Imaging Comet 46P/Wirtanen

Sunday night, December 16th, was the evening of the closest approach of Comet 46P/Wirtanen. The comet was close enough and bright enough with an apparent magnitude between 4.0 and 4.5 that you could see it with the naked eye if you had good, dark skies. Thankfully, that’s just what I have living out in rural Missouri, and we also had crystal clear skies. The comet can be seen between the Pleiades Cluster and Taurus through Wednesday, but as the Moon’s phase transitions from 1st Quarter to Waxing Gibbous, dim objects in the sky will be harder and harder to see. Throughout this coming week, the best time to view the comet will actually be in the very early morning after moonset. You can check out more about where to find and when to view the comet at Sky and Telescope, https://www.skyandtelescope.com/astronomy-news/observing-news/comet-46p-wirtanen-and-moon/.

Comet 46P/Wirtanen passing between the Pleiades Cluster and Taurus.

The image below of Comet 46P/Wirtanen is the result of stacking 63 exposures each with a shutter speed of 20-seconds, ISO 6400, aperture f/8, and a focal length of 200 mm on a Nikon D500 through a Tamron 70-200mm f2.8 riding atop a Celestron 8″ SCT. The telescope was there just to provide the clock drive so that the camera moved with the stars’ diurnal motion. Each raw image file was corrected for dark current, bias current, and the response across the frame was normalized using a flat field image. I’ll write up a tutorial on how to collect these images and why they’re necessary later. The images were then aligned and stacked on the comet, which is why the stars appear as streaks. Since the comet is moving quite rapidly past us, its motion relative to the distant background stars is very noticeable, even in the short timespan of this image set.

An long exposure image of Comet 46P/Wirtanen tracking the comet’s motion rather than the stars’ motion.

You may have noticed the very green color to many of the images of Comet 46P/Wirtanen, including this one. The green color is not a image processing artifact, but is indicative of the comet’s composition. Most comets contain a enough cyanogen (CN) and diatomic carbon (C2). As a comet approaches the Sun and its surface warms, volatile materials such as cyanogen, water, and others begin to vaporize forming the comet’s coma and tail. When cyanogen and diatomic carbon interact with the Sun’s ultraviolet light and fluoresce to create the characteristic greenish glow of many comets.

Voyager II Is Outta Here!

Well, perhaps more precisely, the Voyager II spacecraft is now making its way through the heliopause region.  Unlike planetary magnetospheric boundaries, the boundary between the heliosphere and the local interstellar medium (LISM) has substantial thickness compared to the length scales that Voyager II samples based on its data cadence and its velocity. It will take Voyager II time to make it completely through the region, but the data make it abundantly apparent that one of our oldest operating spacecraft is now set to join its twin as an interstellar traveler having entered the heliopause on November 5 at a distance of 119 AU, 35 AU past the termination shock.

Most of the data gathered by the spacecraft that indicate that Voyager II is now exploring a very different region of space are the sudden drop in the energetic ion intensity (E ~ 1 MeV/nuc) coupled with the simultaneous increase in the galactic cosmic ray intensity (E ~ 100 MeV/nuc). There are strong indicators in the magnetic field data as well. The field is significantly stronger than previously observed, there’s an absence of variation, and the field has a northward (+BN) component. All of this is consistent with what Voyager I saw when it entered the LISM.

Energetic ion and anomalous cosmic rays (ACR) rates as measured by the Voyager II Cosmic Ray Subsystem (CRS).

There are some significant differences between the Voyager I and II observations, though. The timescales of the transitions are very different with the Voyager II data showing a slower transition than as seen by Voyager I. The spectra for energetic hydrogen and helium ions (protons and alpha particles) also showed very little variation at Voyager II and compared to the change seen at Voyager I.

While many of the trends in the data are similar between the two spacecraft, these few and important differences will help the team to further refine our model for the shape of the heliosphere. Early models held that the heliopause was open and teardrop shaped, not unlike the shape of planetary magnetospheres. As the interstellar wind impacts the Sun’s magnetic field, it pushes on and distorts the field forming a tail structure downstream and a bow shock upstream. The data from Voyager I and the preliminary data from Voyager II coupled with remote sensing data from platforms in Earth orbit and on the now destroyed Cassini orbiter around Saturn are leading many to the conclusion that our heliosphere is a closed asymmetric bubble.

Fortunately, both Voyager spacecraft still enjoy good health for their age. The both still have plenty of fuel, but what they are both running low on is power. Solar panels simply won’t work when you’re 100 AU from the Sun. At the distance of the heliopause, you would need over 14,000 times more solar panels to provide the same amount of power that you could produce in Earth orbit. This is why the Voyager spacecraft are powered by radioisotope thermoelectric generators (RTGs). Small lumps of a highly radioactive isotope of plutonium provide a large amount of heat. That heat energy is then converted into electrical energy to provide power to the spacecraft. However, over the past 41 years of flight, the plutonium sources have decayed and cooled, therefore reducing the amount of electricity the RTGs can provide. Both spacecraft have the power they need for the instruments that are still on, but they’re losing power at a rate of 4 W/yr. Some hard decisions will need to be made very soon between turning on heaters, turning off some science instruments, or trying a time sharing scheme. The trouble is every on/off transition can result in an improper commanding sequence on Voyager II that could irreparably damage the spacecraft, and it’s not like we can send a repair crew to service them.

The two Voyagers are the most amazing missions of space exploration ever launched. The amount by which their data has advanced our understanding of the outer solar system and our Sun is beyond anything that any other single mission has provided, and they’re still going! Here’s hoping that we get a few more good years from our now TWO interstellar spacecraft.

The official press release can be found here:
https://www.nasa.gov/press-release/nasa-s-voyager-2-probe-enters-interstellar-space