APOD: NEOWISE of the North (2020 Jul 17)

[APOD Robot]

NEOWISE of the North

Explanation: After local midnight on July 14 comet NEOWISE was still above the horizon for Goldenrod, Alberta, Canada, just north of Calgary, planet Earth. In this snapshot it makes for an awesome night with dancing displays of the northern lights. The long-tailed comet and auroral displays are beautiful apparitions in the north these days. Both show the influence of spaceweather and the wind from the Sun. Skygazers have widely welcomed the visitor from the Oort cloud, though C/2020 F3 (NEOWISE) is in an orbit that is now taking it out of the inner Solar System.

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[orin stepanek]

Neowise days; that's for sure! Pretty!

Went outside early this morning and noticed a
light near the moon; I presumed I was looking
at Venus! Twas a pretty scene also!

[gvann]

the APOD explanation describes the comet as a "visitor from the Oort cloud"; but, with an aphelion of just 544 AU (inbound) it does not seem like it ever gets anywhere close to the Oort cloud. What's the reason why it's believed to come from the Oort cloud? Anyone knows?

[Chris Peterson]

the APOD explanation describes the comet as a "visitor from the Oort cloud"; but, with an aphelion of just 544 AU (inbound) it does not seem like it ever gets anywhere close to the Oort cloud. What's the reason why it's believed to come from the Oort cloud? Anyone knows?

Conventionally, the assumption is that long period comets originate in the Oort cloud. But there is no way to tell for sure, since repeated interactions with the planets scramble the orbital parameters thoroughly. The high inclination of this comet is suggestive of an origin in a spherical cloud, not a disc, but certainly not definitive, as gravitational perturbations can alter the inclination of an orbiting body as well as the semi-major axis.

[Sa Ji Tario]

Inside the polar aurora appears a pillar of light

[XgeoX]

!
Went outside early this morning and noticed a
light near the moon; I presumed I was looking
at Venus! Twas a pretty scene also!

Indeed it was! Reminded me of this...



Eric

[orin stepanek]

!
Went outside early this morning and noticed a
light near the moon; I presumed I was looking
at Venus! Twas a pretty scene also!

Indeed it was! Reminded me of this...



Eric

Actually; more like this:

picture taken from computer images!

[BDanielMayfield]

the APOD explanation describes the comet as a "visitor from the Oort cloud"; but, with an aphelion of just 544 AU (inbound) it does not seem like it ever gets anywhere close to the Oort cloud. What's the reason why it's believed to come from the Oort cloud? Anyone knows?

Conventionally, the assumption is that long period comets originate in the Oort cloud. But there is no way to tell for sure, since repeated interactions with the planets scramble the orbital parameters thoroughly. The high inclination of this comet is suggestive of an origin in a spherical cloud, not a disc, but certainly not definitive, as gravitational perturbations can alter the inclination of an orbiting body as well as the semi-major axis.

A little more info on this, from wikipedia's comet article:

Based on their orbital characteristics, short-period comets are thought to originate from the centaurs and the Kuiper belt/scattered disc[92] —a disk of objects in the trans-Neptunian region—whereas the source of long-period comets is thought to be the far more distant spherical Oort cloud (after the Dutch astronomer Jan Hendrik Oort who hypothesized its existence).[93] Vast swarms of comet-like bodies are thought to orbit the Sun in these distant regions in roughly circular orbits. Occasionally the gravitational influence of the outer planets (in the case of Kuiper belt objects) or nearby stars (in the case of Oort cloud objects) may throw one of these bodies into an elliptical orbit that takes it inwards toward the Sun to form a visible comet.

[heehaw]

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?).

[Chris Peterson]

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.

[neufer]

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.

<<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.