Spotting Comets

Halley’s Comet in 1986

Amongst the coolest and most unpredictable objects to spot in the night sky are comets. Comets come from the outer solar system, and make their trek around the sun. Sometimes, they get spectacularly bright, other times they remain dim. They can make the trip unhindered, or they can break apart as they pass near the Sun. They may even smash into a planet. Let’s take a look at these deep solar system visitors…

Nucleus of 67P/Churyumov–Gerasimenko from the ESA’s Rosetta probe

Comets regularly venture their way into the inner solar system. Some are long-period comets, which can take thousands of years to make a round trip. Others can be short-period comets, which are trapped in the inner solar system and make their trip in just a few years. Comets generally don’t reflect much sunlight, reflecting less light than asphalt generally. What makes a comet bright is its coma and tail.

Halley’s Comet in 1986 imaged by the ESA’s Giotto probe showing the debris being vaporized from the nucleus

The coma is the brighter haze surrounding the nucleus, or center of the comet, and consists of particles being vaporized from the heat of the Sun. The tail, as the name suggests, is the bright trail of debris which follows the comet.

In fact, the debris from comets doesn’t completely disappear. Debris trails from comets cause several meteor showers throughout the year. The Orionids meteor shower in November is thought to be caused from debris from Halley’s Comet, which makes a trip to the inner solar system every 75 years or so.

Comet Hyakutake in 1996

A combination of factors determine a comet’s brightness. First, the closer to Earth that the comet passes, the brighter it tends to be. Another factor is the number of trips a comet has made close to the Sun. A comet that has made multiple passes near the Sun isn’t usually as bright as a comet that has never made a close approach to the Sun. A frequent-passing comet has already lost material and mass from passing near the Sun, and tends to give off less material whereas a comet making its first pass near the Sun has been untouched by heat, and has more material to shed. It can be quite difficult for astronomers to predict the brightness of comets until they get really close to Earth and the Sun due to these factors. Some start out dim and get very bright. Others start out bright, but then are unimpressive as they appraoch. This makes the anticipation of arriving comets greater, since it’s hard to tell which comet will be the next bright one.

Jupiter in 1994 bearing impact scars from fragments of Comet Shoemaker-Levy 9

Sometimes, a comet passes really close to a planet or the Sun. Short-period comets were affected by gravity and captured into the inner solar system. Some comets are trapped by planets, such as Comet Shoemaker-Levy 9. Shoemaker-Levy 9 was captured by Jupiter, broke apart, and slammed into Jupiter in 1994 leaving huge “scars” visible on the planet. Other comets pass too close to the Sun and either break apart, or are vaporized from the heat, similar to Comet ISON in 2013.

Comet Hale-Bopp in 1997

1996-1997 had 2 great comets to see, and was a rarity as far as having 2 bright, naked-eye visible comets in such a close span of time. Both Comet Hyakutake and Comet Hale-Bopp were bright enough to be spotted with the naked eye, and graced our night skies for several months. Although there are several comets each year that pass by Earth, only a small percentage are visible to the naked eye. However, some of these comets can be seen with binoculars or telescopes as they pass by. An example of this was Comet Lovejoy in the winter of 2014. Comet Lovejoy was just barely visible to the naked eye in very dark skies. But for those who knew where to look with optical aid were treated to a greenish-colored comet near the constellation of Orion.

Comet Lovejoy in 2014

Currently, there is a dim comet in our night sky. Comet Catalina is currently passing through the constellation Virgo and moving towards the Big Dipper. Comet Catalina is fairly dim, and can’t be made out with the naked eye yet, as it won’t hit its peak brightness until January 10th. Binoculars or telescopes can make it out as this point.

Comets move quickly across the sky in relation to the stars and planets. They don’t move quickly enough to where their motion is apparent, but if you view a comet on multiple nights, you will see a definite change in position relative to the background stars each night. Therefore, the window of opportunity to spot a comet is measured in months, as opposed to seconds, which is the case with meteors, and minutes, as is the case with eclipses and occultations.

Comet Catalina in 2015

Viewing comets in the night sky is a treat for skywatchers. They’re rare (especially bright ones), and interesting because there’s the chance that you are part of a select group of humans who get to see that particular comet if its a long-period comet that only comes by every several thousand years! Those with large amateur telescopes can spot comets almost regularly (and may even discover a few!). Finding good star charts, suscribing to a good astronomy magazine, and/or visiting astronomy websites can help you find comets, especially the dimmer ones. They’re definitely worth the effort of finding!

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