This flyby will be of some interest because they are boring in close while aiming all their sensing gear forward. They may not survive but we should get our best looksee before that happens.
I would like to know what the dust content actually is, but the constituent I am looking for is elemental carbon which they are unlikely to detect.
Looking at the image of a comet made back in 2004 we are able to make a couple of conjectures. We had always assumed that the comet would be a so called dirty snowball.
This suggests that a process of sublimation is causing volatiles to migrate through to the surface and to form a thick crust leaving an internal core of assembled dust. As the comet approaches the star, the comet charges up presumably carrying off electrons in the dust and volatiles and driving the jets we are seeing.
I do not think that temperature has a lot to do with it at all, since we are seeing a cratering on the surface and I think little sign of flows.
I have other reasons to think that the dust cloud itself is formed from elemental carbon the bulk of which is off the surface and charging in the magnetic field of the sun to create the visible plume effect
The comet is slowly off gassing its volatiles and internal carbon and this helps sustain the plume over many orbits.
Thus an impact from a comet will be of a broken up assemblage of ice and dust that will quickly dissipate leaving traces behind while producing an elemental carbon event in the stratigraphic record.
NASA Spacecraft Hurtles Toward Active Comet Hartley 2
October 15, 2010: NASA's Deep Impact/EPOXI spacecraft is hurtling toward Comet Hartley 2 for a breathtaking 435-mile flyby on Nov. 4th. Mission scientists say all systems are go for a close encounter with one of the smallest yet most active comets they've seen.
Artist's concept of the spacecraft's previous encounter with Comet Tempel 1. [more]
"There are billions of comets in the solar system, but this will be only the fifth time a spacecraft has flown close enough to one to snap pictures of its nucleus," says Lori Feaga of the EPOXI science team. "This one should put on quite a show!"
Cometary orbits tend to be highly elongated; they travel far from the sun and then swing much closer. At encounter time, Hartley 2 will be nearing the sun and warming up after its cold, deep space sojourn. The ices in its nucleus will be vaporizing furiously – spitting dust and spouting gaseous jets.
"Hartley 2's nucleus is small, less than a mile in diameter," says Feaga. "But its surface offgasses at a higher rate than nuclei we've seen before. We expect more jets and outbursts from this one."
EPOXI will swoop down into the comet's bright coma – the sparkling aura of debris, illuminated by the sun – shrouding the nucleus. The spacecraft's cameras, taking high-resolution (7 meters per pixel at closest approach) pictures all the while, will reveal this new world in all its fizzy glory.
"We hope to see features of the comet's scarred face: craters, fractures, vents," says Sebastien Besse of the science team. "We may even be able to tell which features are spewing jets!"
The spacecraft's instruments are already trained on their speeding target.
"We're still pretty far out, so we don't yet see a nucleus," explains Besse. "But our daily observations with the spectrometer and cameras are already helping us identify the species and amounts of gases in the coma and learn how they evolve over time as we approach."
Comet Hartley 2, photographed on Oct. 13 by Science@NASA reader Nick Howes using the 2-meter Faulkes North Telescope in Hawaii
The aim of the mission is to gather details about what the nucleus is made of and compare it to other comets. Because comets spend much of their time far from the sun, the cold preserves their composition – and that composition tells a great story.
"Comets are left-overs from the 'construction' of our solar system," explains Besse. "When the planets formed out of the 'stuff' in the solar nebula spinning around the sun, comets weren't drawn in."
Researchers study these pristine specimens of the primal solar system to learn something about how it formed, and how it birthed a life-bearing planet like Earth.
"These flybys help us figure out what happened 4 1/2 billion years ago," says Feaga. "So far we've only seen four nucleii. We need to study more comets to learn how they differ and how they are the same. This visit will help, especially since Hartley 2 is in many ways unlike the others we've seen."
EPOXI will provide not only a birds-eye view of a new world but the best extended view of a comet in history.
"This spacecraft is built for close encounters. Its instruments and our planned observations are optimized for this kind of mission. When, as Deep Impact, it flew by Tempel 1, it turned its instruments away from the nucleus to protect them from debris blasted up by the impactor. This time we won't turn away."
The EPOXI team will be waiting at NASA's Jet Propulsion Laboratory.
"We'll start diving into the data as soon as we receive it," says Feaga. "We'll work round the clock, on our toes the whole time, waiting for the next thing to come down."
Sounds like it could be intense.
"It's already intense," says Besse. "We're getting more and more data, but at encounter we'll be flooded!"
And that will be only the beginning.
NASA Spacecraft Reveals Surprising Anatomy Of A Comet 06.17.04
Findings from a historic encounter between NASA's Stardust spacecraft and a comet have revealed a much stranger world than previously believed. The comet's rigid surface, dotted with towering pinnacles, plunging craters, steep cliffs, and dozens of jets spewing violently, has surprised scientists.
Image above: This image and diagram show the comet Wild 2, which NASA's Stardust spacecraft flew by on Jan. 2, 2004. The picture on the left is the closest short exposure of the comet. The listed names on the right are those used by the Stardust team to identify features. "Basin" does not imply an impact origin.
+ Click for full image. Image credit: NASA/JPL-Caltech.
Animation: This movie strings together a series of still images of comet Wild 2 taken during Stardust's historic flyby of the comet. + Click for animation. Animation credit: NASA/JPL-Caltech.
"We thought Comet Wild 2 would be like a dirty, black, fluffy snowball," said Stardust Principal Investigator Dr. Donald Brownlee of the
Stardust gathered the images on Jan. 2, 2004, when it flew 236 kilometers (about 147 miles) from Wild 2. The flyby yielded the most detailed, high-resolution comet images ever.
"We know Wild 2 has features sculpted by many processes. It may turn out to be typical of other comets, but it is unlike any other type of solar system body," Brownlee said. He is lead author of one of four Stardust papers appearing in the Fri., June 18, issue of Science. "We're fortunate that nature gave us such a rich object to study."
Stardust images show pinnacles 100 meters tall (328 feet), and craters more than 150 meters deep (492 feet). Some craters have a round central pit surrounded by ragged, ejected material, while others have a flat floor and straight sides. The diameter of one large crater, called Left Foot, is one fifth of the surface of the comet. Left Foot is one kilometer (.62 miles) across, while the entire comet is only five kilometers (3.1 miles) across.
"Another big surprise was the abundance and behavior of jets of particles shooting up from the comet's surface. We expected a couple of jets, but saw more than two dozen in the brief flyby," said Dr. Benton Clark, chief scientist of space exploration systems, Lockheed Martin Space Systems, Denver.
The team predicted the jets would shoot up for a short distance, and then be dispersed into a halo around Wild 2. Instead, some super-speedy jets remained intact, like blasts of water from a powerful garden hose. This phenomenon created quite a wild ride for Stardust during the encounter.
"Stardust was absolutely pummeled. It flew through three huge jets that bombarded the spacecraft with about a million particles per second," said Thomas Duxbury, Stardust project manager at NASA's Jet Propulsion Laboratory, Pasadena, Calif. Twelve particles, some larger than a bullet, penetrated the top layer of the spacecraft's protective shield.
The violent jets may form when the Sun shines on icy areas near or just below the comet's surface. The solid ice becomes a gas without going through a liquid phase. Escaping into the vacuum of space, the jets blast out at hundreds of kilometers per hour.
The Stardust team theorizes sublimation and object hits may have created the comet's distinct features. Some features may have formed billions of years ago, when life began on Earth, Brownlee said. Particles collected by Stardust during the Wild 2 encounter may help unscramble the secrets of how the solar system formed.
Stardust was launched in 1999. It is zooming back to Earth with thousands of captured particles tucked inside a capsule. The capsule will make a soft landing in the
Comets have been objects of fascination through the ages. Many scientists believe they delivered carbon and water, life's building blocks, to Earth. Yet their destructive potential is illustrated by the widely held theory that a comet or asteroid wiped out the dinosaurs.
To view Stardust images on the Internet
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