NASA’s OSIRIS-REx has just arrived at the Bennu asteroid after two years of travelling
through space.
This mission has seemingly gone under the radar for most of the public as it was not
advertised nearly as much as the recent InSight landing on Mars.
But I think you’ll find this mission just as interesting, if not more so than the InSight
lander!
So, what will it do there?
What are its mission goals?
And why is it there in the first place?
I’m Alex McColgan, and you’re watching Astrum, and together we will go through everything
you could want to know about the Osiris-Rex mission.
Let’s start from the beginning.
OSIRIS-REx is part of the New Frontiers NASA program, along with Juno and New Horizons,
which means it is part of a high pedigree if you’ve been following those other missions.
It launched in September 2016, and arrived near Bennu on 3rd December 2018, where it
has begun imaging the surface.
Although Bennu is a near-Earth asteroid, it still took a long time to get there.
This is because Bennu does not orbit along the plane of Earth, and its gravity is incredibly
weak which means Osiris-Rex had to slow down a lot or it would have simply overshot its
mark.
Interestingly, the Osiris team had a pretty good idea of what Bennu would look like long
before Osiris-Rex even launched.
This was because they used radar data to simulate what it would look like.
As Osiris-Rex approached Bennu, the images it sent back were progressively more and more
clear, and the Osiris team saw that their predictions were surprisingly accurate!
Eventually, it got to within 20km of the asteroid, and what it saw was amazing.
This is an actual timelapse of the Bennu asteroid as seen by Osiris-Rex.
If you compare the simulation to the actual photographs, its amazing how accurate their
predictions were.
As you can see, it’s not a particularly smooth asteroid, covered with rough regolith
all over the surface.
It reminds me a lot of the surface of another asteroid that was visited by the JAXA probe
Hayabusa, the asteroid Itokawa, which was more like a clump of rubble than anything
else.
Bennu itself is about 500m in diameter, and has this weird spinning top like shape.
This could be because Bennu has a low gravity but a fast rotation, so regolith has accumulated
there as the asteroid span.
Bennu is a carbonaceous asteroid, but in the less common B-Type classification.
This puts it in the same category as the third biggest asteroid in the solar system, Pallas,
and Bennu could even be the ejecta from an impact early in Pallas’ life.
B-Type carbonaceous also means it has a very low albedo, or that it doesn’t reflect a
lot of sunlight.
Other tests show that it could be an even rarer F-Type asteroid instead, and as such
could have been part of the Polana family of asteroid, meaning it came from the 55km
big asteroid Polana.
In either case, its orbit has changed significantly through various processes and interactions
to bring it close to the orbit of Earth.
Speaking of its orbit, it is one of the most likely asteroids we know of that could impact
Earth, although based on the most recent estimate in 2014, this won’t happen until 2175, and
even then, there is only a 1 in 2700 chance of doing so.
Before Osiris-Rex will go in orbit around Bennu, it first needs to see if there are
any small moons or debris around the asteroid so there isn’t any risk of a collision.
It will do this by going back and forth a few times around the asteroid, also getting
a sense of the gravity of an object so small.
Once the prime orbit has been located, Osiris-Rex will orbit roughly 1.4km above its surface,
mapping out the surface of Bennu in detail, looking for differences in minerals across
the surface of the asteroid.
Interestingly, this means that Bennu will be the smallest world ever to be orbited by
a spacecraft.
This will last for about one year, and what scientists are looking for is the sampling
site.
You see, unlike most robotic NASA missions that don’t return to Earth, Osiris-Rex will,
hopefully carrying samples of the asteroid with it.
Osiris-Rex is unique in that it is a sample-return mission, the first time NASA will be doing
this with an asteroid, although it should be noted that JAXA has done this before with
the Hayabusa mission, and are currently in the process of doing it again with Hayabusa
2.
Once the sampling site has been chosen in mid 2020, Osiris-Rex will drift extremely
slowly towards Bennu to land.
This slow speed is to avoid the use of thrusters as much as possible.
If the thrusters are used close to the surface, the gas from them could contaminate the samples.
The solar arrays will fold into a Y shape upon descent to avoid dust settling on the
panels after the impact with the surface, and to provide more ground clearance should
the spacecraft tip over.
However, the force of the impact should be dissipated by a spring on the arm of Osiris-Rex.
Once it has landed, it will fire a burst of nitrogen gas at the ground, blowing particles
up to 2cm into the sample collection chamber in the arm.
After 5 seconds, the sample collector will close, and the spacecraft will back away from
the asteroid.
Scientists are hoping for about 60g of material.
It will find out how much mass it has in a really clever way, it will do a spin to see
the change in inertia with this added weight.
If it wasn’t able to collect enough samples, it will go in for another try.
Should everything go well at this point,
the container will be stowed by a robotic arm.
By 2023, Osiris-Rex will eject the sample capsule back towards Earth,
where it will be picked up by scientists in the Utah desert.
This is when the sample study part of the mission will begin.
Osiris-Rex will continue to orbit the Sun long into the future.
But why asteroids?
Apart from the curiosity factor, is there anything that actually makes asteroids special?
Well asteroids are remnants of some of the oldest objects in our solar system.
They were there when everything was in the process of forming around the Sun.
To this end asteroids can tell us a lot about the formation of the solar system.
They are the building blocks of planets, and also potentially life!
Asteroids could be the source of water on Earth, and they mysteriously contain organic
materials like hydrocarbons and amino acids.
Actual uncontaminated samples of asteroids will provide insights into this, enabling
us to have a better understanding of the universe we live in.
So hopefully this mission will be as successful as its other New Frontier cousins!
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