Space Science in a Nutshell

Quantum Effects on Asteroids

Radiation pressure … a quantum effect that is known and observed on microscopic scales can also cause (after some time) macroscopic outcomes.

The Arecibo Telescope in Puerto Rico was in service from 1963 to this year, when miscellaneous parts of the telescope collapsed. During its service time the Arecibo Telescope made countless hours of observations. One long-term observation project revealed macroscopic effects by microscopic causes on asteroids. Image Credit: courtesy of the NAIC — Arecibo Observatory, a facility of the NSF

Preface

In 1963 the National Astronomy and Ionosphere Center (NAIC) was founded and an over 300 m diameter radio telescope was build in Puerto Rico: The Arecibo Observatory. For over 50 years this instrument was the largest single-aperture radio telescope in the world. Eventually, an over 500 m diameter dish was setup in China a few ago.

This year, after countless hours of observations, dozens of scientific discoveries and resulting publications the observatory shut down its operations after a cascade of malfunctions, disintegrating events and the collapse of the 900 tons heavy instrument platform that was hold by several metal ropes.

The final collapse and end of the Arecibo Observatory

But instead to mourn for this scientific loss let’s focus in this article on an interesting observation the Arecibo Observatory made: The measurable evidence of the so called Yarkovsky Effect on asteroids.

The Yarkovsky Effect

Around 1900 the Polish-Russian civil engineer Ivan Osipovich Yarkovsky described a theory about thermal effects on small Solar System bodies (like asteroids or meteoroids). However, his theory remained unknown and unnoticed for several decades. In the 70s / 80s the so called Yarkovsky Effect was re-introduced and became also the basic concept for further theories that will be described in future articles.

Let’s take a look at the following schematic drawing that shall help us to understand the effect:

An asteroid (grey) revolves the Sun (yellow) on an orbit (white dotted line). The Sun emits radiation (yellow arrows) that is absorbed and reflected by the asteroid’s surface.

Now let’s consider the rotation of the asteroid around its axis and imagine standing on the surface of this tiny world (in a well protecting space suit). Similar as on our home planet there are different daytimes, like the sunrise, the sunset, the night and so on. An asteroid day depends on the asteroid’s rotation. Some rotate extremely fast and would literally eject you into outer space. A comprehensive list can be found here:

List of fast rotators (minor planets)

During the night, the surface points away from the Sun. The surface cools down and reaches a minimum temperature before dawn. The sun rises and you and your space suit as well as the rocky surface heat up. The radiation input changes, depending on the day time and corresponding Sun elevation above the imaginary horizon you see. It’s noon now; the surface is fully illuminated and from now on, the Sun starts to set and the night starts again …

This rotation, its daytimes and changing heat fluxes cause a non-uniform distribution of the asteroid’s temperature. In simple words:

• The dawn side is cold (indicated with a blue arrow)
• The dusk side is warm (indicated with a red arrow)

The Yarkovsky Effect explained in a schematic drawing: An asteroid (grey) orbits (white dotted line) the Sun (yellow). The asteroid rotates around its own axis causing different daytimes on the asteroid. The absorbed radiation from the Sun (yellow arrows) cause a heating on the day side. At night, the asteroid cools down. This leads to a colder “dawn” and warmer “dusk” side as indicated with a blue and red arrow, respectively. Image Credit: T. Albin.

This temperature difference is linked with different thermal radiation properties. The energy and distribution and consequently linked photon radiation pressure is slightly different between both “dusk” and “dawn” sides. Net, the different radiation pressure components lead to a force that is tangential w.r.t. the asteroid’s path; pointing from the warmer to the colder side. In our example: the resulting force points to the blue arrow direction. The asteroid’s orbital elements change and (in this case) the asteroid moves slowly inwards, towards the Sun. Note: If the asteroid’s rotations would be the other way round, the asteroid would move outwards.

This movement is rather slow. Very slow. Eventually the asteroid gets a set of orbital elements that is e.g., in resonance with one of the major planets. The resonance becomes the dominant effect and alters the orbital properties even more.

(6489) Golevka

Radiation pressure effects on these macroscopic scales … are they even realistic? Are these effects even measurable? Chesley et al. [1] published a paper in 2003 with the first detectable scientific evidence of the Yarkovsky Effect. Their target object: (6489) Golevka:

JPL Small-Body Database Browser

Golevka is an asteroid with a diameter of around 0.5 km and approaches the orbit of our home planet in a minimum distance of around 0.03 AU. From time to time, Earth and Golevka have close non-hazardous encounters. During these encounters Chesley et al. used the Arecibo Observatory to study the asteroid and to perform high precision astrometric measurements. Their observations range from 1991 to 2003, a total span of 12 years. Arecibo sent radio pulses to the asteroid, the asteroid reflected these pulses and Arecibo detected them. Considering the shape model of the asteroid and simulating all perturbations that apply to the asteroid (e.g., gravitational perturbations due to the close encounter with Earth, the strong pull from Jupiter, etc.) they found that the expected and measured position of Golveka deviated. This effect became stronger and more evident over time. In 2003, the radio signals needed 100 µs more run-time than expected. So, the light “travelled” 50 µs longer through space to reach the object (afterwards the radio waves were reflected and 50 µs needs to be added again). These 50 µs correspond to a positional deviation of 15 km. 15 km over a period of 12 years. Consider the cosmic scales and the needed high-performance and -precision instruments that are needed to measure this deviation!

The statistical work did no allow any bias effect, and also the instrument performed well. The only explanation: the asteroid’s orbit was altered by the Yarkovsky Effect over time!

Conclusion

In the meantime several more asteroids, Near-Earth objects (NEOs) etc. have been observed precisely. And more objects show evidence of “Yarkovsky-Effect-Altered” orbits. 17 years ago this theory was verified; around 100 years after it has been postulated. Although the Arecibo Observatory has been shut down recently, its scientific heritage will continue.

Thomas

References

[1] Chesley, S. R.; Ostro, S. J.; Vokrouhlicky, D.; Capek, D.; Giorgini, J. D.; Nolan, M. C.; Margot, J.-L.; Hine, A. A.; Benner, L. A. M.; Chamberlin, A. B.; “Direct Detection of the Yarkovsky Effect by Radar Ranging to Asteroid 6489 Golevka”, Science, vol. 302, no. 5651, pp. 1739–1742, 2003. doi:10.1126/science.1091452.