Leading these discovery efforts are 3 ground-based searches. The Catalina Sky Survey, based at the University of Arizona, has discovered roughly 50 percent of the 20,000 known NEOs. The University of Hawai‘i runs a program called Pan-STARRS (short for Panoramic Survey Telescope and Rapid Response System) that has found about 5,000 objects.

Complementing these two large surveys is ATLAS, the Asteroid Terrestrial-impact Last Alert System, whose 2 telescopes on the island of Maui cover 25 percent of the entire sky every clear night. Those scans serve as an early warning system because they’re able to spot asteroids as small as 30 meters across that are within 1.5 million kilometers (a million miles) of Earth.

Once a NEO is discovered, it’s critical to determine its orbit with enough precision to be able to predict where it’ll be for decades into the future. Many NEO discoveries are followed up by other instruments ranging from huge professional telescopes to modest backyard setups operated by amateur astronomers.

Sometimes, objects in Earth’s vicinity can be observed using radar, particularly the 70-meter-wide dish at NASA’s tracking station at Goldstone, California and the 305-meterwide dish at Arecibo Observatory in Puerto Rico. By bouncing microwaves off the surface of nearby asteroids, astronomers can measure precisely how far away they are and how fast they’re passing by, key factors in refining their orbits.

More Than Just a Dot

While surveys discover what’s out there, follow-up observations can characterize NEOs to determine of what they are made, how fast they spin, and something about their size, shape, and reflectivity. For example, merely recording how an object’s apparent brightness changes with time can yield its spin rate. Astronomers have found that the fastestrotating NEO spins once every 16 seconds, and the slowest takes more than 78 days to finish one round on its axis.

Most observations cannot resolve NEOs, so they appear as star-like dots in images. However, radar-equipped radio telescopes can create “maps” of the radar echoes’ roundtrip time vs. Doppler shift, pseudo pictures that reveal an asteroid’s size, shape, spin rate, surface properties, and even surface features such as craters. The giant Arecibo dish has the most powerful radar system in the world, but it can only see a fraction of the sky. NASA’s Goldstone radar system, though smaller and less powerful, can observe a larger swath of sky. Together, these facilities typically observe roughly 100 NEOs per year.

Two other key characterization assets observe passing objects at wavelengths beyond the range of human vision. NASA’s Infrared Telescope Facility (IRTF) atop Mauna Kea in Hawai`i uses a workhorse instrument called SpeX to record the near-infrared wavelengths reflected from an asteroid’s surface. These spectra reveal the composition of passing NEOs.

The second asset is NASA’s NEOWISE spacecraft. Originally an astrophysics mission dubbed the Wide field Infrared Survey Explorer (WISE), it once observed the entire sky in 4 infrared bands. Once WISE’s coolant was depleted 10 months after launch, it could no longer observe at the longest-wavelength bands, but the spacecraft itself remained healthy and could still observe at 2 shorterwavelength bands in which solar-system objects radiate energy. Astronomers could combine NEOWISE’s measurements with visible-light images from ground-based telescopes to determine the diameters for nearly 200,000 different asteroids and comets, including several hundred NEOs.