Astronomers have long known the universe is expanding—but exactly how fast remains one of the biggest mysteries in cosmology. Different techniques for measuring the Hubble constant stubbornly disagree ...

In this age of Mars rovers, questions about the planet's ancient past have shifted. A growing body of evidence supports the idea that Mars was once warm and wet. Now researchers are focused on the ...

Created by the solar wind — a stream of charged particles constantly pouring out from the sun — the heliosphere acts as a ...