BAN #410: A tale of two asteroids: One that missed us and one that… didn’t

[Hubble image of NGC 3603. Credit: NASA, ESA, R. O'Connell (UVa), F. Paresce (NIA, Bologna, Italy), E. Young (USRA/Ames Research Center), the WFC3 Science Oversight Committee, and the Hubble Heritage Team (STScI/AURA)]

Astro Tidbit

A brief synopsis of some interesting astronomy/science news

[As always, when I post on topics like this, and at the risk of spoilers, let me preface this by saying DON’T PANIC. Everything here works out fine for us and our planet. But that’s why we’re watching the skies in the first place.]

[Artwork of an asteroid passing near Earth, with an actual image of the asteroid Mathilde from the NEAR spacecraft and Earth from Rosetta. Photo: NASA/JPL, ESA / MPS for OSIRIS Team MPS/UPD/LAM/IAA/RSSD/INTA/UPM/DASP/IDA, Phil Plait]

I have a story of a pair of asteroids for you today; the two rocks are unrelated except they both got very near Earth. One, ah, closer than the other. I suppose they’re also related in what they show about how we detect incoming asteroids.

I’m sure you want to hear first about the one that hit us.

The ESA Near-Earth Objects Coordination Centre has the goods on that one. On March 11, 2022, a small rock was detected by astronomer Krisztián Sárneczky

using a 60-centimeter telescope in Hungary. The observer took four images in a row to confirm the object, then at 19:38 UTC reported it to the Minor Planet Center at the Center for Astrophysics on the Harvard campus (note the times here; they’re important). The positions of the asteroid in the separate images can be used to do a fairly complicated calculation that generates the shape of the object’s orbit. That can then be extrapolated to see if it a) intersects Earth’s orbit, and 2) will do so when the Earth is at that spot as well. If so, bang!

A prediction based on four consecutive images is pretty rough, and the orbit determination uncertain. I liken it to baseball:

The farther into the future you extrapolate the orbit the fuzzier it gets. For a given moment of time the future position expands like a cone in front of the asteroid; it could be anywhere in that cone. If that cone includes the Earth then there could be an impact, and the chance is roughly equal to the area of the Earth divided by the area of the cone at that position (as always it’s more complicated than this, but close enough). In this case, after four images the chance of impact was 1%; the Earth’s area was 1% of the area of the possible locations of the asteroid at that time.

In the meantime Sárneczky took more images and reported those at 20:16 UTC, extending the baseline by 40 minutes. That’s usually not nearly enough for a rock that’s millions of kilometers away, but this one was not that far away. It was less than 100,000 km from us. At 20:25 UTC the ESA automated system issued an alert to astronomers with — get this — a 100% chance of impact.

And the impact was predicted to occur in less than an hour! More observatories took images, and the impact time was calculated so closely they could predict where it would hit: north of Ireland.

Sure enough, just two hours after it was discovered, the asteroid now named 2022 EB5 impacted our planet, burning up high in our atmosphere. It was only the fifth such rock to be seen before an actual impact.

The good news is that it was small, no more than 4 meters wide and likely much less, so it burned up harmlessly dozens of kilometers above the ground.

But that’s part of the whole thing: The rock was small, so it was faint, too faint to detect until it was right on top of us. Most rocks this small are actually detected after they pass us! The bigger the asteroid, the more sunlight it reflects, and the easier it is to see when it’s farther away.

For comparison, the Chelyabinsk asteroid that exploded over Russia in 2013 was 19 meters across; hundreds of times the mass of this wee one. So the energy of the impact for 2022 EB5 was comparably smaller. In fact there haven’t been any reliable direct eyewitnesses to it burning up, though I saw some reports on Twitter of people seeing a glow in the sky around the right time. If it were 3 meters across it would’ve had a mass of roughly 30 tons, and if it was moving at 20 kilometers per second (typical for an impact) then the energy released as it burned up would be about 1 kiloton of TNT; big, but not huge. The only real danger from it was debris from the rock raining down over the Norwegian Sea.

So, cool. An impact, but not a dangerous one.

Intermission

Need a quick break from asteroid stuff? Here’s our new puppy Daisy sleeping gracefully in her bed.

[Daisy. Credit: Phil Plait]

I’ll probably post this on my various social media, but figured y’all deserve to see it first.

OK. Moving on.

Which brings me to the other asteroid, called 2022 AE1. It was discovered in January of this year, and is significantly bigger than EB5: Likely around 70 meters wide. Big enough that an impact is dangerous.

And here’s the fun part: At first, right after discovery, it looked like it might impact the Earth on July 4, 2023, with the odds of impact bring about 1 in 2,500. That’s worrisome! I mean, I wouldn’t bet good money on such long odds, but given the stakes here it was worth worrying over.

And then it got worse. Further observations appeared to increase the odds of it hitting. Why?

Remember that probability cone I talked about earlier? At first that cone is very wide, and the Earth occupies very little of its real estate. But then more observations can tighten up the cone, narrowing its opening angle. If the Earth is still in the cone, though, the ratio of its area to that of the cone goes up. So the probability of an impact increases.

Observations had to wait a week for the bright Moon, which washed out the sky near the asteroid. But then, after mid-January, more observations increased the baseline enough that the updated orbit was more certain, and boom. The odds of an impact dropped to essentially zero. The probability cone no longer had the Earth in it, and we can be sure that on July 4, 2023 the rock will slide past the Earth safely, likely missing us by ten million kilometers, over 20 times farther away than the Moon.

Phew.

But this shows us that we need more eyes on the sky. It would be nice to find even small, harmless asteroids a bit longer than two hours before they hit us — getting an alert out to the affected population telling them not to panic would be good — and it’s critical we see the bigger ones even farther out. 70 meters is maybe small enough that simply impacting it with a space probe would break it apart, and if done early enough the debris cloud would expand and most would miss Earth.

Also, we need space-based telescopes to look as well so the Moon isn’t a problem. Happily, NASA is well on its way to building one.

Asteroids and comets are a real threat, though on a day-to-day basis the risk is small. But over a century, even decades, the threat is enough to take seriously. Finding them is step one, and step two is doing something about them. We’re doing fairly well with the former, and getting better every day. As for the latter, well, we’re working on that as well. Let’s hope we get very good at all this before a big asteroid with our number on it is headed our way.

Et alia

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