This week, the skywatchers will experience a flashy double feature: The Geminid meteor shower — one of the year's best — will coincide with an unusually close encounter by an asteroid.
That asteroid? It's called 3200 Phaethon, discovered by a NASA satellite in 1983. With a diameter of about 3 miles, it's the third-largest near-Earth asteroid classified by the space agency as "potentially hazardous."
On Saturday, Phaethon will come within 0.069 astronomical units — about 6.4 million miles — of Earth. That is when NASA plans to take detailed radar images of the asteroid at its Goldstone Deep Space Communications Complex in the Mojave Desert and at the Areceibo Observatory in Puerto Rico.
NASA says this encounter with Phaethon is the Earth's closest since 1974, and the closest it will be until 2093.
And that flyby means good gazing for amateur astronomers, too.
"Hold onto your eyepiece!" says Sky and Telescope, noting that Phaethon will be so bright that asteroid buffs can track it through a 3-inch telescope. "This thing will be scooting along at up 15 [degrees] per day or 38″ a minute ... fast enough to cross the field of view like a slow-moving satellite."
Meanwhile on the meteor shower front, the Geminids are known for the brightness of the individual meteors and their frequency: as many as 120 per hour, according to Space.com.
Most meteor showers occur as Earth passes through the debris trail and orbit of a comet. But December's Geminids are different, because Earth is passing through the debris of an asteroid: Phaethon.
The Geminid shower was first noted in 1862, according to NASA. And the show has only gotten better since then, as Jupiter's gravity has pulled the particles closer to Earth.
While wee hours of the morning are generally the best time to watch for meteors, Sky and Telescope's Bob King says the Geminids offer "an evening matinee":
"You can spot a modest number of meteors visible starting as early as 9 p.m. because the radiant already stands some 30 high in the eastern sky. True, a fair number of shower members are cut off by the horizon at that time, but more of us are likely to go out and share it with our children in the evening as opposed to waking before dawn. Since Geminids travel at moderate to slow speeds and approach us from a low angle at that hour, they can produce brilliant and long-lasting fireballs."
King adds an important note about that 120-meteors-per-hour rate:
"That's the zenithal hourly rate, or ZHR, an idealized number based on observing under a pristine, moonless sky with the radiant at the zenith. Depending on the time you observe and local light pollution, counts will vary. At my observing site, which is handicapped by minor to moderate skyglow, I cut the rate in half to keep expectations realistic. A meteor a minute is certainly nothing to complain about."
This year's arrival of the Geminids and Phaethon is especially welcome because last year, a "supermoon" washed out the meteor shower.
How to best observe the historic show?
Grab a blanket, find a dark place and let your eyes adjust. Then join your fellow earthlings in taking it all in.
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