Comet ISON has arrived, fresh from the icy fringes of our solar system, and it will graze the Sun on November 28. It could provide a magnificent show for astronomers around the world, however, we still don’t know exactly how bright and spectacular this comet will be…
Comet nuclei are like dirty snowballs the size of mountains; but they’re small on the scale of the solar system, and very dark. For comets to become visible, they must be exposed to the Sun’s heat so that frozen water and gas can sublimate, releasing the dust trapped within. This is how comets form their comas — immense clouds of dust and gas that surround the nucleus and glow in the sunlight. As for comet tails, they also consist of dust and gas that extends hundreds of thousands, even millions, of kilometers across space.
When Comet C/2012 S1 (ISON) was discovered, a bit more than a year ago, it was surprisingly bright given its enormous distance from the Sun. Moreover, its trajectory was also exceptional: ISON will pass within 1.2 million kilometers of the Sun’s fiery surface — less than a solar diameter. It was even thought the comet might become as bright as the Moon! That was enough for some to qualify ISON as the “comet of the century.” Though after this somewhat premature declaration was made, the comet subsided and its brightness is now lagging behind initial, optimistic predictions.
Astronomers are not surprised by this behaviour: ISON comes directly from the distant Oort cloud, the farthest region of the solar system populated by billions of deep-frozen comet nuclei. New comets such as ISON, which have never been subjected to the Sun’s heat, have a tendency to dissipate their volatile gases prematurely; after this marked awakening, their activity moderates.
Some astronomers estimate Comet ISON’s nucleus to be between one and four kilometers in diameter — too large to evaporate completely when it nears the Sun, despite the 2500 degree temperatures it will encounter. But its nucleus is not very solid, leading other specialists to think it might break apart due to strong tidal forces. If it does, a lot of dust will be released suddenly; that could produce a spectacular, ephemeral tail like Comet Lovejoy’s, which was visible from the Southern Hemisphere in December 2011, as the photograph that caps this article shows.
Comet ISON: game plan
Until it reaches perihelion, ISON will be visible only during the second half of the night. The best time to see it will be just before dawn, when the sky is still dark as the comet rises above the southeast horizon.
Comet ISON could become visible to the naked eye around mid-November, but only under a sky completely free of light pollution. Otherwise, binoculars will be necessary to observe it. On the days before it grazes the Sun, the comet’s brightness should continue to increase; its tail, which points away from the Sun, will be oriented above the southeast horizon. The comet will be located near the planet Mercury in the morning sky.
ISON will pass closest to the Sun on November 28, around 1:00 P.M. Eastern Time; at this point it might even outshine Venus. Unfortunately, the comet will be too near the Sun to be easily observable. However, that evening after sunset as the sky darkens, we might just be able to see its tail rising almost vertically over the west-southwest horizon.
After rounding the Sun, the comet will retreat in nearly the same direction from which it came. If it survives its death-defying dive, the gap between ISON and the Sun will rapidly increase, and throughout December it will be well above the east-southeast horizon before dawn. After mid-December, ISON will also be visible above the northwest horizon early in the evening.
But the comet will rapidly become fainter as it gets farther from the Sun. It will be closest to Earth on December 26, passing just 63 million kilometers away, and will be observable all night above the northern horizon, among the stars of Draco near the Great Bear and the North Star.
Will ISON be spectacular, or not? The future will tell. Comets are very unpredictable and one can never tell precisely what will happen. We’ll just have to wait and see…
The planets this November
Venus reaches its greatest eastern elongation, 47 degrees to the left of the Sun, on November 1. However, this apparition is less than favourable and the Evening Star will remain low on the southwest horizon during twilight and early evening. Fortunately, Venus’ dazzling brightness pierces the glow of sunset, making it an easy target. Despite circumstances, the planet’s visibility will improve slightly in November; by month’s end, it sets about three hours after the Sun. On November 6, look for a thin crescent Moon 7 degrees to the upper right of Venus.
A small telescope reveals the planet’s phases quite well: They are reminiscent of the Moon. At the beginning of November, the planet appears like a “half-Venus,” which then grows into an increasingly thinner crescent as the month progresses. These spectacular changes evolve throughout December.
Less than two hours after Venus sets, another bright planet appears in the sky: It’s Jupiter. The giant planet rises among the stars of Gemini, in the northeast during mid-evening, and culminates high in the south around 4:00 in the morning. The waning gibbous Moon passes beneath Jupiter on the night of November 21 to 22.
Mars is next to rise, at about one in the morning above the eastern horizon, and is easily visible during the second half of the night. The Red Planet is currently between Regulus, the brightest star in Leo, and the bright star Spica, in Virgo. Look for the lunar crescent to the right of Mars on the morning of November 27.
After mid-November, a third planet joins Jupiter and Mars: It’s the tiny planet Mercury, which rises at dawn in the southeast about an hour-and-a-half before the Sun. This apparition of Mercury is favourable and lasts until mid-December; it will afford a good opportunity to spot the furtive planet.
In closing, a reminder that we return to Standard Time early on the morning of Sunday, November 3: Clocks are set back one hour. There’ll be less light at the end of the afternoon, but more light to start the day with!