Asteroids are the detritus left over from the formation of the Solar System. They never made it into planets or moons and, as such, are time capsules preserved for 4.6 billion years. Get a sample of an asteroid and you have a sample of the building blocks of the Solar System itself.
This is the goal of NASA’s first asteroid sample-return mission, OSIRIS-REX ( and, as it happens, Japan’s Hayabusa-2 mission).
Having launched on 8 September 2016, OSIRIS-REX arrived at asteroid Bennu (1999 RQ36) on 3 December 2018, performing an extensive survey with the aim of returning a sample of the space rock in 2023.
More about OSIRIS-REx:
- OSIRIS-REx captures epic mosaic of asteroid Bennu
- OSIRIS REx discovers water on asteroid Bennu
- Podcast: Jim Al-Khalili, OSIRIS REx, IIAPY 2019, tardigrades on the Moon
Why asteroid Bennu?
Bennu was selected as a target from the 500,000 known asteroids because it is one of only five of that number that fulfil a number of crucial criteria.
Firstly, it is a carbonaceous, or B-type, asteroid that is rich in volatiles potentially containing water and organic molecules.
Secondly, Bennu has a diameter of about 500m, which means it isn’t tumbling through space so quickly that a craft would be unable to safely collect a sample (the smaller an asteroid’s diameter, the faster its rotation).
Thirdly, Bennu is classed as a near-Earth object. It travels close enough to Earth to make a sample-return mission practical, and its orbit has a low eccentricity, which means it never travels too close or too far from the Sun (important when your spacecraft relies, in part at least, on solar power).
Bennu is a potential Earth-impactor and there is a chance (albeit only one in 2,500) that it could collide with Earth sometime towards the end of the 22nd century.
Understanding what makes it tick might one day help save the human race.
OSIRIS-REx’s journey to asteroid Bennu
OSIRIS-REX took two years to reach Bennu, during which time it slingshot around Earth, using our planet’s gravity to accelerate to just under 20,000km/h: the speed it needed to intercept the asteroid, which itself is hurtling around the Sun at over 101,000km/h.
Once the craft was within about 5km of Bennu, it spent 505 days mapping the asteroid’s surface – using its suite of cameras, altimeters and spectrometers to study its chemistry, mineralogy and typography, as well as making a precise determination of its orbit.
NASA will use the latter information to determine how something called the Yarkovsky effect influences the orbit of an asteroid.
As an asteroid orbits the Sun, one side of it is heated by solar radiation. When the asteroid turns and the heated side is plunged into darkness, that heat is radiated into space producing a tiny bit of thrust that, over centuries, can alter its orbit
This is useful information to have when the asteroid’s orbit carries it so close to Earth.
The surface-mapping phase also allowed scientists to determine the safest spot for OSIRIS-REX to perform the sampling part of its mission.
How has OSIRIS REx collected a sample of Bennu?
The collection of a sample of asteroid Bennu is now complete (and yet to be determined wholly successful). OSIRIS-REX deployed its TAGSAM, or Touch-And-Go Sample Acquisition Mechanism, which consists of a cylindrical sampler head attached to a 3m-long articulated robotic arm.
When TAGSAM came into contact with the asteroid’s surface, mission scientists hope, nitrogen was blasted into the head, causing particles to fly off.
A screen inside the head acts like a vacuum bag, catching the dust and storing it in the outer walls of the cylinder.
Sample collected, OSIRIS-REX moves away from Bennu, safely stows the head within the Sample Return Capsule and waits for a window to return home.
OSIRIS REx: the journey back to Earth
Bennu’s elliptical trajectory means that the distance between the asteroid and Earth varies from about 100,000km to 1.9 million km, so OSIRIS-REX will have to wait for Bennu to pass close enough to Earth to begin its return journey.
Once the return window opens in March 2021, OSIRIS-REX will fire its main engines and depart on a trajectory that will intercept the orbit of Earth in September 2023 – at which point the return capsule will be jettisoned and OSIRIS-REX will move into a stable orbit around the Sun.
The capsule will enter Earth’s atmosphere at a speed of just under 45,000km/h, relying on its heat shield to remove 99% of its kinetic energy, before deploying a parachute at an altitude of 3km to make a soft landing in the Utah desert on 24 September 2023.
Analysing the asteroid Bennu sample
It will be opened in sterile conditions to prevent contaminating the sample, which will be analysed down to the atomic level to determine Bennu’s composition and to identify the presence of any organic compounds, such as amino acids and sugars – the building blocks of life.
The question of where we came from has obsessed humanity from its earliest days. Long before the invention of the written word, early humans were sketching creation myths on the walls of caves and imagining how the world around them came to be.
Today we have resources that stretch well beyond mere imagination, but the precise mechanisms that forged the Solar System remain poorly understood.
To get a true insight into the dawn of our existence we need to examine a pristine sample: a nugget of pure, early Solar System untainted by the ravages of time.
For more on the OSIRIX-REx mission, visit www.asteroidmission.org.
Ben Gilliland is a science writer, illustrator and author of How To Build A Universe: From the Big Bang to the End of the Universe.