* Astronomy

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RE: Comet 103P/Hartley 2

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Hyperactive Hartley 2 has a split history

The latest analysis of data from NASA's Deep Impact spacecraft shows that comet 103P/Hartley 2 is hyperactive in terms of the material it spews out, compared to the other comets observed up close to date. The comet also shows surprising diversity - ice on the comet's sunlit surface is found in patches that are isolated from areas of dust. In addition, one lobe of the dog-bone shaped comet may have lost much more of the primordial material from the formation of the comet than the other, suggesting that Hartley 2 was originally two comets that came together in a gentle collision. Mike A'Hearn and Lori Feaga will be presenting their findings at the EPSC-DPS Joint Meeting 2011 in Nantes, France.
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Comet Hartley 2 Leaves a Bumpy Trail

New findings from NEOWISE, the asteroid- and comet-hunting portion of NASA's Wide-field Infrared Survey Explorer mission, show that comet Hartley 2 leaves a pebbly trail as it laps the sun, dotted with grains as big as golf balls.
Previously, NASA's EPOXI mission, which flew by the comet on Nov. 4, 2010, found golf ball- to basketball-sized fluffy ice particles streaming off comet Hartley 2. NEOWISE data show that the golf ball-sized chunks survive farther away from the comet than previously known, winding up in Hartley 2's trail of debris. The NEOWISE team determined the size of these particles by looking at how far they deviated from the trail. Larger particles are less likely to be pushed away from the trail by radiation pressure from the sun.
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Title: WISE/NEOWISE observations of comet 103P/Hartley 2
Authors: James M. Bauer, Russell G. Walker, A. K. Mainzer, Joseph R. Masiero, Tommy Grav, John W. Dailey, Robert S. McMillan, Carey M. Lisse, Yan R. Fernández, Karen J. Meech, Jana Pittichova, Erin K. Blauvelt, Frank J. Masci, Michael F. A'Hearn, Roc M. Cutri, James V. Scotti, David J. Tholen, Emily DeBaun, Ashlee Wilkins, Emma Hand, Edward L. Wright, the WISE Collaboration

We report results based on mid-infrared photometry of comet 103P/Hartley 2 taken during May 4-13, 2010 (when the comet was at a heliocentric distance of 2.3 AU, and an observer distance of 2.0 AU) by the Wide-field Infrared Survey Explorer (Wright et al. 2010). Photometry of the coma at 22 microns and data from the University of Hawaii 2.2-m telescope obtained on May 22, 2010 provide constraints on the dust particle size distribution, dlogn/dlogm, yielding power-law slope values of alpha = -0.97 ±0.10, steeper than that found for the inbound particle fluence during the Stardust encounter of comet 81P/Wild 2 (Green et al. 2004). The extracted nucleus signal at 12 microns is consistent with a body of average spherical radius of 0.6 ±0.2 km (one standard deviation), assuming a beaming parameter of 1.2. The 4.6 micron-band signal in excess of dust and nucleus reflected and thermal contributions may be attributed to carbon monoxide or carbon dioxide emission lines and provides limits and estimates of species production. Derived carbon dioxide coma production rates are 3.5(± 0.9) x 10^24 molecules per second. Analyses of the trail signal present in the stacked image with an effective exposure time of 158.4 seconds yields optical-depth values near 9 x 10^-10 at a delta mean anomaly of 0.2 deg trailing the comet nucleus, in both 12 and 22 {mu}m bands. A minimum chi-squared analysis of the dust trail position yields a beta-parameter value of 1.0 x10^-4, consistent with a derived mean trail-grain diameter of 1.1/{rho} cm for grains of {rho} g/cm^3 density. This leads to a total detected trail mass of at least 4 x 10^10 {rho} kg.

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UMD-Led EPOXI Science Team Publishes Latest Comet Findings in Science

Comet Hartley 2, is in a hyperactive class of its own compared to other comets visited by spacecraft, says a University of Maryland-led study published in the June 17 issue of the journal Science. The comet was visited last fall by NASA's Deep Impact spacecraft during its EPOXI mission. The EPOXI science team's new, in-depth analysis of the images and data taken during the flyby confirms its principal earlier finding that carbon dioxide is the volatile fuel for Hartley 2's ice-spewing jets.
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Tracking Tumbling Comets

A tumbling comet nucleus with a changing rotational rate has been observed for the first time, according to a new paper by a Planetary Science Institute researcher.
These findings, as well as information gleaned from a recent NASA EPOXI spacecraft flyby of Comet 103P/Hartley 2, are expected to offer new insights as researchers strive to better understand comets and the role they could possibly play in aiding human solar system exploration, said Nalin H. Samarasinha, senior scientist at PSI and lead author of a paper titled Rotation of Comet 103P/Hartley 2 from Structures in the Coma (PDF) that appears in Astrophysical Journal Letters.
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Researchers gain new insights into Comet Hartley 2

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At the Heart of Hartley-2, a New Breed of Comet?

At the heart of every comet lies a remnant of the dawn of the solar system. Or is that remnants? Astronomers don't know, but the answer would give them a clearer picture of exactly how comets were born eons ago at the birth of the Solar System. Did thin tendrils of dust and ice get drawn slowly inward and pack themselves into a single, uniform mass? Or did a hodge-podge of mini-comets come together to form the core for a comet of substance?
For Hartley-2, the answer so far is neither.
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Title: Water Production by Comet 103P/Hartley 2 Observed with the SWAN Instrument on the SOHO Spacecraft
Authors: M.R. Combi, J.-L. Bertaux, E. Quémerais, S. Ferron, J.T.T. Mäkinen

Global water production rates were determined from the Lyman-{\alpha} emission of hydrogen around comet 103P/Hartley 2, observed with the SWAN (Solar Wind ANisotropies) all-sky camera on the SOHO spacecraft from September 14 through December 12, 2010. This time period included the November 4 flyby by the EPOXI spacecraft. Water production was 3 times lower than during the 1997 apparition also measured by SWAN. In 2010 it increased by a factor of ~2.5 within one day on September 30 with a similar corresponding drop between November 24 and 30. The total surface area of sublimating water within ±20 days of perihelion was ~0.5 km², about half of the mean cross section of the nucleus. Outside this period it was ~0.2 km². The peak value was 90%, implying a significant water production by released nucleus icy fragments.

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Title: Rotation of Comet Hartley 2 from Structures in the Coma
Authors: Nalin H. Samarasinha, Beatrice E. A. Mueller, Michael F. A'Hearn, Tony L. Farnham, Alan Gersch

The CN coma structure of the EPOXI mission target, comet 103P/Hartley 2, was observed during twenty nights from September to December 2010. These CN images probe the rotational state of the comet's nucleus and provide a ground-based observational context to complement the EPOXI observations. A dynamically excited cometary nucleus with a changing rotational rate is observed, a characteristic not seen in any comet in the past. The lack of rotational damping during the four-month observing interval places constraints on the interior structure of the nucleus.

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Title: The Carbon Monoxide Abundance in Comet 103P/Hartley during the EPOXI Flyby
Authors: H. A. Weaver, P. D. Feldman, M. F. A'Hearn, N. Dello Russo, S. A. Stern

We report the detection of several emission bands in the CO Fourth Positive Group from comet 103P/Hartley during ultraviolet spectroscopic observations from the Hubble Space Telescope (HST) on 2010 November 4 near the time of closest approach by NASA's EPOXI spacecraft. The derived CO/H2O ratio is 0.15-0.45%, which places 103P among the most CO-depleted comets. Apparently this highly volatile species, whose abundance varies by a factor of ~50 among the comets observed to date, does not play a major role in producing the strong and temporally variable activity in 103P/Hartley. The CO emissions varied by ~30% between our two sets of observations, apparently in phase with the temporal variability measured for several gases and dust by other observers. The low absolute abundance of CO in 103P suggests several possibilities: the nucleus formed in a region of the solar nebula that was depleted in CO or too warm to retain much CO ice, repeated passages through the inner solar system have substantially depleted the comet's primordial CO reservoir, or any CO still in the nucleus is buried below the regions that contribute significantly to the coma.

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