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When it comes to astronomy, the approach of “poke it with a stick” is usually not
a very viable option.
You cannot poke stars with a stick, and not much can normally be learned by poking planets.
However, if your stick is actually a high-tech sample collection device, and you target a
cosmological time capsule composed of loose regolith and dust, suddenly we have the setting
for the largest and most scientifically advanced stick poking in human history.
And with the OSIRIS Rex spacecraft’s mission to the Bennu asteroid, that’s exactly what
happened.
I’m Alex McColgan, and you’re watching Astrum.
And today we continue with our third look at the OSIRIS-REx program.
In this video, we explore the moment of truth – the culmination of a journey that spanned
4 years – the first time in history a US spacecraft poked an asteroid.
If you’ve watched my other videos on this topic, you will remember that the OSIRIS-REx
– or the Origins, Spectral Interpretation, Resource Identification, Security, Regolith
Explorer – is a NASA-developed spacecraft that was launched in 2016 with the mission
of collecting and then bringing home to Earth a sample from the near-Earth asteroid known
as 101955 Bennu.
This mission is an important one, as Bennu is an extremely old asteroid.
It is thought to have been around since the birth of the Solar System, and as such is
a treasure trove of information about the Solar System’s formation, and maybe even
the origins of life.
However, while missions have photographed asteroids before and have even collected stardust
from the tails of comets, this was the first time a NASA spacecraft would have ever travelled
to an asteroid to physically collect a sample from its surface.
NASA’s goal was to collect a 60g sample of regolith and dust from Bennu’s surface.
It would do that by entering Bennu’s orbit and slowly descending.
It would be careful not to disturb or contaminate the asteroid’s surface by using its thrusters,
but would drift gently down, extending its long Touch-And-Go Sample Acquisition Mechanism
(or TAGSAM) as it did so.
Then, at the moment it touched the surface of the asteroid, it would release a burst
of Nitrogen gas from its arm, kicking up dust and materials, which would be captured in
its sampler.
OSIRIS-REx would then return home with its sample.
However, that mission was not as simple as it may sound.
Bennu is a relatively small asteroid – only about 490m in diameter – which meant that
there was very little gravity.
While scientists had previously mapped out the rough shape of Bennu to a surprisingly
high degree of accuracy using radar imaging, this low gravity had led them to believe that
Bennu would be made of relatively fine materials.
This was not the case.
Bennu was covered in large boulders, some of which were 58m across.
As such, there were limited places where OSIRIS-REx would be able to touch down to collect its
sample.
And so, OSIRIS-REx spent over a year – 505 days – circling Bennu and mapping out its
surface, choosing the best location for its system-spanning pogo-stick hop.
After careful deliberation, NASA had decided on potentially 4 sites for the sample collection;
Nightingale, Kingfisher, Osprey and Sandpiper.
You might notice a bird theme here.
Bennu itself was named as the result of a contest run by the University of Arizona,
The Planetary Society and the LINEAR project in 2012.
In it, more than 8000 students submitted name ideas.
The winner was a 3rd-grade student, who wanted to name the asteroid after the Egyptian mythological
bird Bennu because he thought that the OSIRIS-REx’s TAGSAM arm looked like the graceful extension
of a heron’s neck, just like the Egyptian God Bennu, meaning it was named after the
craft visiting it rather than the shape of the asteroid itself.
NASA seems to have kept with this bird-like naming convention.
In the end, NASA chose the Nightingale site for OSIRIS’s landing.
This site was set in a crater that seemed to be relatively young and clear of rocks,
which came with numerous benefits.
Not only would landing be easier, but the recently exposed material beneath Bennu’s
surface would likely be an accurate reflection of the asteroid’s internal workings.
With the site selected, OSIRIS began its slow descent.
However, it did not rush in with a single try.
Instead, it did practice runs.
The first took place on the 15th of April 2020.
OSIRIS approached the asteroid to a distance of 65m, before activating its thrusters and
rising again.
Everything worked perfectly, and its thrusters worked just as intended.
Satisfied with this run, scientists sent it in again.
On the 11th of August, OSIRIS approached at a distance of 40m, before once again rising.
Slowly, darting forward and backwards but drawing ever closer.
And on the 20th of October, this heron lookalike went into strike.
Using onboard computer systems to plan its trajectory and perform its operations, as
the lag time for signals from Earth rendered real-time remote control of the spacecraft
impossible, OSIRIS slowly reached out to touch the surface of the asteroid.
It raised its solar panels in a Y shape to keep them out of the way and protected them
from any loose dust that was about to go flying.
It reached out its arm to land.
It touched the surface.
And then, surprisingly, it kept going.
When I think of the surface of asteroids and comets, I imagine them as firm and hard enough
to stand on, as you might see in films such as Armageddon.
But the truth is that – particularly for small asteroids like Bennu – their low gravity
means that their mass is only loosely held together.
What looks like a stable surface is actually loosely held material interspersed with pockets
of nothingness – gaps that mean Bennu is so easy to sink into, if you stepped on it,
you’d likely vanish inside of it.
Like stepping into foam, OSIRIS’ arm sank almost half a metre into the surface of Bennu,
before the thrusters arrested its fall.
Furiously activating its nitrogen gas burst, OSIRIS blasted a hole in the side of the asteroid.
What was expected to kick up a little regolith dust displaced over 6 tons of loose rock,
as all that material, so loosely held under Bennu’s tiny gravity, was catapulted in
all directions.
OSIRIS’ thrusters went into overdrive, burning the most they had ever burned on this trip
to pull them away from the expanding cloud of dust.
Of course, this was all pre-programmed.
NASA had planned for this, although perhaps not to this extreme.
OSIRIS had just made a new massive crater in Bennu.
Upon getting away from the asteroid’s surface, OSIRIS checked its haul.
It had hoped for 60g of material.
Instead, it found it had collected somewhere between 400g and over 1kg of material.
So much material had been collected that a rock got stuck in the flap that closed the
sampler’s head, causing some small amount of the material to get lost.
However, OSIRIS succeeded in transferring the sampler head to a sealed container built
into it.
Its mission had been a huge success.
For a time, OSIRIS stayed watching, flying over the asteroid to image it and see the
outcome of what it had done.
The surface of Bennu was different now.
Even larger materials had moved far.
Just look how far this boulder was thrown – around 12 metres across Bennu’s surface:
However, OSIRIS couldn’t linger forever.
Its sample needed to get back to Earth.
And so, after performing one last flyby on April 7th 2021, it began a long journey home.
And there we have it.
Right now, as of the release of this video, OSIRIS is making its way back through the
void of space.
It is thought that it will arrive back around Earth by September 2023.
Once it reaches Earth’s orbit, it will release its sample, which will fall through our atmosphere
to land in the Utah desert, where it will be collected and examined.
This will mark the culmination of an incredible scientific exercise – collecting a sample
of an asteroid and returning it back in pristine condition to Earth.
But as for OSIRIS itself, this mission was only the beginning.
Having captured a sample of Bennu, this Egyptian heron will be off to slay a new beast – Apophis,
the Egyptian serpent of Chaos, destroyer of light and order.
Or, more accurately, the Apophis asteroid, a 370m diameter asteroid that is believed
to be passing within 50,000km of Earth in 2029.
Let’s hope that the asteroid does not behave in a way that’s too in keeping with its
Egyptian namesake.
Either way, OSIRIS will be heading off to investigate it, once again using its thrusters
to kick up material from the asteroid’s surface for analysis.
To reflect this extension of its mission, OSIRIS has been given a new name.
No longer is it OSIRIS REx.
It is now OSIRIS-APophis EXplorer.
Or, OSIRIS-APEX.
With a name like that, perhaps its greatest discoveries are yet to come.
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