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While I'm strongly supportive of survey astronomy in general... We can already say that we have very high completion of cataloguing near-Earth objects that are

by mapt 1y ago

While I'm strongly supportive of survey astronomy in general... We can already say that we have very high completion of cataloguing near-Earth objects that are anywhere near extinction-event / Chicxulub-sized (~10km), and have a majority of catastrophic / country-killer (~1km), and are digging deeper and deeper into regional / city-killer (~100m) bodies. What we don't have is comets. Comets on long period orbits just aren't readily detectable with this sort of survey unless they're quite close in to the Sun, and I don't think we have great statistics on frequency vs size, size being something that requires very specific radar cross-checking to establish with any confidence. A long-period comet or hyperbolic body has a potential impact velocity much higher than inner system asteroids, and impact energy scales with impact velocity squared.

cogman10 1y ago
Will rubin detect comets? I'd assume not as it seems like they'll only really be visible as they approach the sun (or if they end up blocking a line of stars).
- mapt 1y ago
  The problem is that the difference in optical/NIR brightness (apparent magnitude) between a long-period comet core that's going to hit us in 1000 years, and a long-period comet core that's going to hit us in six months, might be a factor 10^12 (magnitude 10 vs magnitude 40) or worse. Normally brightness drops off with distance squared for light sources, but comets without any tail or halo aren't emitting all that much light, they're reflecting it, and (except for a very brief period) they're about as far from us as they are from the sun. This means that brightness drops with distance to the fourth power. Cometary tails also only offgas a significant amount near the sun. Comet cores are expected to be extremely dark / low-reflectivity due to space weathering producing a carbon coating not unlike chimney-creosote. You can fight this a bit by working in the thermal infrared, which you really need a specific sort of space telescope for. But long-period comets and hyperbolic impactors will be a probabilistic threat for the foreseeable future. I would say "Be thankful that they're so rare", but the data from observatories like Rubin on these bodies during points of their orbit where they're close enough to the sun to actually detect, is necessary to statistically characterize their existence with any confidence.

by mapt 1y ago

cogman10 1y ago

Will rubin detect comets? I'd assume not as it seems like they'll only really be visible as they approach the sun (or if they end up blocking a line of stars).

mapt 1y ago
The problem is that the difference in optical/NIR brightness (apparent magnitude) between a long-period comet core that's going to hit us in 1000 years, and a long-period comet core that's going to hit us in six months, might be a factor 10^12 (magnitude 10 vs magnitude 40) or worse. Normally brightness drops off with distance squared for light sources, but comets without any tail or halo aren't emitting all that much light, they're reflecting it, and (except for a very brief period) they're about as far from us as they are from the sun. This means that brightness drops with distance to the fourth power. Cometary tails also only offgas a significant amount near the sun. Comet cores are expected to be extremely dark / low-reflectivity due to space weathering producing a carbon coating not unlike chimney-creosote. You can fight this a bit by working in the thermal infrared, which you really need a specific sort of space telescope for. But long-period comets and hyperbolic impactors will be a probabilistic threat for the foreseeable future. I would say "Be thankful that they're so rare", but the data from observatories like Rubin on these bodies during points of their orbit where they're close enough to the sun to actually detect, is necessary to statistically characterize their existence with any confidence.