Comet Recovery and Discovery

 

Updated 2017 June 24

 

Contents

 

Introduction

 

Comets can still be discovered by amateur astronomers – both visually and by imaging. The professional robotic surveys do not cover all of the sky all of the time. There are gaps in their coverage which will be described in the section ‘Discovering comets’.

 

This tutorial covers;

- recovering lost comets

- recovering returning comets

- recovering and discovering Kreutz sunskirters

- asteroids that might be comets

- discovering comets

 

Finding comets on SOHO images is explained here

 

Most important of all – confirming (or not) and reporting your potential discovery

 

‘Hunting and Imaging Comets’ by Martin Mobberley and published by Springer is a must read. Please help the British Astronomical Association and order via the Journal section of its website.

 

The use of large, robotic telescopes such as the Sierra Stars Observatory Network (SSON) might be of some help in the detection of faint comets. A project approved via the BAA’s Robotic Telescope Project will provide 50% of the cost. Such observatories are described in the book ‘Remote Observatories for Amateur Astronomers’ by Hubbell, Williams and Ballard published by Springer in 2015.

 

Recovering lost comets

 

Why do comets get lost ?

- their orbits might have changed due to passing close to one of the giant planets (usually Jupiter)

- they may have lost all their volatiles after many orbits and thus become very faint

- they may have disintegrated

- lack of observations (even minor changes to an orbit can add up over many years)

 

This last point is well illustrated on the Minor Planet Center’s website at Dates Of Last Observation Of Comets which shows that many comets have not been observed for five years or more. Note that such comets are not necessarily ‘lost’ but new observations will confirm their existence or not as the case may be. The magnitudes listed indicate that these are targets for CCD imagers rather than visual observers.

 

The Comet Follow-Up Astrometric Program (CFUAP) run by the Italian Organisation of Minor Planet Observers (UAI) uses data from the above mentioned MPC web page to assist observers in selecting comets whose orbits could benefit from being better defined.

 

Recovering returning periodic comets

 

Much as ‘twitchers’ scan the skies for the first Swallow to return to our shores, amateur astronomers hunt for periodic comets returning to the inner Solar System. Seiichi Yoshida’s webpage Comets Waiting for First Observation, Figure 1, lists such comets together with attempts to observe them. Note that negative reports are useful and can also be reported via this facility.

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Figure 1. Comets Waiting for First Observation – Seiichi Yoshida

 

Selecting one of the comets returns orbital elements, a finder chart and a graph of predicted total (m1) magnitude. Clicking on ‘Profile’ lists discovery and returns data.

 

A comet may be earlier or later in its orbit if, for example, it has passed close to one of the giant planets. As shown in Figure 2 the Guide planetarium program will draw a line of variation for a given number of days early or late – in this case 3 for comet 58P. Overlays show a typical CCD frame (Starlight Xpress SXV-H9) and the number of images you would need to obtain to cover all possibilities.

 

Guide LOV

Figure 2. Line of variation - Guide

 

Recovering and discovering Kreutz sunskirters

 

A strategy for recovering/discovering Kreutz comets is described in detail on PP129-132 in ‘Hunting and Imaging Comets’ by Martin Mobberley.

 

Comets passing very close to the Sun fall into two major groups – Sungrazers and Sunskirters. Sungrazers are further subdivided into Kracht, Marsden and Meyer groups which have perihelion distances of less than two solar radii and don’t survive such a close encounter. Sunskirters, or Kreutz group comets, have perihelion distances between six and twelve solar radii and do survive. Returning comets therefore belong to this group. Kreutz comets are most likely fragments of a massive comet which may have entered the inner Solar System and broke up at perihelion between 214 and 467AD. The informal term sunplungers has been used to describe those comets which actually fall into the Sun.

 

At aphelion Kreutz comets would be found (if visible) near Sirius, Figure 3. As they approach the inner Solar System the movement of the Earth in its orbit greatly increases the change in their positions against the background stars (parallax). The search area thus increases from the smaller, inner ellipse to the larger outer ellipse. The shaded box indicates a suggested search area. Blinking images (3 x 30 mins apart) is a good way of detecting objects particularly in dense starfields which can defeat the professional moving object detection software.

 

Kreutz search

Figure 3. Kreutz sunskirter search area

 

The BAA Computing Section website gives positions and other data relating to returning sunskirters. From the main page select ‘What’s observable’ and then click on ‘Add Kreutz search area’. Hovering the mouse near one of the comets shows its name and time from perihelion whilst clicking on it lists relevant data - Figure 4.

 

Figure 4. Kreutz search area – BAA Comet Section website

 

Other features include a finder chart, Figure 5, and a list, Figure 6.

 

kreutz finder

Figure 5. Kreutz comet finder chart - BAA Comet Section website

 

Figure 6. List of Kreutz comets - BAA Comet Section website

 

Asteroids that might be comets

 

Not infrequently a newly discovered object is first reported as an asteroid but subsequently turns out to be a comet. How can we spot such objects ? It might be that the ‘asteroid’ develops a coma and/or a tail and, as more observations are reported, is found to be moving in a comet like orbit.

 

Does it have a coma ?

 

The T3 Project, Finding comets in the asteroid population, is run by the Italian Organisation of Minor Planet Observers (http://asteroidi.uai.it/t3.htm) it’s aim is to discover comets hidden among the asteroid population having a Tisserand parameter respect to Jupiter (Tj) less than 3. T=3 is approximately the boundary between asteroidal and cometary orbits. If a suspected comet does not have any obvious cometary features such as a tail or large coma then examining CCD images as described below - modified version taken from the above mentioned website.

 

‘The strategy is quite simple: one has to image these minor planets in good seeing and the resulting SNR of the stack (normally required for faint objects to achieve a decent SNR and avoid trailing) must be at least 10 but the more the better. With Astrometrica, one has to stack all the images according to the motion of the minor planet and then, if the cometary feature is not obvious, measure the SNR and above all the FWHM, which value must be at least 1.20-1.25 than that of the stars nearby (of similar SNR) for a hypothetical cometary feature. An example, again taken from the T3 website, is shown in Figure 7.

 

t3 example

Figure 7. Comparison of FWHM for a suspected comet and star. UAI T3 Project

 

Unfortunately this project is, to quote Lucca Buzzi, ‘in a sort of quiescent phase’. He is trying to recruit new members so anyone interested in this project should contact him (see http://asteroidi.uai.it/t3.htm).

 

For more on the Tisserand Parameter see;

- http://www2.ess.ucla.edu/~jewitt/tisserand.html

- http://farside.ph.utexas.edu/teaching/celestial/Celestialhtml/node82.html

 

Does the object have a tail ?

 

Asteroids don’t usually have tails and comets only develop them when passing through the inner Solar System. It pays to examine one’s images of ‘asteroids’ very carefully as I did not - Figure 8. Asteroid 2002 EX12 was eventually classified as comet 169/P. If I had looked more carefully at my image, and noticed the faint tail, I might have been the one to make that discovery.

 

Figure 8. The one that got away !!!

 

Is it in an unusual (for an asteroid) orbit ?

 

The object shown in Figure 9 was discovered by La Sagra Observatory (an amateur facility) with an 0.45 m f/2.8 reflector plus CCD. It was originally classified by the Minor Planet Center as an asteroid but subsequently developed a tail and was reclassified as a comet. As can be seen in Figure 10 the object’s orbit is highly inclined and highly eccentric both of which may indicate that the object is a comet rather than an asteroid.

 

The MPC has orbital elements for unusual minor planets many of which have comet like orbits

 

Figure 9. Asteroid 2012 NJ/comet P/2012 NJ

 

Figure 10 Orbit of comet P/2012 NJ (La Sagra)

 

Discovering comets

 

Shigeki Murakami’s web site at http://comet.la.coocan.jp/index.html includes a number of pages relevant to comet discovery;

-          Method of comet hunting - http://comet.la.coocan.jp/method/method-comet-hunt.html

-          Can comet hunters survive? No. 1 - http://www.comet-web.net/~oaa-comet-ml/murakami/tenkai200304_murakami_en.html

-          Can comet hunters survive? No. 2 - http://www.comet-web.net/~oaa-comet-ml/murakami/tenkai200308_murakami_en.html

-          Can comet hunters survive? No. 3 - http://comet.la.coocan.jp/Can_comet_hunters/comet_hunter3.pdf

-          Conference on Comet and Astronomy - http://comet.la.coocan.jp/Conference.html

 

The last mentioned page has a link to ‘A visual discovery of comets is still possible: See stars by heart’

- https://www.youtube.com/watch?v=h9wJfksPYyE&feature=youtu.be

 

Shigeki Murakami was interviewed by Stefan Kurti in 2017 and he describes his comet hunting experiences here.

 

William A (Bill) Bradfield (20 June 1927 - 9 June 2014) was a life member of the Astronomical Society of South Australia (ASSA). His achievement of 18 visual comet discoveries (between 1972-2004) in an amateur capacity is unlikely, given today's technology, to be repeated. After his ultimate discovery of comet C/2004 F4 Bradfield, he gave a talk about it at the May 2004 monthly ASSA meeting.

 

Recently discovered comets are listed on the BAA Comet Section website here, the MPC website here and the Comet Orbit Home Page so worth checking to see if your ‘discovery’ is just that. The professional surveys, including Pan-STARRS and LSST, don’t find them all and, although it is not easy, discoveries are still made by amateurs.

 

What can the professional surveys miss ?

- slow moving comets (=< 3 arc secs/hr)

- comets moving through dense Milky Way starfields

 

Where and when don’t they search ?

- five days either side of full Moon

- close to the Sun

- near the celestial poles (>80º Dec)

- > dec 50º low in NW after sunset, low in NE before sunrise

- July and August (monsoon season in US southwest)

 

So look when and where they don’t and please do read the relevant chapters in Martin Mobberley’s book. The areas searched by a number of surveys, example in Figure 11, can be ascertained on the MPC website at http://minorplanetcenter.net/iau/SkyCoverage.html

 

http://cgi.minorplanetcenter.net/tmp/2457375_11566315.png

Figure 11. Example of professional survey sky coverage.

Figure 12 shows a suggested search strategy for visual observers using binoculars. The sky is scanned either side of a SET SUN AND NEVER WHEN THE SUN IS ABOVE THE HORIZON

 

Figure 12. Suggested search pattern

 

Visual sweeping, using binoculars, is also a great way of learning the night the sky. A DSLR camera is worth experimenting with as is a wide field refractor coupled to a CCD camera. Who knows what else might turn up;

- asteroids

- novae, supernovae

- variable stars in outburst (cataclysmic variables)

- new variable stars

 

It isn’t only the professionals who conduct robotic surveys – amateurs also do, for example;

- Tenagra observatory

- Astronomical Observatory de Mallorca (OAM)

 

Comets can also be discovered on spacecraft images  - see Finding comets on SOHO images

 

Nebulae and galaxies can be mistaken for comets – the reason why Messier produced his famous catalogue – so do check (charts and digital sky survey images) before staking your claim. If you discover a comet you may qualify for the Edgar Wilson award – see http://www.cbat.eps.harvard.edu/special/EdgarWilson.html

 

Verification of and reporting recoveries and discoveries

 

Recoveries

 

These should be reported to the Minor Planet Center in the usual way and to any interested organisation as described in the various sections above. Software such as Astrometrica will generate correctly formatted MPC reports.

 

Discoveries

 

Are you sure it is a comet ?

- verify that it is moving by observing/imaging over a few hours

- use a planetarium program such as Megastar or Guide to search for objects in the area in which you are observing

- access the  MPC Comet checker, MPC NEO Checker, MPC NEO and Comet Checker and MPC Minor Planet checker to check for known objects

- check the ESO Online Digitized Sky Survey for comet like objects e.g. galaxies, nebulae

- contact other observers who may be able to confirm the nature of your potential discovery

 

The MPC Submission Information page contains the following statement with respect to discoveries;

‘Reports for new comets discovered visually (or on SOHO images) should be e-mailed to the Central Bureau for Astronomical Telegrams (cbatiau@eps.harvard.edu). Reports for new comets discovered by other means should be reported to both CBAT and MPC’.

 

The IAU Central Bureau for Astronomical Telegrams page http://www.cbat.eps.harvard.edu/CometDiscovery.html is a useful source of information on how to handle your ‘discovery’.

 

Please do follow the proper reporting procedures as repeated false discovery reports may find you blacklisted. The BAA Comet Section guidelines are at http://www.ast.cam.ac.uk/~jds/comdproc.htm As these are a draft version I would recommend that you contact the Comet Section Director.

 

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