Direct Detections of the Yarkovsky Effect: Status and Outlook

We report the current results on a comprehensive scan of the near-Earth asteroid catalog for evidence of the Yarkovsky effect in the orbital motion of these bodies. While most objects do not have sufficient observational data to reveal such slight acceleration, we do identify 42 asteroids with a ``valid'' detection of the Yarkovsky effect, i.e., those with a signal at least 3 times greater than the formal uncertainty and a value compatible with the Yarkovsky mechanism.

We also identify a special category of non-detection, which we refer to as ``weak signal,'' where the objects are of a size that would permit a clear detection if the Yarkovsky effect is maximized, and yet the orbit is clearly incompatible with such accelerations. The implication is that the Yarkovsky effect is reduced in these cases, presumably due to mid-range obliquity, but possibly also due to size, bulk density, thermal inertia, albedo, or spin rate markedly different from assumptions.

Finally, there are a number of asteroids showing a significant signal for nongravitational acceleration, and yet with a magnitude too great to be attributed to the Yarkovsky effect. We term these ``spurious detections'' because most are due to erroneous optical astrometry, often involving a single isolated night from precovery observations. Some cases may be due to other nongravitational accelerations, such as outgassing, mass loss, or micro-meteoroid flux.