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Back to the Ice Giants?

As data return from New Horizons continues, we can hope that an encounter with a Kuiper Belt Object is still in its future. But such an encounter will, like the flyby of Pluto/Charon itself, be a fleeting event past an object at huge distance. Our next chance to study a KBO might take place a bit closer in, and perhaps we’ll be able to study it with the same intense focus that Dawn is now giving the dwarf planet Ceres. How about an orbiter around Neptune, whose moon Triton is thought by many to be a KBO captured by the ice giant long ago?

The thought is bubbling around some parts of NASA, and was voiced explicitly by the head of the agency’s planetary science division, Jim Green, at this week’s meeting of a working group devoted to missions to the outer planets. Stephen Clark tackles the story in Uranus, Neptune in NASA’s Sights for a New Robotic Mission, which recounts the basic issues now in play. What comes across more than anything else is the timescale involved in putting together flagship missions, multi-billion dollar efforts on the order of our Cassini Saturn orbiter.


Image: Neptune’s moon Triton, as seen by Voyager 2. Credit: NASA/JPL.

Right now Europa is the more immediate priority when it comes to outer planets work, and for good reason, since NASA has already approved a probe to the Jovian moon. Here we’re talking about 2022 as the earliest possible launch date for a spacecraft that would orbit Jupiter and perform repeated close flybys of Europa, a world we need to study close-up because of the evidence for a liquid water ocean beneath its crust and the possibility of life there. Whether such a probe actually flies as early as 2022 is problematic, and so is the launch vehicle, which in a perfect confluence of events could conceivably be NASA’s powerful Space Launch System.

I say ‘perfect’ confluence because the muscular SLS, if it lives up to expectations, would offer more robust mission options not just for Europa but for all the outer planets. Before any of that happens, of course, we have to build and fly SLS. While issues like that remain fluid, the current investigations into still later missions to Uranus and Neptune would seem to be premature, but they have to be, given not just the scientific but the bureaucratic issues involved. The JPL study on Uranus and Neptune ponders missions that could be launched probably in the 2030s, with the expectation of coming up with a mission design that could be used at both of the ice giants, although cheaper options will also be considered.

In any case, space missions begin with preliminary studies that launch a process feeding into the decadal surveys that set priorities for the next ten years of research. The last such survey, coming out of the National Research Council in 2011, gave particular weight to Mars sample return and the Europa probe. Getting an ice giant mission into the next decadal survey is no sure thing, given a strong case to be made for further investigation of Titan, and Clark notes that Venus is also likely to gain support.

So it’s early days indeed for Uranus and Neptune, but conceptual studies are a critical first step toward eventual approval. It’s daunting, and a bit humbling, to realize that if all goes well — if the bureaucratic gauntlet can be successfully run all the way through myriad technical studies and peer review into the budgeting phase and beyond — any mission to the ice giants will arrive half a century after Voyager 2 made our first and only encounters with these worlds. A return with orbiters would give us the opportunity to evaluate the major differences between the ice giants and the larger gas giants around which we’ve been able to conduct orbital operations.


Image: Voyager 2 image of the crescents of Neptune and Triton taken on its outbound path, about 3 days after closest approach. The picture is a composite of images taken from a distance of almost 5 million km as Voyager 2 flew southward at an angle of 48 degrees to the ecliptic after its encounter with Neptune, the final encounter of its journey through the solar system. Credit: NASA/JPL.

It’s often said that we want missions that can be flown within the lifetime of the people who designed them, which is an understandable though mistaken thought. If you were a planetary scientist, would you turn down the chance to work on a Uranus/Neptune mission design even if you were unlikely to see it arrive? Clark quotes William McKinnon (Washington University, St. Louis) on the ice giant probe concept: “An ice giant mission, presumably an orbiter, is, alas, over the horizon as far as my lifespan is concerned, so I salute those who will live to see it!”

Which is what we do, looking forward to the next generation if we can’t complete a task within our own. On the individual level we can say little about the details of our own mortality, so there is no guarantee of survival to mission completion even for relatively nearby targets. We keep building spacecraft anyway. In the case of the outer system, we take decades to design, approve, build and fly spacecraft not in the name of individual ego but of a shared humanity.

The interstellar effort places this principle in even starker terms, for in terms of the physics we understand today, missions to another star will be a matter of, at best, decades and perhaps centuries of flight time. We can hope that rather than turning away from this effort, we continue to probe it with better designs, continuing missions and a determination to keep exploring.


Comments on this entry are closed.

  • Andrew LePage August 27, 2015, 12:42

    Coming on the heels of Tuesday’s 26th anniversary of the Voyager 2 flyby of Neptune, follow up missions to Uranus and Neptune are long overdue. But the real problem will be whether or not these missions ever get approved and funded. The last 40+ years are filled with Uranus mission proposals that never flew including a study back in 1972-74 to adapt the Pioneer 10/11 spacecraft to carry atmospheric probes to Saturn and Uranus.


    A return to Uranus was even considered a fairly high priority in the last Decadal Survey but, while there were a couple of good ideas for return missions to be launched in the early 2020s, no missions were funded. Hopefully I’ll be around to see the next encounters with Uranus and Neptune in the 2040 time frame.

  • steven rappolee August 27, 2015, 12:55

    I hung out with the OPAG meeting online as of Tuesday so missed the JPL study announcement.The JPL study in progress would do well to consider the works of Okutsu Et Al who did several studies on the topic of Cassini Saturn/Titan escape orbits at end of mission
    This was to meet the planetary quarantine requirements but also to perhaps extend the mission
    The Okutsu study might inform the new JPL study in that such multi plant missions with SEP propulsion might allow a flagship mission to perform a Saturn(enceladus) and Uranus mission.
    Titan gravity assist missions have flight times of two decades to Uranus so we do need to study SEP for such a mission to shorten this
    My blog article tries to think up how a two planet mission could be done :)


  • Andrew Palfreyman August 27, 2015, 15:34

    I think it’s a waste of effort to go to Europa and NOT melt through the crust.

  • Brett August 27, 2015, 17:28

    It’s a huge disappointment that an Ice Giant mission is so far out that most of the current planetary scientists will be retired or dead by the time it happens. They can’t even work out some kind of Discovery-class mission to Uranus? Even just putting an orbiter out there with some good instruments would be incredibly useful (maybe they could do something similar to Dawn with an on-board RTG and ion thrusters).

    Ugh. We need orbiters for the Ice Giants, a floating probe for Titan, landers/orbiters for Enceladus and Europa, a balloon probe and surface lander for Venus, and a sample return mission from Mars. The sad thing is, all of this wouldn’t even be that horribly expensive over the next 10-15 years – I’d wager you could probably do it all for $20 billion spread out over multiple years.

  • Alex Tolley August 27, 2015, 20:41

    @Andrew Palfreyman
    Why? It would be very expensive to devise a lander with a method to melt down into the subsurface ocean, when a flight that [with luck] can sample a geyser would give us most of the path finding data we need to plan on a submersible. If geyser samples prove sterile, then an exploration of the ocean would have a lower priority. Better to spend the money on an Enceladus geyser sample mission.

    What I would like to see is a cheap cubesat mission, perhaps using a beamed solar sail, but certainly at least SEP, to do dedicated sample missions to the icy moons using a mass spec and spectrophotometer. I hope that next year’s Lightsail mission stimulates some thoughts in this direction. Could they also reach Triton and do useful path finding data collection there too? A solar sail could get the cubesat to an icy moon and put it in orbit. They are cheap enough to send swarms to allow for failures.

  • Navin Weeraratne August 27, 2015, 21:33

    @Tolley, I think a concern for cubesats is radiation shielding. The Lightsail test had problems with that, even in it’s low orbit. A later generation though of hardened (end perhaps even smaller) ones should follow. There’s a niche for them with asteroid prospecting, and as you’ve pointed out, cheaper probes.

  • Enzo August 27, 2015, 21:33

    According to this article :

    “Argo, the last proposed mission to Neptune, was grounded because NASA didn’t have enough plutonium to power all of its spacecraft, according to one of its designers.
    Candice Hansen of the Jet Propulsion Laboratory says there was a special launch window from 2015 to 2020 that would put Argo at Neptune in a decade thanks to gravity assists from Jupiter and Saturn. That timeline is now impossible to meet. ”

    For the record, Argo is a New Frontiers fly-by mission to Neptune and a KBO.
    There was a huge KBO choice as well as the deflection angle was much much bigger than New Horizon.
    With a flight time of only 8-11 years it could have been at Neptune
    2025-30 without SLS.

    Sounds like yet another victim of NASA’s Mars obsession. In this particular case wasting plutonium on Mars (Curiosity and MSL-2) where solar panels work well to the detriment of missions where plutonium is actually essential.

  • Joy August 28, 2015, 3:06

    How are we meant to puzzle out what is going on with extrasolar mini-Neptunes when we haven’t yet seriously studied the ice giants in our backyard? The ice giant orbiter missions are the obvious next steps in planetary science.

  • J. Jason Wentworth August 28, 2015, 4:01

    Even for exploring “just” the distant planets in our own solar system, we must either become immortal (or more nearly so), or somehow reduce the transit time, preferably while also reducing the mission cost. There are interstellar probe concepts that, in simplified and “de-rated” form, could likely achieve the latter:

    Robert Freitas’ 1 mm diameter x 50 mm long “Needle” starprobes are designed to be launched by an electromagnetic mass driver-type device at one-tenth of the speed of light, after which their nanotechnological payloads would arrange to create a communications downlink to our solar system, after arriving in the target star system and having access to its raw materials. That type of capability doesn’t exist yet, but non-reproducing probes of about this size (and larger) could be manufactured and launched on slower but still very quick (by *interplanetary* travel standards) trajectories, taking just a few months to perhaps half a decade or so to reach worlds in the outer solar system, in “streams” of the probes. If the probes could be mass-produced, especially by automated means, they should be quite cheap per unit.

    The probes could be equipped with instruments, imagers, and laser communicators, by which each probe in the stream would communicate its findings to the probe following it, in a daisy-chain of signal relaying that would ultimately reach back to a “light bucket” telescope in space near the Earth, or perhaps on the Moon. For redundancy, a “multi-strand” stream of probes could be launched; each set of probes would comprise (after launch) a circular or square planar array of probes in space, with the arrays spaced many tens of miles to perhaps thousands of miles apart along the trajectory. Their lasers could emit light through lenses that would create conical beams, so that following probes need not be directly behind those ahead of them in order to receive their laser messages. Also, the probes need not be needle-like in form, but could be made in whatever shape proved most efficient from the standpoints of minimum mass, launch acceleration “g”-resistance, and instrument packaging (short cylinders, discs, cubes, hexagonal prisms, etc.).

    Spinning the probes at launch would permit simple spin-scan imagers to be used, and the probes could be powered by a laser (which would operate at a different wavelength than the probes’ onboard communication lasers) that would be directed through the probe stream from (or near) the launching point near Earth. Or, the probes might utilize long-storage life batteries that would power their instruments only during the flybys. If deep space fields & particles data were desired enroute to the target world, onboard batteries that could be re-charged at intervals by a near-Earth power laser might be practical. The multi-strand probe stream would enable fast “continuous flyby” investigations of the target objects, and multiple probe streams on slightly different paths could fly close by satellites of the planets. The instruments could, if desired or necessary (due to instrument size, mass, power consumption, and/or desired spatial resolution factors), be distributed among the probes, instead of having every probe carry a full complement of instruments. In addition:

    The electromagnetic “Scissors” launch method (which is discussed on pages 147 and 148 of “The Starflight Handbook: A Pioneer’s Guide to Interstellar Travel,” by Eugene Mallove and Gregory Matloff) could also be useful for dispatching such fast interplanetary probes to their targets. Two counter-rotating, tension-stiffened “bolo”-type space tethers might be used to form the Scissors, which would “flick” the probes away like a cherry pit pressed between two fingers.

  • J. Jason Wentworth August 28, 2015, 6:50

    This article also underscores something which is sometimes overlooked even by some SETI enthusiasts, particularly some of the “raised-on-SF” younger ones who aren’t scientists:

    Even if electromagnetic (radio and/or laser) communication was established with another civilization, it would–unless it happened to be just a few light-years away–be a dialogue between societies, not between individual members of their respective societies. Until/unless faster-than-light (or *effectively* FTL) travel becomes a reality, this stark state of affairs will apply even more forcefully to all interstellar space flight, whether by starprobe or starship.

    As Arthur C. Clarke pointed out in “The Promise of Space,” the scholars who ask the questions (of extraterrestrials) via radio or laser links will never live to hear the answers, but they will still be better off than archaeologists, who can never query their long-dead subjects of study at all. But if the only signals we ever receive arrive from far enough away (in time as well as space), the SETI researchers will find themselves in the place of archaeologists, trying to decipher a message from a possibly long-dead race that their great-great-great-great-(perhaps for thousands of “greats”)-grandchildren couldn’t hope to contact, even if they still exist out there.

    This all assumes, of course, that there *is* someone else out there whom we can talk with. I’m all in favor of searching (including for artifacts and/or probes here in our solar system), but it is entirely possible that Earth is the only home of intelligent life–and perhaps even the only place where *any* life exists, period–in the universe. All of the three possibilities (we’re utterly alone, we’re the only intelligent life among other lowly forms of life elsewhere, or we have some or many intelligent “neighbors” somewhere out there) are sobering, and we have to be prepared to accept any of them (and at this stage, *all* of them). Who needs drugs, when just contemplating these things expands the mind at no charge, and with no pharmaceutical side effects? :-)

  • Michael Spencer August 28, 2015, 7:02

    Important to not put too much hope in SLS, which will be stunningly expensive to operate if indeed it ever flies for anything but (very) occasional HSF.

    Now Falcon Heavy, while not as capable in some regard, is a much better horse and will be available years sooner.

  • J. Jason Wentworth August 28, 2015, 8:29

    In reference to Andrew LePage’s fascinating posting (I’ve saved that URL–Thank You!) about the proposed-but-not-flown atmospheric probe-carrying Pioneer spacecraft (to be launched on Titan IIIEs) for Saturn and Uranus missions:

    Those “bad old days” (budget-wise) are a stark reminder of what mis-placed priorities can do to successful, lower-cost programs (the Shuttle ate their portion of the budget). NASA wouldn’t even approve a third Pioneer mission (Pioneer H, using an existing backup to Pioneer 11 that now hangs in the National Air and Space Museum) to be launched aboard an Atlas-Centaur with a solid upper stage, for an Ames-proposed out-of-ecliptic mission. What a waste of flight hardware! (The Soviets were incredulous when told that NASA wouldn’t launch the fully-equipped second Skylab that it had paid so much to procure; it too graces that museum…) Oh well–at least Pioneer 11 did fly well above the ecliptic for several years, returning data on that region of the Sun’s sphere of influence while on its way to its 1979 Saturn flyby.

  • Andrew Palfreyman August 28, 2015, 15:47

    There are technical barriers to building decent starships right now. In any case, there’s no point launching if you don’t know that where you’re headed is likely to include some payback. Therefore telescopes.

    The superior solution is the gravscope at 550+ AU. That’s 10 times further out than Pluto. To make that feasible, we need fast and efficient interplanetary travel. Therefore beamers before telescopes.

    The chief requirement for affordable deployment of a beamer web is cheap launches. Therefore StarTram and Skylon before beamers.

    Once launch, beams and gravscopes are in place, we can figure out where to point the first starship. It will use a combination of beamers and fusion, and magsail for deceleration and shielding.

    If a successful destination is located, this is likely to be between 15-30 LY distant. For this we will need stepping stone infrastructure to cement the bond in both directions. There will be many dark planet-sized bodies between here and there. At a number of these intermediate locations we will install fusion reactors and bidirectional beamer arrays.

    Lather, rinse, repeat, colonise.

  • ljk August 28, 2015, 16:03

    Don’t forget the three whole Saturn V rockets also gracing various museums instead of being at the bottom of the ocean floor in pieces after having sent three more Apollo missions to the Moon.

    Perhaps in an alternate reality.

  • lepton August 28, 2015, 16:05

    Wentworth, I never know that aspect of Pioneer 11’s trajectory, it’s AWESOME!

  • J. Jason Wentworth August 29, 2015, 6:55

    lepton wrote:

    “Wentworth, I never know that aspect of Pioneer 11’s trajectory, it’s AWESOME!”

    Thank you. Pioneer 10 and 11 each also carried an asteroid/meteoroid instrument that consisted of four non-imaging telescopes, which collected optical parameter data on such bodies. If either or both probes could have been targeted to fly by main belt asteroids enroute to Jupiter (as Galileo later did), dedicated missions to asteroids might have come about sooner. Also:

    Until the Ulysses solar polar orbiting mission (which was–also thanks to the Shuttle’s perennial cost overruns–a scaled-back version of the original two-spacecraft, joint ESA/NASA Solar Polar mission, which would have made simultaneous observations of both of the Sun’s poles), Pioneer 11 provided the only data from far outside (north of) the ecliptic. In addition:

    The Pioneer H mission (which the NASA Ames Research Center wanted to use the third, spare spacecraft for), had it flown, would have given us a third Jupiter flyby (with global Jupiter coverage, Jovian satellite observations, *and* close-up observations of either the north or south polar region of Jupiter), in addition to new data from north and south of both the ecliptic and the Sun, in a post-flyby high-inclination solar orbit having a period of five years. (That five-year figure is from the late 1960s Pioneer Jupiter mission study, which is mentioned in Arthur C. Clarke’s book “The Promise of Space.”) Plus, the “stock” (un-modified) outer planet Pioneer spacecraft could have flown yet another type of mission with the help of Jupiter’s gravity, as that study indicated:

    A third option covered in the study–besides the solar system escape and high-inclination solar orbit flight paths–would have used Jupiter’s gravity to cancel out the probe’s velocity so that it would fall straight into the Sun, reaching it (or perhaps whipping past it, if it could be so targeted) about sixteen months after the Jupiter flyby. (I’m admittedly speculating here, but with judicious launch timing, perhaps a Uranus, Neptune, or Pluto flyby could have been worked into the flight plan, if the probe could have performed a close solar flyby instead of falling into the Sun). But even if those things weren’t do-able, the probe could have examined a cross-section of the solar system from Jupiter into the Sun (as well as observing Jupiter and its satellites up close), and at a high rate of data transmission (as Clarke pointed out) due to the ever-decreasing distance as the spacecraft fell sunward. Now:

    When I contemplate all of the important and exciting science that could have been done decades ago with these inexpensive and reliable Pioneer spacecraft, it infuriates me to think that all of these mission opportunities were squandered because that brick-winged monstrosity soaked up so much of NASA’s budget while never even coming close to delivering on the promises of cheaper and more frequent space flights that were made regarding it. Now I understand why the planetary scientists despised the Space Shuttle so much (in the then-U.S.S.R., Dr. Roald Sagdeev, the director of the Soviet space probe effort, said of his nation’s Buran shuttle’s only flight: “It went up and it came down. But it did nothing of scientific value.”)

  • Ashley Baldwin August 31, 2015, 16:13

    Given that “mini Neptunes” and the terrestrial/ice giant divide is one of the big unanswered questions to emerge from burgeoning exoplanet science to say nothing of the fact that such planets seem to be the most common , there has to be some sort of return to either Uranus or Neptune. Ugly reason or not , there just isn’t any money for flagship missions which is what any ice giant orbiter would be , and that before Europa ( and Enceladus ?) are probably higher up the priority list . So that leaves another “New Frontiers/New Horizons” flyby with at least 21st century technology to improve upon Voyager and aiming for a slower pass of Uranus as the nearer target. Come Decdel 2022 the big push will be for a big 2030s Hubble 2 with the 2020s flagship money spent already on WFIRST and possibly Europa Clipper. A lot of time has passed since Voyager so a fly by in the 2030s should bring far more data than with 1970s technology and we really do need some sort of science return from an ice giant. Still plenty of time to work on better launchers till then too.

  • Andrew LePage September 2, 2015, 12:29

    @Ashley Baldwin August 31, 2015 at 16:13

    I recently outlined a possible relatively inexpensive “fast flyby” mission option as part of a larger outer planet exploration for launch in the 2028 to 2032 time frame with encounter dates around 2040:


    While I would love to see flagship-class orbiter missions to Uranus, Neptune and Pluto, I share your assessment that the prospects of actually getting such expensive missions off the ground anytime soon do not look good. Instead of waiting decades more to return to the ice giants (and Pluto!), I figured an inexpensive flyby option using 21st century technology (and subsequent encounters with KBOs) might be the best way of getting at least some new data within my lifetime of these interesting worlds.