See Comet PanSTARRS in March and April

March 2013  :  Dave Huestis

When is the last time you observed a naked-eye comet? For many of you the answer will most likely be 1997. And the comet was the beautiful Hale-Bopp. Here in southern New England Hale-Bopp was well placed for observers during the last week of March into the first week or so of April. In fact, the accompanying image was taken on April 1 from my backyard as I stood in 18-inches of fresh snow!

While there have been many other comets since then, quite a few of them have been better observed from the southern hemisphere. Others were not as bright as Hale-Bopp, or only appeared during a short observing window, like Comets McNaught and Machholz.

Comet Holmes, in late 2007, was an exception. It exhibited the most rapid outburst ever observed, which helped the comet develop the largest coma (halo) in recorded history. Comet Holmes set a record by becoming the largest object in the solar system – amazingly surpassing the diameter of the Sun. It was even a naked-eye object in the light-polluted skies of Ladd Observatory in Providence. While at that time it did not exhibit a tail from our viewing perspective, the sheer size of the comet was impressive.

On any clear night there are tens of comets visible in the night sky. The majority are faint and require large telescopes to view them visually. They can be detected because they shine by reflected sunlight, just like all the planets and moons in our solar system. Once in a while a new comet will be discovered that shows potential for putting on a good show that anyone in a dark sky can observe and appreciate.

Back on June 6, 2011, a new comet was discovered on its journey towards the inner solar system and an encounter with the Sun. This long-period comet (orbital period greater than 200 years) originated in the Oort Cloud, a theoretical spherical cloud or halo of perhaps several trillions of comets encompassing our solar system and extending perhaps up to 465 billion miles from the Sun.

Comets are likened to “dirty snowballs,” a 1950 theory proposed by an old friend of Skyscrapers, the late Dr. Fred Whipple (1906-2004) of Harvard College Observatory. They are some of the leftovers from the creation of the solar system and are irregularly shaped objects composed of ice, small rocks, dust, various gases, and organic compounds. While in their cold and cozy orbit within the Oort Cloud, comets are inactive. Occasionally one of them gets nudged by the gravity of a nearby star, sending the comet on its long journey towards our Sun. Comet PANSTARRS is one of those comets. Named after the 70.7-inch Panoramic Survey Telescope & Rapid Response System on Mount Haleakala in Maui, which discovered it, Comet PANSTARRS could wind up being the best comet in a long time for northern hemisphere observers.

Just how “dirty” are the surfaces of comets? They have the darkest surfaces of any object in the solar system. In fact, Comet Halley, being a very old comet, is so dark it reflects only about four percent of the light that bathes it. I can’t stress enough how dark that is. A little quick research revealed that asphalt reflects seven percent of the light it receives! For those of you who live around New England, think about the snow on the side of the road in mid to late March. It’s full of sand, dirt and small rocks. It’s so dirty you almost want it to snow again just to cover over the unsightly mess. Well, a comet’s surface is many times more “dirty.” 

When a comet reaches the orbit of Jupiter (mean distance of approximately 483,500,000 miles), it starts to feel the influence of solar radiation. The comet reaches what is known as the H₂O turn on point. It begins to heat up. Subsurface ice melts and the pressure forces the material out through cracks or vents on the surface of the comet. Jets of this material spew out into space like geysers (remember the scene in the movie Armageddon?). This process is called outgassing. All this expelled material forms the cloud of nebulous material, called the coma, which envelopes the nucleus.

The solar wind (a stream of particles radiating from the Sun) not only sandblasts loose material off the comet’s surface, but also starts pushing dust, gasses and rocky material away from the comet, forming two tails. One is the dust tail which is usually curved. The dust tail is responsible for producing a trail of debris along the comet’s orbital path. If the Earth happens to pass through this debris we experience a meteor shower. The second tail is the ion tail, comprising gases that always points directly away from the Sun. So unless you observe the comet over a period of time, you really can’t tell if it’s coming or going!

Some comets develop extensive tails, which can be many tens of millions of miles long. What we observe from the Earth depends upon our viewing angle. Astronomers originally had high expectations for Comet PANSTARRS, as they believe this is the first time it has made the plunge towards our Sun. That means there is most likely a lot of loose material on the comet’s surface which will be blown off by the solar wind creating a large and bright coma, as well as a lengthy tail.

However, as I was finishing up this article in mid-February to meet publication deadlines, the magnitude/brightness expectations for PANSTARRS were being forecast downward. Since this was the comet’s first encounter with the Sun, most of the loose surface material was likely stripped off early, making it initially very bright. With that material now absent, the comet has dimmed significantly. It may now be a little fainter than most of the stars in the Big Dipper asterism of Ursa Major, or about the same magnitude as Albireo in Cygnus.

Still, a word of caution about forecasting a comet’s behavior is called for here. David Levy, famed comet discoverer (remember Comet Shoemaker-Levy that impacted Jupiter back in July 1994), is fond of saying, “Comets are like cats. They have tails, and they do precisely what they want.” My sentiments exactly. The only thing predictable about comets is that they are unpredictable.

On Comet PANSTARRS’ inbound race to towards the Sun, it will glide by the Earth on March 5 at a safe distance of about 102 million miles. It will then only be visible from the southern hemisphere. Five days later the comet will reach perihelion (closest approach to the Sun), coming within roughly 28 million miles of our life-giving star. Just before perihelion PANSTARRS will become visible to northern hemisphere stargazers. The comet’s dust and ion tails, as well as the coma, could still be very well developed after this close encounter. (Visit http://www.shadowandsubstance.com/ for a great animation showing the projected path of this comet through the inner solar system.)

On March 8 you might still be able to catch a glimpse of Comet PANSTARRS very low in the west after sunset. You’ll need a completely unobstructed western view, since the comet will be within five degrees of the horizon (half a fist held at arm’s length provides this approximate unit of measure) just after sunset. The tail will extend up and to the south (left). Each successive night the comet will appear a little higher above the horizon and farther to the north (right) as the comet recedes from the Sun and Earth on its journey back into deep space. All the while the comet’s brightness will diminish even further and the tail will shorten as each day passes.

On the 12th an exquisite waxing crescent Moon (only one percent illuminated) will complement the sky scene, although it may be extremely difficult to observe in a bright twilight sky due to its downgraded brightness. However, on the following evening the Moon will be higher in the sky, and the comet’s tail may sweep behind the Moon’s disk. That’s the night you want to be able to snap a few images with your camera.

On the 15th the comet’s tail will point perpendicular to the sunset location. Afterwards the tail will point up towards the right. By March 30 the comet’s tail may extend to a position just below the Andromeda Galaxy. From April 2-4 the nucleus of Comet PANSTARRS will pass by the Andromeda Galaxy. (Keeping in mind that PANSTARRS’ forecast brightness has been significantly reduced, many of the early to mid-March viewing opportunities noted above may be compromised.) 

While a dark sky might still show the comet after April 4 depending on how well it performs, binoculars and telescopes will be able to follow it for some time afterwards as PANSTARRS moves into a circumpolar sky, never setting below our horizon in southern New England. The Shadow and Substance website noted above also has an animation showing the comet’s position against the western horizon through the first week of April.

When Comet PANSTARRS is at its best and a naked-eye object, it will be too low in the sky for most of the local observatories. However, as it rises away from the western horizon it may become accessible. Please check the following local observatory websites to see if any observing opportunities will be available. Accessibility conditions permitting, Seagrave Memorial Observatory in North Scituate is open every clear Saturday night. Ladd Observatory in Providence is open every Tuesday night. Frosty Drew Observatory in Charlestown is open every clear Friday night. Winter hours for Seagrave and Ladd are 7:00-9:00 p.m., while Frosty Drew begins at 6:00 p.m. with no set end time. 

Make every attempt to catch a glimpse of Comet PANSTARRS, for it is not expected to return to the inner solar system for perhaps 110,000 years!

Viewing updates will be forthcoming as warranted by the comet’s performance. 

Keep your eyes to the skies!