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Nov. 15, 2013: What are the odds? On Nov. 18th and 19th not one but two comets will fly by the planet Mercury.
"This is a unique coincidence," says Ron Vervack an astronomer at the Johns Hopkins University Applied Physics Lab and a member of the science team for NASA's MESSENGER spacecraft, "and a golden opportunity to study two comets passing close to the sun.”
On Nov. 18th Comet Encke will pass within 0.025 AU of Mercury, followed a day later by Comet ISON at 0.24 AU (1 AU is the distance between the sun and Earth, 150 million km). The MESSENGER spacecraft, which is orbiting Mercury, will turn its sensors toward the passing comets for a point-blank investigation of both.The double flyby is exciting, says Vervack, but "it makes things a little crazy. We have to rush to complete our observations of Comet Encke, then do it all over again for Comet ISON. Everything is happening at more or less the same time."
MESSENGER was designed to study Mercury, not comets, “but it is a capable spacecraft with a versatile instrument package,” he adds. “We hope to get some great data.” Onboard spectrometers will analyze the chemical makeup of the two comets while MESSENGER's cameras snap pictures of atmospheres, jets and tails. Comet Encke is less well known, but no less interesting. For one thing, it is the source of the Taurid meteor shower, a slow display of midnight fireballs that occurs every year in early- to mid-November. Comet Encke dips inside the orbit of Mercury every 3.3 years, so it is regularly exposed to solar activity.
"We'll be catching Comet Encke just days before its closest approach to the sun (0.3 AU)," Vervack says, "so we get to see it at its most active."
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| Dates acquired: 2013 (UT): Nov. 6, 7, 8 (Encke) and Nov. 9, 10, 11 (ISON) Image Mission Elapsed Time (MET): 26100552, 26190552, 26276500 (Encke), 26330792, 26417208, 26503610 (ISON) Image ID: 1026100552, 1026190552, 1026276500 (Encke), 1026330792, 1026417208, 1026503610 (ISON) Instrument: Wide Angle Camera (WAC) of the Mercury Dual Imaging System (MDIS) WAC filter: Filter 2 (700 nm) Resolution: 38 seconds of arc per pixel for Encke, 76 for ISON Scale: Each tile: 2.85 deg. wide by 0.745 deg. high (Encke), 5.7 deg. wide by 1.49 deg. high (ISON) Of Interest: As the new comet C/2012 S1 (ISON) and the well-known short-period comet 2P/Encke both approach their closest distances to the Sun later this month, they are also passing close to the MESSENGER spacecraft now orbiting the innermost planet Mercury. Just this week, both comets have brightened sufficiently to be captured in images by MESSENGER’s wide-angle camera. This figure shows, on the left, images of Encke on three successive days from November 6 to November 8; on the right, images of ISON are shown for three successive days from November 9 to November 11. Both comets appear to brighten each day (top to bottom). Green circles indicate some of the brighter cataloged background stars in each image; yellow marks bracket the comets. The background stars are fixed in each image from top to bottom, so the motion of the comet relative to the stars is clear in each case. The images have been smoothed slightly to diminish the distracting sharp edges of the pixels. MESSENGER is viewing these comets from a vantage point that is very different from that of observers on Earth. Comet Encke was approximately 0.5 AU from the Sun and 0.2 AU from MESSENGER when these images were taken; the same distances were approximately 0.75 AU and 0.5 AU, respectively, for ISON. More images will be obtained starting on November 16 when the comets should be both brighter and closer to Mercury. The MESSENGER spacecraft is the first ever to orbit the planet Mercury, and the spacecraft's seven scientific instruments and radio science investigation are unraveling the history and evolution of the Solar System's innermost planet. During the first two years of orbital operations, MESSENGER acquired more than 150,000 images and extensive other data sets. MESSENGER is scheduled to continue orbital operations until late March 2015. |
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Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington/Southwest Research Institute |
Ironically, the fact that MESSENGER is designed to study a rocky planet could prove advantageous for the icy comets. MESSENGER's x-ray spectrometer, in particular, could detect signs of ‘comet dirt’.
"We hope to obtain the first definitive detections of x-ray emissions from silicon, magnesium and aluminum," he explains. "If you think of a comet as a dirty snowball, these are elements that make up the dirt. Close to the sun is where we expect the dirt to be vaporized."
In total, Vervack expects MESSENGER to gather 15 hours’ worth of data on Comet Encke and another 25 hours on Comet ISON. With that kind of observing time, discoveries are a distinct possibility.
Vervack says the first images will be beamed back and released to the public within days of the flybys. “There are no guarantees,” he cautions, “but I can’t wait to see the pictures.”
Quelle: NASA