From Venus, we now turn to Comet 2I/Borisov. This interstellar asteroid has been a target of opportunity for pretty much anything that could point at his sublimating form. As we discussed last week, the chemistry of Borisov isn’t identical to what we generally have here in our Solar System, with carbon monoxide appearing in greater amounts than expected. Now, new observations from the Swift mission find that Borisov is 10 times more active than the standard comet. With a total surface roughly twice the size of Central Park, it was able to shed 8 gallons of water per second at its peak output. As it approached the Sun, this water spray dropped in volume, most likely due to surface erosion, rotational changes, and the fragmentation that was eventually observed by Hubble. While still within the observed range of possible comet behaviors, Borisov doesn’t neatly match any category of comet we’ve seen before. This is consistent with it having a different formation history, with colder temperatures and still to be understood other variations. As we’ve said before, we need to see more interstellar asteroids to understand what is, at the galaxy level, the oddity and the average for comets.
While we don’t have any new interstellar comets at the moment, we do have a nice bright regular comet to fill the gap left when Comet Atlas fell apart. Comet Swan was found in data by SOHO’s Solar Wind ANistropies by amatuer astronomer Michael Mattiazzo. This small comet Is on an odd orbit that is tilted 111 degrees relative to the ecliptic plane. This icy body is now bright enough to see in binoculars, and might possibly become an object bright enough to see with the unaided eye. On May 12, this object will pass near Earth on its way in toward its closest approach to the Sun on May 27. This isn’t an ideal geometry, but it does mean there is a good chance Swan won’t fall apart before it gets close to us. For now, all we can do is wait and see … and hope.. Maybe 2020 will give us one bright comet while it systematically takes everything else away.