Update on a Veteran: The Pioneer Spacecraft

Dr. Larry Lasher

They are the emissaries to interstellar space from Planet Earth. They are Pioneers 10 and 11, two success stories for NASA.

Pioneer 10 was launched on March 2, 1972, and as of February 2000, it was twice as far from the Sun as the planet Pluto and traveling toward the constellation Taurus at a speed of 27,380 mph. At that distance, it takes a radio signal well over 20 hours to make the trip between Earth and Pioneer 10.

In February 1999, Pioneer 10 detected a 5% reduction in cosmic rays from a solar flare experienced on Earth in April of 1998. This suggests that Pioneer 10 is still under the influence of the Sun and therefore has not passed the heliopause yet. The solar maximum provides another opportunity this year to see whether the cosmic ray readings on Pioneer 10 after the time lag will reflect this cycle, therefore indicating whether it's still within the heliopause boundary.

This past February, NASA received a weak signal from Pioneer 10. The spacecraft is still returning data from the outer solar system, but it's is so low on power that it can't transmit and maneuver at the same time. If you take a look at the constellation Taurus, know that Pioneer 10 is somewhere out there along your line of sight.

Pioneer 10 will be in galactic orbit for billions of years. It is moving in a straight line away from the Sun at a constant velocity. Until Pioneer 10 reaches a distance of about 1.5 parsecs (309,000 AUs) - about 126,000 years from now - it will be dominated by the gravitational field of the Sun. After that, Pioneer 10 will be on an orbital path in the Milky Way galaxy influenced by the field of the stars that it passes.

Pioneer 11 was launched on April 5, 1973, and made the first flyby of the planet Saturn 21 years ago. The mission of Pioneer 11 ended when its RTG power source was 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 Aquila, and it may pass near one of the stars in the constellation in about 4 million years.

General Information - Power for the Pioneer 10 is generated by the Radioisotope Thermoelectric Generators (RTG's). Heat from the decay of the plutonium 238 isotope is converted by thermoelectric couples into electrical current. The electrical output depends on the hot junction temperature, the thermal path to the radiator fins, and the cold junction temperature. It is the degradation of the thermoelectric junction that has the major effect in decreasing the power output of the RTG. In the 26-year time scale operation of Pioneer 10, the 92-year half-life of the isotope does not appreciably affect the RTG operation. The nuclear decay heat will keep the hot junction temperature hot for many years but unfortunately will not be able to be converted into enough electricity to power the transmitter for much longer.

The wear, pitting, and erosion that Pioneer 10 has sustained are probably over now. The asteroid belt and the severe conditions of Jupiter have already been experienced. Now, Pioneer is in the vacuum of space where the average spatial density of molecules is one-trillionth the density of the best vacuum we can draw on Earth. Pioneer is expected to last an indeterminate period of time, probably outlasting its home planet, the Earth. In 5 billion years, the Sun will become a red giant, expand, envelop the orbit of the Earth, and consume it. Pioneer will still be out there in interstellar space. Erosion processes in the interstellar environment are largely unknown, but are very likely less efficient than erosion within the solar system, where a characteristic erosion rate, due largely to micrometeorite pitting, is on the order of 1 Angstrom per year. An etched plate affixed to Pioneer should most probably survive in a readable state until 100 parsecs from the sun. Accordingly, Pioneer 10 and any etched metal message aboard it are likely to survive for a much longer period than any of the works of Man on Earth.

Even though the spacecraft are leaving the Solar System, their distance from Earth sometimes gets shorter. This is a matter of a hyperbolic escape trajectory, geometry, and relative velocity vectors. The distance from the Sun is always increasing. However, since the Earth travels around the sun faster than the spacecraft moves away from the sun, the spacecraft-earth distance decreases for a few months, then rapidly increases again.

Although Pioneers 10 and 11 are the most famous of the spacecraft in this series, there were others leading up to them. Pioneers 1 through 5 were launched from 1958 through 1960; they made the first thrusts into space toward the Moon and into interplanetary orbit. Pioneer 1 was the first of this type of spacecraft launched by NASA and provided data on the extent of the Earth's radiation belts. Pioneer 2 suffered a launch vehicle failure. Pioneer 3 discovered a second radiation belt around Earth. Pioneer 4 was the first American spacecraft to escape Earth's gravitational pull as it passed within 36,650 miles of the moon. The spacecraft did return data on the lunar radiation environment, although the desire to be the first man-made vehicle to fly past the moon was lost when the Soviet Union's Luna 1 passed by the Moon several weeks before Pioneer 4. Pioneer 5 was designed to provide the first map of the interplanetary magnetic field. The vehicle functioned for 106 days and set a record for the time, communicating with Earth from a record distance of 22.5 million miles. The early Pioneers were exploratory missions that led to intriguing new questions that required more advanced types of spacecraft capable of exploring space to considerable distances within and beyond Earth's orbit. This led to the Pioneer 6 through 9 series that made the first detailed comprehensive measurements of the solar wind, solar magnetic field, and cosmic rays.


How We Communicate with Pioneer 10 - The Pioneer spacecraft is spin-stabilized, spinning at approximately 4.28 rpm, with the spin axis running through the center of the dish antenna. If you were to sit in the spacecraft, looking through a hole in center of the dish antenna with a telescope, you would see the Sun traveling very slowly to the left. The Earth's path would describe a very narrow ellipse (the orbit is seen nearly edge-on) around the Sun. In July the Earth is near the right hand edge of the ellipse, and 6 months later will be near the left-hand edge of the ellipse. The angle to the spacecraft between the left edge of the ellipse and the right edge is less than 2 degrees. In order to communicate with the spacecraft, the Earth has to be within 0.8 degrees of the boresight of the spacecraft antenna. Since the Earth moves by almost 2 degrees, the spacecraft has to be re-aimed at the Earth about twice a year. This is done by a conical scan precession maneuver executed by the spacecraft. The radio signal transmitted from an antenna on Earth is focused and reflected by the spacecraft dish antenna toward a small feed horn located on a tripod which is centered in front of the spacecraft dish antenna, and then conducted to a receiver in the spacecraft. During the scan maneuver, the feed horn is physically moved by 8 inches to one side. A ground command turns on a heater in a bellows filled with liquid Freon. The Freon boils, the bellows expands, and moves a mechanical piston and cam attached to the feed horn mounting plate against a mechanical stop. A micro switch cycles the heater power on and off to keep the feed in the offset position.

With the feed in the offset position, the spacecraft receiver sees the radio signal from the tracking station as varying sinusoidally in amplitude. This error signal contains amplitude and phase information on the pointing angle between the spacecraft spin axis and the Earth and the direction to the Earth during the spin cycle. The minimum amplitude occurs during the spin cycle when the antenna points to the Earth, whereas the maximum occurs when the antenna dish points away from the Earth. The frequency of the modulation is equal to the spacecraft spin rate (4.28-rpm). The error signal is processed on board the spacecraft to calculate the timing requirements for firing the jets at the appropriate instant in the spin cycle to precess the spin axis towards the Earth.

The scan processor averages the modulation over two revolutions of the spacecraft. On the third revolution, it orders two hydrazine thrusters (mounted 180 degrees apart on the rim of the dish antenna) to fire a short pulse of 0.0312 seconds duration. This moves the spacecraft spin axis a tiny amount toward the minimum amplitude value, i.e., the Earth, reducing the amplitude of the modulation by a small amount. This process is repeated each three revolutions, each time reducing the pointing angle error and the modulation amplitude. When the pointing angle is within 0.3 degrees of boresight, the processor terminates the maneuver automatically. Typically, about 20 to 28 pulses are fired. A ground command then executes to turn off the power to the feed offset heater, the gaseous freon re-condenses to pull the mechanism back to the normal centered position, and the maneuver is completed.

Technical Data Courtesy of Dr. Larry Lasher

NASA Pioneer Project Manager