(31345) 1998 PG

Lore & Background

1998 PG orbits the Sun at a distance of 1.2–2.8 AU once every 1,046 days, with an eccentricity of 0.39 and an inclination of 7° to the ecliptic. A precovery image from Palomar Observatory in 1978 extended its observation arc by 20 years before its official discovery. The asteroid has an Earth minimum orbital intersection distance of 0.2354 AU (about 92 lunar distances) and made its closest known approach to Earth on 15 October 1978 at 35,648,680 kilometers. With an aphelion beyond 2.8 AU, it also crosses the orbit of Mars.

In the SMASS taxonomy, 1998 PG is classified as a transitional Sq-type, intermediate between common stony S-type and rarer Q-type asteroids. Its diameter is estimated between 880 and 940 meters, with an albedo of 0.18 to 0.20. A rotational lightcurve obtained in the late 1990s by Hungarian astronomers gave a rotation period of 2.5 hours with a brightness variation of 0.1 magnitude. A second lightcurve in 2000 refined the period to 2.51620 ± 0.00003 hours with an amplitude of 0.11 magnitude.

During the 2000 photometric observations, 1998 PG was identified as a probable asynchronous binary system. Its moon orbits every 7.0035 hours (or twice that period) and has a secondary-to-primary diameter ratio greater than 0.3, corresponding to a diameter of 270 meters. The moon's orbit has a semi-major axis of about 1.4 kilometers. From the asteroid's surface, the moon would appear with an angular diameter of roughly 16.3°, compared to the Sun's 0.5° as seen from Earth.

Reader's Guide

1998 PG is significant as a binary near-Earth asteroid, a class that provides insight into the formation and dynamical evolution of small bodies in the inner Solar System. Its moon, with a diameter about 30% that of the primary, is among the larger satellites relative to its primary for near-Earth binaries. The asteroid's Sq-type spectrum bridges common S-type and rarer Q-type compositions, suggesting a surface that may have experienced space weathering or other alteration processes. Its close approaches to Earth—the closest recorded at 35.6 million kilometers in 1978—and its Mars-crossing orbit make it a dynamically interesting object. The system's rotation and orbital parameters, measured through photometric lightcurves, contribute to the broader understanding of binary asteroid mechanics, including asynchronous rotation and mutual orbit periods. Despite being numbered by the Minor Planet Center in 2001, it remains unnamed as of 2018, a status that underscores the many minor planets still awaiting formal names.

Did You Know?

• 1998 PG is a binary system with a moon about 270 meters in diameter.
• Its rotation period is 2.51620 ± 0.00003 hours.
• The asteroid was discovered on 3 August 1998 by the LONEOS survey at Anderson Mesa Station.
• From the surface of 1998 PG, its moon would appear about 16.3° across—much larger than the Sun as seen from Earth.