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Publication Date: October 11, 2023, 16:26

One of the asteroids orbiting our solar system is known as 'Bennu'. This celestial body, which may hold clues to the origins of the solar system and the birth of life, and which may one day collide with Earth, was visited by NASA's spacecraft 'OSIRIS-REx' three years ago on October 21, 2020.

OSIRIS-REx collected rock samples from Bennu's surface and packed them into a capsule. On September 24, 2023, the capsule was successfully delivered to Earth. Detailed analysis using ground-based equipment is set to begin.

Meanwhile, OSIRIS-REx, renamed 'OSIRIS-Apex', has left Earth once again for a new mission.

The recovery capsule of OSIRIS-REx, which returned to Earth, contains samples such as rocks collected from the asteroid 'Bennu' (C) NASA/Keegan Barber

OSIRIS-REx (OSIRIS-REx) is an asteroid exploration spacecraft developed by NASA and was launched towards the asteroid 'Bennu (Bennu, 1999 RQ36)' in 2016.

Bennu, discovered in 1999, is one of the near-Earth asteroids (asteroids with orbits approaching Earth) belonging to the Apollo group. Its dimensions are 565m x 535m x 508m, and it has a bi-conical shape, as if two cones were stuck together at the base.

Bennu is classified as a 'B-type asteroid' and contains a lot of carbon, organic compounds, and water. Similar asteroids include C-type asteroids like 'Ryugu', visited by the spacecraft 'Hayabusa2', and B-type is classified as a kind of C-type.

The solar system was born about 4.6 billion years ago, and it is believed to have been shaped over a long period of time. In this process, planets like the Earth we live on are thought to have started from small dust particles, which gradually gathered. However, because they melted into a sludge in the process and then solidified, the information about what the original material that made up the planets was has been lost.

On the other hand, asteroids like Bennu have remained almost as they were when they were born. Therefore, it is believed that the material from the time the solar system was formed remains, and furthermore, organic matter and water are considered to be important keys to exploring the origin of life and the birth of the Earth's oceans, attracting the attention of many researchers.

Bennu is also known to pose a future risk of colliding with Earth. Although the probability is extremely small at 0.057% by the year 2300, it is expected that direct visits and exploration of Bennu will contribute to various research to protect the Earth, such as improving the accuracy of orbit prediction, studying ways to divert collisions, and researching whether asteroids can be destroyed.

The asteroid Bennu, photographed by OSIRIS-REx (C) NASA/Goddard/University of Arizona

The name 'OSIRIS-REx' is derived from the initials of 'Origins', 'Spectral Interpretation', 'Resource Identification', 'Security', and 'Regolith Explorer'. Through the exploration of Bennu, it is intended to contribute to the origins of the solar system and life, and the protection of Earth. Incidentally, OSIRIS is the name of the god of the underworld in Egyptian mythology, and Bennu is the immortal spirit bird in the same mythology, and is also considered to be the soul of Osiris.

OSIRIS-REx was developed by NASA, Lockheed Martin, the University of Arizona, and others. At launch, it weighed about 2110kg and was equipped with various types of cameras, an infrared spectrometer, a thermal radiation spectrometer, an X-ray imaging spectrometer, a laser altimeter, etc., and was able to examine the composition and chemical characteristics of Bennu in detail while orbiting it.

And the biggest feature is the 'TAGSAM (Touch-and-Go Sample Acquisition Mechanism)', a device for collecting samples such as dust and stones.

TAGSAM is a device consisting of a 3.35m long robot arm and a 'collector head' sample collection mechanism attached to the end. The spacecraft performs a "touch-and-go" landing on Bennu's surface, and the moment the head touches the surface, it ejects nitrogen gas, causing dust and stones to fly up and be captured in the collection device. NASA likens this mechanism to an 'air-blow vacuum cleaner'.

The imagined landing of OSIRIS-REx on Bennu (C) NASA Goddard's Scientific Visualization Studio

OSIRIS-REx was launched from Cape Canaveral Air Force Station in Florida, USA, on September 9, 2016 (Japan time, same hereafter). A year later, it swung by Earth and entered an orbit towards Bennu, arriving at Bennu on December 3, 2018, and entering an orbit around 1.61km above Bennu on the 31st of the same month.

Until then, observations from Earth had predicted that Bennu's surface was generally smooth and that there were few large rocks. However, when OSIRIS-REx actually observed it, it became clear that there were several large rocks and rugged terrain.

The operation team had planned to develop and operate the spacecraft with the intention of landing within a radius of 25m, but no such large place was found. Therefore, they were forced to devise a new landing plan to accurately land in a narrow place.

As a result of the entire exploration of Bennu, the operation team decided to land in a crater named 'Nightingale' in the northern hemisphere. This place was only about 8m in radius, but it was the only relatively open place in Bennu.

After rehearsals, the touch-and-go to Nightingale was finally carried out on October 21, 2020. The spacecraft's program and TAGSAM worked perfectly, and OSIRIS-REx achieved a perfect touchdown and collected more samples than could fit in. The operation team had thought that it would be good if at least 60g of samples could be collected, but in fact, there were 250g. The operation team carefully closed the lid and sealed it in the recovery capsule.

After that, OSIRIS-REx began preparations for returning to Earth, and on May 11, 2021, it left Bennu and got on the orbit to Earth.

The moment OSIRIS-REx landed on Bennu (C) NASA/Goddard/University of Arizona

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*Please note that this article is based on information available at the time of publication and may differ from the latest information.

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