[T]he most important asteroid discoveries are now being made in twilight, when astronomers are able to look close to the horizon — and close to the sun — for little-known asteroids that orbit inside the orbits of Earth, Venus and even Mercury. That includes the first asteroid with an orbit interior to Venus and one with the shortest-known orbital period around the sun, both of which have been unearthed in the last two years. It also includes "city-killers," asteroids large enough that if they were to impact Earth, the damage would be severe. DECam and another telescope are making it...