Osiris Rex Bennu Sample Return: NASA's Bold Journey into an Ancient Solar System Messenger

Overview

NASA's Osiris Rex mission embarked on a pioneering journey to asteroid Bennu with the goal of imaging its surface, selecting a scientifically compelling sampling site, and collecting a pristine sample to return to Earth. The mission demonstrated a careful, low disturbance approach to a rubble pile world and set new benchmarks for asteroid exploration.

What was learned

Among the early discoveries were hydrated minerals that point to water related processes in Bennu's history, the surprising abundance of surface boulders, and a successful sample return that yielded more material than originally planned. The mission also demonstrated how bringing a sample back allows scientists worldwide to study pre-solar grains, amino acids, and complex organics in pristine conditions, offering insights into the solar system's origins and the building blocks of life.

Introduction

Osiris Rex is NASA's bold New Frontiers mission to Bennu, designed to image a small near Earth asteroid, characterise its surface and chemistry, and most importantly collect a sample for return to Earth. Launched in 2016 as part of a high pedigree program alongside Juno and New Horizons, the mission represented the first attempt by a US spacecraft to physically sample an asteroid and bring material home for laboratory analysis.

Why Bennu and what makes it special

Bennu is a carbonaceous, potentially water bearing body thought to be a time capsule from the era before the Earth formed. Its surface was expected to be a rubble pile with relatively fine material, making sample collection both challenging and scientifically valuable. Mapping Bennu in detail before sampling allowed scientists to pick a site likely to yield a geologically informative sample while minimizing contamination risks.

The Osiris Rex mission profile

The spacecraft entered Bennu orbit at roughly 1.75 kilometers above the surface, the smallest world ever orbited by a spacecraft. Over the course of more than a year Bennu was mapped at high resolution, with spectroscopy revealing hydrated minerals that hinted at the asteroid's aqueous past and its possible connection to Bennu's parent body and its history in the solar system.

Sampling site selection and the sampling event

Although Bennu hosted large boulders, four smaller landing sites were ultimately chosen for sampling Nightingale, Sandpiper, Osprey and Kingfisher. After rehearsals in April and August 2020, Osiris Rex reached Nightingale, extended its sampling arm, and released a burst of nitrogen gas to kick up surface material into the sampler. The result exceeded expectations, collecting well over the 60 g target mass, with estimates of 400 g to over 1 kg of material delivered to the sampler. A rock later caused a minor snag, but the team secured the sample in a sealed container and prepared for the long journey home.

Return to Earth and the handling of the sample

The return capsule traveled about two years and 1.9 billion kilometers to reach Earth in September 2023. Reentry temperatures peaked around 2700 C, and the capsule used a phenolic impregnated carbon ablator system to protect the sample. The first material released outside the inner canister weighed about 70 g, and it took months to carefully reveal the pristine interior. Engineers then designed a specialized tool to open the inner canister inside a nitrogen purged glove box, finally accessing an additional 51 g of material. The total recovered material exceeded the original goal, with more than 70 percent preserved for long term study and distribution to laboratories around the world.

What the Bennu sample revealed

Analyses identified pre-solar grains that illuminate the contributions of ancient stars to the solar system, including significant supernova dust. The samples also contained amino acids and sugars, including ribose and glucose, as well as a polymer like material rich in nitrogen and oxygen. Water bearing clays and minerals such as serpentine and phosphates were found, indicating Bennu's history with water and a possible ancestry from a larger, water-rich body. This paints Bennu as a curator of early solar system chemistry, not a lifeless rock, raising exciting questions about the delivery of water and organics to Earth.

From Bennu to Apophis: mission extension and the Apex phase

Following the Bennu milestone, Osiris Rex received a mission extension and was renamed Osiris Apex. Beginning in 2029 the spacecraft will conduct an 18 month campaign to map Apophis, a 370 meter near Earth asteroid that will pass within roughly 50,000 kilometers of Earth, repeating a close approach that stirs up surface material using thrusters and dipping within 16 feet of the surface to study changes and chemistry at a near surface scale. This extension promises to extend yields of science and demonstrate continued near surface sampling operations on a different small world.

Concluding perspective

The Osiris Rex science program, now Osiris Apex, continues to advance our understanding of how life's building blocks emerged, how water and organics formed in the early solar system, and how to conduct responsible, high fidelity sample return missions for future exploration. The Bennu samples have already expanded our view of the solar system's history and the processes that shaped the planets we know today.