JOIN USAs OSIRIS-REx flew off Cape Canaveral Air Force Station, Florida, on September 8, 2016, it began a journey that could revolutionise our understanding of the early solar system. In the coming seven years, OSIRIS-REx (smartly coined from origins, spectral interpretation, resource identification, security, regolith explorer) intends to fly to an asteroid called Bennu, presently 1.2 astronomical units (AU) away from Earth, and bring back samples of its soil. Asteroids are small, airless, rocky bodies revolving around the Sun that are too small to be called planets. Bennu is an asteroid discovered in 1999 by NASA’s Spitzer space telescope. Radar measurements by ground-based radio telescope suggest that Bennu is carbon rich and measures 1,650 feet across and weighs over 60 million tonnes.
Why Bennu?
The first reason for selecting Bennu is that it is a conglomeration of pristine materials formed at the dawn of the solar system and has remained unchanged over the last 4.5 billion years. Professor Dante Lauretta, the principal investigator of the mission, considers Bennu as a time capsule from the earliest stages of the solar system formation. Theory advocates that life on Earth is a result of the amino acids that may have come from an asteroid or a comet.
Professor Lauretta wants to specifically verify whether Bennu contains higher concentrations of the 20 amino acids that resulted in life on Earth, compared to the dozens that are not found in living organisms. Thus, the analysis of organic materials on asteroids will give scientists an inventory of materials of the early solar system that may have a role in the origin of life on Earth, or potentially elsewhere.
The second reason for selecting Bennu is its orbit time and position. It takes about 14 months to orbit around the Sun and like Earth, its orbit lies between Mars and Venus. Due to this, Bennu passes fairly close to Earth once in every six years. In 2035, it is expected to be the closest, passing within the orbit of the Moon. The gravity of the Earth will greatly influence Bennu’s orbit, which becomes difficult to predict after this encounter.
There is also a small possibility (one in 2,700) that Bennu will collide into Earth sometime in the later half of the next century. Although Bennu is not large enough to cause planet-wide extinctions, a collision would be devastating with a tremendous amount of energy unleashed that is equivalent to 1,450 million tonnes of trinitrotoluene (TNT) and carving a crater almost three miles wide and 1,500 feet deep.
One of the uncertainties in predicting Bennu’s path is that gravity is not the only force acting on it. As the asteroid rotates, the dark surface absorbs sunlight and then radiates heat. The radiated heat pushes the asteroid like a rocket thruster in a mechanism called as Yarkovsky effect. Although this effect is trivial, it can significantly change the course of Bennu’s orbit over a long time. Once the deviation in Bennu’s orbit is known, the chances of Bennu colliding with Earth can be calculated.
The idea of the OSIRIS-REx mission was conceived in 2004 by late Professor Michael Julian Drake, the then director of the University of Arizona’s Lunar and Planetary Laboratory (LPL). Although Professor Drake analysed meteoroids and took part in many of NASA’s interplanetary missions, he firmly believed asteroids held the secrets to the evolution of the solar system and the origin of life on Earth. After being rejected twice, NASA finally decided to approve his idea after a two years of gruelling review, under the New Space Frontiers programme, with a budget of 800 million USD. Four months into the programme when the team started working on the design, it hit a roadblock — Professor Drake’s health deteriorated and he died of liver cancer. Finally, NASA asked the then deputy principal investigator, Professor Dante Lauretta, also from the University of Arizona’s LPL, to step up and lead this ambitious mission.
To achieve its mission, OSIRIS-REx is equipped with a suit of seven instruments developed in collaboration with scientists from the University of Arizona, the Canadian Space Agency, Massachusetts Institute of Technology (MIT), Harvard University and the Lockheed Martin Corporation. Out of the seven, there are three spectrometers working at different wavelengths, a camera suite, one 3D mapping LIDAR and two instruments for collecting and storing the samples.
In its arsenal
The OSIRIS-REx camera suite consists of three cameras — PolyCam, MapCam, and SamCam. PolyCam will image Bennu at high resolution, MapCam searches the area around Bennu for satellites and outgassing plumes and SamCam will continuously document the sample acquisition event and the touch-and-go (TAG) manoeuvre.
The payload also has an OSIRIS-REx laser altimeter (OLA), a scanning light detection and ranging (LIDAR) similar to radar, but uses light instead of radio waves to measure distances. This would collect data to produce three dimensional topographic maps of Bennu and help in selecting sampling site.
The OSIRIS-REx thermal emission spectrometer (OTES) on board will provide mineral and temperature information by collecting infrared spectral data from Bennu. Most minerals have a unique spectrum under infrared light, which can be used to determine the mineral content on the surface of Bennu. In addition, analysis of the temperature at infrared wavelength will be used to comment on the physical properties of the surface. The OSIRIS-REx visible and infrared spectrometer (OVIRS) will split the light received from Bennu into its component wavelengths, much like a prism splits sunlight into a visible spectrum. This helps in identifying mineral and organic material globally and provide spectral maps.
Regolith X-ray imaging spectrometer (REXIS) will determine the elements present and their abundance on the surface of Bennu, thus complementing the onboard mineral mapping provided by OVIRS and OTES. REXIS takes advantage of the fact that solar X-rays and the solar wind interact with regolith, the layer of unconsolidated rocky material covering bedrock on Bennu’s surface. Atoms in the regolith absorb these X-rays, become unstable and emit their own X-rays in turn. The re-emitted X-rays have an energy that is characteristic of the atom from which it comes. This process is called fluorescence. REXIS is a telescope that images the X-ray fluorescence line emission.
The touch-and-go sample acquisition mechanism (TAGSAM) is a mechanical arm that will be extended towards Bennu to help in the collection of samples. Once the sampler head makes contact with the surface, a burst of pure nitrogen gas will push surface dust into the sampler’s chamber. The sample will be stored in the sample return capsule (SRC), which will return to Earth at the end of the mission in 2023.
OSIRIS-REx flies towards Bennu with a bundle of hope of unearthing the mystery of life on our planet. Scientists have always debated that organic molecules and water were carried to Earth by asteroids and comets when Earth was still young and forming. The OSIRIS-REx mission plans to investigate this topic of debate with concrete evidences. If it succeeds in providing a convincible answer to the fundamental question of how life originated on Earth, it will indeed be a second giant leap for mankind.
(The author is with Gubbi Labs, Bengaluru-based research collective)