A Tail of Two Comets

By Riddle, Bob | Science Scope, January 2014 | Go to article overview

A Tail of Two Comets


Riddle, Bob, Science Scope


After the breakup of Coment ISON during perihelion, we can turn our attention to another comet, Comet 67P/Churyumov-Gerasimenko, the focus of the European Space Agency's (ESA's) International Rosetta Mission. The two-part mission involves placing a lander on the comet's surface before perihelion, or the point where it is closest to the Sun, and having the Rosetta spacecraft follow the comet as it orbits around the Sun at perihelion in 2015.

Comet 67P/Churyumov-Gerasimenko was discovered by Klim Ivanovich Churyumov, a Ukrainian astronomer, in 1969 while he was studying photographic plates taken by instruments at the Fessenkov Astrophysical Institute in Kazakhstan. Comet 67P/Churyumov-Gerasimenko has a 6.6-year period, with an orbit that ranges from 5.6 AU (astronomical units) at aphelion (or the point where it is farthest from the Sun, and near the orbit of Jupiter) to 1.2 AU at perihelion.

The comet will approach perihelion passage in November 2015, and the ESA's Rosetta spacecraft will join it before then. The Rosetta spacecraft was launched in March 2004. Over its long voyage toward its rendezvous in May 2014 with the comet, the spacecraft has been given three gravity assists from Earth flybys and one gravity assist from a Mars flyby. In that same time period, the spacecraft flew past two asteroids: Steins in 2008 and Lutetia in 2010. In July 2011, the spacecraft was placed in hibernation mode, which ends on January 20, 2014, in anticipation of the comet encounter in May.

The International Rosetta Mission, like most space exploration missions, has a long history, beginning with its approval in 1993 of a one-year delay in launch that involved changing the target from Comet 46P/Wirtanen to Comet 67P/Churyumov-Gerasimenko. The overall goal for the mission is "to study the origin of comets, the relationship between cometary and interstellar material, and its implications with regard to the origin of the solar system" (ESA 2013). With the goal of unlocking secrets about our solar system, the mission was named after the Rosetta Stone, a tablet discovered in 1799 that was inscribed with ancient Egyptian hieroglyphs, Egyptian demotic script, and ancient Greek. A few decades after its discovery, researchers realized that the inscriptions could be used to unlock and translate the ancient Egyptian language, which had not been understood until then. See Resources for the International Rosetta Mission website, as well as the website for the British Museum, which has housed the Rosetta Stone since 1802.

Quadrantid meteor shower

The Quadrantid meteor shower is one of the best annual meteor showers, and this year, it will not be obstructed at its peak time by reflected light from the Moon brightening the sky. The source of this meteor shower is asteroid 2003 EH1, which is believed to have been part of a comet nucleus that broke apart several centuries ago. The Earth's orbital path and the orbital path of the asteroid are perpendicular to each other, with an intersection somewhat offset from the center, presumably the densest part of the debris cloud (see Figure 1). During January, when the Earth passes through the debris cloud, the peak period is not as long as peak periods during other meteor showers. Nonetheless, this annual meteor shower is one of the best because of the high number of visible meteors. All meteor showers are given a numerical value called the zenith hourly rate (ZHR), which represents the average number of meteors seen per hour. For the Quadrantids, the average could be as high as 100 or more meteors per hour. That's several meteors per minute!

The radiant, or the point on the asteroid from which meteors radiate outward, is near the upper part of the constellation Bootes the Herdsman. To find the radiant, go outside before sunrise on the morning of January 4 and look high above the northeastern horizon for the stars of the Big Dipper. The Big Dipper will be high, nearly overhead, and will appear upside-down with the curved handle aimed toward the east.

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