While all the rest of us were out looking at Comet Neowise earlier this summer with our scopes, binoculars, and—in some places—the unaided eye, Hubble Space Telescope took a close-up look. It orbits high above Earth’s atmosphere.
On August 8th, the telescope focused on the nucleus of the comet. The coma hid the actual surface of NEOWISE, and there’s no way HST could study something that small at a distance of 43 million kilometers. But, that wasn’t what astronomers were looking for. They wanted to check out details of the coma.
What’s to See at Comet NEOWISE?
From Earth, NEOWISE looked like a streak of light across the sky. We saw two tails stretching out from the coma. All that, thanks to the nucleus—a chunk of ice mixed with dust that is about 4.8 kilometers across. Normally, these objects stick to the cold, outer regions of the solar system. That’s where their ices remain safely frozen. But, if they get knocked into an orbit that brings them close to the Sun, things change.
As any comet, including NEOWISE, nears the Sun, it starts to sublimate (like dry ice on a hot sidewalk. That’s due to the increasing heat it experiences. That’s when the coma forms and the dust and plasma tails start to grow. The solar heating warms the surface, and, along with the spinning motion of the nucleus, sets up a chain of events. Pressurized gas from inside the comet blows out through cracks and crevices in the cometary surface and forms jets. They carry gas and dust away from the nucleus.
To distant observers, that action results in a comet with a long flowing set of tails. The dust tail appears mostly white and has a fan-like shape. The plasma tail forms as atoms of gas interact with the magnetic field in the solar wind). It’s usually a dim blue and shows twisted and wavy structure. Savvy observers may remember the amazing images that Rosetta sent back of jets on Comet 67P/Churyumov-Gerasimenko.
In the August 8th images, Hubble shows a pair of jets shooting out from NEOWISE’s nucleus in two opposite directions. They twist as the nucleus spins.
Learning More about Comets
There’s more data in the HST images to be studied. Scientists will use to determine the color of the cometary dust and what changes it goes through as NEOWISE returns to the outer solar system. It’s possible that heat from the Sun affects the properties of dust from the comet. But first, it’s important to understand the original makeup of the comet’s dust and ices. Any changes from that “baseline” information will point out the role sunlight plays in the evolution of cometary dust.
Comets are Probes of Solar System History
Remember that comet nuclei formed early in the history of the solar system. They probably existed well before the Sun was even a “thing”. They remain a great treasury of information about conditions at that time. All that existed of the future solar system was a thick cloud of gas and dust.
NEOWISE isn’t the first comet that HST has imaged, and it surely won’t be the last. It has provided yet another clue to some of the mysteries that still surround comets. There’s still a lot to be gleaned from the latest data, so stay tuned.