By: Alex Zapién
It has been over a year since the New Horizons spacecraft approached Pluto in July 2015. Now, New Horizons continues its path into space much like New Frontier, the project responsible for New Horizons, continues its progress. New Frontier is also responsible for Juno, the space probe that successfully entered Jupiter’s orbit in July of 2016, and for OSIRIS-REx (1). The OSIRIS-REx (The Origins, Spectral Interpretation, Resource Identification, and Security- Regolith Explorer) spacecraft was sent by the National Aeronautics and Space Administration (NASA) to the asteroid Bennu this fall in order to obtain and return asteroid samples for analysis back here on Earth (2).
The OSIRIS-REx spacecraft was launched on September 8, 2016 from Cape Canaveral, Florida and is scheduled to reach Bennu in 2018 (2). The mission’s objective is to retrieve, analyze, and return a carbonaceous asteroid sample in order to gain a deeper understanding of the minerals and organic material found in Bennu and other Near Earth Objects (NEOs) (2). Analysis of the collected data will allow scientists to precisely map the asteroid’s orbit and measure the orbit deviation caused by non-gravitational forces out in space, otherwise known as the Yarkovsky effect (3).
Bennu was chosen out of an initial pool of 500,000 known asteroids and 7,000 NEOs. Researchers narrowed down the pool based on the object’s proximity to Earth, size, and composition (3). The distance from the object to Earth had to be between 0.8 astronomical units (AU) (about 75 million miles/120 million km) and 1.6 AU (about 150 million miles/240 million km) and would ideally have an Earth-like orbit with inclination and eccentricity to make the journey to the object more accessible. In addition, the size of the asteroid was also highly considered. The smaller the asteroid’s diameter, the more rapidly it rotates. Asteroids with diameters less than 200 meters rotate so rapidly they eject loose material from the surface, which would pose a serious hazard for the OSIRIS-REx spacecraft upon contact (3). The ideal asteroid would have a diameter of about 500 meters, which is exactly the size of Bennu’s diameter. Finally, the ideal asteroid needed to have the properties of a primitive, yet extremely old asteroid. This meant having a carbon-rich composition that had not been significantly changed in nearly 4 billion years. The composition of the asteroid is determined based on how the asteroid reflects the Sun’s light. Asteroids that fit this description usually contain amino acids and original material of gas and dust from solar nebula that collapsed to form the solar system, which can tell us about the building blocks of life on Earth as well as the conditions at the solar system’s birth (1, 3).
With the criteria, the choice was narrowed down to five candidates with Bennu ultimately being chosen. Interestingly, Bennu satisfied every criterion but one: distance. Researchers decided choosing Bennu would be more appropriate, as it comes within only 0.002 AU (about 186,000 miles/300,000 km) from the Earth every six years, much closer than the ideal range of 0.8 and 1.6 AU (3). This proximity gives Bennu a high probability of impacting the Earth within the next 200 years. Indeed, NASA scientists have calculated that the odds of Bennu starting an orbit (in the early to mid-2100s) that would lead to a collision with the Earth in the late 22nd century is 1 in 2,700, making it a fascinating NEO (4).
Ultimately, it was Bennu’s composition, size, and hazardous orbit that made it a perfect and accessible NEO.3 If successful, the OSIRIS-REx spacecraft will return a sample to Earth in 2023. The OSIRIS-REx mission will improve our understanding of both the formation of the solar system and the quality and trajectories of asteroids that could collide with Earth (2). We will be able to devise future strategies to mitigate possible Earth impacts from celestial objects, and, more importantly, we will gain a greater understanding of our planet and our solar system. There is no doubt that we will reach a new frontier in science.
Alex Zapién ‘19 is a sophomore in Cabot House, concentrating in physics.
 New Frontiers Program. https://discoverynewfrontiers.nasa.gov/index.cfml (accessed Sept. 26, 2016).
 OSIRIS-REx. https://www.nasa.gov/osiris-rex (accessed Sept. 26, 2015).
 OSIRIS-REx: Asteroid Sample Return Mission. http://www.asteroidmission.org/why-bennu/ (accessed Sept. 27, 2016).
 Wall, Mark. http://www.space.com/33616-asteroid-bennu-will-not-destroy-earth.html (accessed Sept 27, 2016)