Rock Around the Comet Clock with Hubble

These photos, taken on April 4, 2016 over the span of 4 1/2 hours, reveal a narrow, well-defined jet of dust ejected by the comet’s icy nucleus. With a diameter of only about a mile, the nucleus is too small for Hubble to see. The jet is illuminated by sunlight and changes direction like the hour hand on a clock as the comet spins on its axis. Credit: NASA, ESA, and J.-Y. Li (Planetary Science Institute)

Remember 252P/LINEAR? This comet appeared low in the morning sky last month and for a short time grew bright enough to see with the naked eye from a dark site. 252P swept closest to Earth on March 21, passing just 3.3 million miles away or about 14 times the distance between our planet and the moon. Since then, it’s been gradually pulling away and fading though it remains bright enough to see in small telescope during late evening hours.

252P LINEAR looks like a big fuzzy ball in this photo taken on April 30. The comet is located in Ophiuchus and rises in the eastern sky at nightfall. At this scale, the jet shown in the Hubble photos is too tiny to see. See map below to find the comet yourself. Credit: Rolando Ligustri

While amateurs set their clocks to catch the comet before dawn, astronomers using NASA’s Hubble Space Telescope captured close-up photos of it two weeks after closest approach. The images reveal a narrow, well-defined jet of dust ejected by the comet’s fragile, icy nucleus spinning like a water jet from a rotating lawn sprinkler. These observations also represent the closest celestial object Hubble has observed other than the moon.

Want to get a good look at a comet’s tiny nucleus and its jets of vapor and dust? Get up close in the spaceship. This photo was taken by the European Space Agency’s Rosetta probe which has been orbiting Comet 67P/Churyumov-Gerasimenko since the fall of 2014. Credit: ESA

Sunlight warms a comet’s nucleus, vaporizing ices below the surface. In a confined space, the pressure of the vapor builds and builds until it finds a crack or weakness in the comet’s crust and blasts into space like water from a whale’s blowhole. Dust and other gases go along for the ride. Some of the dust drifts back down to coat the surface, some into space to be shaped by the pressure of sunlight into a dust tail.

This map shows the path — marked off every five nights at 11:30 p.m. CDT (4:30 UT) — of 252P/LINEAR along the border of Ophiuchus and Hercules through the end of June. Bright stars are labeled by Greek letter or number. Stars shown to magnitude 8.5. Click to enlarge. Diagram: Bob King, source: Chris Marriott’s SkyMap

You can still see 252P/LINEAR if you have a 4-inch or larger telescope. Right now it’s a little brighter than magnitude +9 as it slowly arcs along the border of Ophiuchus and Hercules. With the moon getting brighter and brighter as it fills toward full, tonight and tomorrow night will be best for viewing the comet. After that you’re best to wait till after the May 21st full moon when darkness returns to the evening sky. 252P will spend much of the next couple weeks near the 3rd magnitude star Kappa Ophiuchi, a convenient guidepost for aiming your telescope in the comet’s direction.

Get oriented on where to look for the comet by first using this map, which shows the sky facing southeast around 11-11:30 p.m. local daylight time in mid-May. Mars and Saturn make excellent guides to help you find Kappa Oph, located very near the comet. Diagram: Bob King , source: Stellarium

While you probably won’t see any jets in amateur telescopes, they’re there all the same and helped created this comet’s distinctive and large, fuzzy coma. Happy hunting!

The full sequence of images of the spinning jet in 252P/LINEAR seen by Hubble. Credit: NASA, ESA, and Z. Levay (STScI)


Bob King

I'm a long-time amateur astronomer and member of the American Association of Variable Star Observers (AAVSO). My observing passions include everything from auroras to Z Cam stars. I also write a daily astronomy blog called Astro Bob. My new book, "Wonders of the Night Sky You Must See Before You Die", a bucket list of essential sky sights, will publish in April. It's currently available for pre-order at Amazon and BN.

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