Shooting Gallery — The universe didn’t like dinosaurs, either

Shooting Gallery

The universe didn’t like dinosaurs, either

Bryan Zepp Jamieson

Oct 26th 2022

OK: Don’t look up. There. I said it.

According to Watchers, a truly excellent site that chronicles natural events, “A newly-discovered asteroid designated 2022 UR4 flew past Earth at a distance of 0.04 LD / 0.00011 AU (17 043 km / 10 591 miles) from the center of our planet at 22:45 UTC on October 20, 2022.” The asteroid is 32 feet long and 14 feet wide. So it passed about 6,500 miles above the surface of the Earth.

That doesn’t sound very close by Earthly standards. It’s a hair more than the distance between Los Angeles and Warsaw, Poland. But by astronomical standards, it’s a gnat’s whisker. The Moon orbits some 25 times further away. The illustration below shows Earth, with a white circle representing the Moon’s orbit, and the trajectory of 2022 UR4 shown as a green line. A close look shows that the flyby was so close the Earth’s gravity affected the course of the asteroid. If the amount of the perturbation is known, then you can calculate the speed the object flew by at (orbital velocity at that altitude is about 5 km/sec). It was going pretty fast. Had it scored a direct hit, it would have caused local, but significant damage. (It was part of a group of asteroids known as the Apollo group, of about 25,000 asteroids. One of them hit near Chelyabinsk, Russia in 2013, injuring 1,500 people and shattering millions of windows).

The Apollo group is the best-known group of asteroids since their orbit intersects ours on a regular basis. There are over 10,000 of them that we know about, and 1,648 are classified as Potentially Hazardous Asteroids, which loosely means “could possibly hit us at some point or other.” A much smaller group, 17, are on the Sentry Risk Table, which means a measurable chance of hitting Earth in the next one hundred years. Most of those are roughly the same size as 2022 UR4. The rest are just cosmic hemorrhoids; annoying at worst, usually not a serious concern.

According to Watchers, “[2022 UR4] is the 91st known asteroid to fly past Earth within 1 lunar distance and the 3rd closest since the start of the year…It is also the 13th closest flyby on record (since the year 1900).”

Of course, direct hits are common. “Shooting stars” are nightly occurrences, and fireballs bright enough to cast shadows pretty much a daily occurrence at some point or another around the globe. Even the Webb telescope had one of its mirrors holed by a grain of dust a few days after it was unfurled. Not enough to affect the operation of the scope, but a reminder that space isn’t empty. (Granted, it’s in a LaGrange point, a gravitational nexus that tends to attract debris)

Our mapping of potential celestial risks is expanding rapidly, but still has a long way to go. “2022 UR4 was first observed at ATLAS-MLO, Mauna Loa, Hawai’i on October 20.” some 24 hours before it flew by. We didn’t spot the one that hit Chelyabinsk at all. Astronomers think that if we’re lucky, we know about perhaps 10% of the potentially hazardous objects; space is big, and many objects have dark, non-reflective surfaces, making them extremely hard to spot.

A few weeks ago, NASA’s Double Asteroid Redirection Test (DART) impacted an asteroid, Dimorphos, and the kinetic energy changed the object’s orbit. It was basically a feasibility test, and showed that we do have at least some limited ability to change the course of a rock aimed at our heads.

This week’s close call shows the need to discover and avert such threats. Granted, as immediate threats to humanity, it isn’t in the same league as climate change, ecological collapse, or the ever-present threat of war, all of which will deeply affect humanity in the next 100 years. There are some existential threats we can’t do anything to avoid: megavolcanoes, a truly huge solar flare (and the sun popped off an X3 flare, big enough to seriously screw things up here, but fortunately pointed away from us, just six weeks ago) or gamma burst from a supernova. “Dinosaur killers” are on that list, but we can address that particular threat with existing technology. We don’t even need Bruce Willis.

But the threat is real, and we need to continue to prepare. The universe doesn’t consider us important, won’t even notice if we’re gone. So it’s up to us.

It would be ghoulishly ironic if we somehow managed to save ourselves from our selves, only to be plastered by an avoidable cosmic hemorrhoid.

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