OASI: C/2012 S1

Comet C/2012 S1 (ISON) is named after the International Scientific Optical Network of telescopes in Russia. Two astronomers, Vitali Nevski and Artyom Novichonok, using a 400 mm reflecting telescope of the network, inadvertently imaged the comet on 21 September 2012. Unfortunately, they failed to notice it in the image and it was left to others to highlight it! After discovery, it was found to have been imaged also by the Mount Lemmon Observatory on 28 December 2011 and the PANSTARRS survey on 28 January 2012.

On discovery, astronomers found the comet to be relatively large and bright and, by assuming it would evolve in the same manner as other relatively large, bright comets, predicted that, in winter 2012-13, it could become as bright as the full moon! At the time of writing (29 August 2013), the comet has recently emerged from conjunction with the Sun and is heading towards perihelion on 28 November 2013.

Just to show that observing sessions don’t always go entirely to plan…! I’ve been planning to image the comet for a while but 17 January was a particularly good date as it was then moving in front of a group of galaxies. Unfortunately, it was ‑7°C in my observatory so rather than freeze to death I submitted a request to the Sierra Stars Observatory Network (http://www.sierrastars.com/) Mt. Lemmon 0.81 m Cassegrain telescope in Arizona. The resulting image (processed from four 120 s exposures) is below. It shows one of those unexpected problems that sometimes crop up: the comet was only 30 arcminutes from Castor in Gemini which caused glare in the image. Luckily, the glare missed the comet!

The comet is still brightening as expected and is probably about mag 15 at the moment. At the time of the image it was 5.1 AU from the Sun and slowly heading west at 36 arcsec/hour. It will remain in Gemini for the next few months before heading east towards perihelion in late November.

On checking the position of the comet again, I found that on 02-03 May it came within 35 arcmin of dwarf planet Ceres and I thought it would be worth trying for an image of the appulse. The best telescope for this task is "T‑11", a 51 cm telescope in New Mexico operated by iTelescope.net. It has a large CCD providing a field of view 36x54 arcmin, perfect for capturing both objects together.

The image below is the result of 5x120 second exposures. During that time Ceres (magnitude 8.8) trailed significantly resulting in the north-south diffraction spikes becoming blurred making it easy to pick out from the stellar background. The comet remains faint even in a 51 cm telescope sited at altitude! At the time of the image Ceres was 446 million km away from Earth and the comet was a further 200 million km beyond that.

Interestingly, Vesta will pass C2012 S1 at the end of May at a separation of approximately 4°. The Dawn spacecraft is now en-route from Vesta to Ceres so I enquired of the Dawn science team whether there were any plans to observe the comet using the spacecraft. The e-mail exchange is below.

From: Rayman, Dr. Marc D (3100)

Sent: 24 April 2013 19:54

To: Mike Harlow

Cc: Joe Wise

Subject: Re: Ask a scientist question - Dawn website

Dear Mr. Harlow,

Your message was forwarded to me, and I appreciate your interest in Dawn.

This is a good question. I think it is really exciting that with spacecraft throughout the solar system, we occasionally have opportunities to view unexpected phenomena. Around the time I first heard of comet ISON, I had considered whether it would be worth having Dawn observe it, but an important part of the reason it would not be is contained in your message. At 1 AU, the distance is too great to be worthwhile. Our instruments are designed for broad mapping of uncharted worlds from orbit.

As an experienced astrophotographer, you may appreciate the details. Our camera has a field of view of about 96 mrad (5.5 deg). The CCD is 1 Mpx (yes, that seems like something our great-grandparents might have used, but space qualified electronics reliable enough for long interplanetary missions tend to be behind terrestrial consumer electronics), so the IFOV is 94 microrad (19 arcsec). At 1 AU, therefore, a pixel is 14,000 km.

Our visible and infrared mapping spectrometer has an IFOV of 250 microrad, so its pixels projected onto the comet would be > 37,000 km.

These instruments served us exceedingly well in exploring Vesta, and they will again when we reach Ceres in 2015, but as you can see, they don't have much to offer for such long-range observations, even of the intriguing comet ISON.

As a secondary point, I should also mention, as I explained in my November 2012 Dawn Journal, we are making a concerted effort to reduce hydrazine expenditures, including minimizing the number of turns. Given the absence of anything apparently worthwhile that Dawn could discern about the comet, observing it would not be a prudent use of resources we want to devote to Ceres.

I hope this answers your question. Thank you again for contacting Dawn!

Yours sincerely,

Marc Rayman

From: Mike Harlow

Sent: Tuesday, April 23, 2013 12:46 PM

To: Joe Wise

Subject: Ask a scientist question - Dawn website

Comet ISON

Just wondering if there are any plans to observe comet ISON from Dawn? Looks like the spacecraft will come within about 1 AU of the comet at the end of May.

Thanks,

Mike Harlow