Approaching December comet may be visible to the naked eye
The comet 46P/Wirtanen (46P) is set to make a close approach to Earth this weekend, and may even be bright enough to be seen by the naked eye. At its closest, 46P will pass a mere 7.2 million miles (11.7 million km) from our planet, making it the 10th closest comet encounter in the modern era.
Wandering comets are chunks of material that have remained relatively unchanged since the formation of our solar system. They represent valuable time capsules that essentially grant astronomers a window into the distant past.
The December comet is no different. The icy relic, which is estimated to measure a little over a kilometer in width, is thought to be made up of ancient rock, dust and frozen gasses. It orbits the Sun once every 5.5 years, and the 2018 pass is expected to be highly visible compared to the previous flyby in 2013 and the following pass in 2024.
46P was once set to be the focus of the historic Rosetta mission. The European Space Agency had planned to launch both probe and Philae lander in early 2003, which would have seen the robotic explorers rendezvous with 46P as it made its closest approach to the Sun in 2013.
However, these plans had to be revised, and a new target chosen in the form of 67P/Churyumov-Gerasimenko when Rosetta's launch date slipped to 2004.
"We had to say good-bye to 46P for Rosetta, but the comet was for so many years at the core of my professional life so it is very emotional that I might have a chance to see it directly with my own eyes," states Gerhard Schwehm, who was a Rosetta project scientist in the 1990s when 46P was selected.
46P will reach the point of closest approach to Earth mere days after perihelion – the point in the comet's orbit at which it is closest to the Sun. Scientists know from observations made of 67P that perihelion is an incredibly active period in the life of a comet, during which the heat pouring out of our star leads to an increase in activity such as outgassing.
The period directly before and after perihelion is an excellent time to make scientific observations. Whilst the brightness of these celestial wanderers is extremely unpredictable, it is thought that 46P will reach magnitude 3, which would make it easy to spot using a telescope or binoculars, and could even render it visible to the naked eye.
Whilst the remote observations to be made of 46P during its close pass could never equal those made by an orbiting spacecraft, they will still offer valuable insight regarding the nature of the comet.
Ground-based telescopes are able to capture a comet and the extended trail of material that follows in its wake in its entirety. Furthermore, when a comet passes as close as 46P will this weekend, it can be observed by a host of powerful telescopes capable of capturing a variety of wavelengths, each of which reveal a different aspect of its nature.
For example, images and data collected by this army of observers could reveal a lot about the processes that drive the movement of dust and gas in the comet's coma.
"At NASA, we normally send spacecraft missions to solar system targets. It's always nice when the target comes to us," comments Dr. Kelly Fast, Program Manager of the Near-Earth Object Observations Program at NASA Headquarters. "This is a wonderful opportunity for comet science, and for the astronomical community around the world who will be studying the chemistry, coma changes, and other processes taking place at Comet Wirtanen."
If visible on December 16 the comet will appear around the Pleiades star cluster, which, also known as the Seven Sisters, appears as a cloudy patch to the naked eye close to the Orion constellation in the night sky.
The comet will be moving relatively quickly across the sky. Because of this, anyone hoping to image 46P using a telephoto lens should opt for shorter exposure times of around 10 seconds.