Near-Earth Asteroid news wrapup

March 7, 2023 Issue #535

The edge-on disk galaxy NGC 5866. Credit: NASA, ESA, and The Hubble Heritage Team (STScI/AURA); W. Keel (University of Alabama, Tuscaloosa)

Astro Tidbit

A brief synopsis of some interesting astronomy/science news

It’s an all-asteroid issue today!

First, something brief: The European Space Agency Near-Earth Object newsletter just came out, and had an interesting note about a newly discovered asteroid called 2023 DW. Its size is unknown, but based on its brightness (bigger objects reflect more light) it’s likely around 50 meters across. About the size of the Tunguska impactor.

It’s on a decently elliptical orbit that takes it from inside the orbit of Venus to just past Earth’s. And here’s where things get a little tricky: According to the currently calculated orbit, it gets pretty close to Earth, only about 75,000 km away. That’s where I start to use adverbs like “pants-wettingly” close. In fact it currently has a 1 in 700 chance of impacting Earth in 2046.

The orbit of asteroid 2023 DW (white) takes it pretty close to Earth (blue; positions are shown as of March 2023) and a future impact can’t be ruled out yet, though is incredibly unlikely. Note that the orbit is slightly tipped to Earth’s, but that perspective is flattened here. Credit: NASA/JPL-Caltech

But Don’t Panic! Here’s the good news: Note that I bolded the word “currently” above. It’s only been observed for a few days so the orbit isn’t well known at all. As I’ve written about many times before, the orbit of an asteroid is calculated based on its measured positions, and you need a lot of them over a good length of time before you can make serious claims about the orbit. Right now the “observing arc” is only 6 days, which isn’t long enough to get good data. The JPL Small-Body Database rates the uncertainty of the orbit as 7, where 0 is a rock solid determination and 9 means the orbit isn’t really known well at all. So we don’t know this thing’s orbit very well.

In circumstances like that it’s very, very likely that further observations will nail down the orbit and show that it will move well outside the position of Earth in 2046. This happens all the time, which is why you probably haven’t heard much about this rock. Asteroid hunters know the score, and they’re not worried. You shouldn’t be either.

I’m writing this just in case the usual scummy doomsday conspiracy vultures start making videos about this asteroid, claiming it will hit Earth blah blah blah, just like they do with every single near-Earth asteroid. Weird how they never make a video saying, “Gee I was wrong about the last one I was trying to scare you about” probably because they’d have to make that video hundreds of times, because they’re always wrong and perpetually scummy about it.

I have opinions about people like that.

Anyway, the irony here is that these sorts of events show just how good we are at finding and tracking asteroids, and how unlikely a big impact really is. Overall it’s a circumstance I think we should be concerned with, but not one we should panic about.

So, speaking of which…

Astronomy News

It’s a big Universe. Here’s a thing about it.

The near-Earth asteroid 2011 AG5 is an elongated potato of a space rock that was thought to be about 140 meters across. That’s an interesting size; it’s the lower limit for the size of an asteroid that NASA is worried about should one hit. At that size the impact would generate a blast of about 100 megatons (twice that of the Tsar Bomba, the largest nuke ever tested) and that would have regional (as opposed to just local) effects. For comparison, the 2013 Chelyabinsk impact was half a megaton and did some light damage to buildings (mostly shattered windows from the shock wave). An explosion 200 times bigger would be bad.

The good news is, while it gets close to Earth, it’s not considered an impact risk. Technically it can get to within about 60,000 kilometers of Earth, which is pretty close, but such a near encounter is unlikely.

The position of asteroid 2011 AG5 on February 3, 2023, when it passed Earth. The ellipse shows the orbit of the Moon. This is seen from an angle, so perspective makes it look closer than it was; it passed by about five times the distance to the Moon from Earth. Credit: NASA / JPL-Caltech

But it does get close, and on February 3, 2023 it passed about 1.8 million km from Earth, close enough that astronomers could ping it with radar using the Goldstone radio antenna in California. The reflected radar waves can reveal the shape of the asteroid (my friend Emily Lakdawalla has a terrific article describing how this works, too), and in this case revealed that AG5 is really elongated:

Radar images of the asteroid 2011 AG5 show it to be highly elongated. Note its rotation over time. Credit: NASA/JPL-Caltech

Like I said: Space potato. It’s one of the most elongated asteroids ever observed using radar, too. This part is interesting: the press release says AG5 is about 500 meters long and 150 meters wide, which is much larger than previous estimates. I’d tend to favor these new numbers; radar is extremely accurate in getting things like an asteroid’s size, whereas previous estimates would’ve used its distance and brightness to determine the size. To do that you have to assume a reflectivity for it; a shiny asteroid can be smaller or a dark one bigger at a given brightness. But knowing that reflectivity (called the albedo) is tough, and the uncertainty in it can change the size estimated by a lot.

So the radar is likely more accurate, making AG5 way bigger than first thought. Good thing it won’t hit us! I’ll note that in 2040 it passes us again at 1.1 million km, and we’ll get an even better look at it then.