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Pasadena CA (JPL) Oct 30, 2010 Exactly one week before the world gets a new look at comet Hartley 2 via NASA's EPOXI mission, observations of the comet by the Arecibo Planetary Radar in Puerto Rico have offered scientists a tantalizing preview. "It kind of looks like a cross between a bowling pin and a pickle," said EPOXI project manager Tim Larson of NASA's Jet Propulsion Laboratory in Pasadena, Calif. "Only it's about 14-thousand-times larger and hurtling through space at 23 miles per second." Scientists using Arecibo's massive radar dish began observations of Hartley 2 on Oct. 24, just four days after the comet made its closest approach to Earth since its discovery in 1986. (On Oct. 20, the comet came within 17.7 million kilometers, or 11 million miles, of Earth). The observations are scheduled to continue through Friday, Oct. 29. During the Nov. 4 flyby, the cameras aboard the EPOXI mission spacecraft will get within 700 kilometers (about 435 miles) of the comet. "Observing comet Hartley 2 from the Earth with radar was like imaging a 6-inch spinning cucumber from 836 miles away," said Jon Giorgini, a scientist at JPL and a member of the Arecibo team that imaged the comet. "Even without all the data in, we can still make some basic assertions about Hartley 2. Its nucleus is highly elongated and about 2.2 kilometers [1.4 mile] long, and it rotates around itself about once every 18 hours. In addition we now know the size, speed and direction of particles being blown off the comet, and we immediately forwarded all this information to the EPOXI team." Just what a celestial pickle means for the EPOXI mission remains to be seen. Mission engineers and scientists are discussing the new findings and what - if anything - they signify for the upcoming comet encounter. Along with Giorgini, observations of comet Hartley 2 were led by Arecibo Obervatory's John Harmon, with contributions by Mike Nolan and E. S. Howell. The name EPOXI itself is a combination of the names for the two extended mission components: the extrasolar planet observations, called Extrasolar Planet Observations and Characterization (EPOCh), and the flyby of comet Hartley 2, called the Deep Impact Extended Investigation (DIXI). The spacecraft will continue to be referred to as "Deep Impact."
Image Citation in full: COMET 103P/HARTLEY J. K. Harmon, M. C. Nolan, E. S. Howell (Arecibo Observatory) and J. D. Giorgini (Jet Propulsion Laboratory) obtained 150-m-resolution radar images and radar doppler spectra for Comet 103P/Hartley on 2010 Oct. 24-27 using the Arecibo Observatory 12.6-cm planetary radar. The images show the nucleus to be a highly elongated, bilobate object with a long-axis dimension of at least 2.2 km.
The images give a preliminary rotation period estimate of 18.1 +/- 0.3 hours, although a less likely period of 13.2 hours cannot be ruled out. The radar cross section of the nucleus is 0.04 km^{2}. The doppler spectra show a broadband echo component from large (> cm-size) ejected grains in the inner coma.
The radar cross section of the large-grain coma is 0.6 km^{2}. This echo component is preferentially redshifted, indicating that the bulk of the grain ejection is in the anti-earthward direction. The characteristic radial velocity dispersion of the grains is 4 m/s.
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NASA Spacecraft Preps For Comet FlybyPasadena CA (JPL) Oct 29, 2010 In one of its final mission trajectory correction maneuvers, the EPOXI mission spacecraft has refined its orbit, preparing it for the flyby of comet Hartley 2 on Nov. 4. The time of closest approach to the comet on that day is expected to be about 7:02 a.m. PDT (10:02 a.m. EDT). Today's trajectory correction maneuver began at 11 a.m. PDT (2 p.m. EDT), when the spacecraft burned its engines ... read more |
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