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Data from Pioneer 10 may determine if it is Still Interior the Heliopause
Data from Pioneer 10 may determine if it is Still Interior the Heliopause
1 May 1999
(Source: Ames Research Center)

Pioneer 10
(Launched 2 March 1972)

Distance from Sun (1 May 1999): 72.51 AU
Speed relative to the Sun: 12.24 km/sec (27,380 mph)
Distance from Earth: 11.00 billion kilometers (6.832 billion miles)
Round-trip Light Time: 20 hours 22 minutes


Sheryl Fullner, librarian of the Nooksack Valley Middle School of Everson, Washington, came up with the idea of commemorating the 27th anniversary of Pioneer 10. The school, with a large immigrant and Native American student body and a school symbol that features pioneers, adopted Pioneer 10 as its school mascot. The anniversary was celebrated with a bagpiper, Sousa marches, a bookmark contest, and the unveiling of a student-painted mural featuring the Pioneer 10 spacecraft. The local newspaper, the Lynden Tribune, ran an article on the mural.

The mission formally ended on 31 March 1997 when funding ended in favor of more scientifically productive Heliospheric missions. However, a waiver was given to operate Pioneer 10 as part of the Lunar Prospector controller training program as long as other NASA missions were not interfered with. Pioneer 10 has continued at a much reduced activity level under those guidelines. We are deeply grateful for the gracious way that the Lunar Prospector staff and the DSN have managed this extra burden on their time. The spacecraft is at a distance of >6.8 Billion miles (>72 AU's) and is the farthest out in the opposite direction to which the Sun moves. Voyager 1 passed Pioneer 10 in mileage out of the Solar System on 17 February 1998 but is travelling in the opposite direction.

The low-power Geiger-Tube-Telescope (GTT) instrument still yields valuable scientific data. We also receive data from the Charged Particle Instrument but only for a few hours each week to conserve battery power on Pioneer 10.


Continuing GTT data from Pioneer 10 during the first part of 1999 will be of special importance in determining whether or not Pioneer 10 is still interior the heliopause.

Neutron monitors on Earth (e.g., at Climax and Goose Bay) recorded a marked and rapid decrease in cosmic ray intensity of about 4 % during April and early May of 1998. If Pioneer 10 is still inside the heliopause, we can expect a decrease in cosmic-ray intensity at Pioneer 10 to occur during early 1999. The approximate 9 month delay from Earth to Pioneer 10 corresponds to the distance of ~72 AU covered by the solar wind assuming a speed of 450 km/s. If Pioneer10 has passed outside the heliopause into interstellar space, then the decrease in cosmic intensity will not be observed at Pioneer 10.

The battery reading is very low - perhaps at a minimum. Pioneer 10 persists longer than ever conceived or expected. Stay tuned!

Pioneer 10 will continue into interstellar space, heading generally for the red star Aldebaran, which forms the eye of Taurus (The Bull). Aldebaran is about 68 light years away and it will take Pioneer over 2 million years to reach it.

SUNWARD PULL!? (See the December 1998 issue of Scientific American)

A team of planetary scientists and physicists led by John Anderson (Pioneer 10 Principal Investigator for Celestial Mechanics) has identified a tiny unexplained acceleration towards the sun in the motion of the Pioneer 10, Pioneer 11 and Ulysses spacecraft. The anomalous acceleration - about 10 billion times smaller than the acceleration we feel from Earth's gravitational pull - was identified after detailed analyses of radio data from the spacecraft. A variety of possible causes were considered including: perturbations from the gravitational attraction of planets and smaller bodies in the solar system; radiation pressure, the tiny transfer of momentum when photons impact the spacecraft; general relativity; interactions between the solar wind and the spacecraft; possible corruption to the radio Doppler data; wobbles and other changes in Earth's rotation; outgassing or thermal radiation from the spacecraft; and the possible influence of non-ordinary or dark matter. After exhausting the list of explanations deemed most plausible, the researchers examined possible modification to the force of gravity as explained by Newton's law with the sun being the dominant gravitational force. "Clearly, more analysis, observation, and theoretical work are called for," the researchers concluded. The scientists expect the explanation when found will involve conventional physics.

Pioneer 11
(Launched 5 April 1973)

The Mission of Pioneer 11 has ended. Its RTG power source is exhausted.

The last communication from Pioneer 11 was received in November 1995, shortly before the Earth's motion carried it out of view of the spacecraft antenna.

The spacecraft is headed toward the constellation of Aquila (The Eagle), Northwest of the constellation of Sagittarius. Pioneer 11 may pass near one of the stars in the constellation in about 4 million years.

Project Manager: Dr. Lawrence Lasher (

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Last Updated: 5 Jun 2001