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Comment by est31 | Hacker News Reader

I've heard that radio astronomy was greatly supported by the military because it can be used to monitor for incoming ICBMs. I guess if Arecibo were still used for such purposes, they wouldn't have let it fall into disrepair. How is such monitoring conducted in this day and age?

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Tracking is mostly done space-based. Because of political issues. In our moon shots, we frequently had 'blackouts' in tracking the Apollo capsules because it was over unfriendly or uncooperative nations who wouldn't share their ground station data.

It was my wife's school friend Marjory Bacon that headed that project. (She was also the person who turned on the Hubble telescope and initiated the first self-diagnostic)

It went thru the journalist filter a million times until unrecognizable.

The first answer is ICBMs are, now a days, mostly an IR optical thing.

The flightpath of most Russia to US ICBMs would never have passed over the observatory, obviously, but it was a major radar technology research site in the early years.

>The first answer is ICBMs are, now a days, mostly an IR optical thing.

Source? This seems unlikely since ground-based visual tracking would be obscured by e.g. clouds. Is ICBM tracking all space-based and visual? (I can see why IR is useful to detect the initial boost, but it would also seem rather useless to track a small, passive object in freefall).

DSP and more recently SBIRS are satellite-based systems, both use IR. If you track the missile to the end of it's burn you'll have a pretty good idea of where it's going after that -- ballistic trajectories are fairly predictable.

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

https://www.lockheedmartin.com/content/dam/lockheed-martin/s...

> small, passive object in freefall

That's probably anomalously hot as it plows back through the atmosphere.

I think radar stealth coatings are sufficiently advanced that they swung the balance back in favor of IR for tracking purposes.

Historically: it was hard / unknown how to make a flying object that resisted reflecting radar

Current: it's hard / unknown how to make a flying object that's resistant to radiating heat

>as it plows back through the atmosphere.

I would assume that if you're tracking it on re-entry that's a little too late.

Depends on what you want to do.

Launch - early warning, max time, low certainty of trajectory

Boost - trajectory and impact refinement

Re-entry - terminal maneuvering, impact prediction, terminal interceptor cueing

I'd imagine you'd have ground-based sensors for terminal interception refinement, but having a wide angle view to cover highly maneuverable glide vehicles would be useful too.

> Current: it's hard / unknown how to make a flying object that's resistant to radiating heat

There are techniques to reduce infrared signature, though:

https://en.wikipedia.org/wiki/Northrop_Grumman_B-2_Spirit#In...

They're mostly talking about the initial launch detection which is indeed IR based because it's going to be nearly impossible to hide the signature of a rocket launch of any useful size. After that flight tracking is a lot of different sources like radar.

The IR systems are space-based and look for rocket plumes in the ascent phase. Once a rocket gets above the troposphere there's no clouds to obscure it. It's an intensely bright IR source that easily stands out from the -60°C surrounding atmosphere.

Just a second of observing the plume's trajectory will tell the difference between and orbital or ballistic one. Ballistic missiles will also be launched over the poles to travel the shortest distance to their targets. So IR plumes over the North Pole either means Santa's upped his present delivery game or there's an incoming ballistic missile attack.

Hmm ok I watched the video someone else linked here, apparently the telescope was used to check for facilities in Russia by checking the reflection of radio waves on the moon. Pretty cool stuff.

The anecdote I heard was that it was because there was no good way of getting data on the USSR over-the-horizon radars - other than the times where their signals would reflect off the Moon and could be analyzed by the US, given good enough antennas.

That doesn't make any sense. OTH radars work by skipping off the atmosphere back to Earth (shortwave style). If the signals escaped to the Moon and back, it wouldn't work OTH.

The troposphere isn't 100% reflective. You're going to have loss, and some of that loss is going to bounce off of the moon.

A week or two ago when HN was discussing the second cable snapping, someone posted this origins history of silicon valley video: https://youtu.be/ZTC_RxWN_xo?t=2983 (the entire thing is about electronics and radar development from the atomic age through the cold war, interesting to watch whole but the timestamp link is the relevant radio telescope part).

Short story: before our nuke strategy shifted to basically all ICBM's and submarines, the plan was to nuke the soviets with fleets of long range bombers. To get the bombers in, though, you needed to know where the air defense radars were. The U2's were good at flying too stealthily and too high for missiles, but not great... one or two did go down creating some, er, light international incidents. So eventually someone figured out big air defense radars bounce off the moon when the moon is overhead, and suddenly there was a lot of CIA money funding radio telescopes like Arecibo and the Stanford dish.

Not directly ICBM related, but the Arecibo telescope detected an ionospheric anomaly in the 1979 Vela incident [0], believed to be an undeclared nuclear weapons test.

[0] https://en.wikipedia.org/wiki/Vela_incident

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