NASA has announced that for the first time, its Neil Gehrels Swift Observatory has been able to track water loss from an interstellar comet as it approached and rounded the sun. The comet that the Swift Observatory was observing was the first confirmed interstellar comet 2I/Borisov traveled the solar system in late 2019. Comet 2I/Borisov is only the second known interstellar visitor discovered two years after Oumuamua passed through the solar system and was discovered by amateur astronomer Gennady Borisov.
The early discovery of the comet gave multiple space and ground-based observatories time for detailed follow-up observations. The first hints of water from the comet were detected in October. As the comet approached the sun, frozen material on its surface, like carbon dioxide, warmed and began converting to gas. When the comet got within 230 million miles of the sun, water began to vaporize and scientists were able to measure its fluctuations using ultraviolet light.
Sunlight breaks apart water molecules, and one of the fragments produced is hydroxyl, which is a molecule composed of one oxygen and one hydrogen atom. Swift was able to detect the fingerprint of UV light emitted by hydroxyl using the Ultraviolet/Optical telescope. Researchers were able to make six observations of Borisov using Swift between September 2019 and February 2020 and note that they saw a 50% increase in the amount of hydroxyl, therefore water, the comet produced between November 1 and December 1.
The team says that at peak activity, the comet shed 8 gallons of water per second, which is enough to fill a bathtub in about 10 seconds. During its entire trip to the solar system, the comet lost almost 61 billion gallons of water. Interestingly, as the comet moved away from the sun, its water loss dropped off more rapidly than any previously observed comets.
That could have been caused by a variety of factors, including surface erosion, rotational change, and fragmentation. The water production measurements helped the scientist calculate the minimum size of the comet at just under half a mile across. At its closest approach to the sun, approximately 55% of its surface was actively shedding material.