Pioneer 10: first probe to leave the inner solar system & precursor to Juno

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The first spacecraft to leave the inner solar system sailed into the asteroid belt 45 years ago today, 15 July 1972, on a mission that would mark many “firsts” for NASA’s exploration of the solar system.  Pioneer 10, the first outer solar system mission, became the first probe not only to leave the inner solar system, but also the first probe to be launched on an escape trajectory from the solar system and the first craft to visit the planet Jupiter.  Today, NASA’s Juno spacecraft continues the exploration efforts of the Giant Planet begun by Pioneer 10 over four decades ago.

Mission proposal and selection:

The Pioneer 10 mission, and its companion, Pioneer 11, began life as part of NASA’s concerted effort to take advantage of a rare outer planetary alignment in the late 1970s and 1980s that would allow a probe to visit all four of the gas and ice giants of the outer solar system.

While Pioneer 10 was never designed to fly this Grand Tour – a mission ultimately completed by the Voyager 1 and Voyager 2 probes – Pioneer 10 was a groundbreaking precursor to those missions, proving that travel through the asteroid belt and Jupiter’s massive radiation field were in fact possible.

The decision to fly this precursor mission through the pair of Pioneer 10 and 11 probes was taken in 1964, with NASA’s Goddard Space Flight Center proposing that the two launches take place in the 1972 and 1973 periods of favorable launch trajectories to Jupiter that only occur every 13 months.

The Pioneer 10 and 11 missions were formally approved by NASA in February 1969 – just three and four years ahead of their planned launch dates.

At the time of mission approval, the probes were known as Pioneer F and Pioneer G before taking on their numerical designations later on.

Construction and scientific experiment/instrument selection:

Unlike the previous Pioneer probes, Pioneers 10 and 11 were specifically designed for exploration of the outer solar system, with enhanced communications systems and hardened radiation shielding to protect their instruments and systems from the damaging radiation fields they would encounter at Jupiter.

Based on formal acceptance and approval of the project in 1969, the traditional bidding process for construction and design of the spacecrafts was curtailed, with NASA awarding TRW the contracts for both Pioneer 10 and 11 in February 1970 – just two years before Pioneer 10 would need to be launched.

As design and construction began, more than 150 scientific experiments were proposed for Pioneer 10, with final scientific instrument selection occurring in early 1970.

In all, 11 instruments were chosen for inclusion on Pioneer 10, including the Helium Vector Magnetometer (HVM), the Quadrispherical Plasma Analyzer, the Charged Particle Instrument (CPI), the Cosmic Ray Telescope (CRT), the Geiger Tube Telescope (GTT), the Trapped Radiation Detector (TRD), the Meteoroid Detector, the Asteroid/Meteoroid Detector (AMD), the Ultraviolet Photometer, the Imaging Photopolarimeter (IPP), and the Infrared Radiometer.

Specifically the HVM was included to help define the structure of the interplanetary magnetic field, to map the Jovian magnetic field, and to provide magnetic field measurements of the solar wind’s interaction with Jupiter.

The Quadrispherical Plasma Analyzer would likewise help detect particles of the solar wind originating from the sun – thus aiding the measurements and detection of particles by the HVM.

The CPI was designed to detect cosmic rays inside the solar system, while the CRT would collect data on the composition of cosmic ray particles and their energy ranges.

For radiation detection, the GTT would allow Pioneer 10 to return data on the intensities, energy spectra, and angular distributions of electrons and protons as the vehicle passed through Jupiter’s radiation belts.

Meanwhile, the TRD would return information on light emitted in a particular direction from particles passing through recording electrons in the energy range of 0.5 to 12 MeV (mega electron volt.

Additionally, as Pioneer 10 was to be the first probe to pass through the asteroid belt, the Meteoroid Detector and the AMD were included to help define the danger micrometeoroids and asteroids posed to probes traversing the belt.

Specifically, the meteorite detectors consisted of 12 panels of pressurized cell detectors that would record penetrating impacts of small meteoroids.

Conversely, the AMD was designed to track close-by objects ranging in size from dust to large distant asteroids.

Also included on Pioneer 10 was the Ultraviolet Photometer, which would help quantify the amount of hydrogen and helium present at Jupiter as well as the amounts that were floating free in space.

The IPP, a unique experiment designed to work in tandem with Pioneer 10’s spin rate, was created to help build a visual image of Jupiter by scanning a narrow 0.03 degree wide band of the planet.

The small observation band would gradually move as Pioneer 10 spun, aiming the IPP at different areas of Jupiter.

Finally, the Infrared Radiometer would collect information on cloud temperature and heat emanated from inside Jupiter.

To power the instruments and the spacecraft, Pioneer 10 was fitted with four SNAP-19 Radioisotope Thermoelectric Generators (RTGs) positioned on two of the three rod trusses of the spacecraft.

At launch, the four RTGs, powered by plutonium-238, provided 155 W of power, decaying to 140 W by the time the spacecraft encountered Jupiter.

Communications with and from the craft were routed through a series of narrow-band, medium-gain, high-gain, and omni-antenna transceivers with a transmission rate from Pioneer 10 of 256 bit/s at launch, dropping to 255.18 bit/s by the time the craft made its closest approach to Jupiter.

Leaving the inner solar system:

As construction began on Pioneer 10, NASA understood that the 1972 launch window for the craft opened on 29 February and closed on 17 March 1972.

Despite only having two years to construct the spacecraft and finish all preparations for launch, the construction company met their goal.

On 3 March 1972 at 01:49:00 GMT, Pioneer 10 lifted off from SLC-36A at the Cape Canaveral Air Force Station, Florida, aboard an Atlas-Centaur launch vehicle.

After the Atlas-Centaur duo did their job, a solid fueled third stage, created specifically for the Pioneer missions, imparted 15,000 lbf of additional thrust to increase Pioneer 10’s overall speed to 51,682 km/h (32,114 mph) – making it the fastest human-made object at the time and the first spacecraft to be launched onto an escape trajectory (though not yet at escape velocity) from the solar system.

More importantly for the mission’s primary objective, Pioneer 10’s velocity was enough to reach Jupiter without any planetary gravity assist maneuver – which at that point had not yet been attempted for interplanetary missions.

The third stage also imparted an initial spin rate of 30 rpm onto the spacecraft, a rotation rate which was reduced to the mission standard 4.8 rpm 20 minutes after liftoff when Pioneer 10 extended its three boom/truss structures.

Just 11 hours after launch, Pioneer 10 passed the orbit of the moon and was safely on a trajectory to Jupiter for an arrival in December 1973.

As the mission was originally conceived, Pioneer 10 was to reach Jupiter in November 1974; however, NASA advanced the craft’s arrival date before its launch to December 1973 to avoid scheduling conflicts with the Deep Space Network and to avoid a period of communication blackouts with the probe when Earth and Jupiter would be on opposite sides of the Sun from one another.

After all of its instruments were turned on and successfully checked out, Pioneer 10 became the first spacecraft to detect helium in the interplanetary medium of the solar system, as well as the first spacecraft to detect ions of sodium and aluminum in the solar wind.

Then, on 15 July 1972, just over four months after leaving Earth, Pioneer 10 became the first craft to exit the inner solar system 45 years ago today.

Passage out of the inner solar system was marked by Pioneer 10’s entrance into the asteroid belt.

Being the first spacecraft to traverse the belt, mission planners extensively planned Pioneer 10’s trajectory so that it would avoid by some 8.8 million km (5.5 million miles) the nearest known asteroid.

At the time, the closest known approach Pioneer 10 made to any asteroid came on 2 December 1972 when the craft passed 307 Nike.

During its passage through the asteroid belt, Pioneer 10’s onboard meteoroid and asteroid detection systems identified no significant variation in dust particles between 10 – 100 μm (micrometers) between Earth and the outer edge of the belt’s defined boundaries.

However, Pioneer 10 did return information regarding a threefold increase in 100 μm to 1.0 mm diameter particles.

The probe found no evidence of objects larger than 1 mm, indicating that those were far less common than thought at the time.

On 15 February 1973, exactly seven months after entering the asteroid belt, Pioneer 10 exited the belt – at which point the craft was less than 10 months away from its big encounter with Jupiter.

Encounter with Jupiter:

Pioneer 10’s encounter trajectory was carefully planned to maximize the information returned about Jupiter’s radiation environment, even at the expense of that environment’s potential damage to some of Pioneer 10’s systems.

On 6 November 1973, while still 25 million km (15.5 million miles) from Jupiter, direct observations of the Jovian system began.

After a series of health checks, mission controllers uplinked 16,000 encounter commands to Pioneer 10 covering the entire 60-day encounter sequence.

The uplinked commands provided the trajectory that would take Pioneer 10 to within three times the radius of the planet.

At the time, controllers believed that was as close as the craft could approach Jupiter and still survive the radiation.

Pioneer 10 crossed the orbit of the outer moon Sinope on 8 November and reached the bow shock of Jupiter’s magnetosphere eight days later on 16 November as confirmed by its instruments via a drop in the velocity of the solar wind.

The craft then passed through Jupiter’s magnetopause on 17 November, with Pioneer 10s instruments confirming that Jupiter’s magnetic field was inverted when compared to that of Earth’s.

By 29 November, Pioneer 10 was still operating flawlessly as its instruments collected data point after data point and image after image of Jupiter.

Over the course of the entire encounter sequence, more than 500 images were collected and transmitted back to Earth, with image quality and resolution exceeding those taken from Earth or Earth orbit on 2 December 1973.

With a trajectory taking Pioneer 10 along the magnetic equator of Jupiter, ion radiation concentration increase dramatically, with a peak flux of electron radiation reaching 10,000 times that of the maximum radiation experienced around Earth.

On 3 December, the radiation began to take its toll on Pioneer 10, with the spacecraft generating several false commands.

Thankfully, Pioneer 10’s controllers had prepared for just such a contingency, and most of the false commands were able to be countermanded by contingency commands to the spacecraft.

However, the radiation-induced false commands did result in the loss of one image of Io and several close-ups of Jupiter.

Nonetheless, the trajectory chosen by Pioneer 10’s controllers allowed the spacecraft to perform detailed observations of Io.

In all, Pioneer 10 discovered that Io’s ionosphere extended 700 km (430 mi) above the moon’s surface and had a density of 60,000 electrons per cubic centimeter on the day side to 9,000 electrons per cubic centimeter on the night side.

Unexpectedly, Pioneer 10 also discovered that Io orbited within a cloud of hydrogen extending 805,000 km (500,000 mi) in width and 402,000 km (250,000 mi) in height.

On 4 December 1973, Pioneer 10 made its closest approach to Jupiter, passing 132,252 km (82,178 mi) from the tops of Jupiter’s clouds.

Despite all the radiation fears, the spacecraft came through the encounter in excellent shape, obtaining detailed and close-up images of the planet, including the ever-evocative Great Red Spot.

Since Pioneer 10, observation of Jupiter’s Great Red Spot has been a prime science target of the probes that have visited Jupiter.

This includes NASA’s current mission at Jupiter, the Juno spacecraft – which captured stunning high-resolution images of the complex storm system in Jupiter’s atmosphere when it performed its latest perijove – time of closest approach to Jupiter during its orbit – on 11 July 2017.

Thanks in large part to the characterization of Jupiter’s radiation environment first begun in-situ by Pioneer 10, Juno is built to withstand Jupiter’s harsh radiation environment and allow the craft to come far closer to Jupiter then Pioneer 10 did.

While Pioneer 10 approached to within 132,252 km (82,178 mi) of the top of Jupiter’s clouds, Juno dives to an impressively close 4,200 km (2,600 mi) above the cloud tops during its perijove science weeps over the planet.

Nevertheless, while the radiation environment around Jupiter is significantly better understood today than it was in 1973, and even though Juno is much more hardened against radiation than Pioneer 10 was, radiation is still the limiting factor for Juno’s mission.

And for Pioneer 10, its mission certainly didn’t end with observation of the Great Red Spot.

Proximity operations to Jupiter increased the craft’s overall velocity to 132,000 km/h (82,021 mph) as the craft swung around the planet, imparting the needed extra velocity kick to allow the craft to escape the solar system – but not before it passed behind Jupiter as viewed from Earth.

As Pioneer 10 passed behind Jupiter, the radio occultation data transmitted from the spacecraft back to Earth allowed for direct measurement of the temperature structure of Jupiter’s upper atmosphere, revealing and inversion between the altitudes with 10 and 100 mbar pressures.

Temperature ranges between -113° to -133°C (-171°F to -207°F) into 10 mbar levels and -163° to -183°C (-261.4°F to -297°F) in the 100 mbar levels were also measured.

Pioneer 10 also established definitively that Jupiter radiated more heat than it received from the sun.

Post-Jupiter life and legacy:

Once on its outward trajectory from Jupiter, Pioneer 10 crossed the bow shock of Jupiter’s magnetosphere a grand total of 17 times due to the shifting nature of the magnetosphere and its dynamic interaction with the solar wind.

The Jovian encounter phase of Pioneer 10’s mission officially concluded on 1 January 1974.

After leaving the Jovian system behind, Pioneer 10 crossed the orbital distance of Saturn in 1976 and the orbit of Uranus in 1979.

On 13 June 1983, Pioneer 10 crossed the orbit of Neptune and became the first human-made object to leave the major planets of the solar system behind.

Nonetheless, NASA officially maintained the Pioneer 10 mission until 31 March 1997.

At the time of the mission’s official conclusion, Pioneer 10 was still the farthest human-made object from Earth at 67 Astronomical Units (AU) from the Sun and was still transmitting coherent data.

This provided engineers an unanticipated ability to study the application of chaos theory (an idea that within the apparent randomness of chaotic complex systems there are underlying patterns that, if understood, can help avoid detrimental actions/commands) to extract coherent data from the fading signal of Pioneer 10.

On 2 March 2002, just one day shy of the 30th anniversary of its launch and at a distance from the sun of 69.419 AU, Pioneer 10 lost its title of farthest human-made object from the sun when it was overtaken by Voyager 1 – which was moving away from the sun 1 AU per year faster than Pioneer 10.

Meanwhile, strong enough signals continued to be received from Pioneer 10 until 27 April 2002.  On this day, the final 33 minutes of clean data routed through the Deep Space Network while Pioneer 10 was 80.22 AU away.

Subsequent signals were too weak to return useful information.

The final signal received from Pioneer 10 arrived on Earth through the Deep Space Network on 23 January 2003 from a distance of ~82.2 AU.

All further attempts to contact the spacecraft were unsuccessful, with the final attempt made on 4 March 2006 – 34 years and 1 day after the craft left Earth on its historic mission.

Today, Pioneer 10 – assuming it hasn’t collided with anything – is ~118.5 AU from the sun and is travelling outward at 2.54 AU per year.

It is currently the second farthest human-made object from the sun – a position it will hold until April 2019 when Voyager 2 overtakes it.

If left undisturbed, Pioneer 10’s trajectory will take it in the general direction of Aldebaran.

[Find related articles about exploration of the outer solar system and beyond here: Voyager 1Voyager 2; Cassini; JunoNew Horizons; and Voyager’s 1 interstellar mission]

(Images: NASA)

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