The Advanced Composition Explorer (ACE) is Earth’s vanguard. Orbiting around a point 900,000 miles away between Earth and our sun, this satellite is ever vigilant, recording the combination of radiation — from the sun, from the solar system, from the galaxy — that streams by. None of this radiation can harm humans on Earth, but the biggest bursts of particles from the sun can flow into near-Earth space causing a dynamic space weather system that can damage satellites and interfere with radio communication transmissions and navigation systems.