A comet is an icy, rocky object in the solar system that is surrounded by gases released from the object when it is heated by the sun. The rocky center is known as the nucleus. The nucleus is surrounded by a volatile atmosphere made up primarily of water vapor, carbon dioxide, ammonia and methane. Some comets orbit the sun in just a few years; others have a period of thousands of years; and some pass by the sun only once before drifting off into interstellar space.
This comet was discovered in March 2022 by the Zwicky Transient Facility telescope on Palomar Mountain in California. Known in the media as the “green comet,” ZTF orbits the sun with a period of 50,000 years. Depending upon its interaction with the sun this time around, it may make another appearance in the year 52,023 (or thereabouts) or it may get thrown out of the solar system altogether.
The green color is due to the light emitted by diatomic carbon atoms that are excited by ultraviolet light from the sun.
The NEOWISE comet is now higher in the west in the evening sky. It’s on its way to the outer reaches of the solar system and won’t return for over 6,000 years. The comet is growing more faint and as it moves farther from Earth. It was high enough in the sky to observe from the observatory last night and photographed through the 4″ refractor telescope.
On Sunday evening, July 19 the sky from Whidbey Island was clear all the way down to the horizon, so we had an excellent opportunity to take a few more images of comet NEOWISE. We headed for the parking lot at Greenbank Farm where we had a good view of the northwestern sky. It would have been nice if the parking lot lights had been off, but since the camera was pointing in the opposite direction, it wasn’t too much of a problem. We had good luck leveling the telescope mount and aligning it with the celestial pole by viewing the North Star through the polar scope. We used Arcturus for a quick one-star alignment. The mount (iOptron CEM25) was tracking very well.
This time I remembered to bring along my intervalometer so I could take exposures longer than 30 seconds. The image below is the result of 13 minutes of total exposure time (5 frames each of 90s, 45s, 15s, 5s and 2s). I had the idea of using the shorter exposures to avoid over-exposing the comet nucleus, but that turned out not to be much of an issue.
We easily see the bright head or ‘coma’ and the fanned out dust tail. Within the coma is the comet nucleus, a small (~ 5km) clump of rock, dust, water ice and frozen gases such as carbon dioxide, methane and ammonia, which can be described as a “dirty snowball” or an “icy dirtball”. The bright coma we see is much larger. From this photo I estimate the coma to be about 230,000 kilometers wide.
If you look carefully, you may see another tail from the coma—straight and angled up slightly relative to the dust tail. This second tail is a result of ionized gas emanating from the nucleus of the comet. Both tails are the result of material coming off of the nucleus. Dust particles drift away and are spread out by pressure from sunlight, producing the prominent dust tail. Various gases are ionized by ultraviolet light from the sun. Being much smaller than dust particles, these ionized atoms and molecules are more strongly affected by sunlight pressure and are propelled along a straighter trajectory.
My estimate of the size of the coma is based on the field of view of my camera and lens. The width of the coma in the image is about 102 pixels, or 2.8% the height of the frame (3648 pixels). The field of view of my telephoto lens is 274 arc-minutes high, so the coma subtends an angle of 2.8% of 274 arc-minutes, or 0.00223 radians. According to NASA, the NEOWISE comet is about 100 million kilometers from Earth. So the width of the coma must be (0.00223)x(100,000,000 km) = 230,000 km, compared to the diameter of the nucleus of only 5 km.
We were in good shape for taking long exposures so we decided to try our luck capturing a spectrum of the comet nucleus. For this, I attached a Star Analyzer 100 diffraction grating onto the front of the telephoto lens.
Star Analyser 100 diffraction grating (Paton Hawksley, UK) attached to a 300mm lens on a Canon 6D DSLR.
The diffraction grating separates the light into its constituent colors. With the Star Analyser in place, the aperture of the lens is reduced to about 10%, so capturing spectra really needs longer exposures, but I settled for a total exposure of 12 minutes.
In the image below, you see what the comet looks like through the diffraction grating. The comet is on the left (its tail streaming upward). The rainbow-colored line to the right is the spectrum of the coma, while the blended colors are from light from the tail. Since the coma is relatively compact, its spectrum is more distinct than for the tail. You can see brightening in the blue, green and yellow parts of the spectrum.
Note the spectra of other stars that happen to be in the field of view.
Five-minute exposure of comet NEOWISE through Star Analyser 100
As a reference, I also captured a spectrum of Vega which was in a good position, almost directly overhead. It is so bright, it doesn’t require a very long exposure. A 9-second exposure was sufficient. Vega is a good reference because it has prominent hydrogen absorption lines, especially the hydrogen beta line at 4861 Angstroms.
To analyze the spectra, I rotated the images to make them horizontal and cropped them.
Cropped spectrum of NEOWISE (5-min exposure)
Cropped spectrum of Vega (9-second exposure)
The next step was to open the Vega spectrum in the spectral analysis program RSpec and calibrate it by identifying the hydrogen beta line. In this way I could associate wavelengths in Angstroms with pixel values along the x-axis. I got a scale factor of 2.23 Angstroms per pixel.
Light intensity vs wavelength for Vega. Peak at the far left corresponds to the undiffracted (0th order) image of Vega.
Next, by opening the spectrum of NEOWISE and using the previously determined scale in Angstroms, we have a profile for the comet.
Spectrum profile for comet NEOWISE
Note that there is considerably more noise in the profile (random vertical variation) due to the fact that the comet is so much dimmer than Vega.
The camera is only sensitive to wavelengths between about 4000 and 7000 Angstroms, so we cropped the plot outside to within this range to produce the following.
I am not certain that I have identified the peaks correctly, but wavelengths for C2 and CN were listed in the RSpec software as typical in comets. ‘C2’ refers to neutral carbon diatomic molecules; ‘(CN)2’ refers to cyanogen. The closest I could find for an element with an emission around 5577 Angstroms was the singly-ionized state of the oxygen atom, which I’ve labeled as ‘OI”. Not sure whether that makes any sense, however. I would invite suggestions from those who know more about spectra than I do.
On Tuesday evening, 2020 we drove to Coupeville and set up the camera near some picnic tables by the wharf, overlooking Penn Cove. At 10:30 pm, it was still too bright to make out the comet, but by 11:00 another person spotted it with binoculars. We had the tracking mount aligned, but slewing to the comet was challenging due to its position low in the north. With the help of another couple, we finally got the comet in the frame of the camera and took several shots. Here is the result of combining four 30-second exposures.
Comet C/2020 F3 was discovered in March 2020 by the NASA spacecraft Wide-Field Infrared Survey Explorer (WISE). In 2013 its mission was focused on near-Earth objects, or NEOs.
In the early morning of July 11, 2020 the comet was visible low in the sky to the northeast. From the Greenbank Farm, we spotted it about 3° above the horizon at 4:00 am. There was an airliner contrail cutting through the comet’s tail, but ten minutes later we had a relatively unobstructed view of the comet.
Photographed through a 300mm lens on a Canon 6D, ISO 400, exposures of 30s and 15s respectively.
This photo of Comet Lovejoy, C/2014 Q2 was taken on the evening of January 14, 2015. The original photo was a 60-second exposure in color, but to increase the visibility of the tail, I converted it to black and white. The tail points directly away from the sun, a result of light pressure on the ionized gas released from the comet.
Here is the corresponding color image. The green glow of the comet’s head results from fluorescing carbon atoms (C2) in ultraviolet light from the sun. For further explanation of the origin of the green color, visit the Planetary Society page. Camera was a Canon 6D on a Takahashi FSQ-106 at f/5.
Comet Lovejoy C/2014 Q2’s orbital period around the sun is roughly 11,500 years. If you miss it on this pass, it will return in about 8,000 years hence.
Comet ISON (C/2012 S1) is visible through a telescope in the early morning hours. I captured this image on the morning of October 6, 2013.
Total exposure time was 24 minutes (twelve 2-minute exposures), tracking the comet, which moved appreciably against the background of stars during that time. This image was processed in a special way to prevent smearing, and to preserve the stars as points of light.
Comet ISON will swing around very close to the sun on November 28. If it survives its fiery encounter, it may be visible on the other side during the evening.
