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The moon that isn't there.


Contrary to predictions, the most distant object known in the solar system solar system, the sun and the surrounding planets, natural satellites, dwarf planets, asteroids, meteoroids, and comets that are bound by its gravity. The sun is by far the most massive part of the solar system, containing almost 99.9% of the system's total mass.  doesn't appear to have a moon. According to according to
prep.
1. As stated or indicated by; on the authority of: according to historians.

2. In keeping with: according to instructions.

3.
 Hubble Space Telescope Hubble Space Telescope (HST), the first large optical orbiting observatory. Built from 1978 to 1990 at a cost of $1.5 billion, the HST (named for astronomer E. P. Hubble) was expected to provide the clearest view yet obtained of the universe.  images released last week, the remote body dubbed Sedna roams the solar system's edge without a partner, making its origin and evolution especially puzzling.

When Michael E. Brown
    Michael (Mike) E. Brown (born June 5, 1965) has been a professor of planetary astronomy at the California Institute of Technology (Caltech) since 2003. He was previously an associate professor at Caltech from 2002-2003 and an assistant professor at Caltech from
     of the California Institute of Technology California Institute of Technology, at Pasadena, Calif.; originally for men, became coeducational in 1970; founded 1891 as Throop Polytechnic Institute; called Throop College of Technology, 1913–20.  in Pasadena and his colleagues announced the discovery of Sedna on March 15 (SN: 3/20/04,p. 179), they were convinced that the so-called planetoid planetoid: see asteroid. , some 13 billion kilometers from the sun, must have an unseen moon. The researchers based their assumption on observations suggesting that Sedna rotates unusually slowly--just once every 20 days. Most bodies in the solar system rotate every 10 hours or so.

    The leisurely rotation can best be accounted for by the gravitational grav·i·ta·tion  
    n.
    1. Physics
    a. The natural phenomenon of attraction between physical objects with mass or energy.

    b. The act or process of moving under the influence of this attraction.

    2.
     tug of a companion, Brown says. "There is no other good scientific explanation for why something would rotate as slowly as 20 day's" he argued during a press briefing on April 14.

    However, he announced that 35 images that his team took with Hubble's Advanced Camera for Surveys The Advanced Camera for Surveys (ACS) is a third generation axial instrument aboard the Hubble Space Telescope (HST). The initial design and scientific capabilities of ACS were defined by a team based at Johns Hopkins University.  show no evidence of a moon.

    It's possible, although not likely, that a moon might have been hiding behind Sedna or was passing directly in front of it, so that it wouldn't have been apparent during the Hubble observations, says study collaborator David Rabinowitz of Yale University Yale University, at New Haven, Conn.; coeducational. Chartered as a collegiate school for men in 1701 largely as a result of the efforts of James Pierpont, it opened at Killingworth (now Clinton) in 1702, moved (1707) to Saybrook (now Old Saybrook), and in 1716 was  in New Haven, Conn. Alternatively, a moon might be too faint for Hubble to have detected it.

    Although the Hubble images were taken in blue light to record the sharpest possible pictures, Sedna still appears blurred. But the new images do reveal that Sedna's diameter is no bigger than three-quarters of that of Pluto. Brown says that now he'd like to schedule Hubble to take images at redder wavelengths, where a moon might appear brighter.

    Scott Tremaine of Princeton University says that one of the most likely explanations is that a moon that's since vanished slowed Sedna. Another planetoid passing close by might have yanked the moon from Sedna's gravitational grasp. Planetary scientist Alan W. Harris of the Space Science Institute in La Canada, Calif., had previously suggested a similar scenario for slowly rotating members of the asteroid belt, a reservoir of rocky bodies between the orbits of Mars and Jupiter.

    Another possibility is that Sedna's moon was smashed to bits by a collision with another object at the solar system's fringes, Brown adds.

    A more mundane explanation for the lack of a moon is that Sedna rotates more quickly than astronomers have considered. The team observed Sedna about once every 24 hours, so a rotation of slightly more than once around in that time would appear the same as the small amount of rotation reported. However Rabinowitz says, "It would require the unusual coincidence that Sedna's rotation period would have to be unusually close to the Earth's rotation period."

    Sedna is now too close to the sun for a good view from Earth, but next fall Brown's team plans to monitor the body for a full night to pin down its rotation speed.
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    Article Details
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    Title Annotation:Puzzle on the Edge
    Author:Cowen, R.
    Publication:Science News
    Date:Apr 24, 2004
    Words:514
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