Title

Spitzer Space Telescope Observations of the Nucleus of Comet 103P/Hartley 2

Authors

Authors

C. M. Lisse; Y. R. Fernandez; W. T. Reach; J. M. Bauer; M. F. A'Hearn; T. L. Farnham; O. Groussin; M. J. Belton; K. J. Meech;C. D. Snodgrass

Comments

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Abbreviated Journal Title

Publ. Astron. Soc. Pac.

Keywords

DEEP IMPACT OBSERVATIONS; CCD PHOTOMETRY; CENTAURS; 46P/WIRTANEN; 9P/TEMPEL-1; TEMPEL-1; SURFACE; CHIRON; EJECTA; TRAILS; Astronomy & Astrophysics

Abstract

We have used the Spitzer Space Telescope InfraRed Spectrograph (IRS) 22-mu m peakup array to observe thermal emission from the nucleus and trail of comet 103P/Hartley 2, the target of NASA's Deep Impact Extended Investigation (DIXI). The comet was observed on UT 2008 August 12 and 13, while 5.5 AU from the Sun. We obtained two 200 frame sets of photometric imaging over a 2.7 hr period. To within the errors of the measurement, we find no detection of any temporal variation between the two images. The comet showed extended emission beyond a point source in the form of a faint trail directed along the comet's antivelocity vector. After modeling and removing the trail emission, a NEATM model for the nuclear emission with beaming parameter of 0.95 +/- 0.20 indicates a small effective radius for the nucleus of 0.57 +/- 0.08 km and low geometric albedo 0.028 +/- 0.009 (1 sigma). With this nucleus size and a water production rate of 3 x 10(28) molecules s(-1) at perihelion, we estimate that similar to 100% of the surface area is actively emitting volatile material at perihelion. Reports of emission activity out to similar to 5 AU support our finding of a highly active nuclear surface. Compared to Deep Impact's first target, comet 9P/Tempel 1, Hartley 2's nucleus is one-fifth as wide (and about one-hundredth the mass) while producing a similar amount of outgassing at perihelion with about 13 times the active surface fraction. Unlike Tempel 1, comet Hartley 2 should be highly susceptible to jet driven spin-up torques, and so could be rotating at a much higher frequency. Since the amplitude of nongravitational forces are surprisingly similar for both comets, close to the ensemble average for ecliptic comets, we conclude that comet Hartley 2 must have a much more isotropic pattern of time-averaged outgassing from its nuclear surface. Barring a catastrophic breakup or major fragmentation event, the comet should be able to survive up to another 100 apparitions (similar to 700 yr) at its current rate of mass loss.

Journal Title

Publications of the Astronomical Society of the Pacific

Volume

121

Issue/Number

883

Publication Date

1-1-2009

Document Type

Article

Language

English

First Page

968

Last Page

975

WOS Identifier

WOS:000269232300004

ISSN

0004-6280

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