September 4, 2016 6:08 pm
Mission seeks to probe near Earth asteroid Bennu and peer back to the beginnings of our solar system.
Today’s guest blog post is by Kirk Long, StarTalk’s newest intern. Kirk is majoring in physics while minoring in mathematics and piano at Idaho State University.
When the OSIRIS-REx mission samples return from Bennu seven years from now, they will be the largest samples returned to Earth from a foreign body since the Apollo missions finished more than 50 years prior. OSIRIS-REx is an ambitious project that aims to help us clarify some of our most fundamental questions. Bennu—the target asteroid OSIRIS-REx is visiting—is a cosmic time capsule from the formative days of our solar system.(1) By surveying and sampling Bennu, we can better understand not only the processes and conditions that have allowed life to exist on Earth, but also our collective future and the chances of life existing elsewhere, both within our solar system and beyond. Bennu is a leftover from the tumultuous beginnings of our solar system, and it’s a perfect target for OSIRIS-REx. Chosen from a candidate pool of more than half a million objects, Bennu is just the right size—about 500 meters across—the right distance—it comes within 0.002 AU of us every six years and has the potential to impact us in the late 22nd century—and the right composition—Bennu is carbon-rich, potentially holding the most basic building blocks of life. There’s a lot we can learn from Bennu, but perhaps most exciting is the data we might glean regarding our own existence. By analyzing Bennu’s composition, we can better piece together what kind of objects would have impacted the early Earth, which can better inform our theories on how life evolved on our pale blue dot.
OSIRIS-REx is not the first time we’ve visited or even retrieved samples from an asteroid, but it is the largest yet in scope and significance—as well as the first U.S. led mission to sample such an object. Even after the advent of space travel, asteroids, comets, and dwarf planets were lower on the exploration food chain. For a long time, the surface and topography of these smaller celestial bodies were a mystery to us. The first probe to visit and retrieve high quality images of an asteroid was Galileo, which flew past the asteroids 951 Gaspra in 1991 and 243 Ida in 1993 on its way to Jupiter. The first landing on an asteroid was performed by NASA’s NEAR-Shoemaker probe when it landed on Eros in 2000, and although landing was the culmination of that mission the lessons learned from surveying and landing on Eros have allowed NASA to be even more ambitious with OSIRIS-REx. There have been a few sample return missions prior to OSIRIS-REx—most notably the Hayabusa mission, where the Japanese Aerospace Exploration Agency successfully collected and returned a sample from a small near Earth asteroid. Although the Hayabusa mission was overall a resounding success and an incredible technological achievement, a small hiccup in the final stages of the mission resulted in less than one gram of material ultimately being collected.(2) If successful, OSIRIS-REx will bring back at least 60 grams of material and has the capacity to retrieve two kilograms (4.4 pounds) for scientific study.(3) Just as OSIRIS-REx has built on the success of past missions, the data gained from OSIRIS-REx will be paramount for future missions seeking to study and return large samples from other celestial bodies.
OSIRIS-REx is scheduled for liftoff on September 8th, 2016, and will launch from Cape Canaveral Air Force Station. If weather or other factors intervene, the current launch window will remain open for 34 days—giving the launch team plenty of breathing room to reschedule liftoff. The probe sits atop an Atlas V rocket, powered by a Russian made RD-180 first stage engine burning a mix of kerosene and liquid oxygen. Strapped to the first stage is a solid rocket booster manufactured by Aerojet, which will further aid the probe’s acceleration through the lower atmosphere. The second stage is powered by an American made RL10 engine, burning a mix of liquid hydrogen and liquid oxygen. Once in space, the cone atop the rocket will split and allow OSIRIS-REx to be released. From there the probe will begin its journey towards Bennu, rocketing away from the Earth at more than 12,000 miles per hour.
(Video courtesy of OSIRIS-REx on YouTube)
OSIRIS-REx will spend the first year of its two-year journey towards Bennu orbiting the sun before it comes back towards Earth, where it will perform a gravity assist maneuver to slingshot on its final trajectory towards the asteroid. Bennu orbits the sun at more than 63,000 miles per hour, and OSIRIS-REx must precisely match both its speed and trajectory in order to be successful. At this speed, OSIRIS-REx could travel from New York to Los Angeles in just over two minutes. If everything goes according to plan, the probe will reach Bennu towards the end of 2018, with surveys of the asteroid scheduled to begin in October of that year. These surveys will last for about a year, as the team builds detailed maps of the surface of the asteroid and its composition. Using these maps, a sample site will be selected and Bennu will begin to approach the asteroid to perform its riskiest maneuver. Instead of landing on the asteroid, NASA has opted to perform a touch-and-go maneuver using a specially designed mechanical arm. The probe will carefully approach the sample site, and once close enough the arm will reach out to touch the surface. The arm will release pressurized nitrogen gas directed at the surface of the asteroid, stirring up loose regolith and collecting it. OSIRIS-REx contains enough nitrogen to attempt three different samplings, hopefully allowing it to bring back up to two kilograms (4.4 pounds) of material. After the sample is collected, the arm will safely store the sample inside the return capsule and OSIRIS-REx will begin its journey back to Earth. The window to leave Bennu opens in March of 2021, and if the spacecraft leaves as intended it will return to Earth in September of 2023—almost exactly seven years from its launch date. When the spacecraft nears Earth it will perform a final realignment to put itself into a stable orbit around the sun and jettison the capsule containing the sample. The sample capsule will reenter the atmosphere at more than 27,000 miles per hour, after which it will finally touch down in the Utah desert on September 24th, 2023.(4)