Just like extended deep-sky objects they can appear dim because their light fans over a large area. Faint-looking comets with large comas are sometimes assigned surprisingly bright magnitudes for this reason. In a common one, called the In-Out method, the in-focus comet is compared to stars of known magnitude that are defocused to match its diameter. When making a magnitude estimate observers use several different variations on a theme. Use averted vision when making your estimate. Magnitude 8 or 10 - it dependsĬomet Hartley's large, diffuse coma can make it tricky to determine exactly how far it extends. Larger instruments with more restricted fields of view may not show the coma as large and lead to fainter estimates. With an expansive field of view and dark skies Comet 103P/Hartley's diffuse coma balloons to 10′ to 12′ across. Recent magnitude estimates on the Comet Observation database (COBS) site show it around magnitude 8–8.5 for observers using large binoculars. Its apparent brightness rather depends on your instrument. The comet passed closest to Earth on September 26th at 56.9 million kilometers (35.3 million miles) and will reach perihelion on October 12th at a little more than one Earth-Sun distance. In contrast, the D/H ratio for Comet 67P/Churyumov-Gerasimenko is more than four times higher. It's thought that asteroids and at least some comets contributed to Earth's water reserves once it cooled down enough for water to pool on its surface. Both have a similar ratio of deuterium (an isotope of hydrogen) to hydrogen, or D/H. You'll be happy to know that after proper filtering you'd find a cup of Comet Hartley's water nearly as tasty as that from your faucet. Curiously, most of the carbon dioxide shoots out the ends of the comet in geyser-like jets while water lofts from its midsection. Outgassing from the core revealed the presence of water ice, methanol (also known as wood alcohol), carbon dioxide, and possibly methane. The comet's diameter is about 1.6 kilometers (1 mile).ĭuring the flyby the spacecraft came within 694 kilometers (431 miles) of the peanut-shaped nucleus. Shiny blocks of material approximately 50 meters high and 80 meters wide litter the surface. Hartley quoted her as saying, "Oh, we must fly, we must all fly to the comet." Take a sipĭuring the November 4, 2010, flyby Deep Space (EPOXI) mission scientist Don Yeomans described Comet Hartley as "hyperactive, small and feisty." The spacecraft imaged numerous jets, especially at one end of the bowling-pin-shaped object, shooting gases and dust in a showerhead formation. When he departed for the U.S., she thought grandpa was leaving to go to the comet. At the time his three-year-old granddaughter wanted to go to the comet so badly she begged him to come along. NASA officials invited Hartley to the festivities during the flyby. Little did he know then that his comet would be targeted for a flyby by NASA's Deep Impact (EPOXI) mission in November 2010, one of just eight comets visited by spacecraft to date. At the time he described it as a faint streak. Malcolm Hartley discovered his namesake in 1986 while examining plates taken with the UK Schmidt Telescope at Siding Spring Observatory in New South Wales, Australia. Jupiter-tamed comets have shorter periods with orbits shaped by the gas giant. Comet 103P/Hartley is a Jupiter-family comet that orbits between Mars and Jupiter with an average distance from the Sun of 3.5 a.u. That makes its current appearance the first for visual observers since 2010–11. The comet last reached perihelion in April 2017 during an unfavorable apparition when it clung near the Sun and never brightened much beyond 12th magnitude. Comet Hartley glows bright green from diatomic carbon (C 2) emission and displays a short tail pointing almost due west in this image from September 26, 2023.Įvery 6.5 years Comet 103P/Hartley swings back into the inner solar system and grows a fluffy coma and silky tail under the Sun's harsh gaze.
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