Sunday, April 16, 2017


OSIRIS-REx is a NASA sample return mission to the near-Earth asteroid 101955 Bennu. It aims to collect a sample from an asteroid whose composition could reveal a great deal about the beginning of the Solar System and the formation and evolution of the Earth. The first asteroid sample return mission was Hayabusa, developed by the Japanese Aerospace Exploration Agency (JAXA). This probe returned about 1,500 microscopic grains from the asteroid 25143 Itokawa. OSIRIS-REx, however, was designed to obtain at least 60 grams of material in the form of macroscopic samples. In addition, Bennu differs enormously from Itokawa in that it is carbonaceous while the latter is siliceous. Further, there is evidence that it is rich in organic and volatile compounds. Bennu is also of interest because its orbit takes it very close to Earth. It was measured to have a small cumulative probability of 0.037% of striking the Earth sometime in the 22nd century. This is due to the present uncertainty as to whether Bennu with pass through a gravitational "keyhole" in its 2135 flyby of Earth that would set it on a collision course. This mission will allow more precise predictions of its trajectory.

Bennu's orbit is slightly larger than Earth's but also more elliptical. As a result, it crosses inside Earth's orbit with every revolution.

The spacecrafts's name stands for Origins, Spectral Interpretation, Resource Identification, Security, Regolith Explorer. This unwieldy acronym contains the four main goals of the mission: to return a sample to Earth that will elucidate the Solar System's origins, to map the asteroid with spectroscopy to learn about its composition and formation, to investigate whether near-Earth asteroids such as Bennu could provide materials as resources for human development, and to discover what impact threat Bennu poses, if any. The word "regolith" describes the layer of loose material at the surface of an asteroid, from which OSIRIS-REx will obtain a sample.

On September 8, 2016, the OSIRIS-REx mission began with a launch from Cape Canaveral. Over a year later, the probe performed a gravity assist at Earth that set it on course to the asteroid Bennu. On June 27, 2018, the sister mission Hayabusa 2 (JAXA's second asteroid sample return) arrived at 162173 Ryugu, another near-Earth asteroid. The techniques and targets of these two missions were selected to complement one another.

During the next few months, the spacecraft made its final approach to the asteroid. Along the way, it performed several asteroid approach maneuvers (AAM's) to lower its relative speed to Bennu. It was essential to achieve an incredibly low relative speed because the asteroid is so small: only 0.3 miles in diameter. OSIRIS-REx was the first spacecraft to ever attempt orbit about such a small object. Therefore, a series of four AAM's in October and November were performed to reduce the relative speed from 1,100 mph down to a minuscule 0.10 mph, about the speed of a sloth!

Meanwhile, the spacecraft had come quite close to its orbiting distance. On November 16, the above image was taken of Bennu from a distance of just 85 miles. On December 3, 2018, OSIRIS-REx "arrived" at Bennu, an occasion marked by the firing of thrusters that left the craft within 5 miles of the asteroid.

Shortly after arriving, the probe's spectrometers indicated that clays on its surface were hydrated, meaning that they had likely interacted with water in the distant past. While no liquid water exists on the surface of Bennu today, this discovery painted an intriguing picture of the asteroid's history. On December 31, another brief thrust put OSIRIS-REx in orbit of Bennu, making the latter the smallest object ever orbited by spacecraft. The gravitational pull of the asteroid is minuscule, only a few millionths of Earth's.

The next phase of the mission was a detailed survey of the asteroid's surface via several close flybys to guide the future sample acquisition. However, it did not take long before scientists discovered surprising features of Bennu that would make the next mission stages challenging. For one thing, the terrain was more rugged than expected; Bennu's surface is densely packed with boulders in many places, meaning that higher accuracy would be necessary for sample collection.

Further, in January 2019, OSIRIS-REx discovered periodic particle ejections from the surface, captured by long-exposure photography above (the contrast of this photo is boosted to capture the debris). Investigations concluded that these events did not pose a significant threat to the spacecraft. In June 2019, the probe entered a second orbit at an altitude of about 2230 feet (680 meters) to obtain even higher resolution images of possible sample collection sites. By August, all but 4 potential sites had been eliminated.

The above image shows the site that was ultimately selected, nicknamed "Nightingale". The final decision took place in December 2019. The size of the spacecraft is superimposed on the image for scale. The site was ultimately chosen because of the fine material and lack of large boulders, maximizing the safety of the sample collection. In addition, Nightingale was located in a crater estimated to be relatively young, giving OSIRIS-REx access to a pristine crust sampled not covered by the debris of eons.

After collecting a sample in 2020 and leaving the asteroid in 2021, the spacecraft will then return the sample to Earth in 2023.