From August to early December, the OSIRIS-REX spacecraft aimed three of its scientific instruments toward Bennu and began to make the first observations of the asteroid's mission. During this period, the spacecraft traveled the last 2.2 million kilometers of its outward journey to reach a point 19 km from Bennu on 3 December. The science obtained from these initial observations confirmed many of the missionary team's observations on Bennu soil and revealed several new surprises.
Members of the mission team, led by the University of Arizona, presented the results at the American Geophysical Union's Annual Fall Meeting, or AGU, in Washington, D.C., on December 10.
In an important finding for the scientific investigation of the mission, data obtained from the two probe spectrometers, the OSIRIS-REx Visible Infrared Spectrometer (OVIRS) and the OSIRIS-REx Thermal Emission Spectrometer (OTES), reveal the presence of molecules containing oxygen and hydrogen atoms linked together, known as "hydroxyls". The team suspects that these hydroxyl groups exist globally through the asteroid in clay minerals containing water, which means that at some point, the rocky material interacted with the water. Although Bennu himself is too small to harbor liquid water, the discovery indicates that liquid water was present at some point in Bennu's body, a much larger asteroid.
"This finding may provide an important link between what we think happened in space with asteroids like Bennu and what we see in meteorites that scientists study in the laboratory," said Ellen Howell, senior researcher at the LPL, . ) and member of the mission's spectral analysis group. "It is very exciting to see these hydrated minerals spread across the surface of Bennu because it suggests that they are an intrinsic part of Bennu's composition, not just sprayed on its surface by an impactor."
"The presence of hydrated minerals through the asteroid confirms that Bennu, a remnant since the beginning of the formation of the solar system, is an excellent example for the OSIRIS-REx mission to study the composition of volatile and organic primitive," said Amy Simon, OVIRS Vice Instrument Scientist at NASA's Goddard Space Flight Center.
In addition, data obtained from OCAMS (OSIRIS-REx Camera Suite) corroborate Bennu's ground-based radar observations and confirm that the original model – developed in 2013 by OSIRIS-REx chief scientist Michael Nolan, now based at LPL, and collaborators – closely predicted the actual shape of the asteroid. Bennu diameter, rate of rotation, inclination and overall shape presented almost exactly as designed.
Soon after the asteroid later named Bennu was discovered in 1999, Nolan's group used the Arecibo Observatory in Puerto Rico to gather clues about its size, shape, and rotation, reflecting the radar waves during one of its approach to Earth, about of five times. the distance between the Earth and the moon.
"Radar observations do not give us any information about the color or brightness of the object, so it's really interesting to see the asteroid up close by the eyes of OSIRIS-REX," Nolan said. "As we get more details, we are finding out where the craters and boulders are, and we were very pleasantly surprised that pretty much every little bump we saw on our radar image actually is actually there."
The mission team used this ground-based Bennu model when designing the OSIRIS-REx mission. The accuracy of the model means that the mission, spacecraft and planned observations have been properly designed for future tasks in Bennu.
An outlier of the planned form model is the size of the large rock near the south pole of Bennu. The soil model calculated that this gravel is at least 33 feet (10 meters) high. Preliminary calculations of OCAMS observations show that the boulder is closer to 50 meters in height, with a width of approximately 55 meters.
As expected, the initial evaluation of the Bennu regolith indicates that the Bennu surface is a mixture of very rocky regions, full of boulders and some relatively smooth regions that do not have boulders. However, the amount of stones on the surface is higher than expected. The team will make additional observations at closer intervals to more accurately assess where a sample can be made in Bennu for later return to Earth.
"Our initial data show that the team chose the correct asteroid as the target of the OSIRIS-REx mission. We have not discovered any insurmountable problem at Bennu so far, "said Dante Lauretta, principal investigator at OSIRIS-REX and professor of planetary science and cosmochemistry at LPL. "The spacecraft is healthy and the scientific instruments are working better than necessary. Now it's time for our adventure to begin. "
"What used to be science fiction is now a reality," said AU Chair Robert C. Robbins. "Our work in Bennu brings us closer to the possibility of asteroids providing astronauts in future missions to the solar system with resources such as fuel and water."
The mission is currently conducting a preliminary survey of the asteroid, flying the spacecraft in passes along the north pole, equator and south pole of Bennu, at a distance of up to 7 km to better determine the mass of the asteroid. This research also provides the first opportunity for the OSIRIS-REx Laser Altimeter (OLA), an instrument provided by the Canadian Space Agency, to make observations now that the spacecraft is close to Bennu. The spacecraft's first orbital insertion is scheduled for December 31, and OSIRIS-REx will remain in orbit until mid-February 2019, when the mission moves to the next stage of the survey. During this first orbital phase, the spacecraft will orbit the asteroid at a distance of 1.4 km 1.2 km from the center of Bennu, setting two new records for the smallest body orbited by a spacecraft. orbit closest to a planetary body by any spacecraft.