Starship Asterisk*

Nov. 4, 2010 | 09:24 PDT | 16:24 UTC
Animation of the five closest-approach Hartley 2 images

<<Here are the things that I notice when I look at them.

The comet nucleus has two lobes. This much we knew already from the radar images. But there is a surprising (to me) narrow and concave neck connecting the two. Furthermore, that neck is very, very smooth. What I think we are seeing here is a contact binary, two main bodies that orbit each other so closely that they are touching. Gravel and dust has flowed into the weird gravitational region between the two lobes, filling it almost as though it were a liquid. I'll bet that smooth neck traces out an equipotential surface. In shape, this comet looks very similar to Borrelly.

http://www.planetary.org/blog/article/00002758/ wrote:

Animation of Deep Impact close-approach images Credit: NASA / JPL / UMD / animation by Emily Lakdawalla

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The Planetary Society Blog
By Emily Lakdawalla Nov. 4, 2010
Animation of the five closest-approach Hartley 2 images

<<Here are the things that I notice when I look at them.

The comet nucleus has two lobes. This much we knew already from the radar images. But there is a surprising (to me) narrow and concave neck connecting the two. Furthermore, that neck is very, very smooth. What I think we are seeing here is a contact binary, two main bodies that orbit each other so closely that they are touching. Gravel and dust has flowed into the weird gravitational region between the two lobes, filling it almost as though it were a liquid. I'll bet that smooth neck traces out an equipotential surface. In shape, this comet looks very similar to Borrelly.

Hartley 2 has lots and lots of jets. It would be a fun activity to use the five images to triangulate and trace the jets back to their sources. I think that work will get easier with more images; I think there are more images in between these, yet to be downlinked.

There are some huge boulders on those lobe ends. That's much like Itokawa, another very small body that's been visited by a spacecraft. Hartley 2 is maybe 4 or 5 times larger (in diameter) than Itokawa, but is still the smallest comet yet visited by a spacecraft.>>

http://en.wikipedia.org/wiki/Roche_lobe wrote:

A binary star enmeshed in a spiraling stream of ejected gas. Artwork copyright 2008 Don Dixon/cosmographica.com

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A three-dimensional representation of the Roche potential in a binary star with a mass ratio of 2, in the co-rotating frame.

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<<The Roche lobe is the region of space around a star within which orbiting material is gravitationally bound to that star. If the star expands past its Roche lobe, then the material can escape the gravitational pull of the star. If the star is in a binary system then the material will fall in through the inner Lagrangian point. It is an approximately tear-drop shaped region bounded by a critical gravitational equipotential, with the apex of the tear-drop pointing towards the other star (and the apex is at the L1 Lagrangian point of the system). It is different from the Roche limit which is the distance at which an object held together only by gravity begins to break up due to tidal forces. It is different from the Roche sphere which approximates the gravitational sphere of influence of one astronomical body in the face of perturbations from another heavier body around which it orbits. The Roche lobe, Roche limit and Roche sphere are named after the French astronomer Édouard Roche.

The droplet-shaped figures in the equipotential plot at the bottom of the figure are called the Roche lobes of each star. L1, L2 and L3 are the Lagrangian points where forces cancel out. Mass can flow through the saddle point L1 from one star to its companion, if the star fills its Roche lobe.>>

http://en.wikipedia.org/wiki/Beta_Lyrae wrote:

Beta Lyrae (β Lyr, β Lyrae) is a binary star system approximately 882 light-years away in the constellation Lyra. Beta Lyrae has the traditional name Sheliak (occasionally Shelyak), from الشلياق šiliyāq, the Arabic name of the constellation Lyra.

Beta Lyrae is an eclipsing semi-detached binary system made up of a B7II primary star and a secondary that is probably also a B-type star. The brighter, less massive star (B7II) in the system has filled its Roche lobe and is undergoing mass transfer with the other star due to its evolutionary state. The secondary, more massive star has an accretion disk due to this mass transfer. This accretion disk blocks our view of the secondary star, making it difficult for astronomers to pinpoint what its stellar type is. The system also contains bipolar jets.

Beta Lyrae changes its apparent magnitude from +3.4 to +4.6 over a period of 12.9075 days. The two components of the main star are so close together that they cannot be resolved with optical telescopes, forming a spectroscopic binary.>>

orin stepanek wrote:
Most of the venting is done on the smaller lobe; with some venting on the large lobe.

http://www.planetary.org/blog/article/00002759/ wrote:
The Planetary Society Blog
Hartley 2's jets By Emily Lakdawalla Nov. 4, 2010

Hartley 2 is an unusually active small comet, a fact made clear by this enhanced view of its night side. Credit: NASA / JPL / UMD

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<<It was a very happy set of scientists, engineers, managers, and administrators who filled the Jet Propulsion Laboratory's Von Karman auditorium this afternoon to do the postgame show on Deep Impact's flyby of Hartley 2. Project Manager Tim Larson remarked that the spacecraft and its navigators could not have performed better; the aimpoint for the flyby was missed by only two seconds in time and three kilometers in distance, which is pretty darned close to the mark. And of course, the images, as I mentioned previously, were spectacular.

I won't have time this afternoon to post a detailed writeup about the press briefing -- look for that tomorrow -- but I thought I'd post just one image from the briefing, the one that got a big "ooooh!" from the auditorium. It was part of co-investigator Jessica Sunshine's presentation on the appearance of the comet nucleus. It's an enhanced view of the part of the comet that is not directly lit by the Sun -- that is, the comet's night side, and the terminator (the day-night boundary). There are so many amazing things about this one photo. It's amazing how many jets there are. It is amazing that you can track the jets right down to where they are erupting from the sunlit surface. It is amazing that there are so many jets on the night side -- comets aren't supposed to do that. The nightside jets are not visible where they sprout from the surface; that's in the dark. You can see them where the dust from the jets has risen above the nightside shadow, into sunlight. You can even see where there are jets beyond the limb, on the back side of the comet, which, lit up by the Sun, show you where the comet's limb is. It's spectacular.>>

orin stepanek wrote:
Could this comet have a limited amount of icy material and will eventually become just another asteroid?

http://en.wikipedia.org/wiki/25143_Itokawa wrote:

Asteroid 25143 Itokawa observed by Hayabusa (Credit: JAXA)

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Asteroid 25143 Itokawa orbit.

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<<Itokawa is an S-type asteroid. Radar imaging by Goldstone in 2001 observed an ellipsoid 630 ± 60 m long and 250 ± 30 m wide. The Hayabusa mission confirmed these findings and also suggested that Itokawa may be a contact binary formed by two or more smaller asteroids that have gravitated toward each other and stuck together. The Hayabusa images show a surprising lack of impact craters and a very rough surface studded with boulders, which were referred by the mission team as being in a 'rubble'. Furthermore, the density of the asteroid is too low for it to be made from solid rock. This would mean that Itokawa is not a monolith but rather a ‘rubble pile’ formed from fragments that have cohered over time.>>

orin stepanek wrote:Most of the venting is done on the smaller lobe; with some venting on the large lobe. Could this comet have a limited amount of icy material and will eventually become just another asteroid?

http://www.planetary.org/blog/article/00002763/ wrote:

To compose this 3D animation of the nucleus of comet Hartley 2, Daniel Macháček used Squirlz Morph to morph among the five images of the nucleus that were released by the Deep Impact team on the day of the flyby, November 4, 2010, in a process explained here. Then Luca Cassioli took sequential frames from the morphed animation and used them as left and right images in this animated GIF. Credit: NASA / JPL / UMD / Daniel Macháček / Luca Cassioli.

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Click to play embedded YouTube video.

Hartley 2 compared to other comets, and in motion 3D
The Planetary Society Blog
By Emily Lakdawalla Nov. 5, 2010

<<Get out your 3D glasses. This cool animation is the result of a group effort among the space enthusiasts at unmannedspaceflight.com, which is now part of the Planetary Society! There's no new Hartley 2 data in here; it was just made, by Daniel Macháček Luca Cassioli, by morphing among the five images that were released yesterday and then using sequential images from the morph animation to make the anaglyphs.

I've composed a montage of all five comets whose nuclei have been imaged directly by spacecraft, to scale with each other, sized for inclusion in your favorite digital presentation software. Hartley 2 is the smallest comet ever visited by a spacecraft!

Five comet nuclei directly imaged by flyby spacecraft are shown here to scale with each other. Credit: Halley: Russian Academy of Sciences / Ted Stryk. Tempel 1 and Hartley 2: NASA / JPL / UMD. Borrelly: NASA / JPL / Ted Stryk. Wild 2: NASA / JPL. Montage by Emily Lakdawalla.

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I get a lot of requests for reuse of these kinds of images. People may absolutely use these in their presentations. I'd love to get an email if you're planning on using it! Please keep the credit line intact -- each image is from a different mission, who should get credit for the data, and Ted Stryk put in a lot of work processing those Halley and Borrelly images. Please contact me first before using it in a publication.>>