Hacker News Reader

Comment by AlphaGeekZulu | Hacker News Reader

The two satellites involved are:

OPS 6182 (10820, 1978-042A): https://www.n2yo.com/satellite/?s=10820

SL-8 R/B (12443, 1981-041B): https://www.n2yo.com/satellite/?s=12443

They are on head-on course. With a probability of about 80 percent they will miss each other. With a probability of more than 20 percent they will impact at a speed of more than 50000 km/h in an altitude of 780 km.

The two objects are not important (an old Russian rocket piece and an old US weather satellite).

But upon impact, the objects will create a large cloud of debris, that might collide with more objects, in the worst case in a chain reaction.

The danger is not falling debris, but that they will produce more damage in their orbit, maybe hitting more essential space equipment in the future.

No need to hide in a bunker.

View on HN

Did they miss? Both those links show them still there, but I suspect they are calculated not real time

They missed

Can somebody give a layman explanation why the debris would stay in the same orbit? I would expect a lot of energy to go into destroying both objects, especially in a head-on collision.

Put quite simply - the energy doesn't go into destroying objects. There's only a very small amount going into heating things up from mechanical deformation [and a bit into spinning], but fundamentally, splitting something in a million pieces just means you have a million pieces that have the same total kinetic energy in orbit.

However, they also exchange kinetic energy & impulse with each other, meaning you get a mix of slower and faster pieces. This is what the Gabbard diagram shows:

https://twitter.com/EU_SST/status/1380443036862316545

Anything that is sped up in the collision (i.e. "steals" energy from another piece) has the same perigee (orbit low point) but a higher apogee (orbit high point.) The reverse happens to anything slowed down / stolen from. That's why the graph looks like open scissors.

Some pieces with lower perigee will friction against earth atmosphere and burn up, but the ones with higher apogee become a serious problem...

(also, for anyone who can't remember to distinguish apogee & perigee: think of your periphery, it's the things close to you :D)

My mental crutch was always Apogee Software. It's more intuitive that someone calls their company "High Point Software" than "Low Point Software".

I think of "peripheral" as being "far from the center of my vision".

Fortunately, years of Kerbal Space Program means the apo/peri prefixes are pretty solidly ingrained.

Ah crap :/

…well I guess the "center of vision" is the center of the orbit ellipse… ;D

(just trying to salvage the mnemonic aid for myself)

My mental crutch is "apotheosis" (the high point of something)

A lot of the debris won't stay in the same orbit. It'll be like a mini-explosion, spraying out debris in every direction, but then add the existing velocity to that. So yes, the debris will be in another orbit. The question is, how much of a change occurs.

In the twitter thread, there is a Gabbard diagram that shows a prediction of the orbits that the pieces will end up in. The satellites currently have a reasonably circular orbit, with apogee (highest point) and perigee (lowest point) about 800km. After the collision, the apogee is likely to be higher and the perigee is likely to be lower, so the orbits won't be circular any more.

The way this happens is - if the debris piece is knocked directly backwards, then its apogee will be about the same, but its perigee will be lower. If it is knocked forwards (yes, that can happen), then the apogee will be raised, and the perigee will be about the same, and the piece could remain in orbit for a long time. If it is knocked up or down, then both the perigee will be lowered and the apogee will be raised. If the perigee is low enough, then the pieces will scrape through the upper levels of the atmosphere, and de-orbit eventually.

It looks like the prediction is for the lowest perigee to be about 200km. It may take a long time for even those pieces to de-orbit.

It doesn't. It splatters into many related orbits.

https://twitter.com/EU_SST/status/1380443036862316545/photo/...

Conservation of momentum. All of the teeny little bits of the two colliding objects still have the exact same average momentum of the two original objects, now there are just hundreds of thousands of them flying off in slightly different directions. The movie Gravity actually gives a fairly good, if overly dramatized, example of what happens when objects collide in space.

Does the debris have a degrading orbit and eventually get pulled earthward and burn up on entry? Or is that chaff out there floating around forever just outside the gravitational pull?

Some of the pieces will have a lower orbit, which might be low enough for them to scrape the atmosphere and bring them down. Some of the pieces will be placed into a higher orbit (although the lowest point won't be raised).

In the case of these objects, at ~800km, the time for orbital decay is often measured in decades.

Try millennia...

Excellent explanation, thanks very much!

Reply on news.ycombinator.com