Voyager 2 took these images of Hyperion as it closed in from 1.2 million kilometres (left) to 500,000 kilometres (right) showing that it is non-spherical. The prominent arcuate ridge, Bond-Lassell Dorsum, was named for the moon's co-discoverers. The ridge was suggestive of a crater comparable in size to the moon's longest dimension, and hinted that Hyperion might be a fragment of a much larger body.

Voyager 2's particles and fields instruments showed the solar wind as strong and gusty, so the bow shock was not expected until near, or maybe even inside, Titan's orbit. After being first sensed early on 24 August at a range of 32 planetary radii, the shock washed back and forth several times during the day, and the spacecraft did not finally meet the magnetopause until midnight, at 18.6 radii, just inside Titan's orbit, which confirmed that when the wind is strong the moon is exposed while on the sunward side of its orbit. Titan was not a priority for Voyager 2, and it approached no closer than 663,400 kilometres - a distance 150 times that of its predecessor. Nevertheless, this was an opportunity for remote-sensing of the north polar region. The Photopolarimeter on Voyager 1 had been crippled during the deep penetration of the Jovian magnetosphere, but Voyager 2 had kept its distance and its instrument had fared better. Despite being partially disabled, it made the surprising discovery that the reflected insolation from Titan is highly polarised, which provided valuable insight into the size of the aerosol particulates in the upper haze layer.

As Voyager 2 finally entered the magnetosphere in the early hours of 25 August, and closed to within 1 million kilometres of Saturn, it initiated its hectic 'near encounter' schedule. Images were streaming into JPL every 3 minutes. As the time for the next one to appear approached, everyone not otherwise engaged turned to the nearest TV monitor. At 04:00 Voyager 2 returned a series of images of Enceladus's intriguingly smooth surface from a range of 800,000 kilometres, then again at 06:00, by which time the range had reduced to 600,000 kilometres. At 09:00 it snapped the co-orbital moonlets of Tethys and Dione, then documented the 'F' ring which, surprisingly, was now regular and free of clumping and braiding. At noon, it returned to Enceladus, then made a start on Tethys. The scan platform was slewing from target to target at its fastest rate of 1 degree per second. At times when the remote-sensing instruments were inactive, the spacecraft rotated to enable the particles and fields instruments to sense the magnetosphere most efficiently.

On the final approach, the imaging ceased and the scan platform fixated upon the star delta Scorpii as this passed behind the rings so that the Photopolarimeter could take measurements every 10 milliseconds.113,114 Monitoring started at 18:18, some 22 minutes prior to the star's contact with the inner edge of the 'C' ring, and ran until 20:40, at which time it cleared the 'F' ring. With such a close-in vantage point, the angular resolution on the sample was much greater than can be achieved from Earth, and the light curve was not complicated by the twinkling effect of the signal passing through the Earth's atmosphere. With a linear resolution of 100 metres, this gave the best profile of the opacity across the rings to date. Instead of the light passing unhindered through a series of gaps, as had been expected, the ceaseless 'flickering' indicated that, despite it being only 100 metres thick, there is very little 'empty' space within the main ring system, even where the highest-resolution imagery seemed to show gaps. The real shocker, however, was that the material extends beyond the 'A' ring. This made Pioneer 11's transit at 2.87 radii all the more remarkable, and elevated concern over whether Voyager 2 would survive its attempt to fly this 'proven' route. In fact, concern was already running high, because in documenting the broad but tenuous 'E' ring Voyager 1 had spotted a section of a narrower ring, later designated the 'G' ring, at 2.83 radii. A re-examination of Pioneer 11's particles and fields data uncovered a hint of it. However, with the Grand Tour reliant upon this ring-plane crossing, there was little option but to trust to luck. The 'window' for the Grand Tour had already closed, and would not reopen for an century and a half, so for this most privileged of generations of planetary scientists, with their only real prospect of reconnoitring the outer Solar System in the fickle hands of Lady Luck, the tension was electric.

At its closest point to Saturn, Voyager 1 had been behind the ring system and had thus inspected the non-illuminated face, but as Voyager 2's ring-plane crossing would be after closest approach, it was able to record the illuminated face at an increasingly 'open' perspective. ''The images of the rings we receive today will be of much higher resolution than anything we've ever seen before,'' E.C. Stone promised. The best Voyager 1 imagery had suggested that there might be as many as a thousand individual ringlets, but the new imagery revealed that there are actually tens of thousands. It seemed that every time the resolution was increased, every ringlet was found to incorporate many finer ringlets. ''This is about as far as imaging can take us,'' B.A. Smith informed the reporters, referring to the 10-kilometre-per-pixel resolution. Only the Photopolarimeter's stellar occultation light curve could identify finer detail. ''We have a superb set of ring data, there is no question about it,'' assured A.L. Lane, that instrument's leader. It had to be recognised, however, that this data pertained only to the narrowest of lines across the system. It would not be easy to infer whether a specific 'dip' in the light curve was due to the starlight being occulted by material in a narrow ringlet or by a single object. With the 'F' ring now looming, a very-high-resolution image was secured from a range of 50,000 kilometres which showed that this comprised one bright strand, two fainter ones and one diffuse band. The brightest strand was shown by the stellar occultation to be

Voyager 2 took this shot of the 'F' ring from 50,000 kilometres with a resolution of a few kilometres per pixel (the best ever) shortly after crossing the ring plane. It shows the bright ringlet and three fainter strands, but no 'braiding'. The light curve from the stellar occultation established that the main strand is itself composed of a dozen fine ringlets.

Voyager 2 took this shot of the 'F' ring from 50,000 kilometres with a resolution of a few kilometres per pixel (the best ever) shortly after crossing the ring plane. It shows the bright ringlet and three fainter strands, but no 'braiding'. The light curve from the stellar occultation established that the main strand is itself composed of a dozen fine ringlets.

composed of a dozen finer ringlets, each only a hundred metres wide.115 Was there no end to the ever-finer subdivisions?

At 21:50 on 26 August, Voyager 2 passed 101,300 kilometres above Saturn's cloud tops. Fifteen minutes later, in preparation for passing behind the planet's trailing limb at 22:26, it boosted its signal strength and ceased to modulate it. The signal took about a minute to fade, and the manner in which it did so provided information on the atmosphere. Although out of sight, the spacecraft was not idle; it pursued its programmed observations, saving the data on magnetic tape for later replay.

Voyager 2 was to cross the ring plane at 22:44 at 2.86 planetary radii, some 32,000 kilometres beyond the 'F' ring. ''I think we're all confident,'' E.C. Stone told the journalists, alluding to the fact that the crossing point was just 3,000 kilometres beyond the only vaguely defined boundary of the 'G' ring. There was wild cheering in the von Karman Auditorium when the signal was re-acquired at midnight. The celebration was pre-empted, however, by the shocking realisation that the computer had ceased scan platform operations. Instead of returning home for a few hours of well-deserved sleep, the engineers remained to find out what had gone wrong.

When the Plasma Wave Spectrometer data was transmitted, this revealed that the ring-plane crossing had not been plain sailing. For several minutes the spacecraft had been immersed in a cloud of plasma, and several times fired its thrusters to correct a drift in its orientation. An analysis suggested that the plasma had been generated by impacts with many thousands of micron-sized dust grains striking the spacecraft at a relative velocity of 10 kilometres per second, and been instantly vaporised.

24 hts

24 hts

Was this article helpful?

0 0
Telescopes Mastery

Telescopes Mastery

Through this ebook, you are going to learn what you will need to know all about the telescopes that can provide a fun and rewarding hobby for you and your family!

Get My Free Ebook

Post a comment