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Ian ReganApril 16, 2019

Voyager Wide-Angle Views of Jupiter

Last month marked the 40th anniversary of the historic Voyager 1 encounter with Jupiter in 1979. Voyager 1 was not the first robotic visitor to Jupiter; Pioneers 10 and 11 flew past the gas giant in 1973 and 1974. But while the Pioneers’ primitive spin-scan photopolarimeters took very good, groundbreaking images of the planet, they couldn’t compete with the TV cameras of the Voyagers.

The cameras, while not at the forefront of technology in the late-1970s, were still a reliable and proven method of capturing images in deep space. They had a pedigree stretching back to the early Mariner probes—not surprising, given that the Voyager program was essentially a continuation of Mariner (the two Voyager probes were developed under the moniker ‘Mariner-Jupiter-Saturn’).

Each Voyager carried a pair of TV cameras, mounted behind telescopes of differing focal length, giving the operators the option of taking images spanning 0.4 degrees (the NAC or narrow-angle camera), or 3 degrees (the WAC or wide-angle camera). In traditional photographic terms, both systems are squarely in the telephoto category.

Most of the valuable photographic data came from the NAC, given the greater resolution it offered. However, the overlooked WAC had two principal roles: to provide context views of the highly localized NAC shots, while also providing continuous photographic surveillance of global-scale features on Jupiter and its moons near the period around closest approach.

Voyager 1

Jupiter is huge. On 17 February 1979, Voyager 1 was no longer able to fit the planet into a single frame of the narrow-angle camera, even though the spacecraft was still a staggering 11 million miles away.

The wide-angle camera was similarly overwhelmed on 3 March 1979, when the planet was 1.4 million miles distant.

Jupiter and Callisto from Voyager 1

NASA / JPL / Ian Regan

Jupiter and Callisto from Voyager 1
This color composite of Jupiter was the last image captured by Voyager 1 before its wide-angle camera could no longer fit the whole of the planet within its 3.2 degree field of view. However, due to pointing inaccuracies, most images that Voyager 1 acquired from this distance were truncated on one edge. Therefore, this is actually a mosaic of two sets of frames taken through orange, green, and violet filters, all taken on March 3, 1979, from a distance of 2.3 million km. Callisto is visible just off to the right of Jupiter's eastern limb, while the Great Red Spot and a large white oval dominate the southern hemisphere.

All Voyager images of Jupiter captured on approach were usually taken with the NAC, and show a near fully illuminated globe, with the morning terminator visible just shy of the planet’s unlit, western limb. As Voyager 1 grew closer to the planet, its trajectory gave its cameras a view of a fully illuminated Jupiter. On 2 March, the phase angle dwindled to as little as 0.2 degrees, analogous to a typical full Moon as seen from Earth.

From that point onward, with closest approach just a few days away, Jupiter presented a markedly different face to the spacecraft—one where the night terminator now became visible on the eastern extent of the gaseous sphere.

I wondered if it would be possible to reconstruct this seldom seen face of Jupiter: a global portrait with the night terminator hugging the eastern limb. At this distance, narrow-angle imagery was all but useless for this purpose and data gathered by the lesser-celebrated wide-angle camera offered the only possibility.

For Voyager 1, the wide-angle images captured over a 24-hour period prior to closest approach were not sufficient, both in spatial and spectral coverage, to compile a representative color mosaic of the planet. A great quantity of frames were trained (understandably) upon the Great Red Spot, leaving rather meager coverage of everything else.

However, I found it possible to combine data collected through the orange, green, blue, and occasionally violet, filters to produce the following mosaic of Jupiter’s western limb, as seen by Voyager on the evening of March 4th, 1979:

Io above Jupiter

NASA / JPL / Ian Regan

Io above Jupiter
The innermost Galliean satellite, Io, coasts above the russet cloud decks of Jupiter. This Voyager 1 mosaic principally consists of 3 color composites made of orange-, green-, and blue-filtered images, all taken with the wide-angle camera on 4 March 1979. Gaps in coverage were filled by other 3-filter composites taken earlier or later the same day. North is to the right. Planet-spacecraft distance was some 1 million km.
Io above Jupiter with shadow below

NASA / JPL / Ian Regan

Io above Jupiter with shadow below
An alternate presentation of data from the same time frame allows us to see the shadow cast by Io onto the cloud decks of Jupiter’s Equatorial Zone.

Io can be seen transiting the planet, as the Great Red Spot comes into view. Given that the three principle WAC color composites used were captured at 17:11, 21:21, and 21:37 (all times UTC), the view is somewhat of a Frankenstein creation, as Jupiter’s rapid rotation ensures that atmospheric features seen here at different latitudes are not shown in proper relation to one another. Despite this drawback, this mosaic gives a very effective if somewhat illusory impression of the face that Jupiter presented to Voyager 1, just 14 hours before closest approach.

Io transiting Jupiter's equatorial zone

NASA / JPL / Ian Regan

Io transiting Jupiter's equatorial zone
Voyager 1 took the images for this composite through orange, green, and blue filters on 4 March 1979 at 21:37 UTC. It was 999,000 kilometers from Jupiter at the time.
Brown barge

NASA / JPL / Ian Regan

Brown barge
A brown barge in Jupiter's north equatorial belt as seen by Voyager 1 at 23:29 on 4 March 1979 from a distance of 889,000 km. Three photos taken through orange, green, and blue filters were combined and adjusted to appear approximately natural color.
Great Red Spot and white ovals from Voyager 1

NASA / JPL / Ian Regan

Great Red Spot and white ovals from Voyager 1
Voyager 1 took the images for this composite through orange, green, and blue filters on 4 March 1979 at 17:11 UTC. It was 1,254,000 kilometers from Jupiter at the time. This frame was used in the preparation of the second and third images in this blog post.
Jupiter belts and zones

NASA / JPL / Ian Regan

Jupiter belts and zones
This Voyager 1 view spans both of Jupiter's orange-tinted equatorial belts and includes a portion of the high northern latitudes. Images composing it were taken through orange, green, and blue filters from a distance of 1,302,000 kilometers. A section of this composite was used to fill in gaps in the second and third images of this blog post.

Voyager 2

Sixteen months later, the second Voyager (wielding a TV camera with marginally greater sensitivity), focused on the Jovian system.

Callisto's shadow on Jupiter

NASA / JPL / Ian Regan

Callisto's shadow on Jupiter
This was one of the last wide-angle views of Jupiter that Voyager 2 snapped before the planet became too big to be photographed in a single frame. Although Jupiter would have fit in one view, the Voyager scan platform did not always achieve perfect pointing, so in this composite Jupiter is truncated where it fell off the edge of the detector. The fuzzy dark spot is the shadow of Jupiter's outermost large moon, Callisto. Voyager 2 took the photos for this composite through orange, green, and violet filters on 8 July 1979 from a distance of 2.17 million kilometers.

In the evening of 8 July 1979, a mere 25 hours before closest approach, Voyager 2’s WAC captured a more comprehensive assortment of images of Jupiter than its twin, allowing a full globe portrait of the planet to be composed, from six 3-filter color composites (either OGB or OGV).

Ganymede and Jupiter Global Voyager 2 Mosaic

NASA / JPL / Ian Regan

Ganymede and Jupiter Global Voyager 2 Mosaic
The images for this composite were taken over a period of 4 hours on 8 July 1979 through orange, green, and blue or violet filters. Because of the long time span and Jupiter's rapid (10-hour) rotation, the cloud positions don't represent any real instantaneous view of Jupiter. Ganymede's shadow falls on the disk of Jupiter. Ganymede was actually much farther from Jupiter as seen by Voyager 2 during this observation; the distance between the two worlds has been reduced for aesthetic expediency. Voyage 2 was about 1.5 million kilometers away from Jupiter during this observation, but only 430,000 kilometers from Ganymede, which is why Ganymede appears so much larger than its shadow.

Again, these observations spanned a non-trivial time period (from 18:32 to 22:35 UTC), so the relative longitudinal positions of various cloud features cannot be trusted.

As an added bonus, the shadow of Ganymede was captured crossing the face of Jupiter; it is shown here as seen by the WAC at 20:04 hrs. In fact, after these images were taken, Voyager 2 turned its suite of instruments toward the moon itself, capturing many images through both camera systems. A wide-angle color composite of Ganymede, taken as context for the higher resolution mosaics, has been added to the Jupiter mosaic to complete this panoramic vista.

Near-global high-resolution color view of Ganymede from Voyager 2

NASA / JPL / Constantine Thomas

Near-global high-resolution color view of Ganymede from Voyager 2
Voyager 2 surveyed Ganymede with its narrow-angle camera around the same time as the wide-angle context frames were obtained. This mosaic courtesy of Constantine Thomas.
Ganymede's shadow

NASA / JPL / Ian Regan

Ganymede's shadow
The shadow of Ganymede transverses the Jovian cloud decks. Voyager 2 took the photos for this composite with its wide-angle camera through orange, green, and blue filters at 18:32 on 8 July 1979 from a distance of 1.53 million km.
Equatorial anticyclone on Jupiter

NASA / JPL / Ian Regan

Equatorial anticyclone on Jupiter
Voyager 2 looked at the region east of the Great Red Spot to take the photos for this composite through orange, green, and violet filters at 21:40 on 8 July 1979 from a distance of 1.4 million km. Sections of this frame were incorporated into the Jupiter and Ganymede global mosaic above.
Great Red Spot and a white oval from Voyager 2

NASA / JPL / Ian Regan

Great Red Spot and a white oval from Voyager 2
The Great Red Spot moves toward the dusk terminator in this Voyager 2 view from a distance of 1.25 million km at 22:57 UTC on 8 July 1979. Images taken through orange, green, and blue filters were combined to create this color view.

While later emissaries to Jupiter, most notably Cassini and New Horizons, captured views of Jupiter from this particular angle, these unique Voyager perspectives have been buried in archived data until now. Long live the Voyagers!

Read more: Io, Ganymede, Callisto, Voyager 1 and 2, Saturn

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Ian Regan
Ian Regan

Contributor for Apollo Lunar Surface Journal
Read more articles by Ian Regan

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