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METEOR MYSTERY: In 1967, NASA's Mariner 4 spacecraft was hit by a surprising flurry of meteoroids--a shower more intense than any Leonid meteor storm. Where did the meteoroids come from? It's been a mystery for 40 years. Now, astronomers may have found a solution. Get the full story from Science@NASA.
PLUTO DECLARED A DWARF PLANET: Today at the International Astronomical Union (IAU) General Assembly in Prague, astronomers decided that the Solar System has eight planets, and Pluto is not one of them. Instead, Pluto is a "dwarf planet."
To be a planet, the assembly ruled, a world must meet three criteria:
(1) It must have enough mass and gravity to gather itself into a ball.
(2) It must orbit the sun.
(3) It must reign supreme in its own orbit, having "cleared the neighborhood" of other competing bodies.
So, e.g., mighty Jupiter, which circles the sun supreme in its own orbit, is a planet--no adjective required. Pluto, on the other hand, shares the outer solar system with thousands of Pluto-like objects. Because it has not "cleared its own neighborhood," it is a dwarf planet.
This decision clarifies the vocabulary of planetary astronomy while simultaneously upturning 76 years of "Pluto is a planet" pop-culture. Will non-specialists heed Pluto's demotion? That remains to be seen. Meanwhile, according to the IAU, the Solar System has eight planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune; and three dwarf planets: Ceres, Pluto and 2003 UB313.
IAU Press Release
IMPACT MOON: In only 30 hours of observing, astronomers at the Marshall Space Flight Center have recently photographed seven explosions of light on the Moon. Each one, they believe, was caused by a meteoroid falling from the sky and hitting the ground. (continued below)
Above: Candidate meteoroid impact sites. [details]
The flashes seen by the Marshall group ranged in brightness from 7th to 9th magnitude, which means they were invisible to the human eye, but easy targets for backyard telescopes. Amateur astronomers are thus invited to join the hunt.
FAQ: The Moon has no oxygen to support fires or explosions. So what causes these flashes of light?
Answer: The source of the light is not combustion, which would require oxygen, but rather the simple heating of rocks and soil at the impact site. Even small meteoroids carry a lot of kinetic energy because they travel so fast, some in excess of 100,000 mph. When they hit the ground, all that kinetic energy gets converted into thermal energy. Rocks and soil are heated to such high temperatures that they glow--hence the flash of light.