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EPOXI at Comet Hartley 2 Michael F. A'Hearn, et al. Science 332, 1396 (2011); DOI: 10.1126/science.1204054 This copy is for your personal, non-commercial use only. If you wish to distribute this article to others, you can order high-quality copies for your colleagues, clients, or customers by clicking here. Permission to republish or repurpose articles or portions of articles can be obtained by following the guidelines here. The following resources related to this article are available online at www.sciencemag.org (this infomation is current as of November 11, 2011 ): A correction has been published for this article at: http://www.sciencemag.org/content/333/6048/1381.2.full.html Updated information and services, including high-resolution figures, can be found in the online version of this article at: http://www.sciencemag.org/content/332/6036/1396.full.html Supporting Online Material can be found at: http://www.sciencemag.org/content/suppl/2011/06/15/332.6036.1396.DC1.html on November 11, 2011 A list of selected additional articles on the Science Web sites related to this article can be found at: http://www.sciencemag.org/content/332/6036/1396.full.html#related This article cites 51 articles, 5 of which can be accessed free: http://www.sciencemag.org/content/332/6036/1396.full.html#ref-list-1 This article appears in the following subject collections: Planetary Science www.sciencemag.org http://www.sciencemag.org/cgi/collection/planet_sci Downloaded from Science (print ISSN 0036-8075; online ISSN 1095-9203) is published weekly, except the last week in December, by the American Association for the Advancement of Science, 1200 New York Avenue NW, Washington, DC 20005. Copyright 2011 by the American Association for the Advancement of Science; all rights reserved. The title Science is a registered trademark of AAAS. RESEARCH ARTICLES Prior remote sensing showed that Hartley 2’s nucleus has an average radius 1/5 that of comet Tempel 1’s nucleus (5, 6), yet it releases more gas EPOXI at Comet Hartley 2 per unit time at perihelion, even when allowing for the smaller perihelion distance of Hartley 2 Michael F. A’Hearn,1* Michael J. S. Belton,2 W. Alan Delamere,3 Lori M. Feaga,1 (1.059 versus 1.506 AU). This puts Hartley 2 in a Donald Hampton,4 Jochen Kissel,5 Kenneth P. Klaasen,6 Lucy A. McFadden,1,7 different class of activity than that of Tempel 1 or Karen J. Meech,8 H. Jay Melosh,9,10 Peter H. Schultz,11 Jessica M. Sunshine,1 any of the other comets visited by spacecraft Peter C. Thomas,12 Joseph Veverka,12 Dennis D. Wellnitz,1 Donald K. Yeomans,6 (fig. S1). The two comets have very different Sebastien Besse,1 Dennis Bodewits,1 Timothy J. Bowling,10 Brian T. Carcich,12 surface topography (Fig. 1), but whether the dif- Steven M. Collins,6 Tony L. Farnham,1 Olivier Groussin,13 Brendan Hermalyn,11 ferent topography is related to the hyperactivity is Michael S. Kelley,1 Michael S. Kelley,14 Jian-Yang Li,1 Don J. Lindler,15 still being investigated. Carey M. Lisse,16 Stephanie A. McLaughlin,1 Frédéric Merlin,1,17 Silvia Protopapa,1 James E. Richardson,10 Jade L. Williams1 The Nucleus Spin state and variations. The rotation state of the Understanding how comets work—what drives their activity—is crucial to the use of comets in nucleus distinguishes the morning from the eve- studying the early solar system. EPOXI (Extrasolar Planet Observation and Deep Impact Extended ning terminator and, in comparison with longer- Investigation) flew past comet 103P/Hartley 2, one with an unusually small but very active nucleus, term coma observations, allows the number and taking both images and spectra. Unlike large, relatively inactive nuclei, this nucleus is outgassing relative strengths of active areas to be determined. primarily because of CO2, which drags chunks of ice out of the nucleus. It also shows substantial Knowledge of the nuclear spin can also put con- differences in the relative abundance of volatiles from various parts of the nucleus. straints on internal distribution of mass in the nucleus, internal energy dissipation, and the mag- omets are the fundamental building blocks The flyby spacecraft carries the High Resolu- nitude of the net torque. of the giant planets and may be an im- tion Instrument (HRI), which combines a visible- The large variations in brightness in Fig. 2, Cportant source of water and organics on wavelength camera with a pixel size of 2 mrad reduced to a measure of the amount of dust Earth. On 4 July 2005, the Deep Impact mis- and a set of filters with a near-infrared (near-IR) leaving the nucleus, show a period of roughly 18 sion carried out an impact experiment on comet (1.05to4.85mm) spectrometer with an entrance hours, but the spacing of peaks in the light curve on November 11, 2011 9P/Tempel 1 (1, 2) to study differences between slit of 10 mrad by 256 mrad, with 512 spatial shows a clear pattern that repeats every three cy- the comet’s surface and the interior. Although pixels along the slit. Spectral maps were created cles. We interpret this [supporting online mate- the impactor spacecraft was destroyed, the flyby by scanning the slit across the comet while tak- rial (SOM) text] as an excited state of rotation, spacecraft and its instruments remained healthy ing a sequence of spectra. The Medium Resolu- with each cycle corresponding to precession of in its 3-year, heliocentric orbit after completion tion Instrument (MRI) has a pixel size of 10 mrad the long axis of the nucleus around the angular of the mission. The Deep Impact flyby spacecraft and a different but overlapping set of visible- momentum vector, with a period of 18.34 hours was retargeted to comet 103P/Hartley 2 as part of wavelength filters (3, 4). at encounter. The pattern of three cycles is due an extended mission named EPOXI (Extrasolar to an approximate commensurability between Planet Observation and Deep Impact Extended Encounter with Hartley 2 this precession and the roll around the long axis www.sciencemag.org Investigation). The closest approach to Hartley 2 was 694 km at with a period of 27.79 hours (55.42 hours is 13:59:47.31 UTC on 4 November 2010, 1 week also possible; the ambiguity does not affect any after perihelion passage and at 1.064 astronom- conclusions in this paper). The orientation of 1Department of Astronomy, University of Maryland, College ical units (AU) from the Sun. Flyby speed was the angular momentum vector is not yet tightly –1 Park, MD 20742-2421 USA. 2Belton Space Exploration Ini- 12.3 km s , and the spacecraft flew under the constrained but is within 10° of being perpen- tiatives LLC, 430 South Randolph Way, Tucson, AZ 85716 USA. comet with a somewhat northward trajectory in a dicular to the long axis. This excited state also 3Delamere Support Services, 525 Mapleton Avenue, Boulder, 4 solar system reference frame. Because instru- implies a nodding motion of the long axis rel- CO 80304, USA. Geophysical Institute, University of Alaska– Downloaded from Fairbanks, 903 Koyukuk Drive, Fairbanks, AK 99775–7320, USA. ments are body-mounted on the spacecraft, the ative to the angular momentum vector, but the 5Max-Planck-Institut für Sonnensystemforschung, Max-Planck- spacecraft rotated to keep the instruments pointed observed near-axial symmetry of the shape lim- Strasse 2, 37191 Katlenburg-Lindau, Germany. 6Jet Propulsion at the comet. Observations of the comet were its this to an amplitude of <1°. The precession Laboratory, 4800 Oak Grove Drive, Pasadena CA 91109, USA. carried out for 2 months on approach (5 Sep- period is increasing at 0.1% per period near 7Code 600, NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA. 8Institute for Astronomy, University of Hawaii, tember to 4 November) and for 3 weeks on de- perihelion, which is an unusually high but not 2680 Woodlawn Drive, Honolulu, HI 96822, USA. 9Lunar and parture (4 to 26 November), during which more unprecedented rate of change for a comet. The Planetary Library, University of Arizona, 1629 East University than 105 images and spectra were obtained. roll period is decreasing. These changes are Boulevard, Tucson, AZ 85721–0092, USA. 10Department of Earth and Atmospheric Sciences, Purdue University, 550 Sta- 11 dium Mall Drive, West Lafayette, IN 47907, USA. Department Fig. 1. Comparison of a of Geological Sciences, Brown University, Providence, RI 02912, smallpartof(left)Tem- USA. 12Department of Astronomy, 312 Space Sciences Build- ing, Cornell University, Ithaca, NY 14853, USA. 13Laboratoire pel 1 with (right)Hartley d’Astrophysique de Marseille, Universitéde Provence and CNRS, 2 at approximately the 13013 Marseille, France. 14Planetary Science Division, NASA same image scale and Headquarters, Mail Suite 3V71, 300 E Street SW, Washington, with nearly identical in- 15 DC 20546, USA. Sigma Space Corporation, 4600 Forbes Bou- struments. (Left) Impac- levard, Lanham, MD 20706, USA. 16Johns Hopkins University– Applied Physics Laboratory, 11100 Johns Hopkins Road, Laurel, tor Targeting Sensor (ITS) MD 20723, USA. 17LESIA, Observatoire de Paris, UniversitéParis image iv9000675, 9.1 m –1 7, Batiment 17, 5 place Jules Janssen, Meudon Principal Cedex pixel .(Right)MRIim- 92195, France. age mv5004032, 8.5 m –1 *To whom correspondence should be addressed. E-mail: pixel . Sun is to the right. [email protected] 1396 17 JUNE 2011 VOL 332 SCIENCE www.sciencemag.org RESEARCH ARTICLES presumed to be due to torques produced by the because the waist is no longer a gravitational low forms that make up the knobby terrain. There is outgassing.