On Dec. 16, Asteroid 3200 Phaethon, the source of the recent Geminid meteor shower, made its closest pass of Earth since its discovery in 1983. Observers had the rare treat that week of watching the Geminid shower — little crumblies of Phaethon striking the atmosphere — at the same time we tracked the asteroid in our telescopes. Its close passage also made for an ideal opportunity for astronomers to image the asteroid with radar.
After several months of downtime after Hurricane Maria struck the island of Puerto Rico, the Arecibo Observatory Planetary Radar returned to normal operation just in time to provide the highest-resolution images to date of 3200 Phaethon. The radar images reveal the asteroid’s ball shape, a large concavity, or depression, at least several hundred meters in extent near its equator, and a conspicuous dark, circular feature near one of the poles. The dark spot could be a crater or some other kind of depression that didn’t reflect the radar beam back to Earth. Arecibo’s radar resolves details on the asteroid as fine as about 250 feet (75 meters) per pixel.
The encounter is the closest the asteroid will come to Earth until 2093, but it came a little closer in 1974 and about half this distance back in 1931 before its existence was known. The radar images indicate Phaethon has a diameter of about 3.6 miles (6 km), which is about 0.6 miles (1 km) larger than previous estimates. Phaethon is the second largest near-Earth asteroid classified as “potentially hazardous.” Asteroids that pass near Earth are classified as potentially hazardous asteroids (PHAs) if they’re 460 feet (140 meters) across or larger and approach within about 4.6 million miles (7.5 million km).
Tracking and characterizing PHAs is a primary mission of NASA’s Planetary Defense Coordination Office. Radar is a powerful technique for studying asteroid sizes, shapes, rotation, surface features and roughness, and for more precise determination of their orbital path, when they pass relatively close to Earth.
Lucky for all involved that the Arecibo Observatory, the most powerful astronomical radar system on the planet, only suffered minor structural damage when Maria swept across the island on Sept. 20. The observatory performed double duty for a time, serving as a base for relief efforts to surrounding communities.