The fast mission counted the water from the interstellar comet Borisov

For the first time, NASA’s Swift Neil Gehrels Observatory tracked the loss of water from an interstellar comet as it approached and circled the Sun. The object, 2I / Borisov, traveled through the solar system in late 2019.

“Borisov does not fit neatly into any class of comets in the solar system, but he does not stand out exceptionally well from them,” said Zexi Xing, a graduate student at the University of Hong Kong and Auburn University in Alabama who led the research. . “There are known comets that share at least one of their properties.”

Comets are frozen groups of gases mixed with dust, often called “dirty snowballs.” Scientists estimate that hundreds of billions of them can orbit around the Sun. However, based on Borisov’s calculated speed and path, it must have come from outside the solar system. The comet is only the second known interstellar visitor, discovered two years after the first object, called ‘Oumuamua, passed through the solar system.

Amateur astronomer Gennady Borisov discovered the comet on August 30, four months before getting closer to the Sun. The early identification gave multiple space and ground observatories time to make detailed follow-up observations. In October, scientists using the Apache Point Observatory in Sunspot, New Mexico, detected the comet’s first water trail. In the following months, NASA’s Hubble Space Telescope imaged Borisov as the comet moved at a speed of around 100,000 miles (161,000 km) per hour.

When a comet approaches the Sun, the frozen material on its surface, such as carbon dioxide, heats up and begins to turn into gas. As it approaches 230 million miles (370 million km) from the Sun, the water vaporizes. Xing and colleagues confirmed Borisov’s presence of water and measured its fluctuations with ultraviolet light.

When sunlight separates the water molecules, one of the fragments is hydroxyl, a molecule made up of an oxygen atom and a hydrogen atom. Swift detects the fingerprint of UV light emitted by the hydroxyl using its optical / ultraviolet (UVOT) telescope. Between September and February, Xing’s team made six Borisov observations with Swift. They saw a 50% increase in the amount of hydroxyl, and thus water, that Borisov produced between November 1 and December 1, which was only seven days from the comet’s closest brush with the Sun.

At peak activity, Borisov dumped eight gallons (30 liters) of water per second, enough to fill a tub in about 10 seconds. During its journey through the solar system, the comet lost nearly 61 million gallons (230 million liters) of water, enough to fill more than 92 Olympic pools. As it drifted away from the Sun, Borisov’s water loss decreased, and it did so more rapidly than any previously observed comet. Xing said this could have been caused by a variety of factors, including surface erosion, rotational change, and even fragmentation. In fact, data from Hubble and other observatories show that pieces of the comet broke in late March.

“We are very pleased that Swift’s fast response time and UV capabilities captured these rates of water production,” said co-author Dennis Bodewits, associate professor of physics at Auburn. “For comets, we express the number of other detected molecules as a relationship to the amount of water. It provides a very important context for other observations.”

Swift’s water production measurements also helped the team calculate that Borisov’s minimum size is less than half a mile (0.74 km) across. The team estimates that at least 55% of Borisov’s surface, an area roughly equivalent to half that of Central Park, actively dumped material when it was closer to the Sun. That’s at least 10 times the active area in most of the comets of the observed solar system. Borisov also differs from comets in the solar system in other ways. For example, astronomers working with Hubble and the Atacama Large Millimeter / submillimeter Array, a radio telescope in Chile, discovered that Borisov produced the highest levels of carbon monoxide ever seen from a comet at that distance from the Sun.

However, Borisov has some features in common with comets in the solar system. Its increase in water production as it approached the Sun was similar to the objects previously observed. Xing and his team also discovered that other molecules in Borisov’s chemical inventory, and their abundances, are similar to homegrown comets. For example, regarding hydroxyl and cyanogen, a compound composed of carbon and nitrogen, Borisov produced a small amount of diatomic carbon, a molecule made of two carbon atoms, and amidogen, a molecule derived from ammonia. About 25% to 30% of all comets in the solar system share that trait.

But Borisov’s combined features challenge location on any known kite family. Scientists are still pondering what this means for the development of comets in other planetary systems.

The team’s results were published in the April 27, 2020 issue of The Astrophysical charts and they are available online.

Swift was developed to study gamma-ray bursts, the brightest bursts in the universe. But for the past decade, Bodewits has used it to learn more about comets as they traverse the solar system. Earth’s atmosphere absorbs most of the ultraviolet light, so scientists must look for the hydroxyl signature from space. And because Swift has a flexible observation strategy and fast reaction time, it can monitor long-term interesting new targets. Borisov’s first five observations consisted of UVOT snapshots taken over 12 hours, and the last was a series of images captured over 24 hours.

“The team did not imagine that the mission would contribute so much to our understanding of planetary science when it was being built,” said Swift principal investigator S. Bradley Cenko at NASA’s Goddard Space Flight Center in Greenbelt, Maryland. “But it is a good example of people finding creative and powerful ways to use the capabilities that exist to do unexpected and exciting science.”