54509 YORP
Earth co-orbital asteroid
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Through Lincoln Near-Earth Asteroid Research, Earth and Spectroscopy
Discovery and naming
YORP was discovered on 3 August 2000 by the Lincoln Near-Earth Asteroid Research (LINEAR) program at the Lincoln Laboratory's Experimental Test Site in Socorro, New Mexico, United States. It was given the provisional designation 2000 PH5, and its discovery was announced in a Minor Planets Electronic Circular on 7 August. At the time of discovery, YORP was located 0.04 astronomical units (AU; 6 million km; 4 million mi) away from Earth. Once its orbit was sufficiently determined, it was numbered 54509 by the Minor Planet Center on 16 February 2003. It is named after researchers Ivan Yarkovsky, John O'Keefe, Vladimir Radzievskij, and Stephen Paddock, contributors to the understanding of the mechanisms behind the YORP effect and after whom the effect is named. The naming was chosen as the asteroid provided the first direct observational evidence for the effect; the name announced in Minor Planets Circular 59387 on 2 April 2007.
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Orbit
YORP orbits the Sun with an average distance, or semi-major axis, of 1.006 AU, completing one orbit in 368.5 days. Due to its orbital eccentricity of 0.23, its distance from the Sun varies from 0.775 AU at perihelion to 1.237 AU at aphelion, crossing Earth's orbit. YORP's orbit is inclined by 1.6° with respect to the ecliptic plane.
YORP is classified as an Apollo asteroid, a subgroup of Earth-crossing near-Earth asteroids (NEAs) with semi-major axes greater than 1 AU. YORP is also classified as an Earth co-orbital; it is in a near-1:1 mean-motion resonance with Earth in a horseshoe configuration. In a horseshoe configuration, the asteroid's orbit librates between the L4 and L5 Lagrange points, traversing the L3 point. Over a cycle lasting about 100 years, YORP librates until it experiences a close encounter with Earth, after which its libration reverses direction until it encounters Earth again. Because of its eccentricity, its encounters with Earth are close, coming within a few lunar distances. For the past cycle, YORP trailed Earth, approaching it at a rate of roughly 3° per year. It encountered Earth in July 2003, where its semi-major axis was raised from 0.994 AU to 1.006 AU. It is currently lagging behind by 3° per year; it will reach the other end of its horseshoe path about 100 years from now.
Possible asteroid group
YORP is the largest member of a candidate asteroid group. More than a dozen NEAs have similar orbital properties to YORP, including 2017 DR109, which also follows a horseshoe trajectory. A 2018 study by Carlos and Raul de la Fuente Marcos concluded that the asteroids are likely dynamically if not genetically related, but the scarcity of spectroscopic data and the poorly determined orbits of several candidate members prevented a definitive confirmation. Several of these asteroids may originate from YORP effect-driven mass shedding of 54509 YORP, tidal disruption events from close Earth and Venus encounters, and collisional disruption.
Physical characteristics
YORP's size and shape has been directly measured through radar observations conducted by the Goldstone observatory on 27–28 July 2001 and the Arecibo Telescope on 27–28 July 2004 and 24–26 July 2005. Various shape models were compared against Doppler images of YORP, with the best-fit model having a diameter of 112.8 metres (370 ft) and dimensions of 149 by 134 by 96 metres (489 ft × 440 ft × 315 ft). YORP has a very irregular shape, with Doppler imagery revealing convex, linear, and concave topography features. Its northern hemisphere is flattened, though it has a prominent concavity.
Rotation
YORP has a retrograde spin, with an axial tilt of 173° relative to its orbit normal. Its spin axis points towards the ecliptic south, lying within 10° of the J2000 ecliptic coordinates (180°, –85°). Observations of YORP's lightcurve, or fluctuations in brightness as it rotates, suggests a sidereal rotation period of 12.17 minutes, which has been corroborated by radar observations. YORP's fast rotation and small size classifies it as a monolithic fast rotator.
In 2007, two studies—one led by Stephen C. Lowry and the other led by Patrick A. Taylor—investigated changes in YORP's rotation period over time, reporting the direct detection of the YORP effect. This was the first direct observational evidence of the YORP effect; previously, evidence of the effect was indirect, manifesting in the anomalous distributions of some asteroids' rotation periods and axial tilts. YORP's lightcurve data from 2001 to 2005 suggests its rotation period is fractionally decreasing by 1.7×10−6 per year. Taylor and collaborators used YORP's shape model to theoretically derive the expected change in its rotation period under the YORP effect. Despite incomplete shape and thermal data, theoretical predictions agreed within an order of magnitude with the observed change in period. Tidal torques from repeated close encounters with Earth were ruled out as an explanation, leaving the YORP effect as the most likely explanation for the change in the asteroid's spin.