Starship Asterisk*

Jets from Comet Churyumov-Gerasimenko

Explanation: Where do comet tails come from? Although it is common knowledge that comet tails and comas originate from comet nuclei, exactly how that happens is an active topic of research. One of the best images yet of emerging jets is shown in the featured image, taken last November by the robotic Rosetta spacecraft in orbit around the Comet 67P/Churyumov-Gerasimenko (Comet CG), and released last month. The overexposed picture shows plumes of gas and dust escaping numerous places from the Comet CG's nucleus as it nears the Sun and heats up. Although Comet CG is currently further out from the Sun than Mars, its orbit will take it almost as close as the Earth this coming August, at which time its jet activity is expected to increase by a factor of about 100. You've likely seen some debris from comet nuclei before but in another form -- when sand-sized bits end their journey through the Solar System by impacting the atmosphere of Earth as meteors.

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This is a haunting and evocative photo of something we don't completely understand. I think seeing meteors in the sky is one of most magical experiences!

Any idea what the surface & sub-surface temperature is when this photo was taken?

Phylae should be levitating soon and get charged up again ?

Ok, we are not accustomed to seeing a brightly lit objects with stars in the background. Has the overexposure in this image revealed background stars or are the flecks of light surrounding the comet from some other source?

FloridaMike wrote:Ok, we are not accustomed to seeing a brightly lit objects with stars in the background. Has the overexposure in this image revealed background stars or are the flecks of light surrounding the comet from some other source?

Don't hold me to this, but I think I read somewhere that those are meters-sized particles coming off the comet.

FloridaMike wrote:Ok, we are not accustomed to seeing a brightly lit objects with stars in the background. Has the overexposure in this image revealed background stars or are the flecks of light surrounding the comet from some other source?

Stars, I think. This image is of a body with a 6% albedo- similar to fresh asphalt or the open ocean. Pretty dark, but still exposed into saturation. Such an exposure should easily capture stars (and I don't know what else the bright spots could reasonably be).

geckzilla wrote:

FloridaMike wrote:Ok, we are not accustomed to seeing a brightly lit objects with stars in the background. Has the overexposure in this image revealed background stars or are the flecks of light surrounding the comet from some other source?

Don't hold me to this, but I think I read somewhere that those are meters-sized particles coming off the comet.

Comets don't produce meter-scale particles, at least not very often. The dust particles produced by a comet are mostly in the micrometer size range (which we see in the jets) with some pebble-sized bodies produced, as well.

Chris Peterson wrote:

geckzilla wrote:

FloridaMike wrote:Ok, we are not accustomed to seeing a brightly lit objects with stars in the background. Has the overexposure in this image revealed background stars or are the flecks of light surrounding the comet from some other source?

Don't hold me to this, but I think I read somewhere that those are meters-sized particles coming off the comet.

Comets don't produce meter-scale particles, at least not very often. The dust particles produced by a comet are mostly in the micrometer size range (which we see in the jets) with some pebble-sized bodies produced, as well.

Yeah, I wish I could remember where I read that, but can't seem to find it. There are rather large bits floating around it, though. Large enough to form isolated dots in an image. A simple sequence of images would easily show whether the dots are in the foreground or whether they are distant stars or even artifacts. But, you know... that image release policy...

Close-up on this APOD shows a lot of "stuff" around those tiny dots of light. Can some knowledgable person say what it is?

This is a 1000% zoom on the two bright spots near the bottom-right part of the comet. Rob

rstevenson wrote:Close-up on this APOD shows a lot of "stuff" around those tiny dots of light. Can some knowledgable person say what it is?

This is a 1000% zoom on the two bright spots near the bottom-right part of the comet.

I'd say JPEG artifacts, especially given the 8x8 structure (which is a function of how JPEG data is encoded).

At least one of the light specs is clearly in front of the comet; the one spaced down a bit below the "throat". Maybe just a pixel misbehaving? Most of the rest of them look like background stars to me.

Mulutunnel wrote:At least one of the light specs is clearly in front of the comet; the one spaced down a bit below the "throat". Maybe just a pixel misbehaving? Most of the rest of them look like background stars to me.

There are many artifacts in this image, and many stars. I was lucky and found a couple dozen stars surrounding the comet of which the brightest are identified. Additional fainter stars formed cleanly identifiable asterisms. In particular in the region near λ Cas had a near perfect match for over a dozen stars. Even though the comet's predicted viewing position wrt Rosetta was off by about 5°(in Cepheus), I couldn't identify any asterisms at all there. For me, it was the patch of stars near λ Cas anchored the comet's position.

geckzilla wrote: Don't hold me to this, but I think I read somewhere that those are meters-sized particles coming off the comet.

Science 23 January 2015:
Vol. 347 no. 6220
DOI: 10.1126/science.aaa3905

Research Article
Dust measurements in the coma of comet 67P/Churyumov-Gerasimenko inbound to the Sun
Alessandra Rotundi, Holger Sierks, Vincenzo Della Corte, Marco Fulle, et al

Abstract

Critical measurements for understanding accretion and the dust/gas ratio in the solar nebula, where planets were forming 4.5 billion years ago, are being obtained by the GIADA (Grain Impact Analyser and Dust Accumulator) experiment on the European Space Agency’s Rosetta spacecraft orbiting comet 67P/Churyumov-Gerasimenko. Between 3.6 and 3.4 astronomical units inbound, GIADA and OSIRIS (Optical, Spectroscopic, and Infrared Remote Imaging System) detected 35 outflowing grains of mass 10−10 to 10−7 kilograms, and 48 grains of mass 10−5 to 10−2 kilograms, respectively. Combined with gas data from the MIRO (Microwave Instrument for the Rosetta Orbiter) and ROSINA (Rosetta Orbiter Spectrometer for Ion and Neutral Analysis) instruments, we find a dust/gas mass ratio of 4 ± 2 averaged over the sunlit nucleus surface. A cloud of larger grains also encircles the nucleus in bound orbits from the previous perihelion. The largest orbiting clumps are meter-sized, confirming the dust/gas ratio of 3 inferred at perihelion from models of dust comae and trails.

The paper now seems to be behind a paywal, geck, but I have a pdf copy on my computer.

M

Thanks, I needed some reassurance tonight that I'm not completely nuts. The abstract is fine. I'm not really that interested in comet clumps at this moment.

geckzilla wrote:Thanks, I needed some reassurance tonight that I'm not completely nuts. ...

I know the sensation! However, if you were to develop a passion for comet clumps, there's a nice blog post at the ESA Rosetta blog, enticingly entitled GIADA’S DUST MEASUREMENTS: 3.7-3.4 AU ...
I liked this bit:

we infer that the bound grains varied from 4 cm to 2 m*...

(*Editor’s note: In a follow-up discussion, I asked Alessandra and Marco about the possible impact – literally – of ‘grains’ up to 2 metres in size on the spacecraft, but there is no need to panic: according to their space density, Rosetta needs to travel for many centuries at a speed of 1 m/s before impacting even just one of them!)

M

So which came first - the chicken or the egg?

If you wonder why I'm talking about that here use the "active topic of research" link.

If this is the tail coming off a comet, why is it apparently pointing toward the sun (off the top of the picture)?

Rusty Brown in Cda wrote:If this is the tail coming off a comet, why is it apparently pointing toward the sun (off the top of the picture)?

This is not the tail. This is dust being ejected, which forms the coma (and is relatively isotropic). The dust tail is swept out of the coma and other ejected particles as they get farther from the nucleus. This close to the nucleus, all the particle motion is determined by the ejection dynamics; solar wind and radiation pressure have an insignificant impact over such a short distance.

geckzilla wrote:Thanks, I needed some reassurance tonight that I'm not completely nuts. The abstract is fine. I'm not really that interested in comet clumps at this moment.

It's a pretty interesting paper. I've had it sitting on my table for a few days, but hadn't gotten to it yet (or the other ones in this issue of Science, dedicated to the Rosetta mission).

Grains are detected using multiple images aligned on stars, with bound and unbound particles identified by their trajectories (which also has to take into account the spacecraft motion).

Meter-sized particles are not being ejected. At perihelion, activity is high enough that some bodies of this size are released, but with ejection velocities too low to allow most to escape. So they end up in bound orbits. Fortunately for the Rosetta spacecraft, the density of such bodies is very low. Several hundred bound grains were detected, with sizes in the centimeter range. Again, these large particles are ejected at a low enough velocity that they don't escape. Particles up to that same size range were observed escaping, as well.

Most interesting was the observation that the most of the scattered light we're seeing off the dust is coming from particles in the 0.1-1 mm size range, not micrometer sized particles. I think that was somewhat unexpected. Everyone is looking to see how all these numbers change as the comet gets closer to the Sun.

DavidLeodis wrote:So which came first - the chicken or the egg?

If you wonder why I'm talking about that here use the "active topic of research" link.

Clearly the chicken, those eggs look fresh

Not on topic but have a question. Is the Rosetta still on board the comet.? Haven't been following of late.

CharliePatriot wrote:Not on topic but have a question. Is the Rosetta still on board the comet.? Haven't been following of late.

Sorry, I meant Phylae, or whatever the heck that spacecraft is named that touched down on the darn thing.

CharliePatriot wrote:

CharliePatriot wrote:Not on topic but have a question. Is the Rosetta still on board the comet.? Haven't been following of late.

Sorry, I meant Phylae, or whatever the heck that spacecraft is named that touched down on the darn thing.

Rosetta is currently orbiting the comet. Philae is still resting on the surface and awaiting more sunlight to reach it as the comet approaches the Sun.

FloridaMike wrote:Ok, we are not accustomed to seeing a brightly lit objects with stars in the background. Has the overexposure in this image revealed background stars or are the flecks of light surrounding the comet from some other source?

I've posted a later image from the Rosetta NAVCAM on the Rosetta thread in Breaking Science News . The text accompanying the image in the ESA Rosetta blog specifically mentions "little white blobs" ... If anyone is interested, have a look-see!

M