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Northern Arizona astronomy: The Neanderthal Comet

February 2023 sky chart. (Graphic/Barry Malpas)

February 2023 sky chart. (Graphic/Barry Malpas)

Comets have been observed and recorded since ancient times by many cultures and in many regions. They orbit around the Sun and are the frozen leftovers from the formation of the solar system, composed of dust, small rocky particles and various ices which can vary from hundreds of feet to tens of miles in diameter.

However, unlike the planets, which have relatively circular orbits, comets have very elongated elliptical (cigar-shaped) orbits which can take several years to many thousands of years to orbit just once.

Comets are mostly found way out in the solar system. Some, known as short-period comets, reside in a wide disk called the Kuiper Belt, which is beyond the orbit of Neptune and take less than 200 years to orbit the Sun. Long-period comets come from the Oort Cloud, the sphere-shaped, outer edge of the solar system that is about 50 times farther away from the Sun than the Kuiper Belt, and can take as long as 250,000 years to make just one trip around the Sun.

As they approach the inner solar system, the solar radiation and solar wind warms them enough that the ices begin to outgas from the nucleus along with the dust, which forms the coma (the glowing head of the comet) along with its ion and dust tails. The coma may be up to 15 times the diameter of Earth, while the tails may stretch beyond one solar-earth distance. If sufficiently bright, a comet may be seen from Earth without the unaided eye; very bright comets are observed about every decade.

The most famous comet is Halley’s Comet, observed by the British astronomer Edmund Halley in 1682, who correctly predicted it would return again in 1757. This comet, with a period of about 75 years, has been found to have been historically identified going back 2000 years and is believed to be the comet shown in the 1066 Bayeux Tapestry.

Another famous comet was Shoemaker-Levy 9 which, under the massive gravity of Jupiter, split up into a total of 21 comet fragments before each crashed into the gas giant in 1994. Occasionally, comets can also get too close to the Sun due to their very elliptical orbits where they get torn apart. Currently there are over 6000 known comets.

There is a comet visible in the evening sky with the designation C/2022 E3 (ZTF). It has a very long orbital period of about 50,000 years, making the last time it appeared close to Earth, the Neanderthals were still running around Europe. Although it was at its closest point to the Sun on Jan. 12, it will be in good position to observe in February. This comet is expected to be of moderate luminescence.

The brightness of comets is very unpredictable, since this depends on the scattering of sunlight from the dust and ice particles in a comet's nucleus and tail. As the material streams away from the nucleus, the density at any time is determined by the rate of sublimation of the ice in the nucleus as it is heated by the Sun. It also depends on the amount of dust that is mixed in with the ice. So, while the future positions in the sky of comets are well known with a high degree of confidence, their future brightness is not.

As shown on the sky chart, comet C/2022 E3 (ZTF) will be visible just north of the bright star Capella in the constellation Auriga on Feb 5. By Feb. 10, it will have moved just north of the planet Mars. On Feb. 15, it will be located in the “wedge” of the constellation Taurus. And on Feb. 20, it will have travelled near the left arm of Orion.

As long as you have a non-light polluted sky, you should be able to observe the comet visually. However, a pair of standard binoculars will enhance the viewing and should make the tail more visible, as well as show the slight greenish hue of this once-in-a-lifetime solar system visitor.

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