You might not have heard, and almost certainly didn’t see, Comet Heinze (C/2017 T1), zip by Earth over the past week and a half or so.  Discovered last year, it was expected to brighten up and make a good showing as a binocular comet through January 2018, but it’s still very dim, at Magnitude 9.5.  It’s actually still out there, but most will never see it.

Comets, as it turns out, are surprisingly common – many new ones are discovered each year, and many of those are bright enough for amateurs to observe.  But through history, comets and their nature have remained among the more elusive topics of solar system understanding.  In fact, back when the solar system was being modeled as concentric transparent shells (we’re talking the Aristotle model here), comets weren’t even perceived to be in space.  They were atmospheric, strange weather phenomena, having more to do with omens than orbits.  How things have changed…

So here is a totally unscientific, totally personal review of some of the most important comets – at least so far as they have helped advance our understanding of them.

Comet Halley (1P/Halley)

Halley’s Comet, 1910 (public domain)

I’m listing Halley first, not because I’m implying it is the most important, or because it is arguably the most well known comet – but because chronologically, it is the first to have been understood for what it is – an object orbiting the Sun, on a regular, periodic, predictable path.   Right there in its designation, “1P”, is an indicator that it was the first comet established to be periodic.  Prior to Halley, astronomers had determined that comets had orbits that took them well beyond the known solar system (i.e. past Saturn).  Georg Dorffel asserted that the comets seen in 1680 and 1681 were actually the same comet, seen before and after its perihelion, with the Sun at the focus of a parabolic orbit. In 1687, Issac Newton applied his new theory of gravity to predict that comets were actually on elongated elliptcal orbits, albeit very near parabolic in shape. British astronomer Edmund Halley determined in 1705 that the comets seen in 1531 and 1607, and then again in 1682, were in fact the same comet, and would return again in 1759.  Unfortunately, Halley died before the comet’s return, but his prediction proved correct and his name is inexorably linked to comets as a result.  During the 1986 passage, Comet Halley was the target of numerous probes:  Two joint Soviet/French (Vega 1 and Vega 2), the European Space Agency’s Giotto, two Japanese probes (Sakigake and Suisei), and the United States’ International Cometary Explorer (ICE) all got a close look at Halley.  This turned out to be fortuitous, since the 1986 view from Earth turned out to be underwhelming.  Halley’s next pass will be in 2061.

Comet Encke (2P/Encke)

Comet Encke, Nov 17, 2013.  NASA.

You might guess from the designation that Encke was the 2nd comet determined to be periodic, and you’d be right!  Encke established a pattern, a recognition that maybe Halley wasn’t unique.  Calculating the orbits of comets seen in 1786, 1795, 1805, and 1818, Encke concluded these were all a single orbiting body, and successfully predicted its reappearance in 1822.  Since the recognition that OTHER comets were periodic, humanity has discovered at least 300 unique orbiting comets – though some of those have broken up and disintegrated in encounters with the Sun.  Encke also has the shortest known orbital period of any comet, at only 3.3 years.

Comet Swift-Tuttle (109P/Swift-Tuttle), and Comet Tempel-Tuttle (55P/Tempel-Tuttle)

These two comets, with periodic orbits of 133 and 33 years, respectively, intersect Earth’s orbit almost exactly, making them, at times, EXTREMELY nearby and bright objects, and even potential collision hazards.  The reason I include them here, though, is that the dusty trails left by these two comets form the source of two of our more prominent regular meteor showers here on Earth – the Perseids (Swift-Tuttle), and the Leonids (Tempel-Tuttle).  In 1867, Giovanni Schiaparelli realized that the orbits of these comets intersected the orbit of Earth at locations that happened to coincide with increased meteor activity, and realized that the orbiting dust and debris from the comets’ tails were the source.  This correlation was a new idea, at the time – now all regular showers are attributed to periodic comets.  (The Orionids are derived from Halley).

Comet Hale-Bopp (C/1995 O1)

Comet Hale-Bopp, March 1997.  NASA

Hale-Bopp has a couple records to its credit.  First, it is the largest comet we’ve seen, with a nucleus roughly 60km in diameter (Halley is only 10km wide), and, probably as a result, it holds the record for the longest period of naked-eye visibility of any known comet.  In 1996 and 1997, Comet Hale-Bopp was visible without magnification for a whopping 18 months.  It was a great show, but one we won’t be able to witness for another 2,500 years.

Comet Shumaker-Levy 9 (D/1993 F2)

Jupiter shows the scars from Shumaker-Levy 9 impacts in this NASA image.

By the time this comet was discovered in 1993, it had already passed close enough to Jupiter (in 1992) for tidal forces to fracture its nucleus.  Shumaker and Levy discovered several nuclei in close proximity, and calculations of its orbit revealed that it was actually orbiting Jupiter, not the Sun, and had likely been captured by the giant planet in the 60’s or 70’s.  Excitement grew when astronomers realized that, in its fractured form, it would actually impact the planet in July 1994.  Thus, Shumaker-Levy 9 became the first object ever to be observed impacting another solar system object.  Twenty-one fragments of up to 2km across slammed into Jupiter’s upper atmosphere, burning and exploding as they went, leaving scars that were visible for months.  The impacts also enabled us to briefly peer through the cloud tops and, using spectroscopy, gain insight on the chemical makeup of Jupiter’s lower atmosphere.  Given that the first chunk impacted Jupiter with the energy equivalent to 600-times Earth’s global nuclear arsenal, this event also marked a significant increase in public awareness in – and funding to investigate – the danger of Near-Earth Objects (NEOs) that might pose a similar impact hazard to our own planet.

Comet Tempel 1 (9P/Tempel)

The cloud generated by Deep Impact on Tempel 1 was surprisingly bright.  NASA image.

An otherwise unremarkable, dim comet, Tempel 1’s short orbital period of 5.5 years made it a target for a very ambitious plan.  NASA’s Deep Impact spacecraft rendezvoused with Tempel 1, and intentionally fired an impactor into its surface on July 4, 2005, with the intent of revealing its internal composition.  The comet was revealed to be more dusty, and less icy, than predicted, but with a fine particulate more akin to talcum powder than sand.  It was revealed to be very porous, and likened to a snow bank, and contained chemicals known to form only in the presence of liquid water.  The dust ensuing dust cloud made imaging of the crater very difficult for Deep Impact – so NASA re-tasked an existing probe to make a second visit to Tempel 1.  The Stardust probe, which had been sent to fly through the tail of Comet Wild 2 in 2004, engaged in a 2011 rendezvous with Tempel 1 on a mission called, appropriately enough, New Exploration of Tempel 1 (NExT), where it imaged the nucleus and was able to capture photos of the Deep Impact crater.  Tempel 1 is the only comet, thus far, to have been visited twice, on two different orbits, by different spacecraft.  (Halley was visited by numerous probes in 1986, but all within a span of several weeks on one of its visits to the inner solar system).

So there you have it!  A veritable who’s who in comets.  Like most objects in the solar system, our explorations have revealed that our broad generalizations of comets have their limitations.  Each of these bodies is different, with its own formation history, makeup, and characteristics.  We know far more than those in Aristotle’s time, but comets continue to be somewhat unpredictable in terms of how much dust and gas they will shed, how bright they will be, and the length of their visible tails.

Comet Heinze, and its disappointing, persistent faint-ness, is only the latest comet to surprise us.  Given our track record of discovery, though, there are likely to be several more observable comets in 2018, and we will likely learn something from each one!

Get Out There!
Troy
http://www.flying-squirrel.org