Introducing Comet Hartley 2

The upcoming close approach of Comet Hartley 2 should be the cometary event of the year. If the comet behaves as it did during past returns, we should expect it to be a faint naked eye comet in October and November.

Officially designated Comet 103P/Hartley, it is sometimes known by its original moniker of Comet Hartley 2. A member of the Jupiter family of comets, the comet returns to perihelion once every 6.5 years. Its current orbit takes it to within 1.059 AU of the Sun (just beyond earth’s orbit) and as far out as 5.88 AU (about 60 million miles beyond Jupiter’s orbit).

103P was first seen on photographic plates obtained on 1986 March 15, 17 and 20 by Malcolm Hartley at the Siding Spring Observatory in Australia. At the time, the comet was a faint 17th magnitude object and 9 months past perihelion. At its first two predicted returns in 1991 and 1997 the comet brightened to a nice 8th magnitude object easily visible in small telescopes and binoculars. The 2004 return was poorly placed and the comet could not be seen near perihelion. The current return, the 5th observed, is easily the best. In fact, there will be no better apparitions of Hartley 2 this century.

Before 1971, the comet was located on orbits with much larger perihelia. As a result, the comet never got bright enough to be discovered. Though the comet was probably very active during the 1973, 1979 and 1985 returns, perihelion passage occurred on the other side of the Sun and out of view from Earth. Hence the reason the comet was not seen earlier. Had Malcolm Hartley not happened upon it in 1986, the comet would have undoubtedly been found in 1991.

This year Hartley 2 reaches perihelion on 2010 October 28 at a distance of 1.059 AU from the Sun. Closest approach to Earth occurs on 2010 October 20 at 0.121 AU from Earth (11.3 million miles or 18 million km). Such close approaches by comets are uncommon though 2 have occurred in the past 15 years (C/Hyakutake in 1996 at 0.102 AU and 73P/Schwassmann-Wachmann in 2006 at 0.079 AU) and 2 more are predicted over the next 5 years (45P/Honda-Mrkos-Pajdusakova in 2011 at 0.060 AU and 209P/LINEAR in 2014 at 0.056 AU).

The comet is already a nice sight in small telescopes. The 2 images below were taken by myself with an ASA 8″ astrograph operated by Light Buckets, LLC. For more on LightBuckets and how to use their fleet of 3 Rodeo, New Mexico based telescopes (also a 24″ and 14.5″) see their site at

Many observers have reported estimates of the brightness of the comet over the past few months. Thanks to organizations like the International Comet Quarterly, Comet Observation Database and the CometObs mailing list, we are able to monitor the brightness behavior of the comet and compare it with its behavior from past returns.

The following 2 plots show the visual and CCD magnitude estimates for the current return as well as the 1991 and 1997 returns. Each is a plot of reduced heliocentric magnitude (magnitudes normalized to geocentric and heliocentric distances of 1 AU and a phase angle of 0° ). The correction to 0° was made with the comet dust phase function of Joseph Marcus. The top plot is phase-corrected heliocentric magnitude versus the log of the heliocentric distance while the next plot is versus time from perihelion.

The next plot takes a best fit of the 1997 data and converts it to the expected brightness of the comet in 2010 (after adjusting the heliocentric and geocentric distances and the phase angle). Luckily the perihelion distance in 1997 (1.032 AU) and 2010 (1.059 AU) are similar. Unfortunately the perihelion distance in 1991 was significantly smaller at 0.953 AU so it was excluded.

A quick glance at the plots show that the comet appears to be running fainter than in 1997. This was very apparent in August when the comet was almost 2 magnitudes fainter than predicted. In September it appears to have almost  “caught” up to its expected brightness. My CCD data produced magnitude of +8.1 and +8.2 on the 12th and 15th, respectively. But is this really the case? As the images earlier in the post show, this comet is very large and diffuse. My CCD images from Sept 12 and Sept 15 revealed a coma 26′ and 23′ across, respectively. The apparent shrinkage of the coma between the 2 nights may have more to do with the increasing moonlight than a real change. My CCD derived magnitudes are also running brighter than the visual estimates. Though the visual estimates are catching up with the CCD derived estimates. Most likely what is happening is that much of the very large and very low surface brightness coma is not being seen by the visual observers, and possible also not being entirely measured in the CCD images. The panel below shows how the diameter of the coma can changed just by stretching the contrast on  my Sept 15 images.

Three different contrast stretches from my Sept 15 8" LightBuckets image. Note how a change in contrast dramatically changes the apparent size of the coma. Credit: Carl Hergenrother.


The CCD observations (and perhaps some of the visual too) are also hampered by the very dense Milky Way star field the comet is currently passing through. For my observations, I re-observed each field on a later night without the comet. The brightness of the background stars in the photometric aperture were then measured and subtracted from the earlier (comet + stars) measurement. This will only become more of a problem as the comet grows larger on the sky as its continues to approach Earth.

Based on the 1997 data, the comet should reach a peak brightness of magnitude +4.5 a few days before perihelion (last week of October). Unless all current observers are underestimating the brightness of the comet (possible but, hopefully, unlikely) the comet is running a little fainter this return. Based on this a peak brightness of magnitude +5.0 might be more realistic. Still such a large diffuse comet will not be an “easy” magnitude +5.0. Unlike this summer’s Comet C/2009 R1 (McNaught) which had a very small coma and appeared star-like to the naked eye, Hartley 2 may grow to as large as a degree across. If true, the comet will only be a naked eye object to those observing from the darkest sky. For the rest of us, the comet will be a nice, large binocular object.

I’ll post updates as well as new images here and at the Cometary Science Center/International Comet Quarterly.

As always if you’d like to share your observations and/or images with my readers, send them to <> or just leave a comment on the blog.

Resources for finding the comet include:

The 103P/Hartley page at the Cometary Science Center/International Comet Quarterly.

The 103P/Hartley page at Seiichi Yoshida’s Comet pages.

A finder chart for Comet Hartley 2 can be found at Comet Chasing.

A nice collection of images can be found at the VdS-Fachgruppe Kometen (Comet Section of Germany) and Seiichi Yoshida’s Comet Homepage.


  1. Hi Carl,
    Nice piece, in particular the brightness analysis.
    However, you write: ‘In fact, there will be better apparitions of Hartley 2 this century.’
    I suppose you mean: ‘In fact, there will be NO better apparitions of Hartley 2 this century.’

    best regards,

  2. Thanks for sharing Carl, this is going to be a very interesting campaign indeed.
    Here you’ll find my observation of Sept the 13th, now I’m awaiting for the next dark period to follow this nice comet both with my 0.4 m RCT and telephoto lenses.

    Best regards
    Juan Lacruz

  3. Pingback: astronomy

Comments are closed.