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Comment by andrewl | Hacker News Reader

A similar event in or near a populated area today might be interpreted as a nuclear attack.

By lay-observers, probably. But probably not by the systems actually designed to detect nuclear explosions, because those systems can distinguish nuclear explosions from other sorts of very large explosions by looking for characteristic emissions signatures, like x-ray emissions and the double flash: https://en.wikipedia.org/wiki/Bhangmeter

pretty sure the main focus is on detecting launches instead of explosions (detecting explosions doesn't help very much with holding up the end of the MAD bargain, so to speak)

https://en.wikipedia.org/wiki/Space-Based_Infrared_System

They definitely detect the explosions themselves, not just missile launches (remember that ballistic missiles are only one method of several for delivering nuclear weapons). This is for monitoring compliance with nuclear test bans, as well as just generally knowing what is going on. If a nuclear weapon goes off for any reason, the US government and other nuclear powers obviously want to know about it. These systems would also be able to distinguish non-nuclear explosions such as bolides from nuclear explosions, which is important for MAD if you don't want an accidental war. The systems to do so are now built into GPS satellites.

https://en.wikipedia.org/wiki/Vela_(satellite)

https://en.wikipedia.org/wiki/Nuclear_MASINT#Space-based_Nuc...

https://en.wikipedia.org/wiki/Global_Positioning_System#Mili...

The entire world would have detected the meteor that large and been warned

https://en.wikipedia.org/wiki/Chelyabinsk_meteor

> The Chelyabinsk meteor was a superbolide that entered Earth's atmosphere over the southern Ural region in Russia on 15 February 2013 at about 09:20 YEKT (03:20 UTC). It was caused by an approximately 18 m (59 ft) diameter, 9,100-tonne (10,000-short-ton) near-Earth asteroid that entered the atmosphere at a shallow 18.3 ± 0.4 degree angle with a speed relative to Earth of 19.16 ± 0.15 kilometres per second (69,000 km/h; 42,690 mph).

> [...] The asteroid had a total kinetic energy before atmospheric impact equivalent to the blast yield of 400–500 kilotons of TNT (about 1.4–1.8 PJ), estimated from infrasound and seismic measurements. This was 26 to 33 times as much energy as that released from the atomic bomb detonated at Hiroshima.

If it had been at a slightly sharper angle, or slightly larger, or moving slightly faster, and if it had hit the right location, it could have destroyed a city. Fortunately it disintegrated in the upper atmosphere, so it only blew out windows across a large area as it burned brighter than the sun across the sky.

There was no warning. The Tunguska rock, if it was a rock, was only a few times wider. I don't think we can reliably detect those, either.

Do you actually need to have detected it as it was coming in to be able to say 15 minutes later "oh that was a meteor" detection and forensics are different... it's just really if the forensics happen fast enough to avoid a counter strike. Eg if some nuclear capable countries capitol just evaporated with no nuke early warning systems going off and no one could look up and see the massive trail coming in and had not streamed it on the internet first.

There is space surveillance network tracks things in orbit. I don't think that covers all of the sky and could miss a rock.

There are early warning satellites that look down for ballistic missiles. I don't know if the infrared detectors would see the rock. They are probably best as negative signal with no rocket being detected.

The main signal that wasn't nuclear weapon would be the size and location. Nuclear weapons don't explode as high as Chelyabink meteor. Nuclear weapons aren't as big as Tunguska event. Also, hitting a city is unlikely, hitting one dead on is really unlikely. There would also no nuclear fallout detected.

Space-based nuclear weapons surveillance systems monitor for X-ray burst and the characteristic double-flash pattern. Neither would be present in the case of a meteor, which would rule out a nuclear detonation immediately. Infrared systems likely would detect the meteor entering but the projected flight path would be, well, unreasonable for a missile.

Detecting incoming asteroids, especially small ones, is really hard. The recent 1-meter meteor that exploded over Europe, was only the seventh instance of a successful detection.

https://www.telegraph.co.uk/science/2023/02/13/asteroid-expl...

But it is the big ones, we are interested in, because they can do a lot of damage and not only fireworks.

Tunguska was a lot bigger than a metre across. Even the Chelyabinsk meteorite was a lot bigger, 18 m diameter, 9 000 tonne. So if we cn detect 1 m meteors I think we should be able to detect Tunguska scale objects.

We may notice some small things, but that doesn't mean that we've found all large things.

According to https://www.nasa.gov/planetarydefense/faq we only know about 40% of the rocks sized 140 meters+ that cross Earth's orbit. If one of the 60% we don't know about came at us from the direction of the Sun, our odds of seeing it before impact are fairly small.

The Wikipedia page for the Tunguska event says it was “a stony asteroid about 50–60 metres (160–200 feet) in size.” How far out would something that size be detected? This is very much out of my area.

The other mentioned much smaller one has a whole article on it https://en.wikipedia.org/wiki/2023_CX1

This one was 1 m wide and it seems like it was detected about 6 hours before coming down.

But, as someone mentioned in another comment, the fact that we can detect a 1 m objects doesn't mean that we can detect all of them, far from that. Same for larger ones, we just have more chances of detecting those than 1 m-sized ones.

No, we routinely get hit by rocks we didn’t see coming.

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