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Making an impact

What created such a titanic scar? Probably a dwarf planet. NASA astronomers now calculate that the Borealis impacting body was 1,200 miles across-that's bigger than Pluto, the so-called ninth planet.

Imagine a crater wider than the continental United States-the mind boggles at the magnitude of the explosion that must have created it. Detonating all the nuclear weapons stockpiled now on Earth-and those already exploded since the Trinity Test in New Mexico in 1945-all at once, and within a compact "ground zero" area, would be like a few sputtering Fourth of July firecrackers compared to the ancient Borealis explosion on Mars.

When did the ultra-violent Borealis event occur? NASA researchers think it must have taken place around 3.9 billion years ago.

What's in the Sky: During the pre-dawn hours of July 11, look for the giant planet Jupiter in the south, located well above the horizon. The gas giant's four large moons, called the Galilean moons or the Cosmica Sidera-after Renaissance patron Cosmo Medici-were first discovered by Italian astronomer Galileo Galilei in 1610. "(The moons) make their journeys and orbits with a marvelous speed around the star of Jupiter, like children of the same family," Galileo wrote. The waning gibbous Moon also shines near Jupiter. See sky map, courtesy of J. Kirk Edwards.

Lou Varricchio, M.Sc., lives in Vermont. He is a former NASA senior science writer and is currently a member of the NASA/JPL Solar System Ambassador program in Vermont.

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