In this episode of First Principles, we'll learn about asteroids, the telescopes that find them, and the satellites that protect us against them. ✧ In 2004, astronomers discovered a new, 60-million-ton asteroid hurtling towards Earth. They scrambled the world's best telescopes, traced its path in the sky as accurately as they could, and concluded that the asteroid, 99942 Apophis, had a 2.7% chance of causing "regional devastation" here on Earth. Today, that estimate has been revised down. We now know that Apophis will make its closest pass on Friday, April 13th, 2029, missing Earth but only just barely: the asteroid will pass closer than satellites in geostationary orbit, and will be visible to the naked eye. Exactly how we know that — and what we would do if we determined an asteroid really *was* heading our way — is the focus of today's episode. In this episode, we'll visit the telescope that discovered Pluto, one of the biggest optical reflecting telescopes in the world, and the factory floor of @AstroForge, a company building satellites for asteroid mining and planetary defense. Special thanks to @MattGialich, whose connections to Lowell Observatory made this episode possible — and whose meteorite collection is the envy of space nerds everywhere. ✧ 00:00 — Asteroids & Apophis 1:08 — Pluto Discovery Telescope Overview 1:57 — Pluto Discovery, Tracking 2:34 — Pluto Discovery, Optics 3:38 — Pluto Discovery, Sensor 5:06 — Lowell Discovery Telescope Overview 5:24 — Lowell Discovery, Tracking 7:08 — Lowell Discovery, Optics 8:07 — Lowell Discovery, Sensor 9:35 — Driving the Lowell Discovery Telescope 11:26 — The @AstroForge Satellite Factory 13:46 — How AstroForge Deflects Asteroids 14:49 — AstroForge 1.5 kW Laser Demo