On July 14, 2015 NASA’s New Horizons spacecraft will make its closest approach to the dwarf planet Pluto after a long voyage of 9½ years. Even though humanity has yet to see what the instruments of New Horizons will discover about this distant world and its family of moons, now is as good as time as any to start thinking seriously about the next mission to Pluto. Given the infrequent availability of low-energy launch opportunities and the long flight times to Pluto coupled with the lengthy process of proposing planetary missions, gaining the required support and funding as well as actually building the hardware, it will likely be decades before we visit Pluto again.
Trajectory Options
There are any number of potential options available for realistically launching a usable payload to Pluto using currently available propulsion technology. The simplest trajectory option for reaching Pluto is a direct flight from Earth. And since the positions of the Earth and Pluto line up to allow favorable launch windows about once every year, it is the most frequently occurring option. While a direct flight from Earth to Pluto is technically feasible, it is among the least attractive options, however.
Direct flights to Pluto have large launch energy requirements and trip times of a decade or more. In fact, among the backup launch options considered for the current New Horizons mission was a direct trajectory to Pluto with the penalty of up to an extra five years of travel time to reach its target. As an added complication, since Pluto is moving farther from the Sun in its highly elliptical orbit since reaching perihelion in 1989 and will be moving farther out of the ecliptic plane after crossing it in 2018 as a result of its high orbital inclination, the launch energy requirements and travel times for the direct flight option will only continue to increase in the years to come making it a less favorable option as time goes on.
The Jupiter gravity assist (JGA) trajectory of the New Horizons mission to Pluto. (NASA/JHUAPL/SwRI)
One means of decreasing the launch energy requirements and trip times for a Pluto mission is to employ gravity assisted trajectories where one or more flybys of various planets are used to boost the velocity of the spacecraft. The simplest of these options, and the one employed by the current New Horizons mission, is a Jupiter gravity assist or JGA. But because of the slow motion of Jupiter around the Sun, the proper alignment between Jupiter and Pluto for the JGA option only occurs for a couple of years every 12.5 years. According to the Mission Design Center’s trajectory browser maintained by NASA Ames Research Center, there are very favorable launch windows for JGA trajectories to Pluto late next year and again in late 2016. Unfortunately, this is insufficient time to mount a new mission to Pluto.
The next favorable launch opportunity to Pluto using a JGA trajectory comes at the end of 2028 and the beginning of 2029. One of the better options currently listed in the trajectory browser calls for a launch on December 31, 2028 with a C3 launch energy of 120 km2/s2. Just to get a sense of the size of this launch energy requirement, this C3 is about an order of magnitude larger than is typically required for missions to Venus or Mars. Despite its large size, this is lower than the C3 of about 166 km2/s2 for New Horizons launched in January 2006 because the mission could not be ready in time for the more energetically favorable launch window in December 2004. As a result, even if a follow up mission to New Horizons used the same Atlas V launch vehicle and Star 48A kick stage, it could launch a heavier payload towards Pluto than the 478-kilogram New Horizons spacecraft if it were to use this trajectory option.
A diagram of a potential JGA trajectory for the next mission to Pluto. 1) Launch from Earth in Dec 2028, 2) Jupiter flyby in May 2030 and 3) Pluto encounter in Jan 2039. Click on image to enlarge (NASA/Ames Trajectory Browser)
After launch, a mission following this JGA trajectory would continue on to flyby Jupiter on May 11, 2030 with a closest approach altitude of 2.08 Jupiter-radii or just inside the orbital distance of Io. This would offer a better vantage point to observe Jupiter and its moons than was afforded by the more distant 32 Jupiter-radii encounter of New Horizons in 2007. The encounter with Pluto would take place about 8½ years after leaving Jupiter on January 28, 2039 at a relative velocity of 18 km/s (compared to New Horizons’ encounter velocity of 11 km/s) for a total trip time of 10.1 years (compared to New Horizons’ 9.5 year trip time). This encounter speed is far too fast for an orbit insertion to be practical using a chemical propulsion system. While there are options to decrease the encounter velocity, this trade comes at the cost of a significant increases in travel time (on the order of up to a decade or more). So unless there are some huge breakthroughs in the next decade or so in nuclear-based propulsion (coupled with an equally huge breakthrough in the levels of funding available for planetary missions), any follow on mission to New Horizons is probably going to be another flyby with a decade-long time of flight.
There are also other trajectory options to reach Pluto that could be available. There are Venus-Earth gravity assist (VEGA) options to decrease the launch energy requirements substantially for the JGA option (allowing a smaller launch vehicle to be used and/or an increase in the payload mass) but these options would either increase the already lengthy total trip time by a couple of years or more or increase the encounter speed with Pluto. There are also likely to be low-launch energy VEGA trajectory options that do not entail a JGA that occur more frequently and could be launched earlier in the 2020s but they remain to be worked out in detail. Energetically favorable options that would include a Saturn gravity assist, which occur less frequently than JGA opportunities, will not be available again until the early 2040s – too late for our purposes.
Given that it took over a decade and a half with several cycles of planning and proposing before a Pluto mission was finally approved and launched, it will likely be the late 2020s before a follow on mission is finally launched. While earlier launch dates are possible, I am assuming that a late-2028 launch date for a JGA trajectory is the more probable option.
Mission Justification
So why should we be thinking about a return to Pluto? Having been an observer of planetary missions for decades, it is a safe bet that for every question New Horizons answers, scientists will come up with more questions about Pluto and its family of moons that will need to be addressed by a new mission. But beyond this, there is the simple fact that Pluto will likely be a very different place in 2039 than the one New Horizons will observe in 2015. Pluto will have traveled through 41° of its orbit during this time resulting in a noticeable progression of its seasons. While New Horizons will arrive at Pluto in the late spring of the northern hemisphere, in 2039 it will be early summer and about 8 Earth-years after the sub-solar point had reached as high as 60° north latitude due to the 120° tilt of Pluto’s axis of rotation.
Comparison of maps of Pluto based on Hubble Space Telescope images acquired in 1994 (top) and 2003 (bottom). Click on image to enlarge, (NASA/ESA/M. Buie – SwRI)
In addition to this change of season, Pluto’s distance from the Sun will have increased from 32.8 AU to 38.8 AU during the intervening years resulting in a 28% decrease in the amount of heat the dwarf planet receives from the Sun. Because of this, it has been believed by many astronomers that Pluto’s atmosphere may begin to “collapse” or freeze out by 2020 forming fresh deposits of volatile ices on the unlit southern polar cap. Getting New Horizons to Pluto before this date was an important driver in the choice of trajectory for this mission.
New models of the seasonal evolution of Pluto’s atmosphere that agree better with measurements of how it has changed over the past quarter century suggest, however, that Pluto’s atmosphere will not collapse as predicted and might even thicken towards a maximum value around 2040 (near the time of this proposed Pluto follow on encounter) depending on the dwarf planet’s inventory of volatile ices and the properties of the atmosphere. No matter which of the models developed over the last 30 years proves to be correct, it is clear that Pluto’s atmosphere and surface deposits vary on times scales of decades as a result of the interplay of the seasons and the variations in its distance from the Sun. A new set of close up observations of Pluto almost a quarter of a century after New Horizons’ will provide vital clues about the seasonal changes on Pluto. And given the long orbital period of this distant world, this will be humanity’s only chance to observe Pluto during this crucial phase of the northern summer for another 248 years.
In addition to changes on Pluto that would be observed, a follow on mission to this distant world will also benefit from almost a quarter century’s worth of advances in technology and remote sensing techniques. The instrument payload of New Horizons, as impressive as it is, did not include everything scientists would have wished and was limited by the available payload capabilities of the launch vehicle chosen for a less-than-ideal launch window to Pluto. Advances in technology coupled with a more favorable launch window in 2028/29 should allow a more massive and much more capable set of miniaturized instruments to be carried by New Horizons’ successor. Advances in data storage and space communication technologies should also allow more data to be collected and transmitted back to Earth faster than is possible with New Horizons.
Diagram of the New Horizons spacecraft with its instruments labelled. A follow on mission to Pluto would benefit from over 20 years of advancements in technology and remote sensing techniques. (NASA/JHUAPL/SwRI)
Depending on the available payload margin and, just as importantly, the project’s budget, it may prove possible to carry several lightweight but very capable sub-probes with masses on the order of tens of kilograms (or less!) that the main spacecraft could deploy weeks or months before its 2039 Pluto encounter. These lightweight sub-probes could be directed to make observations of Pluto, Charon or its other moons at closer range or under different viewing conditions than might be possible with the main spacecraft. This tactic would add flexibility to mission planning as well as the quantity and quality of the scientific data returned. While a traditional entry probe would be of little use in the thin atmosphere of Pluto (which has an estimated surface pressure on the order of a few microbars), a properly equipped sub-probe could be aimed to fly maybe as close as tens of kilometers above Pluto’s surface to directly sample its atmosphere.
An important secondary objective of the New Horizons mission has been a close encounter with an object in the Kuiper Belt. While astronomers have finally identified a suitable target, this process took much longer than expected. Given the difficulties with locating such a target even after years of searching, more time and effort should have been allotted to this important task. With the added benefit of a quarter of century of extra time as well as New Horizons’ initial reconnaissance of any real hazards near Pluto, there is a much higher likelihood of identifying one or more potential targets even better than New Horizons’ among the outer planets or beyond in the Kuiper Belt that a new Pluto mission could study at close range adding to the science return of its mission.
With all these factors taken together, there would seem to be a justification for launching a second flyby mission to Pluto around 2028 with an encounter date of about 2039. Considering the time it took from proposing the first mission to Pluto to the time that mission is actually about to reach this distant world, now is a good time to start thinking seriously about a second mission to Pluto. Besides, on a purely personal note, I have been waiting over 44 years for an encounter with Pluto – ever since I found out it was proposed as a target for NASA’s original Grand Tour of the Outer Planets mission cancelled in 1972. I probably do not have another 44 years left to wait for another mission to Pluto and this proposal might represent the last chance to observe this distant world at close range again in my lifetime.
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Note
An updated version of this article which also examines options to add Uranus and Neptune as targets of a broader series of missions to the outer solar system can be found here:
“What about the next Pluto mission?”, The Space Review, Article #2787, July 13, 2015 [Article]
Related Reading
“A Io-Europa Sample Return Mission”, Drew Ex Machina, March 27, 2014 [Post]
General References
Robert Farquhar and S. Alan Stern, “ Pushing Back the Frontier: A Mission to the Pluto-Charon System”, The Planetary Report, Vol. 10, No. 4, pp. 18-23, July/August 1990
Yanping Guo and Robert W. Farquhar, “New Horizons Mission Design”, Space Science Reviews, Vol. 140, No. 1-4, pp. 49-74, 2009
C.B. Olkin et al. “Pluto’s Atmosphere Does Not Collapse”, arXiv:1310.6284, Submitted January 26, 2013 [Preprint]
Mission Design Center Trajectory Browser, NASA Ames Research Center [Web site]
New Horizons: NASA’s Pluto-Kuiper Belt Mission, Johns Hopkins University Applied Physics Laboratory [Web site]
I think it will be centuries before another Pluto mission. The reason is simply that the scientific justification isn’t strong enough. There are much more interesting cases for exploring Jupiters moons, the Saturn system and we have never orbited Uranus or Neptune. Studies of Plutos changing atmosphere probably will be done with the next generation of large telescopes (>20m) from earth, which are going to be available in the next decade or two. And finally, a flyby of Jupiter within the orbit of Io includes a high risk of radiation damage to the probe. Nevertheless, I am really looking forward to New Horizons Pluto flyby and the certainly great images and other scientific data it will send back.
Centuries?? We are a century away from the first winged flight. A century away from the discovery of, and practical application for, radio-waves. A century away from bringing the fledgling utility and concept of electricity into our cities.
I can say to with a level of certainty that hundreds of years will not pass before
humanity turns to take a follow up look at Pluto. If we are still in an era of US/NASA led exploratory chemical launches using gravity assist maneuvers from earth based launch-pads 300 years from now, I’d assume we entered another 275 year period of feudalism, and had just emerged from the second dark age.
If humanity doesn’t begin to appreciate the return on scientific investment within the next two centuries, and start spending more than 1% of GDP of exploratory sciences, we’re in real Jeopardy as a species. If we spent just 30 % of our military budget on Sciences over the past 2 decades, we could have afforded to fund, build and deploy a self-sustaining settlement on Mars the size of a small city. That’s not speculation, that’s an understatement.
“30 % of our military budget on Sciences over the past 2 decades,”. Hey about spending 30% of our non-military budget! I like space exploration but I also like security.
I am extremely excited about New Horizons, but if the choice afterwards is to return to Pluto or explore a different dwarf planet beyond Neptune, I’d choose the latter. I salivate at the possibility of exploring Haumea or Sedna before I die much more so than returning to Pluto again. Of course, in an ideal world where budgetary concerns aren’t an issue, we would do both.
Well, to be honest Pluto would be highly interesting but I think there should be more missions to Uranus and Neptune. These are giant planets that have been explored only once and for a few days. I would suggest first to send Cassini-type orbiters to these two planets to carry out an in-depth study of their atmosphere and rings, plus a detailed mapping and study of their moons. There are worlds like Miranda (Uranus) and especially Triton (Neptune) that are really interesting from the perspective of planetary geology. Even a lander for Triton would be awesome. I wonder if I will see something like this in the next 3-4 decades… and I am not very optimistic.
Drew, good article. Considering hardly anybody is currently advocating a Uranus or Neptune orbiter (which I think will have to happen some day), I have to wonder if anyone has even considered using Neptune to slow a probe approaching Pluto? The slowing would come from passing in front of Neptune in its orbit (gravity assist to reduce speed) and/or passing through some of Neptune’s outer atmosphere (aerobrake). Of course other planets could be used for Uranus and Neptune orbiters when alignment is right. Even if the reduction in speed is not enough to enter Pluto orbit, it would help prolong the encounter.
There are some groups studying Uranus and Neptune missions (the European ODINUS studies immediately come to mind) but more and more teams are talking about avoiding JGA mission options considered by several recent failed proposal attempts that would have had launch dates in the early 2020s and are looking at much later launch dates with target arrival times in the 2040s to even past mid-century.
As for your idea of using a Neptune encounter (e.g. a flyby possibly in combination with an aerobraking maneuver), my gut is telling me that there would be no advantage to the strategy over a slower and longer direct approach to Pluto with a couple of decade trip time and a chemical propulsion system. And I suspect that an ion propulsion system would be easier and quicker to develop than aerobraking technology (at least for the aggressive kind of aerobraking required for really large delta-vs that make a difference). Still, it would DEFINITELY be worth some detailed trade and trajectory studies to see if this kind of approach or some variation has some hidden advantage! 🙂
It is truly unfortunate the current administration does not share the enthusiasm for US manned or unmanned space exploration.
You need to get your facts straight: NASA’s budget during the first six fiscal years of the current administration (FY2010 to FY2015) is 10% HIGHER than its budget during the last six fiscal years of the previous administration ($107.5 billion vs $97.4 billion) despite the budget deal made with Congressional Republicans a couple of years ago to cut discretionary spending in order to stop a government shutdown. That being said, I feel that NASA’s budget should be MUCH larger than it is today. The lack of “enthusiasm for US manned or unmanned space exploration” is hardly unique to the current administration and is the fault of the leaders we have been electing and their policies going back decades.
The current administration killed at least for the foreseeable future, manned US missions back to the moon and beyond. Let alone reducing NASA to hitchhiking its way to the International Space Station. Kennedy inspired a nation to reach for the stars.
Once again, you need to get your facts straight: It was the previous administration with the consent of Congress that cancelled the Space Shuttle program before a replacement was ready to fly and then chronically underfunded the Constellation program to the point where the current administration was forced to cancel it before any more money was wasted on the program that was guaranteed to fail.
Just a reminder, the purpose of the comments on my web site is to discuss space-related topics. I do understand that occasionally that means talking about the effects of politics on space but purely political comments will be deleted. There are plenty of other places on the web to spout partisan political nonsense and would suggest that you take such discussions there.
I don’t see much use for another Pluto mission beyond the current New Horizons. As you point out Pluto’s atmosphere may add a certain amount of dynamic change and there are probably nitrogen geysers like on Triton but the amount of difference will likely be trivial.
I’d actually like to see another Uranus mission, maybe during the northern hemisphere solstice so that the hidden half of the moons could be imaged. (During the recent Uranus equinox would have been preferred but alas a missed opportunity)
When we return to Pluto it will be with an orbiter and a lander. But don’t hold your breath on that one. I only see a few opportunities for it to happen: Advanced propulsion system allows Pluto mission in a fraction of the time or China or India select it as a way to demonstrate their technical prowess.
Ceres the other dwarf planet to be visited in 2015 is a more likely candidate for revisit. I’m betting it will be more interesting (and possibly more water rich with more potential for life than Mars.