This video is sponsored by Magellan TV.
Welcome back to Launch Pad, I'm Christian Ready your friendly neighborhood astronomer
and it's an amazing time here on planet Earth because NASA's OSIRIS-REx spacecraft is bringing
home a can of asteroid! There's probably some other stuff going on but this one has really
held my attention, I can tell you.
Anyway, OSIRIS-Rex stands for Origins, Spectral Interpretation, Resource Identification, Security,
Regolith Explorer. It's a mouthful, but the name symbolizes what the mission is all about.
Asteroids are the leftover rubble from the formation of the solar system. They've played
a role in the formation of our home planet while at the same time playing a role in the
extinction of up to 99% of the species ever lived on Earth.
To date, we know of roughly a million asteroids, most of which are concentrated in a belt between
the orbits of Mars and Jupiter. But despite their reputation as being giant hunks of planet-killing
space debris, many asteroids remain largely undisturbed since they formed over 4.5 billion
years ago.
So, having a sample of an asteroid gives us a look into our solar system's past.
And, on October 20th, OSIRIS-Rex went and grabbed a huge sample of the asteroid Bennu
and will be bringing it home for us to study in unprecedented detail in 2023.
But doing so wasn't as easy as first thought. Instead of the soft, sandy surface that the
science team had expected, Bennu turned out to be a world with large rocks and boulders
strewn everywhere. This required a lot of clever modifications to the mission.
So, while we're waiting for OSIRIS-Rex to return, I thought it would be fun to talk
a little bit about this mission and how a team of insanely talented scientists and engineers
came up with the largest celestial smash and grab since the Apollo program.
But first, I'd like to thank Magellan TV, who are very kindly sponsoring today's video.
Magellan is a new documentary streaming service featuring content on History, Art, Nature,
and of course, Space and Science.
I especially enjoyed "Day of the Asteroid", an episode in Magellan's Cosmic Journeys series.
Even though I'm not losing sleep over an imminent asteroid impact, this episode dives into the
risks asteroids pose to Earth, and the methods scientists are developing to deflect a potential
impactor.
New programs are added weekly and can be watched on Roku, AmazonFireTV, AppleTV, Google Play,
and iOS.
Best of all, Magellan TV has a special offer just for my viewers. By signing up at the
link below, you’ll get an extra month of MagellanTV absolutely FREE. Please make sure
to use the link in the description of this video.
OSIRIS-Rex marks the fourth sample return mission from a comet or asteroid.
The first was in 2004, when NASA's Stardust spacecraft flew through the coma of Comet
Wild 2. It extended what is essentially a "tennis racquet" device filled with a substance
called aerogel. As Stardust flew through the coma, tiny particles of dust were trapped
in the aerogel, and the samples were returned to Earth in 2006.
In 2005, the Japanese Space Agency JAXA sent its Hyabusa spacecraft to the asteroid Itokawa,
which a small rubble pile about 350 meters across. The mission was plagued with problems,
and they even lost communications with the spacecraft at one point.
But after they recovered the spacecraft, a small sample was collected, and its return
capsule landed in Australia in June 2010.
Hyabusa was deemed a "successful failure", in that they saved the spacecraft and managed
to return a sample. But the sample size was much smaller than hoped, and that put a limit
on the amount of science that could be done afterward.
But JAXA learned a great deal from the Hyabusa mission, and in 2018, the improved Hyabusa2
spacecraft arrived at the asteroid Ryugu. Ryugu looks a lot like Bennu. Both asteroids
have a shape that kind of resembles a toy top. However, Ryugu is larger than Bennu and
seems to have much less water.
Anyway, after deploying some rovers to recon the surface, Hyabusa2 collected samples from
two separate regions of the asteroid. It's currently on its way home and will be returning
its samples to Earth in December 2020.
But four years ago, OSIRIS-Rex embarked on its journey to Bennu. Like any piece of real
estate, this particular asteroid was chosen for its location. Bennu's orbit crosses Earth's.
And yes, it qualifies as a potentially hazardous asteroid as well. Although there's no chance
of it hitting Earth anytime soon.
But not only is Bennu close, it's also thought to be relatively primitive and unprocessed.
That makes it rich in carbon, organic compounds, and water-bearing clays - exactly the stuff
astronomers think might have been brought to Earth by asteroids a couple of billion
years ago.
Finally, Bennu was chosen because it was thought to be sandy and therefore easy to sample.
There were several reasons for this assumption. Infrared data from NASA's Spitzer Space Telescope
showed that its surface rapidly heats up when exposed to sunlight and quickly cools at night,
much like beach sand here on Earth.
Additionally, RADAR observations from the Arecibo and Goldstone telescopes suggested
the surface was smooth.
And, it's equatorial ridge was thought to be caused by loose material piling up at the
equator as the asteroid spun.
So, by every remote sensing measurement, Bennu should have been covered in loose, easy-to-grab
regolith.
It wasn't.
When OSIRIS-Rex arrived at Bennu in August 2018, they found it looked an awful lot like
Ryugu, but smaller - about the size of a tall building.
This meant that Bennu would have a much weaker gravitational field than Ryugu. So weak in
fact that solar radiation pressure can push OSIRIS-Rex off course if not corrected. So,
orbiting the asteroid was going to be trickier than thought.
But as OSIRIS-Rex surveyed Bennu, the real challenge of the mission became apparent.
Instead of a sandy beach with a few boulders here and there, Bennu was a pile of spacecraft-killing
boulders with a handful of small sand traps.
However, spectroscopy of the surface revealed the rocks contained water, which makes Bennu
a hydrated asteroid. Despite Bennu's hazardous terrain, OSIRIS-Rex had come to the right
place.
The mission team were caught by another surprise. They noticed small particles less than a centimeter
across were ejecting off the surface. And this was either due to outgassing as it was
warmed by sunlight or by small micrometeorid impacts. At first, there was concern this
could pose a hazard to the spacecraft, but the velocities of the particles were so slow
it soon became clear there was no danger the spacecraft would get obliterated by debris.
But the fact that Bennu was an active asteroid turned out to be a good thing. Some of the
ejected particles settled into a slow, lazy orbit around the asteroid. By tracking the
precise motions of those particles, the team could map out Bennu's gravitational field
to a much higher level of precision than they thought would be possible!
That turned out to be critical for navigating into a much smaller sample area than planned.
The spacecraft used its laser laser altimeter to make a highly detailed map of the surface
down to an elevation of about 5 centimeters.
That allowed the team to identify four possible sample sites.
A small crater called Nightingale turned out to have the greatest abundance of fine-grained
material.
Starting in early 2020, OSIRIS-Rex made a series of low-altitude passes to image Nightingale
at a resolution as low as 3.5 mm per pixel.
By examining Nightingale from multiple phase angles, the team built up a robust 3D model
of the landing site, down to individual pebbles less than a centimeter across.
This would be the site of the Touch And Go maneuver, or TAG.
In this maneuver, the spacecraft’s Touch-And-Go Sample Acquisition Mechanism - or TAGSAM - arm
is extended. Upon contact, TAGSAM fires a canister of nitrogen gas that stirs up regolith
which gets trapped in the outer rim. It's kinda like a reverse vacuum cleaner.
Seems simple enough, but the Nightingale site is much smaller than the mission team bargained
for. Instead of the planned 50 meter TAG zone, Nightingale's sample area is just 8m across.
That's about the size of a few parking spaces.
So, the team had to come up with an all-new method of autonomously navigating to the site,
making sure it was in the right place and collecting a sample without hitting a boulder.
The system was called Natural Feature Tracking.
During approach, NFT compares images from the navigation cameras to a detailed map of
the target area to identify all possible hazards around the site.
When the spacecraft reaches an altitude of 5 meters, the NFT system would predict the
contact point on the surface and compare it to the hazard map.
If it found it was going to come down on an unsafe area, the spacecraft would execute
a wave-off burn.
In order to navigate the very subtle changes in Bennu's gravitational field, the team also
fine-tuned OSIRIS-Rex's maneuvering system to make velocity changes with a precision
less 1 mm/second.
As a result, OSIRIS-Rex would be able to make a safe contact with an area that is 10 times
smaller than it was originally designed for!
At least, that was the plan. To make sure the system would work, the team conducted
two TAG rehearsals, first in April, and then again in August 2020. Each rehearsal brought
the spacecraft closer to Nightingale than it had come before. These rehearsals allowed
the team to test and verify all of the systems necessary for a successful TAG attempt.
Finally, after nearly two years of surveys, modeling, and rehearsals, OSIRIS-Rex attempted
the TAG maneuver on October 20th, 2020.
Starting from its 1-km orbit, TAG began with a propulsive burn to break orbit, and transit
towards the vicinity of the Nightingale site.
At an altitude of about 125 meters above Nightingale, the spacecraft performed the checkpoint burn
to put it on an intercept trajectory with the surface.
About 10 minutes later, 45 meters above Nightingale, thrusters fired again to perform the Matchpoint
burn. This burn zeroed out the spacecraft's horizontal velocity relative to the surface,
thus "matching" the horizontal velocity of Nightingale, while descending vertically.
10 minutes later, the TAGSAM made contact with the surface, the nitrogen canister fired,
and the spacecraft backed away from Nightingale exactly as planned.
And there was much rejoicing.
The next step was to verify how much material was collected before making the decision to
either stow the sample or attempt another TAG maneuver at one of the backup sites.
OSIRIS-REx had two more N2 bottles onboard in case the sample didn't have enough mass.
But measuring mass in space is a little tricky when you don't have a gravity field.
I mean sure, Bennu has gravity, but at about 1-ten-millionth of a g. It's simply too weak
to be of any use.
So the team came up with an ingenious plan to weigh their sample by measuring the change
in the spacecraft's moment of inertia.
Moment of Inertia describes how mass is distributed around a rotating body. So, prior to the TAG
event, the spacecraft extended its TAGSAM arm and rotated to measure its moment of inertia
with an empty.
Then, the same maneuver was going to be performed a second time after collection. The difference
in the moment of inertia after collection would reveal how much mass had been collected.
But things didn't work out that way, for a really good reason.
Once the team got a look at the TAGSAM head, they could clearly see that it was full. In
fact, it was filled so much, it was leaking regolith!
So the verification maneuver was canceled and the sample was stowed in the Sample Return
Capsule.
After a small tug test to verify everything was secure, the arm detached from the sampler
head and the return capsule was closed.
So yay, sample collected! All they gotta go now is return it to Earth. To that end, OSIRIS-Rex
is scheduled to depart Bennu in March 2021 and begin its two-and-a-half year cruise back
to Earth.
On September 24, 2023, the Sample Return Capsule will jettison and re-enter Earth's atmosphere
before landing in Utah. And hopefully, we'll see a TAGSAM head full of asteroid.
Some of the regolith will be shard with NASA's international partners, including JAXA, who
in turn will share some of its Ryugu haul with NASA. Hyabusa2 is going to return its
sample to Earth next month so get your electron microscopes ready.
Now if everything goes well with OSIRIS-Rex's sample return, what then? Well, the spacecraft
will certainly be out of the sample collection business, but if the spacecraft is otherwise
healthy, mission scientists propose placing it into orbit around the asteroid Apophis
in 2029.
Apophis is a 370 meter near-Earth asteroid that's predicted to come as close as the ring
of geostationary satellites around Earth.
After Apophis' flyby, OSIRIS-Rex will be in a position to rendezvous with the asteroid.
Sending OSIRIS-Rex on an extended mission to Apophis would be much less expensive than
launching a new spacecraft.
You know, 2020 has been a really, awful year in many respects, but it's nice to see something
like this "just worked" after some very clever people figured out how to pull it off. It's
amazing what we can accomplish when we work together.
My thanks as always to my Patreon supporters for keeping Launch Pad Astronomy going, and
I'd like to welcome Andrew Waldie my newest patron, as well a thank you to Anna for her
Intergalactic level support and to Michael Dowling, Steven J Morgan, and Morrison Waud
for their Cosmological level support. If you'd like to support Launch Pad for the price of
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for a bit, as much as we can - stay healthy, and stay curious, my friends.