SPACEWATCH®: Following up Near-Earth Objects (NEOs) to help Determine their Orbits
SPACEWATCH®, which pioneered using CCDs to survey the sky for NEOs, currently conducts Near Earth Object (NEO) follow-up observations. To improve planetary defense capabilities by reducing the uncertainty in NEO orbital elements, we conduct full-time rapid astrometric follow-up observations of high priority NEOs as the sole users of the Lunar and Planetary Laboratory’s Spacewatch 1.8-m observatory and the Steward Observatory’s 0.9-m telescope on Kitt Peak. Additionally, we conduct astrometric follow-up with Steward Observatory’s Bok 2.3-m telescope during bright time with the Spacewatch Cassegrain Camera (SCC).
Our highest priority targets for NEO astrometric follow-up are virtual impactors (VIs) and Potentially Hazardous Asteroids (PHAs). PHAs are ≳140 meters in diameter with Earth Minimum Orbit Intersection Distances (EMOIDs) ≲ 0.05 au. VIs have sufficiently uncertain heliocentric orbital parameters such that at least one orbit solution predicts an Earth impact within 100 years. PHAs pose a greater hazard due to their size, but the majority do not have orbits in which the asteroid could impact Earth itself. VIs pose a greater impact risk due to their real (but low) probability of impact. Currently, only ~1% of NEAs on the JPL Sentry risk list of VIs are “large” (>140 m). It is particularly important to minimize the orbit uncertainties for VI PHAs to rule out (or in) possible impacts.
Spacewatch has observed a majority of the newly discovered NEOs that are or were on JPL’s VI impact risk list since October 16, 2019. According to the PDS SBN, from Sept. 1993 through March 2025, the 1.8-m is third in making the first observation for follow-up MPECs, sixth in follow-up MPECS, and fifth over all types of MPECs. It is fifth in MPECs for making the first follow-up observation over the past year. The 0.9-m is sixth in discovery MPECs and eighth in precovery MPECs from September 1993 through March 2025.
