FTN helps astronomers detect the ‘YORP effect’ for the first time

Astronomers have seen an asteroid change the rate at which it spins for the first time, and shown that it is due to a theoretical effect predicted but never before seen. Observations from FT North on Maui formed a part of the data used by astronomers at Queens University Belfast to demonstrate the existence of the ‘YORP effect’.

The international team of scientists from Europe and the United States used a range of telescopes to find that the asteroid is rotating faster by 1 millisecond every year, and is caused by heating of the asteroid’s surface by the Sun. Eventually it may spin faster than any known asteroid in the solar system.

The Yarkovsky-O’Keefe-Radzievskii-Paddack (YORP) effect is believed to alter the way small bodies in the solar system rotate. The effect occurs when photons from the Sun bounce off the irregular surface of the asteroid generating a force (no such force would be generated on an ellipsoid with a totally smooth surface). By analogy, if one were to shine light on a propeller over a long enough period, it would start spinning.

Although this is an almost immeasurably weak force, astronomers believe it may be responsible for spinning some asteroids up so fast that they break apart, perhaps leading to the formation of binary asteroids. Others may be slowed down so that they take many days to rotate once. The YORP effect also plays an important role in changing the orbits of asteroids between Mars and Jupiter, including their delivery to planet-crossing orbits. Despite its importance, the effect has never been seen acting on a solar system body, until now.

Using extensive optical and radar imaging from Earth-based observatories, astronomers have directly observed the YORP effect in action on a small near-Earth asteroid, known as (54509) 2000 PH5.

Fig 1: Asteroid 2000 PH5 imaged at optical wavelengths using ESO’s 3.5m New Technology Telescope in Chile on August 27, 2003. The asteroid can be seen moving relative to the background stars.

Shortly after its discovery in 2000, it was realized that this asteroid would be the ideal candidate for such a YORP detection. At just 114m in diameter, it is relatively small and so more susceptible to the effect. Also, it rotates very fast, with one day on the asteroid lasting just over 12 Earth minutes, implying that the YORP effect may have been acting on it for some time. With this in mind, the team of radar and optical astronomers undertook a long term monitoring campaign of the asteroid with the aim of detecting any tiny changes in the spin-rate.

Over a 4 year time span, Stephen Lowry, Alan Fitzsimmons and colleagues at Queens University, Belfast, took images of the asteroid at a range of telescope sites including the 2.0m Faulkes Telescope North in Hawaii, and measured the slight brightness variations as the asteroid rotated.

Over the same time period, a team led by Patrick Taylor and Jean-Luc Margot of Cornell University used the Arecibo Observatory in Puerto Rico and the Goldstone facility in California to observe the asteroid using radar. They were able to reconstruct a 3-D model of the asteroid’s shape, with the necessary detail to allow a theoretical YORP value to be calculated.

Fig 2: Radar images obtained at the Arecibo facility in Puerto Rico on July 28, 2004, covering one full rotation of asteroid 2000 PH5 (columns 1 and 4).

With careful analysis of the optical data, the asteroid’s spin rate was seen to steadily increase with time, at a rate that can be explained by YORP theory. Critically, the effect was observed year after year. Furthermore, this result was elegantly supported via analysis of the combined radar and optical data, as it was required that the asteroid be increasing its spin-rate at exactly this rate in order for a satisfactory 3-D shape model to be determined.

Fig 3: The observed spin-rate was seen to change from year to year (black dots). The solid curve is the expected theoretical YORP strength derived from the 3-D shape model (from Lowry et al. [1]).

So what will happen in future? Lowry et al. performed detailed computer simulations and found that the asteroids orbit about the Sun could remain stable for up to 35 million years in the future, allowing the spin-rate to be reduced to just 20 seconds, faster than any asteroid spin-rate ever seen. This exceptionally fast spin-rate could force the asteroid to reshape itself or even split apart, leading to the birth of a new binary system.

YORP effect publications:

[1] Stephen C. Lowry, Alan Fitzsimmons, Petr Pravec, David Vokrouhlicky, Hermann Boehnhardt, Patrick A. Taylor, Jean-Luc Margot, Adrian Galad, Mike Irwin, Jonathan Irwin, and Peter Kusnirak (2007). Direct Detection of the Asteroidal YORP Effect, Published online in Science Express (www.sciencemag.org).

[2] Patrick A. Taylor, Jean-Luc Margot, David Vokrouhlicky, Daniel J. Scheeres, Petr Pravec, Stephen C. Lowry, Alan Fitzsimmons, Michael C. Nolan, Steven J. Ostro, Lance A. M. Benner, Jon D. Giorgini, Christopher Magri (2007). Spin Rate of Asteroid (54509) 2000 PH5 Increasing due to the YORP Effect, Published online in Science Express (www.sciencemag.org).