101955 Bennu

101955 Bennu (provisional designation 1999 RQ₃₆) is a carbonaceous asteroid in the Apollo group discovered by the LINEAR Project on 11 September 1999. Scientists think it might come close to Earth in the future, making it an important object to study. Bennu is named after Bennu, an ancient Egyptian mythological bird linked to the Sun, creation, and new beginnings.

Bennu is about 490 meters wide, roughly the size of a big football field. It has been studied closely using special telescopes like the Arecibo Observatory and the Goldstone Deep Space Network.

The asteroid was the focus of the OSIRIS-REx mission. This spacecraft was launched in September 2016 and reached Bennu two years later. It mapped the asteroid's surface to find a good spot to collect samples. In October 2020, OSIRIS-REx touched Bennu's surface and gathered material. The sample was brought back to Earth in September 2023, and scientists are still learning from this space treasure.

Discovery and observation

Bennu was discovered on September 11, 1999, during a survey of nearby space rocks by the Lincoln Near-Earth Asteroid Research (LINEAR) team. It was given a special temporary name, 1999 RQ₃₆, and was classified as a near-Earth asteroid. Scientists watched Bennu closely using special cameras at the Arecibo Observatory and the Goldstone Deep Space Network when it came near Earth on September 23, 1999.

Naming

The name Bennu was chosen from over eight thousand ideas sent in by students from many countries who took part in a contest called "Name that Asteroid!" run by the University of Arizona, The Planetary Society, and the LINEAR Project in 2012. A third-grade student named Michael Puzio from North Carolina suggested the name after the Egyptian mythological bird Bennu. To Puzio, the OSIRIS-REx spacecraft, with its long TAGSAM arm, looked like this bird, which is often shown as a heron.

Its features are named after birds and bird-like creatures from myths.

Physical characteristics

Bennu has a shape like a spinning top and spins in the opposite direction compared to its orbit. Its surface is covered with many large rocks, with the biggest one being about 58 meters across. These rocks have lines of minerals formed by water that once flowed on a larger object from which Bennu came.

Bennu has a ridge around its middle, where tiny particles have gathered because of its fast spin—about once every 4.3 hours. Scientists have found that Bennu is spinning faster over time due to the way it releases heat while moving through sunlight.

Observations show Bennu is about 484 meters wide and has very little reflection, making it dark. Its inside is loose and porous, like a pile of rubble. Bennu contains water in its minerals, which was detected before the spacecraft reached it. Scientists believe Bennu holds a lot of water, possibly up to 1% of its weight.

Photometry and spectroscopy

Observations showed Bennu spins every 4.29 hours and is a type of carbon-rich asteroid. It has special minerals that usually form when water is present.

Water

Scientists say Bennu is rich in water. Before reaching it, they could already detect water from far away. The spacecraft confirmed this and estimated Bennu may hold a large amount of water—enough to make up about 1% of its weight. This water might be hidden just below the surface.

Activity

Bennu sometimes shoots out small rocks and particles. Scientists think this happens because of heat cracking the surface, water escaping, or small space rocks hitting it.

Surface features

All the features on Bennu are named after birds or bird-like figures from myths. The place where scientists collected samples was named Nightingale, after a type of bird. This spot was chosen because it looked fresh and safe for the spacecraft to land.

Origin and evolution

The material that makes up Bennu came from a much larger body that broke apart—a kind of small planet or planet-like object. Its minerals and atoms originated in old stars that died, called red giants and supernovae. This material came together 4.5 billion years ago when our Solar System was formed.

Bennu's basic makeup was set during the first 10 million years of the Solar System. It likely started in the inner asteroid belt as a piece from a larger body. It has been in a path close to Earth for about 1–2.5 million years. The paths of Bennu changed over time due to the influence of large planets like Jupiter and Saturn, as well as a natural effect called the Yarkovsky effect. Some scientists think Bennu might have come from a comet because of its properties.

Orbit

Bennu travels around the Sun once every 1.19 years. Earth sometimes gets very close to Bennu's path, sometimes coming as near as about 480,000 kilometers. Bennu passed very close to Earth in 1999 and again in 2005. The next very close passes will be in 2054 and 2060, which may slightly change Bennu's path.

Possible Earth impact

Bennu is a big space rock, and scientists have studied how likely it is to hit Earth. Early studies thought there might be a small chance of Bennu hitting Earth between 2169 and 2199. Later studies improved these estimates. The chance of Bennu hitting Earth between 2175 and 2196 was estimated to be about 0.037%. If it did hit Earth, it would release a lot of energy.

The latest studies extended the predictions to the year 2300 and slightly changed the risk rating. Bennu's path is affected by the gravity of many other asteroids.

2060/2135 close approaches

In 2060, Bennu will pass very close to Earth—about 750,000 kilometers away. This is much closer than the Moon. In 2135, Bennu will come even closer to Earth, but there is no chance it will hit us then. This close pass in 2135 might change Bennu's path in ways that could affect future close approaches.

2182

The biggest possible chance of Bennu hitting Earth is on September 24, 2182, with about a 1 in 2,700 chance. Bennu would need to pass through a very small area in space during its 2135 approach for this to happen. There are also small chances in 2187 and 2192. Overall, there is about a 1 in 1,800 chance Bennu could hit Earth between 2178 and 2290.

Long term

Scientists used computer simulations to study Bennu's path far into the future. They found Bennu is likely to eventually fall into the Sun, with about a 48% chance. There is also a chance it could be thrown out of the inner solar system, or hit another planet like Venus or Earth.

Meteor shower

Bennu might be the source of a weak meteor shower that happens around September 25 each year. The meteors would be hard to see and would only appear once in an hour or so.

Exploration

OSIRIS-REx

Main article: OSIRIS-REx

The OSIRIS-REx mission, part of NASA's New Frontiers program, was launched on 8 September 2016 to visit the asteroid Bennu. After traveling for two years, the spacecraft reached Bennu on 3 December 2018. One week later, scientists shared exciting news: they found signs that liquid water may have once been present on Bennu.

In October 2020, OSIRIS-REx touched the asteroid and collected samples. The spacecraft left Bennu in April 2021 and brought the samples back to Earth. On 24 September 2023, the samples landed in Utah. Scientists quickly began studying the material and found interesting substances, including some that might help us understand how life could begin.

Selection

Bennu was chosen for the OSIRIS-REx mission because it is close to Earth and has the right conditions for a sample-return mission. It has loose material on its surface and may contain important carbon materials from the early Solar System.

Returned samples

In September 2023, OSIRIS-REx returned about 120 grams of material from Bennu. The samples are dark and contain various particles, from tiny dust to small rocks. Scientists found hydrated minerals, organic compounds, and even some building blocks of life, such as amino acids and nucleobases. These discoveries help us learn more about Bennu and the early Solar System.