Pioneer 10: A tour of the planets

Pioneer 10 was the first probe to successfully achieve close-range exploration of Jupiter, and also the first probe to successfully achieve close-range exploration of the outer planets (Jupiter, Saturn, Uranus and Neptune). Its success has accumulated valuable experience for subsequent outer planet probes. The starting point of mankind's entry into the era of outer planet exploration is Pioneer 10, and it deserves the name "Pioneer". Its success created the first oasis in the desert of outer planet exploration.

Written by | Wang Shanqin

After the launch of artificial satellites, humans began to launch unmanned probes to closely explore the planets in the solar system. Among these planets, the exploration of the "outer planets" (Jupiter, Saturn, Uranus and Neptune) is more challenging because: first, they are farther away than the "inner planets", so they require more powerful rockets and more advanced orbit control technology; second, because they are farther away, the radiation of sunlight near them is weaker, so other energy sources besides solar cells are needed to provide power for the instruments.

Humans did not retreat because of these difficulties. After the concerted efforts of scientists and engineers, humans finally successfully launched a series of probes, achieving the great feat of close-range exploration of four outer planets, which made a qualitative leap in human understanding of outer planets. Pioneer 10 was the first in this series of probes, and also the first to succeed.

An artist's impression of Pioneer 10 and Jupiter. Image credit: Rick Guidice

Origin: A once-in-200-years opportunity

In 1964, Gary Flandro (1934-), an aerospace engineer at the Jet Propulsion Laboratory (JPL) of the National Aeronautics and Space Administration (NASA), pointed out that by the end of the 1970s, Jupiter, Saturn, Uranus and Neptune were located on the same side and almost in a straight line. If a probe could be launched in that direction a few years in advance, the probe could pass by these four giant planets in turn around 1980 and detect them closely by flyby.

Using this plan, the probe can not only fly past the four outer planets in sequence, but also the gravitational acceleration effect of each planet will increase the speed of the probe every time it flies past, thereby saving a lot of fuel and nearly half of the flight time.

This situation only occurs once every 175 years, so this opportunity is very precious. For this reason, NASA launched a plan to explore the outer planets. After discussion, experts initially planned to launch four probes, two of which would explore Jupiter, Saturn and Pluto, and the other two would explore Jupiter, Uranus and Neptune. This is the famous "Planetary Grand Tour" project. [Note 1] At that time, Pluto was still listed among the "nine planets", so it was also selected as one of the core observation targets of the "Planetary Grand Tour" project.

In order to provide valuable experience for the Grand Tour project, NASA's Ames Research Center (ARC) proposed the Galactic Jupiter Probes project in 1964. The project will launch two identical probes to pass through the asteroid belt and explore Jupiter. One of the two probes will serve as a backup for the other - if one fails, the other will complete the mission of the former.

In February 1969, NASA approved the project. The two probes of the project were named "Pioneer 10" and "Pioneer 11" shortly before they were launched. They were numbered from "10" because NASA had launched several Pioneer probes since 1958, and their numbers were 0, 1, 2, 3, 4, P-1, P-3, 5, P-30, P-31, 6 (A), 7 (B), 8 (C), 9 (D) and E. These probes were used to explore the moon and study the properties of the sun, some were successful and some failed.

Pioneer 10 and Pioneer 11 were originally designated F and G. They were the first and second probes in the Pioneer series to explore the outer planets. All Pioneer probes were operated by the ARC Pioneer Team.

An artistic conception of the appearance of Pioneer 6-13. On the far left are Pioneer 6, 7, 8, and 9, second from the left are Pioneer 10 and 11, and third from the left and far right are Pioneer 12 and Pioneer 13, which explored Venus. Image source: NASA
Structure and Instruments

Pioneer 10 is an unmanned spacecraft. It has the following major components: power supply, propulsion and attitude control system, scientific instruments and antenna.

In order to solve the problem of insufficient solar energy in the outer solar system, Pioneer 10 installed four radioisotope thermoelectric generators (RTGs), which use compressed plutonium-238 oxide balls, so this power source is also called a plutonium nuclear battery. They are placed on two three-bar trusses with an angle of 120 degrees between the two trusses, far enough away from the instruments on the probe, like the two tentacles of a snail. When it was just launched, the heat generated by the RTG could produce about 155 watts of electrical power. Due to the decay of radioactive materials, the power of the RTG will continue to decrease, but when it flies to Jupiter, its output power is still 140 watts, and the probe only needs 100 watts of electrical power for normal operation.

Two of the four radioisotope thermoelectric generators for Pioneer 10 and 11 are placed on an extended truss. Image credit:
https://history.nasa.gov/SP-349/ch3.htm

Pioneer 10 was equipped with six MR-103 hydrazine thrusters, which were arranged in three pairs to change speed, control attitude and adjust rotation speed. The 36 kg of propellant in the thrusters was placed in a spherical box with a diameter of 42 cm.

The instruments on Pioneer 10 include: Helium Vector Magnetometer (HVM), Quadrispherical Plasma Analyzer, Charged Particle Instrument (CPI), Cosmic Ray Telescope (CRT), Geiger Tube Telescope (GTT), Trapped Radiation Detector (TRD), Meteoroid Detectors, Asteroid/Meteoroid Detector (AMD), Ultraviolet Photometer, Imaging Photopolarimeter (IPP) and Infrared Radiometer.

Figure: Pioneer 10 (also Pioneer 11) structure diagram. The main antenna (high-gain antenna) is not marked in the middle is the medium-gain antenna. Image source: NASA, Vectors by Mysid; translated by Wang Shanqin

The radio communication system of Pioneer 10 includes a 2.7-meter-diameter parabolic high-gain antenna and a medium-gain antenna. The antenna is used to receive signal commands sent by the Deep Space Network (DSN) station on Earth and send the obtained data to the DSN.

From the names of the instruments, we can find that most of them will be used to detect cosmic rays, various charged particles, plasma, magnetic fields, meteors, and asteroids. The rest of the instruments are responsible for taking ultraviolet, visible light, and infrared images.

The imaging polarimeter on Pioneer 10 consists of a small telescope with an aperture of only 2.54 cm (1 inch) and two detectors. The two detectors are matched with red and blue filters respectively. After the light enters the telescope, it passes through the filters and then forms an image on the detector. Combining the images obtained by the two detectors, it is possible to synthesize the target's almost true color.

After all instruments and parts were installed, the length of Pioneer 10 (from the medium-gain antenna to the tail of the spacecraft) was 2.9 meters, and the maximum diameter (i.e. the diameter of the high-gain antenna) was 2.7 meters. Before launch, the mass of Pioneer 10 was 258 kilograms.

Pioneer 10 nears completion. Image credit: NASA Ames Research Center

Pioneer 10 launches

On March 3, 1972 (UTC), Pioneer 10 was launched on the Atlas-Centaur rocket. This rocket was customized with a powerful third-stage solid engine for Pioneer 10, which could accelerate the probe to 14.4 kilometers per second, which was one of the important guarantees for Pioneer 10 to fly to Jupiter.

Pioneer 10 launches aboard an Atlas-Centaur rocket. Image credit: NASA Ames Research Center (NASA-ARC)

Because of its huge speed, Pioneer 10 entered interplanetary space in just 19 minutes. Eleven hours later, Pioneer 10 passed the moon, becoming the fastest man-made celestial body up to that time.

The rocket also caused Pioneer 10 to spin at 60 rpm around the axis of symmetry of the high-gain antenna after liftoff. As the three trusses (the third truss is used to house the helium vector magnetometer) were extended, its rotation speed was reduced to 4.8 rpm, and it will continue to rotate at this speed. One purpose of the rotation is to control stability, and another purpose is to enable the probe to change the direction of the telescope or detector near the target area to perform imaging or measurement over a larger area.

Within 10 days of launch, the instruments on Pioneer 10 were activated one after another. While traveling through interplanetary space, Pioneer 10 became the first probe to detect interplanetary helium atoms and detected high-energy aluminum and sodium ions from the solar wind.

Artist's impression of Pioneer 10. Image credit: NASA/Don Davis

Just 12 weeks after launch, Pioneer 10 passed through the orbit of Mars and headed for the asteroid belt, a region between the orbits of Mars and Jupiter that is so densely packed with asteroids.

On July 15, 1972, Pioneer 10 entered the asteroid belt, becoming the first spacecraft to enter the asteroid belt.

On August 7, 1972, Pioneer 10 detected a shock wave produced by a violent solar wind burst at a distance of 2.2 astronomical units (330 million kilometers) from the Sun, providing important data for the study of solar physics.

During its passage through the asteroid belt, Pioneer 10 was not hit by large dust particles, indicating that the interior of the asteroid belt is very empty. During this period, Pioneer 10 accurately determined the density of dust particles of different sizes in the asteroid belt and measured the intensity of the light ("zodiacal light") formed after the dust particles in interplanetary space scattered sunlight.

On February 15, 1973, Pioneer 10 safely passed through the asteroid belt and headed for Jupiter, having flown about 435 million kilometers.

Pioneer 10 flyby of the Jupiter system

On November 6, 1973, Pioneer 10 was 25 million kilometers away from Jupiter. The Pioneer team issued a command to start testing its imaging system and then successfully obtained images of Jupiter. In the process of approaching Jupiter, it photographed a large number of crescent-shaped Jupiters; as it was about to enter the shadow area of ​​Jupiter, it saw that the "crescent" was getting thinner and thinner.

Pioneer 10 took multiple images of Jupiter during its close approach, showing a clear change in the phases of the moon. Image credit: NASA

On November 26, 1973, the number of solar wind particles detected by Pioneer 10 decreased sharply, and the temperature increased by about 100 times, which means that it reached the edge of Jupiter's magnetosphere and began to enter Jupiter's magnetosphere. At the edge of Jupiter's magnetic field, the solar wind hits the magnetosphere, forming a bow shock wave; the magnetosphere's blocking of the solar wind causes the solar wind speed to be greatly reduced. On this day, the Pioneer team received 12 images of Jupiter taken by Pioneer 10. A day later, Pioneer 10 passed through the top of Jupiter's magnetosphere.

On November 29, 1973, Pioneer 10 passed through the orbits of all of Jupiter's outer satellites. Starting on December 1, Pioneer 10 was close enough to Jupiter to take pictures of Jupiter of a quality that surpassed the best images of Jupiter that could be obtained by Earth-based telescopes at the time.

On December 3, 1973, Pioneer 10 began its flyby of the Jupiter system. It flew past Callisto (1,392,300 km away), Ganymede (446,250 km away), Europa (321,000 km away), and Io (357,000 km away) at 12:26:00, 13:56:00, 19:26:00, and 22:56:00 on the same day.

The orbit of Pioneer 10 from December 2 to 6, 1973, and the positions of Jupiter and its four satellites when it flew past the Jupiter system on December 4, 1973. Image credit: Tomruen; translated by Wang Shanqin

Images of Ganymede taken by Pioneer 10 showed that the latter has a low albedo in the center and near the south pole, and is brighter in the north pole.

Ganymede as imaged by Pioneer 10 on December 3, 1973. Image credit: NASA

Pioneer 10 was always too far away from Europa, so people could not analyze enough details from the images of Europa it took. However, the images it obtained still showed that Europa has a relatively high overall albedo and has some relatively wide dark areas. These features were further confirmed by other probes later.

This image of Europa was taken by Pioneer 10 on December 3, 1973. Image credit: NASA

At 02:26:00 on December 4, 1973, Pioneer 10 reached its perigee (the point in an object's orbit around Jupiter that is closest to Jupiter), 132,252 kilometers from Jupiter's cloud tops. At this time, its speed was 35 kilometers per second. Ten minutes later, Pioneer 10 crossed Jupiter's equatorial plane. About 78 minutes later, it passed behind Jupiter (relative to the direction of the Earth's line of sight at the time) to conduct a radio masking experiment.

Jupiter as captured by Pioneer 10 during its flyby. The black dot is the shadow of Io. Image credit: NASA

During the close approach to Jupiter, the radiation intensity of Jupiter to Pioneer 10 once reached about 10 times the expected intensity. The strong radiation seriously interfered with multiple instruments of Pioneer 10, causing them to temporarily malfunction one after another and causing a large number of command errors. Fortunately, a few minutes before the system was about to be completely scrapped, the radiation intensity suddenly decreased, and the Pioneer team also corrected most of the erroneous commands through emergency commands.

Afterwards, the Pioneer team analyzed the reason for the sudden decrease in radiation and found that this was because Jupiter's magnetic field is a toroidal magnetic field surrounding the equator and it swings. This caused Jupiter's magnetic field to no longer cover Pioneer 10 from a certain point in time, and the latter thus escaped death.

Although Jupiter's strong radiation interfered with Pioneer 10, 6 of the 11 instruments on it continued to work normally, and the imaging system sent back about 500 images of Jupiter and some of its satellites to Earth. The highest resolution of these images reached 320 kilometers per pixel.

On January 1, 1974, Pioneer 10 ended its mission to explore the Jupiter system and began its interstellar mission. [Note 2] On March 31, 1997, Pioneer 10 ended all its missions.

Pioneer 10 was once the most distant man-made object from the Sun. On February 17, 1998, Voyager 1 surpassed Pioneer 10, when both were about 69.419 AU (10.41 billion km) from Earth. From that time until now, Pioneer 10 is the second most distant man-made object from the Sun. In April 2023, Voyager 2 will surpass Pioneer 10, making it the third most distant man-made object from the Sun.

The last data it sent back was received on April 27, 2002, when it was 80.22 AU from Earth. The last weak signal it received was received on January 23, 2003. On February 7, 2003, the ground could no longer contact it.

Metal Plate for Aliens

A gold-plated aluminum plaque containing some important information was placed on Pioneer 10. This was the first time that humans placed an information plaque on a probe to let aliens know about the Earth. Pioneer 11, Voyager 1, and Voyager 2 all adopted a similar scheme, and the plaque on Pioneer 11 was exactly the same as the plaque on Pioneer 10. [Note 3]

The plaque on Pioneer 10 weighs about 120 grams, and its width, height, and thickness are 22.86 cm, 15.24 cm, and 1.27 mm, respectively.

The nameplate carried by Pioneer 10. Image credit: NASA

On the upper left of the sign is a diagram of the superfine structure transition of a hydrogen atom. When the spin direction of the electron in a hydrogen atom changes, the period of the photon emitted is 0.704 nanoseconds, [Note 4] 1 nanosecond is equal to 1 billionth of a second. This information can allow aliens to understand one of the ways humans measure time. At the midpoint of the line connecting the two hydrogen atoms, there is a very short vertical line, representing the binary 1.

The center of the multiple lines on the left side of the sign represents the solar system. The longest line extending from this center point to the right represents the relative distance between the sun and the center of the Milky Way; the other 14 lines spreading out from the center represent the relative distance between the earth and 14 pulsars. These 14 lines are composed of binary numbers, representing the rotation period of the corresponding pulsar. Aliens can find the corresponding pulsar through the period and thus determine the position of the solar system.

The figures on the plaque represent the outlines of an adult male and female on Earth. The male's raised right hand is a gesture of friendliness. The symbol between the woman's head and feet is a binary 8, indicating that the woman is about 8 feet (168 cm) tall. Partially overlapping the figures is the outline of Pioneer 10, which is strictly proportional to the size of the human.

At the bottom of the sign is the solar system, from left to right are the Sun, Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune and Pluto; the curve represents the flight path of Pioneer 10: starting from the Earth, flying past Jupiter and leaving the solar system. The symbols above or below the planets represent their relative distances from the Sun in binary digits, in units of 0.1 times the average radius of Mercury's orbit.

The nameplate placed on Pioneer 10. Image credit: NASA/HQ

Now, Pioneer 10 is heading towards the star Aldebaran (α Taurus), a red giant star about 65 light-years away from the solar system (1 light-year is nearly 10 trillion kilometers), and it will take Pioneer 10 more than 2 million years to reach it.

In 2019, someone [Note 5] used the data on the position, speed and direction of stars obtained by the Gaia satellite to infer a more "optimistic" result: about 90,000 years later, Pioneer 10 will fly past the dwarf star HIP 117795, and the closest distance to it will be 0.75 light years; this is the closest star it can encounter in the next few million years. However, 0.75 light years is about 50,000 times the distance between the earth and the sun (150 million kilometers), and the planet where the aliens live cannot be that far away from the parent star. Therefore, it will take a longer time for Pioneer 10 to be intercepted by aliens (if there are any).

The Moon occults Aldebaran. Image credit: Christina Irakleous

Fruitful results and great significance

Pioneer 10 created many firsts: it was the first probe to explore an outer planet, it was the first probe to cross the asteroid belt, it was the first probe to explore the Jupiter system at close range, it was the first artificial celestial body to reach the third cosmic velocity and thus leave the solar system, it was the first probe to use nuclear batteries, it was the first outer planet probe to achieve gravity-assisted acceleration and orbit change, and so on.

Its exploration of Jupiter has realized the dream of mankind to observe Jupiter at close range and has achieved fruitful results. From the beginning of the observation to the end of the observation, in nearly 60 days, the Pioneer team issued about 16,000 instructions to let Pioneer 10 perform various observation tasks. During this period, it crossed the bow shock of Jupiter's magnetosphere 17 times, took about 500 images of Jupiter and its satellites, measured or observed various properties of Jupiter's magnetosphere, radiation belt, magnetic field, atmosphere, gravitational field, etc., greatly deepening mankind's understanding of the Jupiter system and opening the precedent for various aspects of Jupiter research.

The success of Pioneer 10 accumulated valuable experience for probes such as Pioneer 11 (exploring Jupiter and Saturn), Voyager 1 (exploring Jupiter and Saturn), Voyager 2 (exploring Jupiter, Saturn, Uranus and Neptune), Galileo (exploring Jupiter), Juno (exploring Jupiter), Cassini-Huygens (exploring Saturn and Titan), and New Horizons (exploring Pluto), becoming their forerunner.

To commemorate the contribution of Pioneer 10, the United States Postal Service issued a commemorative stamp with Pioneer 10 as the theme on February 10, 1975.

The Pioneer 10 stamp, issued in 1975. Image source: US Post Office; Hi-res scan of postage stamp by Gwillhickers

Pioneer 10 launched the era of human exploration of the outer planets, creating the first oasis in the desert of outer planet exploration. Since then, more oases have emerged as other outer planet probes have achieved success.

Notes

[Note 1] The original meaning of Grand Tour in English refers to a tour of the entire European continent by Europeans or British people, and its meaning corresponds to the Chinese word “Grand Tour”.

[Note 2] As early as when it passed through the asteroid orbit, the Pioneer team had chosen a path that would allow it to accelerate and change its trajectory by using the "gravitational slingshot" effect generated by Jupiter's strong gravity when it flew past the Jupiter system, and fly away from the solar system. Pioneer 10 realized this plan when it flew past Jupiter, and it thus became the first outer planet probe to achieve a gravity acceleration and trajectory change plan.

[Note 3] The idea of ​​placing a metal plaque on Pioneer 10 and 11 was first proposed by Eric Burgess (1920-2005). He hoped that advanced intelligent life (aliens) would intercept probes roaming in space in the future and learn from the plaque that there is another group of intelligent life in the universe living on Earth in the solar system, and that they would know information related to Earth and humans. Burgess told Carl Sagan (1934-1996) about this idea. Sagan was very interested in this idea and formally applied to NASA to implement this plan. NASA approved this idea. Sagan and Frank Drake (1930-2022) collaborated to design the plaque. The illustrations on the plaque were designed by Sagan's then wife Linda Salzman Sagan (1940-).

[Note 4] The wavelength of the electromagnetic wave generated by this transition is 21.106 cm and the frequency is 1420.405 MHz, which corresponds to a period of 0.704 nanoseconds.

[Note 5] Bailer-Jones, Coryn AL & Farnocchia, Davide, Future Stellar Flybys of the Voyager and Pioneer Spacecraft, Research Notes of the American Astronomical Society, 2019, 3, 59. An extended version of this paper is available at: arXiv:1912.03503 (
https://arxiv.org/abs/1912.03503)

This article is supported by the Science Popularization China Starry Sky Project

Produced by: China Association for Science and Technology Department of Science Popularization

Producer: China Science and Technology Press Co., Ltd., Beijing Zhongke Xinghe Culture Media Co., Ltd.

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