Comet Hartley Comes Home Again; Gegenschein-gazing

Every 6.5 years Comet 103P/Hartley swings back into the inner solar system and grows a fluffy coma and silky tail under the Sun's harsh gaze. The comet last reached perihelion in April 2017 during an unfavorable apparition when it clung near the Sun and never brightened much beyond 12th magnitude. That makes its current appearance the first for visual observers since 2010–11. Comet 103P/Hartley is a Jupiter-family comet that orbits between Mars and Jupiter with an average distance from the Sun of 3.5 a.u. Jupiter-tamed comets have shorter periods with orbits shaped by the gas giant.

Malcolm Hartley discovered his namesake in 1986 while examining plates taken with the UK Schmidt Telescope at Siding Spring Observatory in New South Wales, Australia. At the time he described it as a faint streak. Little did he know then that his comet would be targeted for a flyby by NASA's Deep Impact (EPOXI) mission in November 2010, one of just eight comets visited by spacecraft to date. NASA officials invited Hartley to the festivities during the flyby. At the time his three-year-old granddaughter wanted to go to the comet so badly she begged him to come along. When he departed for the U.S., she thought grandpa was leaving to go to the comet. Hartley quoted her as saying, "Oh, we must fly, we must all fly to the comet."

Take a sip

During the flyby the spacecraft came within 694 kilometers (431 miles) of the peanut-shaped nucleus. Outgassing from the core revealed the presence of water ice, methanol (also known as wood alcohol), carbon dioxide, and possibly methane. Curiously, most of the carbon dioxide shoots out the ends of the comet in geyser-like jets while water lofts from its midsection. You'll be happy to know that after proper filtering you'd find a cup of Comet Hartley's water nearly as tasty as that from your faucet. Both have a similar ratio of deuterium (an isotope of hydrogen) to hydrogen, or D/H. It's thought that asteroids and at least some comets contributed to Earth's water reserves once it cooled down enough for water to pool on its surface. In contrast, the D/H ratio for Comet 67P/Churyumov-Gerasimenko is more than four times higher.

The comet passed closest to Earth on September 26th at 56.9 million kilometers (35.3 million miles) and will reach perihelion on October 12th at a little more than one Earth-Sun distance. Its apparent brightness rather depends on your instrument. Recent magnitude estimates on the Comet Observation database (COBS) site show it around magnitude 8–8.5 for observers using large binoculars. With an expansive field of view and dark skies Comet 103P/Hartley's diffuse coma balloons to 10′ to 12′ across. Larger instruments with more restricted fields of view may not show the coma as large and lead to fainter estimates.

Magnitude 8 or 10 — it depends

When making a magnitude estimate observers use several different variations on a theme. In a common one, called the In-Out method, the in-focus comet is compared to stars of known magnitude that are defocused to match its diameter. Faint-looking comets with large comas are sometimes assigned surprisingly bright magnitudes for this reason. Just like extended deep-sky objects they can appear dim because their light fans over a large area. That's what may be the case with 103P/Hartley. Observers with larger instruments are estimating closer to visual magnitude 9.5–10. Of course, the best way to find out what the comet is doing is to take a look yourself.

I last observed it on September 14th at magnitude 10.0 with a 5′ coma and DC (degree of condensation) of 3, meaning a light-to-moderately condensed object with a brighter core centered in a diffuse coma. A Swan band filter that passes emissions from diatomic carbon worked well to enhance the coma's contrast. The comet should reach peak brightness early this week and fade about half a magnitude by month's end. It's on the move across the constellation Gemini, crosses into Cancer on October 20th, and slides into Hydra on Halloween. That makes it a morning object best viewed from 1 a.m. local time until dawn. Hartley should be visible in a 6-inch or larger telescope (or large binoculars) under a dark sky without strong moonlight. We'll have to make do with a little moonlight during October's first week, but the Big Green Cheese will scram after that, leaving a relatively dark window from about October 7–23.

Clowning around

I want to point out an especially close pairing of the comet with one of the sky's brightest planetary nebulae. On the morning of October 12th it will brush up against 9.7-magnitude NGC 2392 in Gemini, also known as the Clown Face Nebula. From the Midwest, the comet passes about 12.5′ to its northwest and just 7′ away for observers on the West Coast. You'll easily lasso them in the same low-to-medium magnification field of view. It will also make for an excellent imaging opportunity. Comet Hartley isn't the only bright morning comet. Comet 2P/Encke, now around magnitude 8, is quickly wending its way east through Leo and Virgo low in the eastern sky before the start of morning twilight. Check the October 2023 issue of Sky & Telescope magazine for a map and details.

Gimme some gegenschein

October is also one of the best times to see the gegenschein or "counterglow." If you're up at 1 a.m. hunting for Comet Hartley take a few minutes to track the gegenschein down. The gegenschein looks like a faint, diffuse patch of light about 8°–10° wide and 4°–6° high centered on the ecliptic in Pisces below the Great Square of Pegasus. The best time to see this ghostly glow is when it crosses the meridian around 1 a.m. local daylight time. Early autumn is an ideal time because that section of the ecliptic stands high above the horizon haze at the "midnight" hour and the similarly bright Milky Way doesn't get in the way.

The gegenschein is related to the zodiacal light, which coincidentally is making its best appearance in the predawn sky for mid-northern latitude observers right now. Look for a cone of softly glowing light — broader at its base and tapering with increasing altitude — towering from the eastern horizon just before the start of dawn. Its magnificent, airy presence is the glow of interplanetary dust from comets, asteroids, and even Mars along and near the ecliptic plane forward-scattered by sunlight. Most of the material lies between Earth to just beyond the orbit of Mars.

Bright oasis along the dusty trail

At its tapered end the zodiacal light continues along the zodiac (ecliptic) as the much fainter zodiacal band, which encircles the night sky. From the pristine skies of western Oklahoma I've seen the entire band except near the horizon where light absorption masks it. The gegenschein is a brighter splotch of hazy light directly opposite the Sun within the band. When the Sun dips lowest below the horizon at local midnight (1 a.m. daylight time) the gegenschein stands due south and highest in the sky at what is essentially the "full Moon" point of the ecliptic on that particular night.

Each particle at that location acts as a microminiature version of the full Moon. Lit face-on by the Sun with shadows tucked out of view in the rear they create a region of enhanced brightness within the zodiacal band.

Plan your gegenschein outing fairly early in the month. As the Earth circles the Sun the full-Moon point shifts eastward from night to night. That wouldn't be a problem except for Jupiter. In early October it's about 27° due west of the counterglow's center. That shrinks to 13° by month's end. Glare from the planet will make it difficult to see at that time. Best mornings are October 7–21 from about midnight until 2 a.m. when it will be just you and the owls.