by neufer » Sat Jul 18, 2020 9:48 pm
Chris Peterson wrote: ↑Fri Jul 17, 2020 10:54 pm
heehaw wrote: ↑Fri Jul 17, 2020 10:15 pm
It has always baffled me how it can possibly be that comets, coming from FAR out in the solar system, can possibly manage (so many of them!) to get so DAMED close to the Sun! (Is it just that we miss most of those that don't?).
What is "close"?
A distant object is barely bound to the Sun; it takes very little energy to knock it into a high eccentricity orbit, which means it gets within a few AU of the Sun. Of course, it's also true that most comets that are outside the inner system go unnoticed because they aren't bright enough to be recorded.
https://en.wikipedia.org/wiki/Oort_cloud wrote:
<<The Oort cloud is a theoretical cloud of predominantly icy planetesimals proposed to surround the Sun at distances ranging from 2,000 to 200,000 AU.>>
- Angular momentum grows as the square root of orbital radius.
Angular momentum of a parabolic orbit is sqrt(2) times that of a corresponding circular orbit.
For an Oort cloud planetesimal in a circular orbit at 20,000 AU to drop down to a perihelion of 100 AU it must lose 90% of its angular momentum.
For an Oort cloud planetesimal in a circular orbit at 20,000 AU to drop down to a perihelion of 1 AU it must lose 99% of its angular momentum.
While there should, indeed, be ~10 times (= 10%/1%) as many Oort cloud planetesimal that drop down to a perihelion of 100 AU (or less) than drop down to 1 AU (or less) they must fill a volume a million times greater.
Hence, the density of visible comets with small perihelions is abnormally enhanced over expectations.
[quote="Chris Peterson" post_id=304232 time=1595026479 user_id=117706]
[quote=heehaw post_id=304229 time=1595024143]
It has always baffled me how it can possibly be that comets, coming from FAR out in the solar system, can possibly manage (so many of them!) to get so DAMED close to the Sun! (Is it just that we miss most of those that don't?).[/quote]
What is "close"?
A distant object is barely bound to the Sun; it takes very little energy to knock it into a high eccentricity orbit, which means it gets within a few AU of the Sun. Of course, it's also true that most comets that are outside the inner system go unnoticed because they aren't bright enough to be recorded.[/quote][quote=https://en.wikipedia.org/wiki/Oort_cloud]
<<The Oort cloud is a theoretical cloud of predominantly icy planetesimals proposed to surround the Sun at distances ranging from 2,000 to 200,000 AU.>>[/quote]
[list]Angular momentum grows as the square root of orbital radius.
Angular momentum of a parabolic orbit is sqrt(2) times that of a corresponding circular orbit.[/list]
For an Oort cloud planetesimal in a circular orbit at 20,000 AU to drop down to a perihelion of 100 AU it must lose 90% of its angular momentum.
For an Oort cloud planetesimal in a circular orbit at 20,000 AU to drop down to a perihelion of 1 AU it must lose 99% of its angular momentum.
While there should, indeed, be ~10 times (= 10%/1%) as many Oort cloud planetesimal that drop down to a perihelion of 100 AU (or less) than drop down to 1 AU (or less) they must fill a volume a million times greater.
Hence, the density of visible comets with small perihelions is abnormally enhanced over expectations.