4 ms·

Mmmh, differential equations for N-bodies system have notorious tendencies towards chaos and/or easily getting out of chaotic attraction basins. For that reaso

Mmmh, differential equations for N-bodies system have notorious tendencies towards chaos and/or easily getting out of chaotic attraction basins. For that reason, I'm not entirely sure that playing cosmic bowling with stuff floating around in the solar system is such a good idea ...

ohyoutravel 4y ago
I suppose the calculus might change when it’s between the earth getting smashed and the potential unintended consequences of cosmic bowling.
baq 4y ago
'bowling' is hardly a good analogy here, though. it's more like pea shooting at huge rocks, though in this case it turned out that the rock in question isn't held together particularly well.
xvedejas 4y ago
When the relevant timescales are short, and most of the bodies are small (so, weakly interacting), this is not so big an issue mathematically.
ghaff 4y ago
That probably assumes that cosmic bowling doesn't happen all the time and that we're disrupting some finely-tuned system that has kept earth (mostly) safe for billions of years.
QuadmasterXLII 4y ago
Goin on the hn-gell-man chart
ealexhudson 4y ago
More like cosmic pool, and we have Dave Cinzano Bianco Lister.
- JaimeThompson 4y ago
  Lie Mode Who?
spenczar5 4y ago
Hi, I work on N-body orbit determination software for asteroid detection and planetary defense (for example, https://github.com/moeyensj/thor https://github.com/moeyensj/thor). Ask me anything. You're right that orbit determination is unstable over long runs, and very sensitive to initial conditions. But the good news is that the solar system is big. Most trajectories do not intersect with the earth's location. So perturbing an orbit that we think has a small chance of hitting us tends to be safe. Also, of course, after deflection, any object is going to be monitored really closely for a long time.
- pageandrew 4y ago
  Since DART crashed into Dimorphos, changing its orbit around Didymos rather than its orbit around the sun, isn’t there almost no cause for concern because Didymos is still on the same non-intersecting orbit around the sun? Is the concern that Dimorphos’s slightly different orbit could pull Didymos slightly off course in its orbit around the sun, thereby causing it to potentially intersect with Earth at some point in the future, where it wouldn’t have prior to DART. Can we quantify how much Dimorphos’s new orbit has affected Didymos’s orbit? Is that calculable?
- nighthawk454 4y ago
  Hey, this is a bit tangential, but is Barnes-Hut still used in modern n-body simulations? Some of the math has huge crossover with machine learning and dimension reduction, but it’s rarely traced back to the source in literature. Thanks! (I’ll also check out the GitHub repo you linked but I haven’t read it fully yet)
- Hendrikto 4y ago
  Maybe a bit off topic, but: * What language is that software written in? * What was the hardest problem you had to solve, while working on it?
  - spenczar5 4y ago
    Mostly Python and Fortran. See for example https://github.com/oorb/oorb https://github.com/oorb/oorb. The hardest problems are always the social ones. How do you get uptake of a new method, how do you get funding, how do you politely tell a collaboration they are doing the wrong thing, etc. But if you mean pure technical stuff - the hardest problem I had to solve was rethinking some of the inner loops of the THOR algorithm. The problem was essentially to speed up a Hough transform in 6D space. Lots of time spent profiling CPU cache timings to get that fast.

ohyoutravel 4y ago

I suppose the calculus might change when it’s between the earth getting smashed and the potential unintended consequences of cosmic bowling.

baq 4y ago

'bowling' is hardly a good analogy here, though. it's more like pea shooting at huge rocks, though in this case it turned out that the rock in question isn't held together particularly well.

xvedejas 4y ago

When the relevant timescales are short, and most of the bodies are small (so, weakly interacting), this is not so big an issue mathematically.

ghaff 4y ago

That probably assumes that cosmic bowling doesn't happen all the time and that we're disrupting some finely-tuned system that has kept earth (mostly) safe for billions of years.

QuadmasterXLII 4y ago

Goin on the hn-gell-man chart

ealexhudson 4y ago

More like cosmic pool, and we have Dave Cinzano Bianco Lister.

JaimeThompson 4y ago
Lie Mode Who?

spenczar5 4y ago

Hi, I work on N-body orbit determination software for asteroid detection and planetary defense (for example, https://github.com/moeyensj/thor https://github.com/moeyensj/thor). Ask me anything. You're right that orbit determination is unstable over long runs, and very sensitive to initial conditions. But the good news is that the solar system is big. Most trajectories do not intersect with the earth's location. So perturbing an orbit that we think has a small chance of hitting us tends to be safe. Also, of course, after deflection, any object is going to be monitored really closely for a long time.

pageandrew 4y ago
Since DART crashed into Dimorphos, changing its orbit around Didymos rather than its orbit around the sun, isn’t there almost no cause for concern because Didymos is still on the same non-intersecting orbit around the sun? Is the concern that Dimorphos’s slightly different orbit could pull Didymos slightly off course in its orbit around the sun, thereby causing it to potentially intersect with Earth at some point in the future, where it wouldn’t have prior to DART. Can we quantify how much Dimorphos’s new orbit has affected Didymos’s orbit? Is that calculable?
nighthawk454 4y ago
Hey, this is a bit tangential, but is Barnes-Hut still used in modern n-body simulations? Some of the math has huge crossover with machine learning and dimension reduction, but it’s rarely traced back to the source in literature. Thanks! (I’ll also check out the GitHub repo you linked but I haven’t read it fully yet)
Hendrikto 4y ago
Maybe a bit off topic, but: * What language is that software written in? * What was the hardest problem you had to solve, while working on it?
- spenczar5 4y ago
  Mostly Python and Fortran. See for example https://github.com/oorb/oorb https://github.com/oorb/oorb. The hardest problems are always the social ones. How do you get uptake of a new method, how do you get funding, how do you politely tell a collaboration they are doing the wrong thing, etc. But if you mean pure technical stuff - the hardest problem I had to solve was rethinking some of the inner loops of the THOR algorithm. The problem was essentially to speed up a Hough transform in 6D space. Lots of time spent profiling CPU cache timings to get that fast.