Asteroids are some of the most ancient objects in the Solar System, relics left over from the time when the planets first started forming and evolving. For this reason, scientists are very interested in them, since they can provide clues as to how this process occurred. Most asteroids orbit the Sun in a broad belt between Mars and Jupiter, but they can be found elsewhere in the Solar System as well. NASA’s OSIRIS-REx spacecraft is now en route to one of these asteroids, called Bennu, which it will study and then bring a sample back to Earth. While on the way there, however, OSIRIS-REx will also be searching for other asteroids, called Trojans. These have regular orbits which place them either just before or just behind a planet, including Earth. The spacecraft will be on the lookout for some of these Trojans near Earth this month as it travels toward Bennu.
This month, from Feb. 9-20, OSIRIS-REx will be in an ideal location to search for more of these objects. During those 12 days, the spacecraft will use its MapCam imager to scan the region where Trojans are most likely to be found.
“The Earth-Trojan asteroid search provides a substantial advantage to the OSIRIS-REx mission,” said OSIRIS-REx Principal Investigator Dante Lauretta of the University of Arizona, Tucson. “Not only do we have the opportunity to discover new members of an asteroid class, but more importantly, we are practicing critical mission operations in advance of our arrival at Bennu, which ultimately reduces mission risk.”
The same instruments will also be used at Bennu, so this presents a good opportunity to test them out beforehand.
Trojans are known to accompany six planets – Venus, Earth, Mars, Jupiter, Uranus and Neptune. They remain in stable orbits either leading or following the planet in its own orbit, 60 degrees before or after the planet. Since the orbits are stable, there is no fear of collision with the planets. So far, only one Earth Trojan asteroid has been found – 2010 TK7 – found in 2010 by NASA’s NEOWISE project. Scientists think there are more however, since they are difficult to detect due to appearing close to the Sun as seen from Earth. Jupiter, for example, has 6,000 known Trojans. Discovering more of them near Earth would be an exciting find for planetary scientists.
“That would be the most fascinating thing we could discover,” said Lauretta.
OSIRIS-REx will also be on the lookout for any small moons circling Bennu. Ground-based telescopes so far have not found any larger than about 65 feet across, which is the smallest they could detect.
“That’s a substantial object and we’d obviously want to know that it’s there and plan our operations accordingly,” Lauretta said.
OSIRIS-REx is the first U.S. mission to travel to an asteroid and then return a sample back to Earth. It will approach Bennu, a carbonaceous asteroid, in August 2018 and start surveying the asteroid in October, which will last over one year. The four phases of the survey include:
- Preliminary Survey searches for asteroid plumes and natural satellites, and also measures the Yarkovsky acceleration of Bennu.
- Orbital A allows the Flight Dynamics Team to transition from star-based navigation to landmark-based navigation using images of Bennu’s surface.
- In Detailed Survey, several instruments work together to map Bennu and determine its global spectral, thermal, and geological properties.
- Orbital B continues to map Bennu at higher resolutions, with a focus on candidate sample sites. At the end of Orbital B a sample site will be selected.
Then, in July 2020, the spacecraft will attempt to take a sample from the asteroid’s surface material, which it will then return to Earth, in September 2023.
The main objectives of the mission include:
- Return and analyze a sample of Bennu’s surface
- Map the asteroid
- Document the sample site
- Measure the orbit deviation caused by non-gravitational forces (the Yarkovsky effect)
- Compare observations at the asteroid to ground-based observations
OSIRIS-REx was launched on Sept. 8, 2016. According to Jim Green, director of the agency’s Planetary Science Division, “One of the amazing things about OSIRIS-REx is that it addresses all of the science goals that planetary science has at NASA: how the Solar System works, how it formed, how it informs our understanding of life in Solar System and elsewhere, and how we can understand the resources and hazards of asteroids and better characterize those.”
There are three main reasons why Bennu was chosen for a sample-return mission:
- Proximity to Earth
The closest asteroids to Earth are Near-Earth Objects (NEOs). As their name suggests, NEOs are objects that orbit within 1.3 AU of the Sun. For a sample return mission like OSIRIS-REx, the easiest asteroids to visit will travel between 1.6 AU and 0.8 AU. The ideal asteroid will have an Earth-like orbit, with low eccentricity, and inclination. At the time of asteroid selection in 2008, there were over 7000 known NEOs, but only 192 had orbits that met these criteria.
Asteroids with small diameters rotate more rapidly than those with large diameters. With a diameter less than 200 meters, an asteroid spins so rapidly that the loose material on its surface (regolith) can be ejected from it. The ideal asteroid will have a diameter larger than 200 m so that a spacecraft can safely come into contact with it. This size requirement reduced the number of candidate asteroids from 192 to 26.
Asteroids have been divided into different types based on their chemical composition. The most primitive asteroids are carbon-rich and have not significantly changed since they formed nearly 4 billion years ago. These asteroids contain organic molecules, volatiles, and amino acids that may have been the precursors to life on Earth. Of the 26 asteroids left on the list, only 12 had a known composition, and only five were primitive and carbon-rich.
Another reason for the interest in Bennu is a concern also shared for other near-Earth asteroids – unlike Trojans, Bennu is in an orbit which could possibly cause it to collide with Earth sometime in the 22nd century. OSIRIS-Rex will study Bennu’s physical and chemical properties, which would be critical to know in case an impact mitigation mission was necessary.
On Jan. 17, it was announced that OSIRIS-REx had successfully completed its first Deep Space Maneuver (DSM-1) (Dec. 28, 2016). This engine burn prepares the spacecraft for an Earth gravity assist this coming fall as it continues its two-year journey to Bennu. The burn resulted in a 964 miles per hour (431 meters per second) change in the spacecraft’s velocity, utilizing 780 pounds (354 kilograms) of fuel.
“DSM-1 was our first major trajectory change and first use of the main engines, so it’s good to have that under our belts and be on a safe trajectory to Bennu,” said Arlin Bartels, deputy project manager at NASA’s Goddard Space Flight Center in Greenbelt, Maryland.
Before the launch, artists and other space enthusiasts were invited to submit artwork to be included onboard the spacecraft.
“We’re thrilled to be able to share the OSIRIS-REx adventure with people across the Earth, to Bennu and back,” said Lauretta. “It’s a great opportunity for people to get engaged with the mission early and join us as we prepare for launch.”
As also noted by Bill Nye, “You’ll be part of humankind’s exploration of the Solar System – How cool is that?”
See also Lauretta’s latest blog entry for more information about the Trojans. The OSIRIS-REx mission is managed by NASA’s Goddard Space Flight Center and the spacecraft was built by Lockheed Martin Space Systems. More information about OSIRIS-REx mission is available on the mission website.
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