The solar system is a dangerous place. One hazard is impacts from rocky debris traveling tens of thousands of miles an hour. Large fragments can tear a spacecraft apart, but even the smallest pieces gradually take their toll, eroding spacecraft surfaces and components. Dust-sized particles shed from comets and asteroids and smaller than a single grain of sand, traverse the solar system at speeds reaching 40,000 miles an hour. Our best knowledge of these tiniest meteoroids comes from measurements made near Earth. Now, a clever new technique using the LISA Pathfinder spacecraft is measuring this population of particles at nearly a million miles away. Launched in 2015, LISA Pathfinder is a technology demonstrator for the space-based detection of gravitational waves led by the European Space Agency and with participation by NASA. The full observatory would work by tracking reference objects called proof masses that move under the influence of gravity, which means the spacecraft must reduce all other forces to an astonishing degree. Tests show LISA Pathfinder is the most precise tool for measuring accelerations ever flown, reducing non-gravitational forces to levels required to detect passing gravitational waves. Because LISA Pathfinder is extremely sensitive to its environment, it's also sensitive to those tiny nudges from micrometeoroids. These impacts would be reflected in motions of the proof masses, how the spacecraft thrusters fired to counteract the impact, and other data. Using software originally written to sift out the the faint signatures of gravitational waves in ground-based detectors, the science team found several impacts. When it is struck, the impact both pushes and twists the spacecraft. Onboard sensors detect this change and fire the Pathfinder's ultra-precise thrusters to compensate. At full power, each of these only generates about as much force as the weight of a mosquito. Knowing what it took to restore the spacecraft's orientation gives scientists information on the original direction of the particle, its momentum, and where it struck the spacecraft. LISA Pathfinder is located at Earth-sun L1, a gravitational balance point about million miles from Earth's sunward side. This is essentially unexplored territory for understanding the solar system's dust distribution. By comparing how often Pathfinder is hit, and the particles' origins, scientists can refine their models of solar system composition and formation. By improving our understanding of the solar system's dust environment, LISA Pathfinder will also help pave the way for future giant telescopes whose large optics could be degraded by these impacts. It's an intriguing side benefit from testing spacecraft technology that will one day be used to observe merging black holes billions of light years away. [Music] [Beeping] [Beeping] [Beeping]