OSIRIS-REx: Sampling the Early Solar System
On the morning of October 20, 2020, in the event that planetary scientists claim as their personal Super Bowl, NASA’s OSIRIS-REx spacecraft made contact with its long-awaited destination: the asteroid Bennu. The moment represented the culmination of over four years of flying through space, traveling more than 500 million miles from Earth to touch the rocky surface of Bennu for a few brief seconds.
Making contact with an asteroid during a global pandemic presented unique obstacles in an already challenging mission. A limited group of essential scientists and engineers gathered at Mission Control in matching black O-REx face masks, communicating across plexiglass walls while others participated online.
Luckily for O-REx, scientists had already completed most of the hard work. The spacecraft had spent over a year orbiting Bennu to survey and analyze the surface, sending images, temperature data, and information about Bennu’s chemistry back to Earth. The O-REx team used this data to map out the asteroid’s surface and select the best site for sample collection. This step proved critical; Bennu, as it turns out, has a body covered in hazardous boulders – features that pose a damage risk to O-REx and are not suitable for sampling.
The team ultimately chose a landing site named Nightingale – an area of fine-grained material about the size of a basketball court. Nightingale sits near Bennu’s cold north pole, appearing like a black sandbox surrounded by a halo of rocks. Dangerous terrain bounds all sides of the destination. If you could scoop your hand into the ground at Nightingale, you would pull up small pebbles and fine-grained sand, dark dust escaping around your fingers.
Launched in 2016, O-REx is NASA’s first asteroid-sampling mission, preceded only by two Japanese spacecrafts that sampled asteroids earlier in 2020 and are currently on their way back to Earth.
The interest in this dusty space rock stems from a desire to learn about the formation and evolution of the Solar System. Bennu fits into a particular class of asteroids deemed carbonaceous near-Earth asteroids – a geologist’s label for asteroids containing large amounts of carbon molecules. Owing to this high carbon content, Bennu resembles a massive, lumpy block of coal to the naked eye, reflecting only four percent of the light that hits it. The relatively luminous planet Earth, on the other hand, reflects about 30 percent.
So, why spend nearly a billion dollars and over four years traveling to a super old black rock in space? The answer lies in Bennu’s geologic history. Carbonaceous asteroids like Bennu are of particular significance because they have not undergone any extreme, composition-altering changes. This history means that on and below Bennu’s dark surface survive chemicals and rocks from, and potentially before, the initial creation of the Solar System nearly 4.6 billion years ago.
Though pieces of asteroids (relabeled meteorites) have bombarded Earth’s surface since the planet’s formation, the violent, hot descent to Earth’s surface alters the chemistry of their minerals and muddies the indications of the meteorite’s origins. In contrast, each mineral on Bennu is a relic of former worlds – of dust ejected from dying stars, of particles that gathered to form the Sun, of amino acids that may have led to life on Earth.
While the form of minerals is never truly static, Bennu is one of the closest bodies in the Solar System to a time capsule, preserved through time as it moved through the vacuum of space. Asteroids like these may have created the early Earth – a planet whose beginning and development stems from the accumulation of rock – cosmic dust and boulders converging together in space billions of years ago – and whose aggregate material created the gravity that holds us to Earth. For NASA scientists, the goal of sampling Bennu is clear: study the past to discover our origins and shed light on our future.
To bring material back to Earth, O-Rex first needed to pull off a successful sampling event. Because hundreds of millions of miles separate Earth and Bennu, it takes over 18 minutes for a signal to travel between the two, leaving O-REx to carry out the sampling process autonomously. After sending a “GO” command to begin the event, the team left the mission's success to the spacecraft.
As O-REx descended on Nightingale, its camera continuously captured images and compared them with pictures taken earlier of the target region, allowing it to track if it was safely following its predetermined route. O-REx sought to protect itself first and foremost, fully prepared to retreat away from Bennu if it deemed the landing site too hazardous upon approach. To reach Nightingale, the spacecraft flew a challenging route, coasting near rocks like “Mount Doom” – a boulder reaching a two-story building's height.
As it neared the surface, O-REx’s robotic arm – about the length of three and a half baseball bats – extruded below the spacecraft, awaiting contact with the asteroid. With the words “touchdown declared” ringing through Mission Control, the team had confirmation that the arm safely reached Bennu’s surface. After a brief moment for sighs of relief, they waited for the next signal indicating successful sampling.
Sample acquisition essentially consists of a brief high-five between Bennu and the end of O-REx’s arm. During just six seconds of contact time, thrusters perturb the asteroid’s dusty surface and the arm releases a burst of nitrogen gas, kicking up surrounding grains in a cloud of debris. The sampling device collects some of these particles – roughly aiming for 60 grams (about the mass of a full-size candy bar) – and O-REx swiftly backs away from Bennu.
Once the news of a completed sampling reached Mission Control, an uproar of applause followed, emotions conveyed through squinted eyes peeking above face masks and personal pod dance parties.
There remained only one final step before calling O-REx and its new sample to come home: making sure the arm grabbed a whole candy bar worth of material. If the team discovered the spacecraft collected 40 grams or fewer, they would need to send O-REx to a new sampling site.
A couple of days after gathering the sample, the team prepared for measurement. They aimed to quantify a change in the spacecraft’s moment of inertia – a physicist’s way of describing an object’s resistance to rotational acceleration. The concept is simple enough: if a person spins with their arm extended while holding a string with a ball attached, they can sense the ball's mass by the tension in the string. In O-REx’s case, it has an asteroid sample rather than a ball. With its arm extended, the spacecraft slowly spins, and the team observes how the sample affects the rotation.
In a phrase not often used in science, the team learned they had “too much success,” discovering the sample was overflowing the storage container. While a surprising result, the hefty collection has not caused any issues and is currently safely stored, set for its journey to Earth.
In March 2021, OSIRIS-REx will begin its trip home, bringing with it evidence for scientific detective work – a forensic effort to piece together the deep past using the dust of a distant, dark space rock.