A comet has been making numerous headlines lately; however, don’t expect to go outside and see it for yourself. Comet 3I/ATLAS is still an extremely faint target, only around magnitude 16 at the moment (about 10,000 times fainter than the faintest make-eye star and five times fainter than dwarf planet Pluto) but is expected to grow slightly brighter by the end of October as it makes its closest approach to the Sun. What has made this comet so noteworthy is its trajectory. It is moving incredibly fast, around 137,000mph when discovered in July. With this speed and trajectory, it must have originated outside of our solar system. In addition, it is moving so fast that even the gravitational pull of the Sun, which holds planets in their orbits from billions of miles away, will not be able to hold onto it – it is making a one-way trip through our solar system. This is the third interstellar object that has been discovered (hence the designation “3I”). The first was Oumuamua, discovered by the Pa-STARRS telescope in late 2017, and the second was comet 2I/Borisov, which was discovered in 2019 by amateur astronomer Gennadiy Boris. The later showed distinct cometary activity, making it the first known interstellar comet, while the former lacked the telltale signs of comet, so its exact nature (comet or asteroid) is not well-determined.
During the evening of August 16, Comet 3I/ATLAS was imaged with Vanderbilt Dyer Observatory’s 24” Seyfert Telescope. The full animated sequence shows the comet as a small smudge moving rapidly against the background stars over a period of 30 minutes. In this case, the comet would have moved around 60,000 miles over the half hour, one-quarter of the distance between the Moon and Earth. Even with the telescope’s large mirror, the comet remained hard to photograph. Longer exposures would have made objects in the image brighter, but the high velocity of the comet would have resulted in an elongated smudge. In addition, the comet was moving through the southern constellation Scorpius, which appeared very low in the sky. As one observes at lower angles, we are observing through much more of the thickest part of the atmosphere. The humid conditions and surrounding light pollution, coupled with increased air mass, made fainter objects even harder to detect. The images were also acquired during the Dyer’s August Bluebird on the Mountain concert, so stray light from the observatory grounds had a say in the image quality. Passing haze also interfered near the end of the image sequence. Still, observatory director Billy Teets was pleased as punch to be able to even get one image of the interstellar visitor.
Comet 3I/ATLAS will soon be lost to the glare of the setting Sun. By the end of September, the extremely faint body will set soon after Sun. It will make its closest approach to our Sun at the end of October and then quickly make its way to the morning sky by the first part of November. It will still be extremely faint (only about 14th magnitude); however, if you have dark skies, a decent-sized telescope, and want to try your hand at some astrophotography on an exceedingly rare type of object, opportunities will still await you!
Credit: NASA/JPL-Caltech