Voyager at Pluto? Alternative Histories

With New Horizons in hibernation as it pushes on toward MU69, it’s worth remembering how recently our knowledge of the Kuiper Belt has developed. Gerard Kuiper did not predict the belt’s existence, though he did believe that small planets or comets should have formed in the region beyond the orbit of Neptune (he also thought they would have been cleared by gravitational interactions long ago). And I always like to mention Kenneth Edgeworth’s work in a 1943 issue of the Journal of the British Astronomical Association, discussing the likelihood of small objects in the region. We could easily be calling the area the Edgeworth/Kuiper Belt, as I occasionally do in these pages.

Which takes me back to the Voyager days. It wasn’t until 1992 that astronomers discovered 15760 Albion, the first trans-Neptunian object detected after Pluto and Charon. Back in 1980, when controllers were deciding on adjustments to the trajectory of Voyager 1, Pluto was an option, as New Horizons PI Alan Stern has pointed out. The spacecraft could have reached Pluto in the spring of 1986, not long after Voyager 2’s flyby of Uranus in January of that year. That spectacular double-header was ruled out when the Voyager team chose to study Titan instead.

Image: The Voyagers’ paths through the planetary system. What if Voyager 1 had been pointed toward Pluto? Credit: NASA.

The choice to proceed with the Titan close pass occurred at a time when we simply didn’t have any information about the extent of what would soon be called the Kuiper Belt, or realize that Pluto itself could be considered a Kuiper Belt object. Interestingly, Pluto was almost exactly the same distance from the Sun in 1986 as Neptune was when Voyager 2 flew by it in 1989. Had it been sent Pluto’s way, Voyager 1’s encounter would probably have been a success.

Voyager’s ultraviolet spectrometer, Stern tells us, was not a match for the far more sophisticated Alice instrument carried on New Horizons, and the latter also brought a dust impact detector to bear, along with more powerful radio science equipment to study atmospheric temperature and pressure, and greatly improved mapping cameras. But Voyager would have brought a magnetometer, a wider range of plasma instruments, and an ability to send back data at a rate 10 times that of New Horizons. It would also have been looking at Pluto then orbiting equator-on to the Sun, as opposed to the high-latitude illumination New Horizons dealt with in its close flyby of 2015.

‘What if’s’ are always fun, and Stern looks at another in his latest PI Perspective, asking this time whether Voyager might have been able to explore the Kuiper Belt as New Horizons is now doing. Here the answer is more definitive. Without a target list, it’s difficult to see how Voyager could have made a flyby — we knew next to nothing about objects beyond Pluto as Voyager entered the region. When 15760 Albion was discovered, Voyager 1 had all but crossed the Kuiper Belt, while Voyager 2 was deep inside it. We were pulling data from within the Kuiper Belt, but lacked the ability to locate a specific KBO for a potential encounter. Writes Stern:

…with very few known KBOs at the time, and certainly no small ones known close to Voyager’s trajectory, it would have been impossible to put together a Kuiper Belt target observing list. But even had the team been able to somehow craft such a list, Voyager’s cameras used older-technology Vidicon detectors instead of the charge-coupled devices (CCDs) that LORRI [New Horizons’ Long Range Reconnaissance Imager] uses (and are found in most digital cameras). As a result, Voyager’s imagers were not anywhere near as sensitive as those aboard New Horizons, and they could not have detected faint KBOs like the telescopic LORRI can.

Image: New Horizons is the fifth spacecraft to traverse the Kuiper Belt. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Magda Saina.

Moreover, the Hubble telescope was at that time lacking the advanced wide-field camera that would later allow it to detect KBOs as small as MU69, so we simply would have had no target to aim for. Stern goes on to point out that Voyager 1 traveled well above the plane of the Solar System, while Voyager 2 was well below it, meaning the spacecraft were outside the great bulk of the KBO population. While Pluto was a Voyager possibility, a small KBO was definitely not.

Voyager’s spectrometers, magnetometers and charged-particle detectors have given us priceless data about the heliosphere and the Kuiper Belt, but New Horizons is now studying the area more intensively, viewing many KBOs from a distance, taking Kuiper Belt plasma and dust measurements with its SWAP, PEPSSI and SDC instruments, and homing in on MU69, which it will reach on January 1, 2019. The road ahead is exciting, but it’s essential that we keep working on a New Horizons successor, a craft designed from the outset to study the local interstellar medium. What surprises will this next generation of spacecraft convey?