With their existence known since ancient times, comets are possibly one of the most widely observed class of cosmic targets. Naked eye comets are relatively rare and often unpredictable. Indeed, the only known comet to make unaided eye appearances repeatedly with every passage by earth is the famous Halley’s comet, which makes an approach approximately once every 75-76 years. With its last appearance in 1986, Halley’s comet will make its next approach of the inner solar system in 2061.
Comets can in fact be of two main types – short period or long period comets. Short period comets, such as Halley’s comet, are defined as those completing one complete orbit around the sun in less than 200 years. Extremely bright comets are normally long period comets – comet Hyakutake, which passed by earth in March 1996, has an orbital period of approximately 76,000 years. One of the brightest comets to ever be observed, comet Hale-Bopp, passed perihelion on April 1, 1997 and was visible for 18 months to the unaided eye. With a previous orbital period of 4,200 years, a close encounter by Jupiter in 1996 has changed Hale-Bopp’s orbital path enough to reduce its orbital period to around 2,500 years. In both cases, and as is always the case with such long period comets, their initial discovery was carried out just months before they reached the inner solar system, meaning that such comets might be approaching the inner solar system, unknown to us, at any time.
Currently approaching the inner solar system and brightening rapidly is another comet – Comet C/2019 Y4 Atlas. Discovered on December 28, 2019, it is currently very close to naked eye visibility from excellent dark skies. Comet Atlas is currently crossing Mars’s orbit in its journey towards the inner solar system. Although this comet could reach an apparent magnitude as high as -6, which would be brighter than Venus in our skies, many comets tend to fizzle out or reach a significantly lower magnitude than expected. It will pass within around 117 million kilometres of earth and will have an extremely close encounter with the sun – at less than 38 million kilometres, it will be well within Mercury’s orbit.
The comet appears to be following the trajectory of the Great Comet of 1844, with some speculating that a much larger comet fragmented along this path in the past, leaving behind a number of comet fragments in the same orbit, following a 6,000-year orbital path around the sun. Although this comet might indeed put on a spectacular show, especially for Northern hemisphere observers, comets tend to behave in a particularly erratic fashion and it remains to be seen whether it will reach its potential brightness around the end of May.
Josef Borg is currently a PhD student within the Institute of Space Sciences and Astronomy, University of Malta, and also the President of the Astronomical Society of Malta.
Sound bites
• ESA puts four science missions on hold during pandemic: The European Space Agency announced last Tuesday it is suspending regular operations of four of its missions, including one recently launched spacecraft, to reduce staffing needs during the coronavirus pandemic. ESA said it was putting into a “temporary standby” mode the four-satellite Cluster space science mission in earth orbit, the Solar Orbiter spacecraft, the ExoMars Trace Gas Orbiter and Mars Express orbiter.
https://spacenews.com/esa-puts-four-science-missions-on-hold-during-pandemic/
• It might rain iron on this Hot Jupiter: When astronomers discovered the gas giant WASP-76b in 2016, about 640 light-years from earth in the constellation Pisces, they already knew it was exotic. WASP-76b circles its star in only 43 hours, making it a “hot Jupiter”. Now, a group of astronomers led by David Ehrenreich (University of Geneva, Switzerland) has found that this planet might be even stranger than we thought, hosting clouds and fog made of iron. The research appears in Nature.
https://skyandtelescope.org/astronomy-news/iron-might-rain-hot-jupiter/
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Did you know?
• Comet Shoemaker-Levy 9 collided with Jupiter in 1994. After its discovery in 1993 by astronomers Carolyn and Eugene Shoemaker and David Levy, it was noted that the comet was orbiting Jupiter after the gas giant planet had captured it on its journey through the solar system. The comet stirred great anticipation after its orbital path was calculated, with a collision deemed extremely likely. In July 1994, the fragmented comet collided over the course of several days with the gas giant, producing large blemishes visible from earth.
• Comets are responsible for most periodic meteor showers. With the exception of the Geminid meteor shower, all of the prominent periodic meteor showers occur as a result of comet dust trails crossing earth’s orbit. As the earth orbits around the sun, it crosses specific points in space where a comet would have crossed in the past, and where dust particles left behind by the comet are still orbiting the sun in the comet’s wake. One famous example is the Perseid meteor shower, which occurs between August 12 and 13 and is a result of comet Swift-Tuttle.
For more trivia, see: www.um.edu.mt/think