Anomalies in scientific data can sometimes lead to a richer understanding of the underlying principles involved. Einstein was able to explain the difference between the Newtonian description of Mercury’s orbit and subsequent observations by applying his developing theory of General Relativity. Add the curvature of spacetime to the Newtonian picture and the problem of a tiny discrepancy in Mercury’s perihelion precession can be resolved.
This anomaly briefly changed our view of the Solar System. Originally, the astronomer Urbain Le Verrier had thought it could be explained by the presence of another planet — Vulcan — closer than Mercury to the Sun, but reported sightings of Vulcan were found to be spurious. Einstein’s work solved the precession problem. In a letter to his close friend Michele Angelo Besso, Einstein would write: “In these last months I had great success in my work. Generally covariant gravitation equations. Perihelion motions explained quantitatively… you will be astonished.”
Image: Urbain Le Verrier ((1811 – 1877). Misreading an anomaly in Mercury’s orbit caused him to believe in a planet that didn’t exist. But his prediction of the existence of Neptune showed him to be a master of Newtonian celestial mechanics. Portrait by Felix Henri Giacomotti.
Astonished indeed, but that’s General Relativity for you. As for anomalies, whether they’re a window into revised physical principles or something completely mundane, they’re well worth studying. A new paper by John Anderson (JPL) and Michael Martin Nieto (Los Alamos National Laboratory) looks at four anomalies that have grown out of astrometry, the precise measurement of the position of objects in space. All may have conventional explanations, but we need to investigate to find out whether or not we need to tweak our theories.
The four anomalies:
- Earth Flyby: Earth flybys of the Galileo spacecraft in 1990 and 1992 seem to indicate an anomalous acceleration whose effects were also traced by the Near Earth Asteroid Rendezvous (NEAR) mission during its Earth gravity assist in 1998. And again in 2005 an anomalous acceleration was observed during an Earth flyby by the Rosetta spacecraft. A 2007 Earth flyby by Rosetta, however, produced no reported detection of such an anomaly. Is the effect related to distance? The second Rosetta flyby may have been too far from Earth (5322 kilometers) to produce it. Usefully, we’ll have the chance to study this again, for Rosetta makes a third trip past Earth in November of this year at a much closer range, a gravity assist maneuver that could produce more evidence of the apparent effect.
- The Astronomical Unit: The distance between the Earth and the Sun can be measured down to a level of three meters, making it what the authors call “…the most accurately determined constant in all of astronomy.” The anomalous effect is that, according to at least one 2004 study, the AU appears to be increasing, whereas it should be decreasing. An increase in the Sun’s mass could explain the anomalous result, but it would involve too much mass to be likely, the mass of some 40,000 comets with a mean radius of 2000 meters. If the reported AU increase withstands further scrutiny, it is intriguing indeed.
- The Pioneer Anomaly: This best-known of the four oddities involves an anomalous acceleration that appears to be acting on both the Pioneer spacecraft, an acceleration directed approximately toward the Sun. Possible explanations involve radiant heat from the spacecraft themselves, and Anderson, who has noted this possibility in earlier papers, seems to suspect that it’s the case. But the paper notes other possibilities, from drag from dark matter to a modification of inertia.
- The Moon’s Orbit: Studies of Lunar Laser Ranging data from 1970 to 2008 show slight changes in the eccentricity of the Moon’s orbit. The anomalous change is not yet understood.
We study such phenomena in hopes of learning one of two things. If any of the these anomalies helps us unlock a new principle, we’ve obviously expanded our understanding of the universe, with benefits to our expansion into space. If we learn that any or all of them can be explained by currently understood physics, we’ve then added to the solidity of those theories and can feel renewed confidence in applying them. Either way, we win, but only future studies will tell us which results apply here.
The paper is Anderson and Nieto, “Astrometric Solar-System Anomalies,” to be published in Proceedings of the IAU Symposium 261 and available online.