posted 03-18-2016 04:11 PM               

The first and bigger of the two comets will be visible Monday to the naked eye in the southern hemisphere, as long as city lights are far away. Stargazers in the United States will probably need only binoculars to see the bigger comet in late March. Scientists, however, are bringing out the big guns. The Hubble Space Telescope, the powerful ground-based Gemini telescopes and others will be trained on the celestial visitors, which will provide an extraordinary close-up of objects usually glimpsed only at a distance.

"This is one for the record books," says Michael Kelley of the University of Maryland, who's never heard of two comets approaching close to Earth a day apart. "It's a fantastic opportunity for professionals to learn more about comets, and if you have a chance to try to find them … it's a fantastic chance to see part of history as it happens."

The first member of the pair, known as comet 252P/LINEAR, is a bright green color from the carbon gas it's puffing out, says the University of Maryland's Matthew Knight. 252P will slide past Earth at a distance of roughly 3 million miles. That's well beyond the moon but near enough to put 252P in the top 10 of closest-approaching comets.

posted 03-25-2016 06:46 PM               

Comet Flying by Earth Observed with Radar and Infrared

Astronomers were watching when comet P/2016 BA14 flew past Earth on March 22. At the time of its closest approach, the comet was about 2.2 million miles (3.5 million kilometers) away, making it the third closest comet flyby in recorded history (see "A 'Tail' of Two Comets"). Radar images from the flyby indicate that the comet is about 3,000 feet (1 kilometer) in diameter.

The scientists used the Goldstone Solar System Radar in California's Mojave Desert to track the comet. "We were able to obtain very detailed radar images of the comet nucleus over three nights around the time of closest approach," said Shantanu Naidu, a postdoctoral researcher at NASA's Jet Propulsion Laboratory in Pasadena, California, who works with the radar team and led the observations during the comet's flyby. "We can see surface features as small as 8 meters per pixel.

"The radar images show that the comet has an irregular shape: looks like a brick on one side and a pear on the other," Naidu said. "We can see quite a few signatures related to topographic features such as large flat regions, small concavities and ridges on the surface of the nucleus. "

According to the new radar observations, comet P/2016 BA14 appears to spin around its axis once every 35 to 40 hours.

Vishnu Reddy, of the Planetary Science Institute, Tucson, Arizona, also observed comet P/2016 BA14 using the NASA Infrared Telescope Facility (IRTF) on Mauna Kea, Hawaii. Data collected (infrared spectra) indicate that the comet reflects less than 3 percent of the sunlight that falls on its surface. Comet nuclei are as dark as fresh asphalt. However, infrared spectra can often yield clues to the makeup of these primitive denizens of the solar system.

posted 03-29-2016 07:34 AM               

The first and bigger of the two comets will be visible Monday to the naked eye in the southern hemisphere...

On March 22, BA14 came within 2.2 million miles of Earth, that's only 9 times the Earth-moon distance. Interestingly, it flew past Earth only a day later than another comet, 252P/LINEAR, which is a well-known comet having been discovered way back in 2000. Estimates of 252P's nucleus size put it at around 250 meters (820 feet) wide and BA14 was presumed to be around half that size. Due to their similar orbits, astronomers believe both objects were once likely part of the same object that broke up some time in the past.

However, as BA14 made its Earthly close encounter last week, NASA's Goldstone Solar System Radar in California's Mojave Desert was able to spend 3 nights observing the object, revealing its spin and true size. This hunk of ancient icy space rock spins once on its axis every 35 to 40 hours is 1 kilometer (3,000 feet) wide! This is a lot bigger than previously estimated...