Starship Asterisk*

by **podkayn** » Wed Dec 28, 2011 6:42 pm

No one else seems to have noticed that the words "likely had" are in italics for no clear reason. Is this possibly supposed to be a link, but the wrong code was entered? Can the link be added, please? I'd hate to think I was missing a link in such a fascinating APOD!

by **zloq** » Mon Dec 19, 2011 8:51 am

BMAONE23 wrote: I had heard of a comet coming in so I had been watching the SOHO movies and waiting. I noticed the small fragment enter the frame and thought "what a non-event" Thankfully I checked back a couple of days later for the real Lovejoy. Granted it was no McNaught but I noticed the second fragment then. I visit the SOHO site almost daily. You can find many microcomets (sungrazers) as they end their life. Lovejoy was the first I had seen to pass so closely and survive

Well - nice job, and it looks like it paid off. I still haven't seen much discussion of these 3 (?) other fragments. I was puzzled because I couldn't tell when the fragmentation might have happened since they were relatively far apart. Sekanina has several publications on this topic, including:

http://iopscience.iop.org/0004-637X/576 ... /fulltext/

His conclusion is that the Kreutz comets are due to fragmentation very far from the sun, I guess somewhat randomly on either side of perihelion, whereas other sungrazers tend to break up much closer to perihelion. There have been many fragment pairs seen by SOHO but I'm not sure how many have shown this many fragments in the same clump. The comet splits off a fragment randomly, then some time later that fragment randomly splits off another fragment - etc. - in a runaway process that generates nearby pairs. So it may be that the fragmentation seen here happened a long time ago and far away.

As for Hubble imaging it - it looks like Hubble has imaged Venus but never Mercury due to its proximity to the sun - even though it can be >25 degrees away. Lovejoy will break 20 degrees away around Dec. 23, so maybe Hubble will get a view a bit later.

zloq

by **BMAONE23** » Sun Dec 18, 2011 5:31 pm

zloq wrote:(snip)

Did you notice that earlier fragment yourself, or did you see mention of it on the web?

OK - I checked Phil Plait's blog and he has mention of a companion - but it's a different one described at:

http://sungrazer.nrl.navy.mil/index.php ... hday_comet

So I guess at least one companion is acknowledged - but there are at least two others.

(snip)

zloq

I had heard of a comet coming in so I had been watching the SOHO movies and waiting. I noticed the small fragment enter the frame and thought "what a non-event" Thankfully I checked back a couple of days later for the real Lovejoy. Granted it was no McNaught but I noticed the second fragment then. I visit the SOHO site almost daily. You can find many microcomets (sungrazers) as they end their life. Lovejoy was the first I had seen to pass so closely and survive

by **Guest** » Sun Dec 18, 2011 11:28 am

zloq wrote: I automatically thought this was some kind of artifact, but it sure looks real to me - especially when something similar shows days earlier. I haven't seen anything on the web discussing leading fragments - but at the same time I haven't seen anything by astronomers talking about them at all. So - I look forward to an explanation. The spots get bright and fade out somewhat independently of the comet - as if they have their own brightening curves.

The comet is part of the Kreutz group, so it is thought to be a fragment of a large progenitor comet anyway. It seems likely that there would be many small fragments accompanying it then too.

by **zloq** » Sun Dec 18, 2011 10:33 am

BMAONE23 wrote: I believe you are seeing a leading shard of the comet.
There appears to be another leading shard that followed the same path but 2 days prior
see this mpeg file from 12-13 2011
the fragment appears pretty close to the sun and vanishes by the end of the vid
it also appears in the closer C2 movie

I automatically thought this was some kind of artifact, but it sure looks real to me - especially when something similar shows days earlier. I haven't seen anything on the web discussing leading fragments - but at the same time I haven't seen anything by astronomers talking about them at all. So - I look forward to an explanation. The spots get bright and fade out somewhat independently of the comet - as if they have their own brightening curves.

Did you notice that earlier fragment yourself, or did you see mention of it on the web?

OK - I checked Phil Plait's blog and he has mention of a companion - but it's a different one described at:

http://sungrazer.nrl.navy.mil/index.php ... hday_comet

So I guess at least one companion is acknowledged - but there are at least two others.

As for the speed of the comet - I haven't seen a number on the web, but assuming a parabolic orbit I get a max velocity of around 550 km/s. For comparison, Mercury is around 48 km/s. At that speed and at a height above the surface of 120,000 km, it would go around the sun in about 150 minutes in a circular path, or NY to LA in under 8 seconds. About Mach 1600. This seems like a natural thing to talk about in describing the comet's flight around the sun, so I'm surprised only the height above the surface gets mentioned. Assuming my numbers are in the ballpark.

Most of the orbit is south of the ecliptic, but for about 4 hours during the swing around the sun it is north of the ecliptic. It's orbiting in a plane roughly through the center of the sun, so it has to have part above the ecliptic and part below. If the orbit were tilted on its side perpendicular to ecliptic, then half would be above and half would be below.

Amateur astronomers have been imaging the thing in daylight after its trip around the sun - which is pretty amazing, especially since it wasn't even expected to survive. I'm not sure how bright it will be around dawn - but it favors observers in the southern hemisphere. Looks like this is comet is a big deal for many reasons, and it would be great if it sustains a tail as it gets far enough from the sun to be imaged against a dark sky. I'm not sure how far away from the sun it needs to be for Hubble to get a shot.

zloq

by **Case** » Sun Dec 18, 2011 8:14 am

Here are two links to some high res movies (1024x1024):

Going in: http://science.nasa.gov/media/medialibr ... whoosh.mov
Notice the striped tail, quite like McNaught (C/2006 P1).

Exit: http://science.nasa.gov/media/medialibr ... merges.mov
Notice the tail wobbling up and down!

by **Ann** » Sun Dec 18, 2011 6:12 am

Not only today's APOD finds Comet Lovejoy interesting. Sweden's most prestigious daily, Dagens Nyheter, topped its internet edition on Saturday evening with Comet Lovejoy's daredevil journey. Dagens Nyheter used several headlines, among them "Comet Challenges the Sun". I liked it!

Here is the (short) article. Here the headline reads, "Comet 'Lovejoy' Showed Little Respect for the Sun".

Anyway, I thought it was fun that the most important news item for Swedish online newspaper readers was judged, by Sweden's most prestigious daily, to be the fearless little comet and its "boxing match" with the Sun!

Today's APOD is nice, too, so thank you for that, LASCO, SOHO Consortium, NRL, ESA, NASA and APOD editors!

Ann

by **BMAONE23** » Sat Dec 17, 2011 8:15 pm

Guest wrote:
somebody wrote:In the video LovejoyC3_gap_best.mov, accessible via the link in the text, you can see a small dot ahead of the comet, following the same trajectory. Is this a part of the same comet, that split-off in the past?
I have noticed the dot as well. May be it is an optical effect of the camera or another small comet in the same trajectory.
In the last frame, after the comet has already passed the sun, one can see a dot preceding again.
If it is a small comet, it shouldn't survive the close encounter with the sun though. So the dot in the last frame could be a cosmic ray.

I believe you are seeing a leading shard of the comet.
There appears to be another leading shard that followed the same path but 2 days prior
see this mpeg file from 12-13 2011
the fragment appears pretty close to the sun and vanishes by the end of the vid
it also appears in the closer C2 movie

by **superbcpoet** » Sat Dec 17, 2011 8:09 pm

This should be the final nail in the coffin of the 'dirty snowball' theory of comets.

by **Mactavish** » Sat Dec 17, 2011 7:30 pm

Chris Peterson wrote:
The encounter may have reduced the eccentricity slightly,...

Here’s one for you physicists: The encounter probably would have reduced the mass of the comet (being “burned” off or otherwise). This should have an effect on the relative gravity between the comet and the sun. Would this not affect the size of the orbit and perhaps also the eccentricity?

by **zbvhs** » Sat Dec 17, 2011 7:21 pm

It's a matter of absorption time. By zipping through the Sun's corona at high speed at perihelion, the nucleus wasn't in it long enough to be seriously damaged. Fire walkers don't get burned for the same reason: they don't walk too long in one place.

by **Guest** » Sat Dec 17, 2011 6:12 pm

somebody wrote:In the video LovejoyC3_gap_best.mov, accessible via the link in the text, you can see a small dot ahead of the comet, following the same trajectory. Is this a part of the same comet, that split-off in the past?

I have noticed the dot as well. May be it is an optical effect of the camera or another small comet in the same trajectory.
In the last frame, after the comet has already passed the sun, one can see a dot preceding again.
If it is a small comet, it shouldn't survive the close encounter with the sun though. So the dot in the last frame could be a cosmic ray.

by **Chris Peterson** » Sat Dec 17, 2011 5:15 pm

nstahl wrote:My intuition after watching a couple of them is that the plane containing the comet's path and the sun is below us, slanting up to the sun.

That is correct. The comet's inclination is 136°, meaning it is in a retrograde orbit on a plane inclined at 46° to the ecliptic, with the actual orbit always south of the ecliptic.

And of course if it got close enough to the sun it could have been knocked a little out of that plane by some sort of turbulence, but surely not a lot.

The encounter should have had very little impact on the inclination. What I'd expect to change is the eccentricity, and the time-critical elements. Before the encounter, this comet had an eccentricity very close to 1, meaning it was either in a parabolic or hyperbolic orbit (open), or in a very long period elliptic orbit. The encounter may have reduced the eccentricity slightly, meaning the comet could now be in a closed orbit, and return in some thousands of years.

by **nstahl** » Sat Dec 17, 2011 3:59 pm

Don't miss seeing at least one of the movies.

My intuition after watching a couple of them is that the plane containing the comet's path and the sun is below us, slanting up to the sun. And of course if it got close enough to the sun it could have been knocked a little out of that plane by some sort of turbulence, but surely not a lot.

by **apache32** » Sat Dec 17, 2011 2:29 pm

As many I saw this before the post and there have been many minor grazers ahead of lovejoy. Who can do the math and tell me what fantastic speed Lovejoy had to hit at perihelion? Any guess to its mass?

by **OverlordE** » Sat Dec 17, 2011 1:33 pm

This is one tough comet. I wonder how big the parent was and if it would have been visible in the twelfth century.

by **orin stepanek** » Sat Dec 17, 2011 1:26 pm

The little comet that could!

by **somebody** » Sat Dec 17, 2011 10:15 am

In the video LovejoyC3_gap_best.mov, accessible via the link in the text, you can see a small dot ahead of the comet, following the same trajectory. Is this a part of the same comet, that split-off in the past?

by **bystander** » Sat Dec 17, 2011 6:11 am

http://asterisk.apod.com/viewtopic.php?f=29&t=26198

by **APOD Robot** » Sat Dec 17, 2011 5:06 am

Comet Lovejoy: Sungrazing Survivor

Explanation: Like most other sungrazing comets, Comet Lovejoy (C/2011 W3) was not expected to survive its close encounter with the Sun. But it did. This image from a coronograph onboard the sun-staring SOHO spacecraft identifies the still inbound remnants of the tail, with the brilliant head or coma emerging from the solar glare on December 16. The Sun's position, behind an occulting disk to block the overwhelming glare, is indicated by the white circle. Separated from its tail, Comet Lovejoy's coma is so bright it saturates the camera's pixels creating the horizontal streaks. Based on their orbits, sungrazer comets are thought to belong to the Kreutz family of comets, created by successive break ups from a single large parent comet that passed very near the Sun in the twelfth century. Most have been discovered with SOHO's cameras, but unlike many sungrazers, this one was first spotted by Australian astronomer Terry Lovejoy from an earth-based observatory. Comet Lovejoy is estimated to have come within 120,000 kilometers of the Sun's surface and likely had a large cometary nucleus to have survived its intense perihelion passage. Remarkable videos of the encounter from the Solar Dynamics Observatory can be found here.

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