Look up at the night sky tonight, October 21, to witness the peak of the Orionids meteor shower. If the sky is clear and if you can find an ideal area with little to no light pollution, you might be able to see 10 to 20 meteors per hour. The best time to catch the show will be after midnight. To be sure, don’t expect the sky to look like the sixth seal has just been opened, as the meteors will be faint and fast. But if you’re out for an excuse to enjoy the stars and make a few wishes, this is a good night to lay out a blanket.
THE PHANTOM TRAIL OF HALLEY’S COMET
So what’s going on up there, anyway? You’re probably familiar with Comet Halley, perhaps the most famous of all the comets. It passes through the inner solar system every 75 years or so, most recently in 1986. As the comet travels its orbit, it leaves behind rock and dust particles. The accumulation of debris over millennia, pushed around by solar radiation and pulled by gravity, results in the Earth crossing through the phantom trail of Halley’s Comet twice a year. As the comet’s debris meets—and is vaporized by—the Earth’s atmosphere, you get what are colloquially called “shooting stars.” In October, the resultant shooting stars are called the Orionids; in May, they are called the Eta Aquariid. (The showers are named for their most closely associated constellations: Orion and Aquarius.)
As for Comet Halley, it will again be easily visible in 2061. It is the first comet to have been studied up close by a spacecraft: the Giotto mission in 1986, which was the European Space Agency’s first deep space mission. The photo, taken during that mission, shows the nucleus of Halley’s Comet. The probe flew as close as 370 miles to the comet’s nucleus, revealing it to be a dark peanut-shaped object just nine miles long and six miles wide.
Image credit: European Space Agency
WHO WAS HALLEY?
Edmond Halley was a British astronomer who lived from 1656 to 1742. He did not discover the comet that would eventually bear his name; it had been documented since at least 239 BCE. Rather, and more impressively, in 1705 he published calculations of its orbit using new rules of gravity and motion put forth by his friend and colleague Isaac Newton. (What had previously been thought to be multiple comets turned out to be just the one.)
Halley is also responsible for one of the most ambitious science projects in human history: the experiment to finally determine the size of the solar system. In 1716, Halley issued a proposal calling for the astronomers of the world to observe and carefully measure the transit of Venus, which would occur in 1761 and 1769. The pairing, occurring roughly once every century, would see Venus cross directly in front of the Sun. By taking precise observations of the transit from different vantage points around the world, the varying tracks would allow scientists to determine the Earth’s distance from the Sun. Venus could, in other words, be used as a “celestial yardstick.”
Halley drafted his ambitious plan knowing that he would be long dead by the time the transit of Venus took place—a testament to the higher calling that is science, a quest for human knowledge much larger than any one person, career, or lifetime. Those who answered Halley’s rally cry were equally motivated—and fearless. Astronomers traveling from Europe to the Indian Ocean, for example, would need to sail hostile waters, navigate wars and warships, and fight with ship captains over course headings and celestial navigation.
As a result of the project, not only did astronomers work out with some accuracy the size of the solar system, but through the scores of expeditions it spawned around the world, human knowledge of the world’s flora, fauna, and weather was greatly expanded. (The enormity of the challenge of this science experiment was recounted beautifully in Andrea Wulf's Chasing Venus: The Race to Measure the Heavens.)
This week, while watching meteors streak by, give a moment’s thought to Halley and the men and women who figured out why the stars are “falling like rain.”
Another view of an Orionid meteor, courtesy of NASA:
Image credit: NASA