NEW YORK (EarthSky): NASA’sOSIRIS-REx spacecraft arrived at asteroid Bennu on December 3, 2018. One week later – on December 10 – NASA announced the craft has already made a significant discovery. It has found hydrated minerals on Bennu’s surface – thatis, evidence for previous (ancient) water on the rocky little world.
Just to clarify, this
discovery doesn’t mean Bennu has liquid water – either inside or on its surface
– right now. Instead, it shows that Bennu’s parent body, probably a much larger
asteroid that Bennu broke off of a long time ago, did have water.
The Origins, Spectral
Interpretation, Resource Identification, Security-Regolith Explorer –
OSIRIS-REx – spacecraft reached Bennu on December 3 after traveling 1.4 million
miles (2.2 million km) during the approach phase of the mission, from
mid-August to early December. As of now, the spacecraft is only 12 miles (19
km) away from Bennu! As would be expected, this means that OSIRIS-REx has been
sending back some stunning close-up images of this little world, which was
first discovered in 1999. During the next preliminary survey of the asteroid,
the spacecraft will pass over Bennu’s North Pole, equator and South Pole as
close as 4.4 miles (7 km) to the surface.
OSIRIS-REx is also
NASA’s first asteroid sample-return mission, which will collect samples of the
surface regolith and bring them back to Earth for further study.
The mission team is
elated at what has been seen and discovered so far, said Humberto Campins, a
University of Central Florida planetary scientist, professor of physics and
member of the OSIRIS-REx Science Team:
We’re very excited. The
images are spectacular and spot on, what we expected thanks to predictions made
with the instrumentation at the Arecibo Observatory in the late 90s and early
2000s. We will spend a year and a half mapping Bennu and have to wait until mid
2020 to collect the sample, but it is pretty amazing to actually see it now.
Christmas came early.
So what about the
water? The data comes from two spectrometers, the OSIRIS-REx Visible and
Infrared Spectrometer (OVIRS) and the OSIRIS-REx Thermal Emission Spectrometer
(OTES), which revealed the presence of the hydrated minerals – molecules that
contain oxygen and hydrogen atoms bonded together, known as “hydroxyl groups” –
that exist globally across the asteroid in water-bearing clay minerals.
This means that Bennu’s
rocky material once interacted with water, millions or billions of years ago.
Bennu itself is too small to have ever hosted liquid water, so where did the
water come from? Scientists think that the liquid water was present on Bennu’s
parent body, a much larger asteroid that Bennu broke off of. Perhaps something
like the dwarf planet Ceres – the largest asteroid in the main asteroid belt –
which also shows evidence of liquid water in the past. As explained by Amy
Simon, OVIRS deputy instrument scientist at NASA’s Goddard Space Flight Center
in Greenbelt, Maryland:
The presence of
hydrated minerals across the asteroid confirms that Bennu, a remnant from early
in the formation of the solar system, is an excellent specimen for the
OSIRIS-REx mission to study the composition of primitive volatiles and
organics. When samples of this material are returned by the mission to Earth in
2023, scientists will receive a treasure trove of new information about the
history and evolution of our solar system.
Bennu itself is very
interesting appearance-wise, looking kind of like a rough diamond. Data
obtained from the OSIRIS-REx Camera Suite (OCAMS) corroborated ground-based
telescopic observations of Bennu, confirming the original model developed in
2013 by OSIRIS-REx Science Team Chief Michael Nolan and his colleagues. That
model suggested just such a shape for the asteroid, which turned out to be
pretty much correct, along with the diameter, rotation rate and inclination.
According to Anne
Virkki, a research scientist at Arecibo Observatory in Puerto Rico:
The amazing Bennu
images coming out now look strikingly similar to the shape model derived from
Arecibo radar data in 2013.
Arecibo played a big
role in the planning of the mission, said Campins:
obtained from radar characterization of this asteroid at Arecibo was critical
in mission target selection and supported OSIRIS-REx science definition and
mission planning. Arecibo’s radar data gave us two main advantages. It
minimized the uncertainty in the shape of the asteroid and its orbit, which
help reduce risk and increase the likelihood of a successful mission.
Nolan also commented on
don’t give us any information about colors or brightness of the object, so it
is really interesting to see the asteroid up close through the eyes of
OSIRIS-REx. As we are getting more details, we are figuring out where the
craters and boulders are, and we were very pleasantly surprised that virtually
every little bump we saw in our radar image back then is actually really there.
Bennu’s surface overall
is very rocky, with a mix of boulder-filled regions and a few smoother regions
that lack boulders. There are actually more boulders than had been expected
however. One boulder in particular near the south pole stands out – literally –
at about 164 feet (50 meters) in height and 180 feet (55 meters) wide.
The mission is going
well so far, as noted by Dante Lauretta, OSIRIS-REx principal investigator at
the University of Arizona, Tucson:
Our initial data show
that the team picked the right asteroid as the target of the OSIRIS-REx
mission. We have not discovered any insurmountable issues at Bennu so far. The
spacecraft is healthy and the science instruments are working better than
required. It is time now for our adventure to begin.
OSIRIS-REx is currently
conducting a preliminary survey of the asteroid, flying over Bennu’s north
pole, equator and south pole as close as 4.4 miles (7 km). This will help
scientists determine Bennu’s mass – important information needed for the
spacecraft’s insertion into orbit, since mass affects the asteroid’s
gravitational pull on the spacecraft – as well as the asteroid’s internal
structure and composition. The OSIRIS-REx Laser Altimeter (OLA), an instrument
contributed by the Canadian Space Agency, will also now be able to make its
OSIRIS-REx is scheduled
to enter orbit around Bennu on December 31, 2018, and it will remain in orbit
until mid-February 2019. After that, it will leave orbit in order to initiate
another series of flybys for the next survey phase.
During its first
orbital phase, the spacecraft will pass very close to the asteroid, from a
range of 0.9 miles (1.4 km) to 1.24 miles (2.0 km) from the center of Bennu.
This will set a new record for both the smallest body ever orbited by a
spacecraft and the closest orbit of a planetary body by a spacecraft – a
Bottom line: The study
of Bennu has just begun, but there are already significant discoveries being
made, among them evidence for previous (ancient) water. It’s a world never
explored before – until now – but scientists will be learning much about its
origin and evolution – and asteroids in general – over the months and years