Gravity Works! (Well, Sort of)

"Is there something we don’t know about GRAVITY?”* was the cover story for the March 2009 issue of Astronomy. This piqued my interest since I recall reading an article last year about the Pioneer spacecraft not being where they were expected to be (of course, I neglected to bookmark it and now can’t find it. Help, anyone?), and I was curious if this article expanded on the Pioneer anomaly.

The author, Dr. John D. Anderson (formerly of NASA’s JPL and now a consultant for the Juno and Rosetta space missions), began with a very brief historical overview of gravitational theories, from Newton to Einstein. To date, Newtonian and Einsteinian (sp?) gravity has worked quite well for astronomers. However, there have been some curious anomalies.

The first two anomalies mentioned are relatively minor. First, the Moon’s orbit around the Earth is becoming more elliptical.

“[James G.] Williams [of Caltech’s JPL] reported in 2006 that the distance difference between the Moon’s farthest and closest approach to Earth is increasing nexpectedly by about 0.2 inches (6 millimetres) per year. ...

“After accounting for changes expected from tidal friction, Williams still has a residual change in the Moon’s orbit that he can’t explain.”

Second, the value of the Astronomical Unit (AU) is increasing by about 7 metres (or 0.000 000 047%) per century. IMO, these two anomalies can be considered trivial since the changes are so extremely small that one could consider them, and their effects, negligible.

Dr. Anderson then focused on the Pioneer Anomaly, that there seems to be some mysterious and unexpected force slowing down the probes:

“The tracking data showed that, in 1998, the Pioneer 10 spacecraft was about 36,000 miles (58,000 kilometres) short of what we expected, based on the laws of motion given by Newton and Einstein.. …

“The situation with Pioneer 11 is similar. …we have determined the spacecraft is about 3,700 miles (6,000 km) short of its predicted distance. It is as if something is putting the brakes on both spacecraft by just the right amount that they both change their speed at the same amount in the same interval of time.”

The final anomaly mentioned deals with “anomalous orbital energy changes of six spacecraft that flew by Earth for gravity assists… These changes do not obey Newton’s law of gravity.” These spacecraft include Galileo, Rosetta, and NEAR Shoemaker. The unexpected speed increases were small (from 1.8 mm/s for Rosetta to 13.5 mm/s for NEAR). At first glance, the data appears to suggest an inverse proportional relationship between magnitude of speed increase and closest approach to Earth (the closer the approach to Earth, the greater the magnitude of speed increase).

Are there explanations for these anomalies using already understood physical mechanisms? A proposal to explain the Pioneer anomaly is heat radiating from the Pioneer spacecraft. However, Dr. Anderson notes that if this were the case, then the rate of deceleration would have decreased instead of staying constant as observed for 11 years. With respect to the fly-by anomaly, currently understood effects have been found to be negligible or only account for a small fraction of the speed increase.

So is a new theory of gravity required? Dr. Anderson focused on one proposal - modified Newtonian dynamics, or MOND. Anderson notes that MOND “is not a theory, but a modification that simply fits the data.” The modifications are due to another anomaly, this one at the galactic scale.

“Since the 1970s, astronomers have known that the outer regions of galaxies appear to violate Newton’s gravity laws. We would expect the stars in a galaxy’s outer regions to travel in elliptical orbits with their orbital speeds decreasing with increasing distance. However, this is not observed. Instead of decreasing, the speeds are essentially constant over a wide region of the galaxy.”

The current theory in play to explain the above effect is “that the galaxies actually contain large quantities of unseen material [dark matter] that hold the stars in the galaxy.” Anderson acknowledged that if MOND is correct, dark matter would be superfluous and discarded.

I found Dr. Anderson’s article interesting and informative. First, I find astronomy very interesting (personal interest). Second, the article never made over-reaching and/or grandiose claims. Dr. Anderson simply stated the facts and the direction of research being done, but reported them in a manner that grasped my attention and interest. Finally, the article served as another reminder that we don’t know everything about our world and universe. Current theories, as well as they may perform, represent an incomplete view of our world and universe. Alternative thinking should not only be scrutinized, but welcomed.

*The article capitalized GRAVITY.