As NASA’s Juno spacecraft approaches Jupiter, there has been growing interest among space enthusiasts in the views its camera will return. JunoCam, as it is called, is a solid state camera built by Malin Space Science Systems designed to provide views of Jupiter through RGB and a methane-band filters. Although it was included on the Juno mission primarily for educational and public outreach purposes, JunoCam will provide scientifically useful images with a maximum resolution better than any Earth-based telescope can provide. Juno’s polar orbit will also allow Jupiter’s polar regions to be viewed from nearly overhead producing images superior to those created by earlier missions like Voyager and Galileo whose trajectories were fairly close to the Jovian equator.

While the public and science community are eagerly awaiting these new images of Jupiter, Juno will not be the first spacecraft to provide us with a clear view of this planet’s poles. The first nearly overhead views of Jupiter’s polar regions were actually acquired over 41 years before Juno reached Jupiter by NASA’s Pioneer 11 mission.

The First Good Look at the Poles

Pioneer 11 and its near twin, Pioneer 10, were spin-stabilized spacecraft built by TRW with launch masses of 258 kilograms each – only a fraction of Juno’s 3,625-kilogram launch mass. Launched on March 3, 1972 and April 6, 1973 using Atlas-Centaur rockets fitted with TE-M-364-4 solid motor kick stages to boost their injection velocities to reach Jupiter, Pioneer 10 and 11 were simple yet robust spacecraft tasked with performing an initial reconnaissance of Jupiter and its environment ahead of the more ambitious Voyager missions set for launch in the summer of 1977. Assuming Pioneer 10 met its objectives at Jupiter, an option existed to send Pioneer 11 on an alternate trajectory past Jupiter which would allow it to explore other regions of Jupiter’s huge magnetosphere and send it on towards Saturn to make the first flyby of that planet before the Voyagers’ encounters.

Pioneer 10 and 11 carried virtually identical suites of instruments with a total mass of 30 kilograms to measure radiation, magnetic fields and micrometeoroids. The pair of probes also carried a simple two-color imaging photopolarimeter (IPP). Unlike the more advanced JunoCam, Pioneer’s IPP consisted of a 2.5-centimeter telescope with a pair of detectors fitted with either a red and blue filter. The IPP used the spin of the spacecraft in combination with a stepper motor to change the telescope’s pointing to scan the scene to build up an image slowly one line at a time. By combining these data, approximately true-color images of Jupiter and Saturn could be generated with an instantaneous field of view of 0.5 milliradians – a bit better than JunoCam’s 0.67 milliradians. That translates into an image scale of about 500 and 670 kilometers per pixel at a range of one million kilometers for the Pioneer IPP and JunoCam, respectively.

With the successful completion of the Pioneer 10 flyby of Jupiter on December 5, 1973, Pioneer 11 was free to take a different trajectory past Jupiter the next year. Instead of following a trajectory near Jupiter’s equator as Pioneer 10 had done (along with most other Jupiter probes in the decades to follow), Pioneer 11 followed a trajectory inclined about 50° to Jupiter’s equator which brought the craft to about 42,000 kilometers above the giant planet’s cloud tops – about a third of the 132,000-kilometer flyby distance of Pioneer 10. Even though Pioneer 11 would fly much closer to Jupiter and its powerful radiation belts, its highly inclined flyby trajectory actually decreased the total radiation dose the craft received compared to its predecessor. In addition to allowing Pioneer 11 to observe Jupiter and its powerful magnetosphere at much higher latitudes, this inclined flyby trajectory would hurl Pioneer across the solar system and towards an encounter with Saturn five years later.

During its approach towards Jupiter from the southern hemisphere in December 1974, Pioneer 11 was able to make detailed observations of Jupiter’s famous Great Red Spot. The best image acquired about five hours before closest approach at a range of 545,000 kilometers with the probe over 31° south latitude provided an excellent view of the storm at an image scale of 237 kilometers per pixel. These images of the Great Red Spot would be the clearest until the arrival of the Voyagers four years later.

At 5:02 GMT on December 3, 1974, Pioneer 11 passed behind Jupiter and 20 minutes later made its closest approach. Because Pioneer 11 could only transmit data in real time, images or other data could not be taken at this time. After emerging from behind Jupiter as viewed from the Earth at 5:44 GMT, Pioneer 11 was hurled high above Jupiter’s northern polar regions. During this time Pioneer concentrated on acquiring fields and particle data as well as IR measurements as it had done during the inbound leg of its journey.

Pioneer’s first post-encounter image was acquired at around 9:27 GMT with the spacecraft over 52° north latitude. At a range of 435,000 kilometers, it provided the first and clearest view of Jupiter’s polar regions with an image scale of 152 kilometers per pixel. This image and the ones that followed showed that Jupiter’s fairly regular bands of clouds clearly seen at lower latitudes gave way to a more complex flow circulation pattern filled with individual vortices and storms hundreds of kilometers across.

While the images acquired by Pioneer 11 provided the first clear views of Jupiter’s northern polar region, it was essentially only a snapshot in time. JunoCam, which should return color images with resolution comparable to the IPP near the poles but with better overall quality, should allow the clouds of Jupiter’s polar and other regions to be viewed repeatedly during Juno’s 37-orbit, two-year primary mission letting scientists know how its clouds change over time. With future missions to the Jovian system likely to focus mainly on Europa and Jupiter’s other large moons locked in their equatorial orbits, JunoCam’s observations along with those made over four decades earlier by Pioneer 11 will be our only clear views of Jupiter’s high latitudes for quite some time to come.

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Related Video

Here is a collection of NASA archival footage and animations related to the Pioneer 11 mission.

Related Reading

“The Future That Never Came: Pioneer Saturn/Uranus Probes”, Drew Ex Machina, August 26, 2015 [Post]

General References

Richard O. Fimmel, James Van Allen and Eric Burgess, Pioneer: First to Jupiter, Saturn and Beyond, NASA SP-446, 1980

Charles F. Hall, “Pioneer 10 and Pioneer 11”, Science, Vol. 188, No. 4187, pp. 445-446, May 2, 1975

C.J. Hansen et al., “JunoCam: Juno’s Outreach Camera”, Space Science Reviews, DOI: 10.1007/s11214-014-0079-x, 2014

Emily Lakdawalla, “What to Expect from JunoCam at Jupiter”, Planetary Society Blogs, June 9, 2016 [Post]