asteroid experienced a reshaping due to the YORP effect.