The question came up in class regarding the so-called "Pioneer Anomaly" with respect to gravitation theory.

I recommend that you look up the following papers if you'd like to learn more:

Study of the Pioneer anomaly: A problem set
Slava G. Turyshev, Michael Martin Nieto, and John D. Anderson
Am. J. Phys. 73, 1033 (2005)

The Pioneer spacecraft
Steven Morris
Am. J. Phys. 74, 373 (2006)

Study of the anomalous acceleration of Pioneer 10 and 11
John D. Anderson, Philip A. Laing, Eunice L. Lau, Anthony S. Liu, Michael Martin Nieto, and Slava G. Turyshev
Phys. Rev. D 65, 082004 (2002)

You can look these up from the website <http://scitation.aip.org/ajp/> for the first two, and <http://prd.aps.org/> for the third. (Tell me if you have questions about how to do this.) The first article is the best place to start. The second article is a letter written in response to the first, followed by the authors' comments to that letter. The third article would make for an ambitious reading.