|
|
The Planetary Society Blog
By Emily Lakdawalla
Pluto and... Voyager?
Sep. 29, 2006 | 16:48 PDT | 23:48 UTC
by David Seal
Are you sick of hearing about Pluto: Planet or No Planet? (Sounds like bad TV.) I am. But every other guest blogger has written a Plutonian entry since the IAU decree, and who am I to break the streak?
I was at a social function of sorts a few houses down the street last week and ran into Doug Griffith, former Voyager engineer and Magellan project manager, and he divulged rather wistfully that Pluto was a possible target of the Voyager spacecraft. Pluto? Really?? This piqued my curiosity (which is usually a bad idea). So let's take a stroll back in time and pursue this interesting subject, and let's start with the Grand Tour.
Flinging a probe into deep space to the outer planets posed unique difficulties that seemed insurmountable in the infancy of America's exploration of the Solar System. Even with the most powerful rocket in the U.S. arsenal in the 60s, the Saturn V, it would have taken thirty years to reach Neptune on a Hohmann transfer, and it was nearly impossible to conceive - at the time - of both a project team and spacecraft that would survive that long given the limitations of budgets, politics, and early spaceborne hardware.
Gravitational assists provided the answer. The concept had been around for centuries; Isaac Newton laid the foundation in 1687 in his Principia Mathematica, fully detailing the mathematics of the two-body gravitational problem. Hints of the potential of the three-body problem (e.g. the gravity assist, Sun + planet + spacecraft) was evident as early as the late 18th century, when Anders Lexell computed the orbit of a near-Earth comet discovered in 1770. He showed that the comet had made a close encounter with Jupiter in 1767, flinging it closer to the Earth where it was first discovered, and predicted it would re-encounter Jupiter again in 1779 and be expelled from the inner Solar System. Curiously, this comet which now bears his name is the closest known approach to Earth by any comet (though asteroids have come much closer) at about eight million kilometers (about eight times further than the Moon). Clearly, a trajectory had been altered significantly - twice - without rocket motors.
Interplanetary trajectory design began in earnest after the launch of the first U.S. satellite, Explorer 1 in January of 1958. Just months later, the Department of Defense in collaboration with JPL was directed to develop and launch probes to study the moon. Michael Mintovitch, who came to JPL in 1961, is credited with being the first person to thoroughly explore the concept of gravity assist transfers between multiple bodies in any order. It became clear in the early 60s that these maneuvers were a practical way to send a spacecraft of significant mass anywhere in the Solar System. As far as I can tell, this was first used on Mariner 10, which used a Venus flyby to reach Mercury, and Mercury itself to reencounter Mercury twice more; though Pioneer 11's flyby of Jupiter was around the same time.
A few years later, Caltech grad student and JPLer Gary Flandro made a careful study of Mintovitch's work and became convinced that it held the key to the outer Solar System. In July of 1965 (perhaps earlier, in his journals) he discovered a rare alignment in the 1980s of the gas giants that would enable a reasonably-timed mission exploring all four - provided a launch could be made in the late 1970s. This was a once-in-a-lifetime opportunity. He published the following year in Acta Astronautica (then called Astronautica Acta, actually) and the "Grand Tour" - and that was his own term - was born. Voyager's Grand TourThe Voyagers' Grand Tour as originally conceived at JPL. Credit: Flandro, G. A., Astronautica Acta 12, No. 4, 1966. |
I was pleasantly surprised at finding this article in the JPL library. The first sentence of his abstract is "Contrary to popular belief, indirect ballistic trajectories involving close approach to one or more intermediate planets need not require longer flight duration than is characteristic of direct transfer orbits." It's amazing to see a statement like that about a mission design technique that is the backbone of every interplanetary mission launched by mankind. I never did very well in history in school, probably because it was hard for me to understand how it applied to me, but this stuff is really cool.
In his paper, Flandro describes transfers to Jupiter and Saturn, Jupiter and Uranus, Jupiter and Neptune, Jupiter and Pluto (!!!), and the Voyager grand tour of Jupiter, Saturn, Uranus and Neptune (figures for the last two are attached). The available launch years vary slightly, but generally covered 1975 through 1981, with the middle years having better performance. Flandro's article caused quite a stir, and interest in a mission spread widely and rapidly, reaching the status of a media event with widespread public interest. And after a long string of problems with budgets, priorities, and politics, the Voyager program was finally approved by NASA in 1972. But the Grand Tour, or "Thermoelectric Outer Planet Spacecraft" suffered elimination by budget cuts when the perceived development cost for long-life spacecraft was deemed not affordable. In response, scientists and engineers, principally among them Roger Bourke, then supervisor for JPL's Advanced Projects Group, worked madly to propose a lest costly alternative, the Mariner Jupiter/Saturn '77, project, which met with approval and went forward. (Cassini faced a similar threat in the mid-90s, where we removed the scan platforms to save cost.)
However, there was a lot more to MJS 77 than met the eye. As Roger Bourke says, "we outfoxed them: we built the Jupiter-Saturn-Uranus-Neptune opportunity into the MJS '77 mission design. Engineers 1, Bureaucrats 0." Lou Friedman mused to us recently, "It is too bad we never got to Uranus and Neptune - probably they would have been interesting. (Smile).
Voyager is a little like the Mars Exploration Rovers -- they never die." JPLers today think
they're pretty clever sometimes, but the guys back then were pretty darn clever too. (I totally need to party with Roger some day.) Years after launch, engineers reconfigured V2's mission to extend its lifetime and capabilities towards a full Grand Tour.
Anyway, back to Pluto. There were no options for a Jupiter-Saturn-Uranus-Neptune-Pluto tour; Pluto just wasn't far enough along in its orbit (as you can see from the image) to be reachable from Neptune - it actually was behind Neptune, and closer to the Sun! But Flandro found a Jupiter-Pluto transfer with a flight time of only seven years, and later on a Jupiter-Saturn-Pluto transfer was developed as a potential option, reaching Pluto about nine years after Saturn. Compare these to a minimum energy direct transfer to Pluto of 46 years! Jupiter-assisted Path to Pluto. Credit: Flandro, G. A., Astronautica Acta 12, No. 4, 1966. |
And again, we come to the "balance this versus that" problem. Even though they reconfigured Voyager 2 for a Grand Tour, the mission was sold on the exploration of Jupiter and Saturn. In order for Voyager to go on to Pluto, the encounter trajectory with Saturn would have been further constrained. No close flyby of Titan, for example, would have been possible. And even then, Titan was a key target. As Charley Kohlhase recalls, "we also wanted a second crack at Titan in case Voyager 1 failed.... We would have given up the Voyager 2 Jupiter-Saturn-Uranus-Neptune for another Jupiter-Saturn (with Titan)." Ellis Miner - who worked on Cassini with me for a number of years - also commented: "as we were approaching Saturn with Voyager 2, we could have gone directly to Pluto or we could engineer encounters of Uranus and Neptune. We didn't even know that Pluto had a moon at that time, but it wouldn't have made any difference. The combination of Uranus and Neptune were deemed far more important than a single flyby of Pluto. If we were making that choice today, I believe the choice would be the same."
What would you have done? Sacrificed key objectives of your primary mission for a chance at Pluto? Nine years later? Banking on the (then highly uncertain) possibility that your spacecraft would still be operating? Doing what they did, and leaving Pluto for a later mission seems like the right call to me. What would you do?
References and thanks: email thread and conversations with Charley Kohlhase, Lou Friedman, Roger Bourke, and Jeremy Jones;"Voyager's Grand Tour" by Henry C. Dethloff and Ronald A. Schorn; and "NASA's Voyager Missions" by Ben Evans.
Thanks to Emily for giving me a chance to try this out; it's been a lot of work, but great fun. I'm even more impressed by her blogging capabilities now that I know how hard it is!
Comment Script
Comments
This comment form is powered by GentleSource Comment Script. It can be included in PHP or HTML files and allows visitors to leave comments on the website.
|
|
[ what is this? ]