The Pioneer Anomaly: The NASA Whodunit

William Atkins
Thursday, 29 March 2007 08:25

Science - Space

Microsoft Office 365

Could it be due to conventional physics, an unknown feature of gravity, observational error, gravitational forces from an unidentified source, drag from the interplanetary medium such as dust, solar wind, and cosmic rays, something exotic like dark matter, or some other yet-to-be explained facet of physics?

Whatever it is, it’s causing a lot of astrophysicists to shake their heads in bewilderment, especially at the Monday, March 26, 2007 Seventh Annual Isaac Asimov Memorial Debate at the American Museum of Natural History. There, the Pioneer Anomaly was discussed by astrophysicists who have been pondering the question for some time, and will probably do so for some time in the future.

The Pioneer spacecraft were meant to pass the outer planets and eventually leave the solar system. However, they are on a trajectory that is different from what was originally calculated. This Pioneer Anomaly, or Pioneer Effect, is a mystery to astrophysicists.

However, recently recovered data and telemetry from the Pioneer spacecraft are helping scientists answer this perplexing question.

astrophysicists are analyzing 30 years of tracking data from Pioneer 10 and 20 years of data from Pioneer 11, taken when microwaves were bounced off both spacecraft while they were between 20 and 70 AU from the Sun. One AU is the mean distance between the Sun and the Earth—about 150 million kilometers (93 million miles). So far, the analysis has shown a small but definite acceleration directed toward the Sun. It is called an anomalous Doppler frequency drift. (Doppler refers to the shortening or lengthening of waves due to travel toward or away, respectively, from the sensing device.)

The amount of the drift is calculated as (8.74 ±1.33) x 10^-10 meters per seconds^2. When this amount is inserted into their trajectories, both spacecraft are about 400,000 kilometers (240,000 miles) closer to the Sun than what was expected. Many scientists believe this deviation can be explained with everyday physics such as a thrust leak or heat coming off of the spacecraft’s nuclear power sources (what are called radioisotope thermal generators (RTGs). However, so far nobody has been able to offer a plausible answer. So, the Pioneer Anomaly continues to interest physicists.

Scientists say that similar effects were found on the Galileo and Ulysses spacecraft. Galileo was sent to study Jupiter in 1989. It arrived at Jupiter on December 7, 1995. It was sent into a destruct trajectory into Jupiter on September 21, 2003 after successfully completing its mission. Ulysses was launched in October 1990. It arrived at Jupiter in February 1992. Since then, it has been successfully researching Jupiter, comets, and other celestial bodies. Its mission has been extended into 2008.

The New Horizons spacecraft is now traveling toward the dwarf planet Pluto. Astrophysicists are also interested to see if the Pioneer Anomaly shows up here, too.

The Pioneer Explorer Collaboration (PEC) is a coordinated effort by many scientists to analyze all available historical data from the two Pioneer spacecraft with regards to the Pioneer Anomaly. The Home Web page of PEC is: http://www.issi.unibe.ch/teams/Pioneer/.

Pioneer 10 was launched from Cape Canaveral Air Force Station on March 2, 1972 and Pioneer 11 was launched on April 6, 1973. Pioneer 10 was the first spacecraft to travel through the asteroid belt and to make direct observations of the planet Jupiter, while Pioneer 11 was the first spacecraft to explore the planet Saturn. Pioneer 10 is currently in the outer edges of the solar system, within the Sun’s heliopause. The last communications with Pioneer 11 occurred in November 1995. The last signals from Pioneer 10 occurred in January 2003. As of February 6, 2007, Pioneer 10 was about 92.12 AU from the Sun.

NASA Information about Pioneer 10 is found at: http://nssdc.gsfc.nasa.gov/nmc/tmp/1972-012A.html, and information about Pioneer 11 is viewable at: http://nssdc.gsfc.nasa.gov/nmc/tmp/1973-019A.html.