As Comet NEOWISE quickly becomes a memory for many I was glad to see that astronomers took the opportunity to train the Hubble Space Telescope on the spectacle. NEOWISE became the brightest comet visible from the Northern Hemisphere since Hale-Bopp in 1997. After passing closest to the sun on July 3 it turned back toward the outer solar system and won't be back in our neighborhood again for nearly 7,000 years.
Comets often break apart due to thermal and gravitational stresses when they closely approach the sun, but this closeup image from Hubble released yesterday suggests that it remains intact. The ball of ice and dust called the nucleus is only about 3 miles (4.8 km) across, too small to be directly imaged by the Space Telescope. Instead, the photo captures the inner portion of the vast cloud of gas and dust estimated at 11,000 miles (18,000 km) across that envelopes the nucleus. If you look closely you can just make out other small structures poking from the comet's center.
Within the cloud are a pair of broad, cone-shaped jets shooting from the nucleus in opposite directions. These emerge when solar heating sublimates ice below the surface, turning it into a cloud of expanding vapor. The gas seeks release and finds it by squeezing through cracks and holes in the comet's surface. Like Old Faithful Geyser (live stream link here), the material blasts out as narrow cones laden with dust and gas instead of scalding water.
Like every other body in outer space comet nuclei spin. In the case of NEOWISE, once every 7.6 hours. The rotation caused the expelled dust to curve into broader, fan-like structures resembling a bowtie in this photo. Astronomers are using Hubble images like this one to examine how the color of the comet changed as it moved away from the sun. They hope to explain how solar heating affects the contents and structure of the dust in order to infer its original properties. I well remember how yellow the comet appeared through mid-July, but as its distance from the sun increased that color faded and was replaced by the blue-green glow of fluorescing carbon and cyanogen (related to cyanide) gases.
https://vimeo.com/347565673400,000 images were used to create this video of 67P/Churyumov-Gerasimenko when the European Rosetta spacecraft chased and then orbited the comet from 2014-2016. There are lots of great closeups of jets and flying dust!
Comet NEOWISE lies about a fist above the waxing crescent moon in Virgo this evening (August 22), but at magnitude 7.5 it's only a faint blotch in binoculars. Moonlight will soon scrub it from view for binocular users, but observers with a 6-inch or larger telescope will track it into the fall. NEOWISE is on a very long journey. In a few thousand years it will reach its greatest distance from the sun at more than 66 billion miles or 21.5 times farther than Pluto is at the moment. In that cold, remote spot light takes more than 4 days to travel from comet to Earth.
The solar system is a big place. Fortunately, small things can make a big difference here. NEOWISE was one of them. For a little while the comet inspired us to look up and savor the moment.