Remembering Comet Holmes

Twelve years ago today, Comet Holmes (17P) brightened from magnitude 16.5 to 2.6, forming a right triangle with Mirfak (α Persei) and δ Persei, opposite to Algol. Here is what I wrote in The Sky This Week at that time.


TSTW 10/25/07

Comet Holmes Bursts on the Scene!

Who ever said astronomy isn’t exciting? Sure, much of what we observe in the cosmos seems predictable and unchanging—but then something unexpected happens and we are scrambling to get a front-row seat and our lives are thrown into an exhilarating tizzy for a few hours or days. Whether it be an unexpected auroral display, a meteor fireball, a nova, supernova, or comet, the result is the same: it is exciting to be an astronomer, to be attuned to a universe that existed long before we were born and that will be here long after we are gone. That, to me, is comforting.

Very early Wednesday, October 24, a 16th-magnitude short-period comet presently in Perseus by the name of Holmes brightened about 14 magnitudes from 16.5 to 2.6 in little more than 12 hours: a brightness increase of 363,000 times! While such a cometary outburst was unexpected, it is not unprecedented. From time to time, solar heating (greatest when a comet is near perihelion) must cause pressure to build up inside a comet as subsurface ices volatilize. Eventually, the pressure builds up until it explodes through the surface of the comet, spewing gas and dust into space and exposing fresh material to solar radiation. Sometimes, this process is so violent that the comet breaks into multiple fragments.

Comet Holmes (17P) is one of the so-called “short period” comets, meaning it orbits the Sun in less than 200 years or has been observed at more than one perihelion passage. Comet Holmes orbits the Sun every 6.9 years, ranging from just inside the main part of the asteroid belt (2.1 AU) to the orbit of Jupiter (5.2 AU). No doubt Comet Holmes’ original orbit has been substantially altered by the gravitational influence of Jupiter. Comet Holmes is presently 2.5 AU from the Sun (230 million miles) and 1.6 AU from the Earth (150 million miles), having just passed perihelion on May 4, 2007.

Comet Holmes was discovered during its last outburst, which occurred on November 6, 1892 by English amateur astronomer Edwin Holmes (1839-1919). It was observed again in 1906, but was then lost until being recovered in 1964. It has since been observed near perihelion at every return.

The recent outburst of Comet Holmes may be one for the record books. I am not aware of any other comet outburst being recorded where the comet brightened by as much as 14 magnitudes in less than a day! Fortunately, the first two nights after the outburst the sky was beautifully clear here. The first night, October 24, Comet Holmes looked like a star to the unaided eye. In binoculars, it looked like a tiny yellow or orange planetary nebula, only slightly bigger than a star, and of uniform brightness. The following night, October 25, it still looked like a star to the unaided eye, but in binoculars it was larger than the previous night. The total brightness had not diminished. In the telescope, the comet was truly spectacular, made all the more amazing considering how the comet was only 43° away from the closest full moon of the year! The round coma contained a bright off-center fan-shaped wedge with a brilliant tiny pointlike nucleus. There was definitely evidence of concentric, spiraling shells of material opening outward from the center of the coma to the outermost parts of the coma.

You have just got to get out to see this comet! And as often as possible! Here is an ephemeris for Dodgeville for the coming week.


TSTW 11/1/07

Comet Holmes (17P)

Comet Holmes slowly moves towards Mirfak this week, an impressive binocular and telescopic object in Perseus. It is easily visible to the unaided eye, too, as a small fuzzball on the Capella-side of Perseus.

Sunlight and solar wind particles are hitting the comet on the north-northeast side, and photographs show the comet is sharp edged there. The opposite, south-southwest side is ragged, with ionized gas streamers spreading out in that direction in long-exposure photographs.

Whatever tail the comet has is pretty much hidden behind it, as our viewing angle (known as the phase angle) diminishes from 15° to 13° this week. The phase angle is the Sun – Comet – Earth angle. A phase angle of 0° would mean we are looking directly down the tail (least favorable, maximum foreshortening). A phase angle of 90° would mean we are looking perpendicular to the tail (most favorable, no foreshortening).

Prime time for observing the comet is pretty much all night, with the comet transiting the celestial meridian at 2:05 a.m. CDT at the beginning of the week, and at 12:30 a.m. CST by the end of the week. Look at it every clear night, because surprising changes can and do occur. Don’t miss it! It may be a while until something like this happens again. The last time Comet Holmes went into a major outburst was 115 years ago!

Comet Orbital Elements

The orbit of a comet can be defined with six numbers, called the orbital elements, and by entering these numbers into your favorite planetarium software, you can view the location of the comet at any given time reasonably near the epoch date. The epoch date is a particular date for which the orbital elements are calculated and therefore the most accurate around that time.

Different sets of six parameters can be used, but the most common are shown below. Example values are given for Comet Holmes (17P), which exhibited a remarkable outburst in October 2007, now almost 12 years ago.

Perihelion distance, q

This is the center-to-center distance from the comet to the Sun when the comet is at perihelion, its closest point to the Sun. For Comet Holmes, this is 2.05338 AU, well beyond the orbits of both the Earth and Mars.

Orbital eccentricity, e

This is a unitless number that is the measure of the amount of ellipticity an orbit has. For a circular orbit, e = 0. A parabolic orbit, e = 1. A hyperbolic orbit, e > 1. Many comets have highly elliptical orbits, often with e > 0.9. Short-period comets, such as Comet Holmes (17P), have more modest eccentricities. Comet Holmes has an orbital eccentricity of 0.432876. This means that at perihelion, Comet Holmes is 43.3% closer to the Sun than its midpoint distance, and at aphelion Comet Holmes is 43.3% further away from the Sun than its midpoint distance.

Date of perihelion, T

This is a date (converted to decimal Julian date) that the comet reached perihelion, or will next reach perihelion. For example, Comet Holmes reached perihelion on 2007 May 5.0284.

Inclination to the Ecliptic Plane, i

This is the angle made by the intersection of the plane of the comet’s orbit with the ecliptic, the plane of the Earth’s orbit. Comet Holmes has an inclination angle of 19.1143°.

Longitude of the ascending node, Ω

The intersection between the comet’s orbital plane and the Earth’s orbital plane forms a line, called the line of nodes. The places where this line intersects the comet’s orbit forms two points. One point defines the location where the comet crosses the ecliptic plane heading from south to north. This is called the ascending node. The other point defines the location where the comet crosses the ecliptic plane heading from north to south. This is called the descending node. 0° longitude is arbitrarily defined to be the direction of the vernal equinox, the point in the sky where the Sun in its apparent path relative to the background stars crosses the celestial equator heading north. The longitude of the ascending node (capital Omega, Ω) is the angle, measured eastward (in the direction of the Earth’s orbital motion) from the vernal equinox to the ascending node of the comet’s orbit. For Comet Holmes, that angle is 326.8532°.

Argument of perihelion, ω

The angle along the comet’s orbit in the direction of the comet’s motion between its perihelion point and its ascending node (relative to the ecliptic plane) is called the argument of perihelion (small omega, ω). For Comet Holmes, this angle is 24.352°.


If all the mass of the Sun and the comet were concentrated at a geometric point, and if they were the only two objects in the universe, these six orbital elements would be fixed for all time. But these two objects have physical size, and are affected by the gravitational pull of other objects in our solar system and beyond. Moreover, nongravitational forces can act on the comet’s nucleus, such as jets of material spewing out into space, exerting a tiny but non-negligible thrust on the comet, thus altering its orbit. Because of these effects, in practice it is a good idea to define a set of osculating orbital elements which will give the best positions for the comet around a particular date. These osculating orbital elements change gradually with time (due to gravitational perturbations and non-gravitational forces acting on the comet) and give the best approximation to the orbit at a given point in time. The further one strays from the epoch date for the osculating elements, the less accurate the predicted position of the comet will be.

For example, the IAU Minor Planet Center gives a set of orbital elements for Comet Holmes that has a more recent epoch date than the one given by the JPL Small-Body Database Browser. The MPC gives an epoch date of 2015 Jun 27.0, reasonably near the date of the most recent perihelion passage of this P = 6.89y comet (2014 Mar 27.5736). JPL, on the other hand, provides a default epoch date of 2010 Jan 17.0, nearer the date of the 2007 May 5.0284 perihelion and the spectacular October 2007 apparition. For the most accurate current position of Comet Holmes in your planetarium software, you’ll probably want to use the MPC orbital elements, since they are for an epoch nearest to the date when you’ll be making your observations.