Yesterday, pieces of asteroid impacted Earth… on purpose

September 25, 2023   Issue #621

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Space news

Space is big. That’s why we call it “space”

Yesterday (September 24, 2023), at 08:52 Mountain time, and after a trip of nearly 2 billion kilometers in total, the OSIRIS-REx Sample Return Capsule landed in the Utah desert, carrying its precious cargo of pristine samples of the asteroid Bennu.

OSIRIS-REx — which stands for Origins, Spectral Interpretation, Resource Identification, Security, Regolith Explorer — launched to visit the small 500-meter-wide asteroid Bennu on September 8, 2016. It arrived at Bennu on December 31, 2018, and began its long mission of imaging, mapping, and, most importantly for today’s news, getting a sample of rocks from its surface.

On October 20, 2020, OSIRIS-REx descended to the surface of Bennu and blasted it with cold nitrogen gas. This dislodged the surface material that was then collected in the Touch-and-Go Sample Acquisition Mechanism, or TAGSAM, a device about the size of a snare drum designed to stow the samples. Originally, the plan was to get 60 grams of asteroid rocks, but from later analysis engineers think they collected more than 200.

There’s even video of that!

And here’s another view:

In April 2021 the spacecraft left Bennu, and began its journey back to Earth… more or less. Just hours before touchdown, while it was still some 100,000 km from Earth, OSIRIS-REx released the Sample Return Capsule (or SRC), sending it on a trajectory toward Utah. The spacecraft itself performed a thruster burn to move away so that it would not impact Earth (more on this in a sec…).

Coming in over the Pacific Ocean, the SRC used Earth’s atmosphere to slow itself from orbital speeds down to where a drogue parachute could deploy to stabilize it, then a main parachute to slow it to just about 18 kilometers per hour (the NASA live stream is archived, and you can watch it; I have that link cued to start right as the SRC entered Earth’s atmosphere). It then descended into the Utah desert where it landed safely.

After a few minutes, mission personnel checked it to make sure it was safe, and then it was collected and put aboard a mobile “clean room” to prevent contamination with earthly particles. It will then be taken to NASA's Johnson Space Center in Houston, Texas, where it will be safely stored and scientists can analyze them. Folks there have a pretty good history of preserving samples from space.

And that, THAT, is the whole point. Bennu is a weird little rock. It’s what’s called a rubble pile asteroid, meaning that it’s basically a pile of rocks held together by their own gravity. Astronomers think almost all small asteroids are like that, shattered in place by collisions with other asteroids over the eons. We don’t understand these asteroids as well as we’d like, so having samples to study in a lab is absolutely critical.

Why? Bennu is also what’s called a potentially hazardous asteroid, or PHA, meaning it’s larger than 140 meters across and its orbit takes it within 7.5 million kilometers of Earth. Over time, asteroids like this could impact our planet and do significant damage (don’t worry about Bennu, though; it’s safe from impact for at least 1.5 centuries). If we want to prevent these asteroids from hitting us we must understand their composition and structure.

… and the spacecraft’s not done. OSIRIS-REx as a mission is over, but it’s been reborn as OSIRIS-APex, for Origins, Spectral Interpretation, Resource Identification, Security, Apophis Explorer. Apophis is another asteroid that is also on a potentially hazardous orbit; discovered in 2014 it was found to be coming extremely close to Earth in 2029 and for a time it was though it might hit us in 2036 or possibly 2068. Both impact times are firmly ruled out now, but this has already been a very well-studied asteroid, so redirecting OSIRIS-Apex to orbit it and study it up close is a chance to tie together all those past observations with ones made in situ.

But more than preventing an impact, the scientific value of the Bennu samples is priceless. Asteroids like Bennu have been essentially unaltered chemically since the solar system itself formed 4.6 billion years ago. Obtainining samples of means being able to directly investigate the history of the solar system, to see what conditions were like when the planets themselves were first forming. Also, we know that Earth was pummeled by rocks like Bennu for much of its existence. While astronomers are still trying to figure out the details of this, it’s likely that asteroids brought a significant amount of water to Earth over this time, as well as carbon and simple molecules that were the precursors for life.

In other words, we may exist because asteroids like Bennu impacted Earth billions of years ago.

So yes, this is important work. For millennia, the biggest questions we could ask were the purview of philosophy and religion. How did this all begin? How did we get here? How did life on Earth begin?

With science — like what we’ll learn from the OSIRIS-APex Sample Return Capsule with its hold full of space rocks — we’re going to get those answers.

Et alia

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