The solar system is a dynamic place, but some corners of it have remained virtually unchanged for 4.5 billion years. These are the asteroids – the rubble left over from the formation of the planets. Among them, a small, diamond-shaped rock named 101955 Bennu became the target of one of NASA’s most ambitious robotic explorers: OSIRIS-REx.
The mission was not just about visiting a rock; it was about retrieval. OSIRIS-REx (Origins, Spectral Interpretation, Resource Identification, Security-Regolith Explorer) was designed to travel to Bennu, map it in unprecedented detail, touch its surface, and bring a sample back to Earth.
Why Asteroid Bennu? A Window into the Past
Of the over one million known asteroids in our solar system, why did scientists choose Bennu? The answer lies in its composition and its orbit.
Bennu is a B-type asteroid, a rare subclass of carbonaceous asteroids. It is effectively a fossil floating in space. While planets like Earth melted, differentiated, and changed over billions of years, Bennu has remained a pristine “time capsule.” It contains carbon-rich organic molecules and water – bearing minerals that existed before Earth was even formed.
By studying Bennu, scientists hoped to answer a fundamental question: Did asteroids deliver the molecules of life – water and organic compounds – to the early Earth?
Furthermore, Bennu is a Near-Earth Object (NEO) with a high probability of impacting Earth late in the 22nd century (specifically, a 1 in 2,700 chance in the year 2182). Understanding its orbit and structure is vital for planetary defense.
The Journey and the Challenge of Micro-Gravity
Launched in September 2016 aboard an Atlas V rocket, OSIRIS-REx spent two years chasing Bennu. Upon arrival in December 2018, the spacecraft faced a unique challenge: gravity.
Bennu is tiny—only about 500 meters (1,640 feet) wide. Its gravity is so weak (about 1/100,000th of Earth’s) that orbiting it is incredibly difficult. The solar wind and thermal pressure from the Sun can push a spacecraft out of orbit. To stay close, OSIRIS-REx had to perform a delicate dance, breaking the record for the closest orbit ever flown by a spacecraft.
The “Rubble Pile” Surprise
Before arrival, scientists predicted Bennu would have smooth, sandy “beaches” of regolith where the probe could easily land. Instead, the high-resolution cameras revealed a nightmare landscape. Bennu was a “rubble pile”—a loose collection of boulders held together by weak gravity, rugged and covered in massive rocks that posed a severe danger to the spacecraft.
This discovery forced the mission team to rewrite their plans. Instead of a wide landing zone, they had to program the spacecraft to target a tiny site named “Nightingale,” surrounded by hazardous boulders the size of buildings.
The TAG Maneuver: Touching the Void
On October 20, 2020, the mission reached its climax. The procedure was called TAG (Touch-And-Go). Unlike a traditional landing, OSIRIS-REx did not use landing legs. Instead, it extended a robotic arm called the TAGSAM (Touch-And-Go Sample Acquisition Mechanism).
The plan was simple but risky:
- Descend slowly to the surface.
- Tap the asteroid with the sampling head.
- Blast nitrogen gas into the surface to stir up dust and rocks.
- Catch the flying debris in the filter.
- Back away immediately.
The maneuver worked—perhaps too well. The surface of Bennu turned out to be surprisingly fluid, behaving more like a “pit of plastic balls” than solid rock. The sampling arm sank nearly half a meter into the asteroid. The gas blast captured so much material that the container’s lid was jammed open by rocks, and material began leaking into space. The team had to rush to stow the sample capsule to prevent further loss.
The Return and the Treasure Inside
After a long cruise back to Earth, the OSIRIS-REx sample return capsule separated from the main spacecraft and streaked through the atmosphere on September 24, 2023. It touched down in the Utah desert, marking the end of a seven-year journey.
When scientists finally opened the canister at the Johnson Space Center, the results were staggering. The mission goal was to collect 60 grams of material. OSIRIS-REx returned 121.6 grams of dust and rocks—the largest asteroid sample ever collected by the U.S.
Scientific Breakthroughs
Initial analysis confirmed the team’s highest hopes:
- High Carbon Content: The sample is nearly 5% carbon by weight, essential for organic chemistry.
- Water: The clay minerals in the sample contain water locked inside their crystal structure, proving that asteroids like Bennu could have brought oceans to Earth.
- Magnesium Phosphate: A bright white crust found on the rocks suggests Bennu experienced water-rich episodes in its parent body’s past.
From OSIRIS-REx to OSIRIS-APEX
The sample capsule is safely on Earth, but the spacecraft’s journey isn’t over. Having dropped off its precious cargo, the main probe fired its thrusters to divert past Earth. It has been renamed OSIRIS-APEX (Apophis Explorer).
Its new mission? To orbit the famous asteroid Apophis during its ultra-close flyby of Earth in 2029. The spacecraft will study how Earth’s gravity deforms and reshapes the asteroid, continuing the legacy of exploration that began at Bennu.