By April both spacecraft were already halfway to Jupiter and, about two months later, Voyager 1, still approximately 265 million kilometers from Jupiter, began returning photographs of the planet that showed considerable detail, although less than could be obtained with telescopes on Earth. Both the imaging (TV) and the planetary radio astronomy instruments began observing Jupiter, and, by October 2, 1978, officials announced that “the polarization characteristics of Jupiter’s radio emissions have been defined. In the high frequencies, there is consistent right-hand circular polarization, while in the low frequencies, there is a consistent left-hand circular polarization. This was an unexpected result.” In addition to scientific studies of the interplanetary medium and a first look at Jupiter, the plasma wave instrument, which studies waves of charged particles over a range of frequencies that includes audio frequencies, was able to record the sound of the spacecraft thrusters firing, as hydrazine fuel is decomposed and ejected into space. The sound was described as being “somewhat like a 5-gallon can being hit with a leather-wrapped mallet.”
On December 10, 1978, 83 million kilometers from Jupiter, Voyager 1 took photographs that surpassed the best photographs ever taken from ground-based telescopes, and scientists were anxiously awaiting the start of continuous coverage of the rapidly changing cloud forms. These pictures, together with data from several ground-based observatories, were carefully scrutinized by a team of scientists at JPL to select the final targets in the atmosphere of Jupiter to be studied at high resolution during the flyby. The observatory phase of Voyager 1’s journey to Jupiter was about to begin.
As Voyager 1 approached Jupiter, the resolution of the images steadily improved. In October 1978, at a distance of about 125 million kilometers, the image was less clear than would be obtained with an Earth-based telescope. [P-20790]
By December 10, the spacecraft had moved to a distance of 85 million kilometers, and the resolution was about 2000 kilometers, comparable to the best telescopic images. [P-20829C]
On January 9, 1979 (c), at a distance of 54 million kilometers, the image surpassed all ground-based views and approached the resolution of the Pioneer 10 and 11 photos. [P-20926C]
In this, taken January 24 at a distance of 40 million kilometers, the resolution exceeded 1000 kilometers. [P-20945C]