Why has a team from NASA just spent more than two years looking for the ideal parking spot on an asteroid called Bennu? Find out why the OSIRIS-REX mission is so important and what scientists are hoping to learn from analysing the rocks and dust collected from the asteroid surface when it arrives back on Earth.

OSIRIS-REx is NASA’s multi-year mission to land on asteroid Bennu and collect samples to yield new insights into the origins of life on Earth.

A spacecraft that left Cape Canaveral in 2016 has just touched down – a full four years later – on a small asteroid known as Bennu. Fulfilling its mission by collecting some rocks and dust samples, it will now spend the next three years transporting these back to Earth for analysis.

For the uninitiated, this NASA mission may seem like a lot of effort to go to, just to get some dust samples from an orbiting lump of rock, but studying asteroids offers a unique look into the origins of the planets in our solar system.

After four years of tracking the unmanned spacecraft’s progress – including two years spent analysing the best parking spot on Bennu – in October NASA’s OSIRIS-REx mission team beamed several commands to the probe and guided its safe descent toward the asteroid.

Cheers echoed around mission control when the signal came back confirming the rock and dust samples had been snagged and the OSIRIS-REx was ready to make its return journey.

Caption: OSRIS-Rex touches down on asteroud Bennu sending thousands of pieces of rock flying

Time capsule of our planetary past

The OSIRIS-REx mission takes its names from the Egyptian god Osiris, but is also a clever abbreviation for the full mission moniker: ‘Origins, Spectral Interpretation, Resource Identification, Security, Regolith Explorer’.OSIRIS-REx is part of NASA’s New Frontiers program which consists of several space operations designed to further our understanding of the solar system and how it formed – and the Bennu asteroid samples are expected to yield some important new insights.

Asteroids offer a treasure trove of information and knowledge to scientists. They are considered the ‘leftovers’ from the solar system’s formation, which did not form into planets, and many asteroids remain largely unchanged since the formation of our solar system over 4 billion years ago.

Studying them is like looking in a time-capsule in terms of planetary formation, and they offer many clues about where organic molecules originated from which resulted in the eventual formation of life on Earth. This reflects the other reason OSIRIS-REx was such a fitting name for this ambitious mission: Osiris was also the god of life and resurrection.

Why pick asteroid Bennu?

NASA considered many different asteroids for exploration in the mission’s early planning stages, but eventually settled on an asteroid formally known as 101955 Bennu. Bennu was named after another Egyptian deity, a mythological bird who is also associated with life and re-birth.

Considered a NEO (Near Earth Object) because its path around the Sun brings it into close proximity to Earth, Bennu was chosen as the primary target due to this proximity and because of its physical properties. It is classed as a carbonaceous asteroid, meaning it holds a large amount of carbon – an essential ingredient for life.

Studies by Earth-based telescopes revealed Bennu to be roughly 500m across in diameter, and spectral analysis revealed the presence of a mineral called magnetite – usually formed around the presence of water.

Caption: OSIRIS-Rex collected samples from a somewhat-smooth site on Bennu known as Nightingale Crater. The spacecraft is shown for scale.

Tricky parking for spacecrafts

There were some special challenges for the OSIRIS-REx mission – one of the most notable is that it has to bring samples back to Earth while most missions into the solar system never make the return journey.
Another challenge was finding a good parking spot on Bennu’s rocky surface.

Space agencies may make space exploration look easy, but landing on an asteroid to collect samples is no easy feat. One difficulty is the relatively tiny size of asteroids mean they have extremely low gravity and can be difficult to orbit. Too much acceleration by a spacecraft could cause it to not be captured by the asteroid’s weak gravity, and too little could cause it to crash into it.

Asteroids can also be extremely rocky and made mostly of loose material, so finding a smooth enough surface to land on can be extremely difficult. However, it has been achieved before, by Japan’s space agency JAXA. JAXA successfully landed on asteroid Itokawa in 2005 with their Hayabusa spacecraft, and repeated the success with the follow-up mission Hayabusa2 on asteroid Ryugu. A lot of valuable information was learnt from these two missions that helped formulate the OSIRIS-REx mission, and the USA and Japan agreed to share and collaborate on information and research between their missions.

The difficulty in finding a suitable landing site is why OSIRIS-REx spent over two years in orbit around Bennu analysing the surface in detail, searching for the right spot. Several sites were discovered to be fairly smooth and boulder-free, and the mission settled on a site known as ‘Nightingale’ to collect samples from. NASA conducted several test-runs of the mission and instructed OSIRIS-REx to descend to the surface to ensure the spacecraft was working properly, coming as close as 40m above the surface in the final test.

One of the exciting early discoveries of this mission was finding the presence of water molecules on Bennu – the key ingredient for life – along with many other organic compounds that came as a surprise. After years of development, preparation, planning and tests by the NASA team, OSIRIS-REx successfully touched down on Bennu at 11:36am on 21 October 2020 and collected its precious rock samples from the asteroid surface.


Caption: A mosaic of Bennu’s surface.

Asteroid analysis begins in 2023
The collected rock samples have been placed into a delivery capsule that was stored on the exterior of OSIRIS-REx, and will spend the next three years making their way back to arrive on Earth in September 2023. On arrival the sample capsule will be ejected from the spacecraft to re-enter Earth’s atmosphere and is designed to protect the precious material from the intense heat of re-entry before deploying a parachute.

The capsule will land in the Utah desert in the United States where it will be collected for analysis by teams in both the US and Japan. Much of the rock material will be distributed to scientific research organisations around the world to be studied and analysed for years to come. Rocks may not seem very important in the scale of things, but scientists hope that samples from Bennu will provide new insights into how our solar system formed and how life once began on Earth.