Astronomers are hot on the trail of a new class of cosmic curiosity that's not too small, not too big, but somewhere in between — an intermediate-mass black hole.

It's just a calibration image, but this early view from NASA's Lunar Reconnaissance Orbiter shows off the stunning potential of its mapping cameras.

Ground controllers have finally ended the 18½-year mission of a plucky probe that taught scientists virtually everything they now know about the Sun's polar regions.

Newton's laws of gravity have about a 60% chance of wreaking havoc in the inner solar system. Einstein's corrections lower these chances to about 1%.

Now we know for sure. The cosmic-ray particles that bombard Earth from deep space originate in very exotic places: the shock waves in supernova remnants.

Two NASA spacecraft, launched last week, have successfully swung past the Moon. One is getting ready to study it; the other is a few months away from crashing into it.

Planetary scientists are crazy about Saturn's most active moon but can't agree on what powers the towering plumes gas and particles erupting from near its south pole. New findings, published this week, hint that the water vapor might be slowly evaporating from a salt-laced ocean in deeply buried caverns.

Astronomers shed new light on our picture of the early Universe.

Thanks to a full-court press by a cardiologist with a passion for astronomy, the American Medical Association has taken a stance in the fight to make outdoor lighting more benign to humans — and to the stars above.

A picture perfect Whirlpool galaxy bodes well for the scientific promise of the newly launched Herschel Space Telescope.

New insights, announced this week, help explain why solar activity has been in the doldrums for an unexpectedly long time.

Two U.S. spacecraft are on their way to survey the Moon as never before — and to settle the debate over whether water lies frozen at the lunar poles.

Astronomers discover the best evidence to date that some brown dwarfs have their own weather patterns.

Protoplanetary disks around three young stars turn out to have large central holes, which were presumably cleared by still-growing Jupiter-mass planets. But there’s a problem: the stars are too young.