At the end of the 19th century, astronomers discovered that the perihelion of Mercury (its closest point to the sun during its orbit) was slowly advancing in a way that could not be explained by Newtonian physics. The anomaly was small, so tiny that most believed that a simple explanation would soon be found. It turned out, of course, that Mercury’s orbit was a clue to the existence of an entirely new physics concept, called general relativity, which fundamentally changed our understanding of the universe.

If there’s a lesson there, it is to pay attention to small anomalies.

Which is why a list prepared by John Anderson at the Jet Propulsion Laboratory in California and Michael Nieto at Los Alamos National Laboratory in New Mexico makes for interesting reading. The two scientists have compiled details on four unexplained anomalies that astronomers are currently scratching their heads over (I’ve added a fifth). Let’s take a look.

1. First up is the flyby anomaly, which we’ve discussed on the arxivblog here, here, and here. This is the discovery that spacecraft flying past Earth experience a small but significant change in acceleration, an effect that has been studied in detail by Anderson and shown to be real rather than an artifact of some kind.

2. Next is the slow but sure increase in the length of the astronomical unit (AU), a unit of measure roughly equal to the mean distance from Earth to the sun. This has been calculated as 149,597,870,700 meters, plus or minus three meters, and it is by far the most accurately determined constant in astronomy, the data coming from distance measurements between Earth and various Mars orbiters and landers dating from 1976 to the present. However, these data indicate that the AU is increasing by about 15 centimeters per year.

One explanation is that the mass of the sun is increasing (there is a mathematical relationship linking the AU to the mass of the sun). In fact, the mass of the sun ought to be decreasing because of mass loss to solar radiation and the solar wind. To explain the increase in the AU, the sun would have to be increasing its mass by 10^18 kilograms per year. That’s equivalent to swallowing a good-sized planetary moon or about 40,000 comets per year. Surely we’d have noticed that.

3. Then there is the Pioneer anomaly, the small but steady slowing of the Pioneer spacecraft as they move out of the solar system. Nobody has been able to satisfactorily explain what’s pulling them back, although there has been no shortage of attempts.

4. Finally, Anderson and Nieto point to the increase in eccentricity of the moon’s orbit, as measured by laser-ranging measurements between 1970 and 2008. These data show that the moon’s apogee and perigee have increased in distance by about 3.5 millimeters per year.

5. Anderson and Nieto could add a fifth puzzle to their list: the recent anomalies measured in Saturn’s orbit.

Does that add up to evidence for new physics, or just a poor application of the physics we do know? Whatever the answer, these are problems worth watching.

Ref: arxiv.org/abs/0907.2469: Astrometric Solar-System Anomalies