David Vokrouhlicky

DETECTION OF SEMIMAJOR AXIS DRIFTS IN 54 NEAR-EARTH ASTEROIDS: NEW MEASUREMENTS OF THE YARKOVSKY EFFECT

We have identified and quantified semimajor axis drifts in near-Earth asteroids (NEAs) by performing orbital fits to optical and radar astrometry of all numbered NEAs. We focus on a subset of 54 NEAs that exhibit some of the most reliable and strongest drift rates. Our selection criteria include a Yarkovsky sensitivity metric that quantifies the detectabilityofsemimajoraxisdriftinanygivendataset,asignal-to-noisemetric,andorbitalcoveragerequirements. In 42 cases, the observed drifts (∼10 −3 AU Myr −1 ) agree well with numerical estimates of Yarkovsky drifts. This agreement suggests that the Yarkovsky effect is the dominant non-gravitational process affecting these orbits, and allows us to derive constraints on asteroid physical properties. In 12 cases, the drifts exceed nominal Yarkovsky predictions, which could be due to inaccuracies in our knowledge of physical properties, faulty astrometry, or modeling errors. If these high rates cannot be ruled out by further observations or improvements in modeling, they would be indicative of the presence of an additional non-gravitational force, such as that resulting from a loss of mass of order a kilogram per second. We define the Yarkovsky efficiency fY as the ratio of the change in orbital energy to incident solar radiation energy, and we find that typical Yarkovsky efficiencies are ∼10 −5 .

EVOLUTION OF DUST TRAILS INTO BANDS

We use numerical simulations to investigate the production of dust trails by asteroid disruption events. Our work shows that asteroid trails evolve into pairs of dust bands over time. Coherent trails typically survive several tens of kyr before evolving into complete bands after ~1 Myr. The transition timescale depends sensitively on the location of the source breakup event in the main asteroid belt. Bands develop more efficiently from sources in the middle/outer belt than in the inner belt, which may not produce observable pairs of bands at all. The infrared structures produced by recent disruption events (<1 Myr) are characterized by a complicated and changing set of incomplete arcs and cusps. Their geometry depends both on the observers position and on the sources location in terms of heliocentric distance and inclination to the ecliptic. We postulate that the broad orphan trails named C and D by Sykes in 1988 might have been produced by the formation of the Datura asteroid family 450 ± 50 kyr ago. Additional work will be needed to test this link.

Yarkovsky and YORP effects

Interesting problems in science usually have a long and complex history. It is rare, though, that they have a prehistory or perhaps even mythology. Yet, until recently this was the case for the Yarkovsky effect. Ivan O. Yarkovsky, a Russian civil engineer born in a family of Polish descent, noted in a privately published pamphlet (Yarkovsky, 1901; Beekman, 2006) that heating a prograde-rotating planet should produce a transverse acceleration in its motion and thus help to counterbalance the assumed drag from the then-popular ether hypothesis. While this context of Yarkovsky’s work was mistaken and he was only roughly able to estimate the magnitude of the effect, he succeeded in planting the seed of an idea that a century later blossomed into a full-fledged theory of how the orbits of small objects revolving about the Sun are modified by the absorption and reemission of solar energy. It is mainly Ernst J. Öpik who is to be credited for keeping Yarkovsky’s work alive and introducing it to western literature, long after the original pamphlet had been lost (Öpik, 1951). Curiously, at about the same time, similar ideas also started to appear in Russian regular scientific literature through the works of Vladimir V. Radzievskii and his collaborators (Radzievskii, 1952). While Radzievskii was also the first to consider the effects of systematic photon thrust on a body’s rotation, his concept was based on a variable albedo coefficient across the surface (Radzievskii, 1954). However, there is no strong evidence of large enough albedo variations over surfaces of asteroids or meteoroids. Stephen J. Paddack and John O’Keefe pushed the idea forward by realizing that irregular shape, and thermal radiation rather than just reflected sunlight, will more efficiently change the meteoroid’s spin rate. Thence, the Yarkovsky-O’KeefeRadzievskii-Paddack (YORP) effect was born as an alter ego of the Yarkovsky effect little more than half a century after Yarkovsky’s work (see Paddack (1969), Paddack and Rhee (1975), and Rubincam (2000) for a summation of the history and coining of the terminology). Radzievskii’s school also briefly touched upon a concept of a radiation-induced acceleration of synchronous planetary satellites (Vinogradova