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When Will We See an Ice Giant Orbiter?

With NASA announcing that its Discovery program would fund both Davinci and Veritas, two missions to Venus, it’s worth pausing to consider where we are in the realm of Solar System exploration. This is not to knock the Venus decisions; this is a target that has been neglected compared to, obviously, Mars, and we’ve kept it on the back burner while exploring Jupiter, Saturn and, with a fast flyby, Pluto/Charon. With budgets always tight, the axe must fall, and fall it has on the promising Trident.

Discovery-class involves small-scale missions that cost less than $500 million to develop. The Trident mission would have delivered imagery from Triton that upgraded the 1989 images from Voyager 2, useful indeed given the moon’s active surface, and we might have learned about the presence of a subsurface ocean. I should also mention that we lost IVO when the four candidate missions were pared down to two. IVO (Io Volcano Observer) had a strong case of its own, with close flybys of the tortured geology on the most volcanically active body in the Solar System.

So on to Venus, but let’s consider how the next few decades are shaping up. We have flown orbital missions to every planet in the Solar System other than the two ice giants, and it’s worth considering how many questions about those worlds were suggested by the Voyager 2 flybys of Uranus and Neptune. Imagine if all we had of Saturn were flyby images, conceivably missing the active plume activity on Enceladus. What kind of startling data might an ice giant orbiter return that Voyager 2 didn’t see in its brief encounters?

The ice giants are a class of planet that, as the 2013 Planetary Science Decadal Survey stated “are… one of the great remaining unknowns in the solar system, the only class of planet that has never been explored in detail.” A Uranus Orbiter and Probe was, in fact, the third-highest priority large-class mission named by the report, but it’s clear that we won’t have such a mission in time for the 2030-2034 launch window needed (more on this in a moment). Despite that, let’s switch the focus to Uranus because of a short report from the 2020 Lunar and Planetary Science Conference that Ashley Baldwin forwarded.

There are all kinds of reasons why Uranus makes an interesting target. In addition to its status as an ice giant, Uranus has both a ring system and unusual moons, with five major satellites that may be ocean worlds and in any case show dramatic surface features. The seventh planet also sports a major tilt in both rotational and magnetic axes, and a wind circulation structure that is little understood. In the absence of a major orbiter mission, the brief paper Ashley sent examines the issues involved in sending a much smaller New Frontiers class orbiter with faster turnaround.

Image: Uranus’ moon Miranda sports one of the strangest and most varied landscapes among extraterrestrial bodies, including three large features known as “coronae,” which are unique among known objects in our solar system. They are lightly cratered collections of ridges and valleys, separated from the more heavily cratered (and presumably older) terrain by sharp boundaries like mismatched patches on a moth-eaten coat. Miranda’s giant fault canyons are as much as 12 times as deep as the Grand Canyon. This image was acquired by Voyager 2 on Jan. 24, 1986, around its close approach to the Uranian moon. Credit: JPL.

Back to that launch window I mentioned earlier. The 2030-2034 timeframe for Uranus would allow the needed Jupiter gravity assist that would get the payload to target before it reaches equinox in 2049. This is an important point: We’d like to see the northern hemispheres of the satellites — Voyager 2 could not see these — and after equinox they will once again become dark. A New Frontiers-class orbiter might just make the deadline, but it’s hard to see such a mission being funded in time. NASA now says the next opportunity to propose for the fifth round of New Frontiers missions will be no later than the fall of 2024.

New Horizons is a New Frontiers-class mission, as is OSIRIS-REx and Juno, all the subject of competitive selection through the program, which focuses on medium-scale missions that cost less than $850 million to develop. Within that cost envelope, a Uranus orbiter is a tricky proposition. The total mission duration cited in the paper is fourteen years because of the flight design life of the needed Multi-Mission Radioisotope Thermoelectric Generators (MMRTGs). Thus the baseline is a two year mission in orbit at Uranus with mapping of the entire system, all completed by Uranus spring equinox in 2049, “enabling different illuminations of the satellites and seasonal orientation of the planet and magnetosphere than observed by Voyager 2.”

Other issues: How to achieve orbital insertion at Uranus? Aerocapture seems a reasonable possibility and would have to be considered. The paper cites a 60-kg payload including five instruments along with radio science capabilities, and goes on to note that power is the most limiting constraint on a mission like this under New Frontiers cost limits. Here’s what the paper says about the power question:

…addressing power within cost is the primary obstacle to the feasibility of a NF Uranus orbiter mission. Previous Ice Giant mission studies have resulted in architectures requiring >350 W-e end-of-life power, which requires six MMRTGs. Owing to the relative inefficiency and significant cost of MMRTGs, any design should attempt to reduce the needed end-of-life power; this will have significant impact on both the spacecraft and orbit design as well as the communication subsystem and payload.

And of course we have this:

Other design considerations that place significant constraints on the feasibility of a NF Uranus orbiter include deep-space communications (specifically the power required for downlink) and radiation shielding mass.

Not an easy task. But this is what we face as we look beyond the current selections in the Discovery program. We’d all like to see an orbiter around both ice giants, but given the realities of time and budget, the likelihood of getting one around either before mid-century is slim. Eventually it will get done, and new technologies will make for a more efficient design and a more comprehensive mission. Sadly, the timeframe for seeing all this happen stretches a long way ahead.

Many of us find this frustrating. But the overview is that the exploration of the Solar System and the push beyond is a civilizational project that dwarfs human lifetimes. The things we can accomplish today build the basis for projects our children will complete. We push the limits of what we have, drive technology forward, and refuse to stop trying.

The paper is Cohen et al., “New Frontiers-class Uranus Orbiter: A Case For Exploring The Feasibility of Achieving Multidisciplinary Science With a Mid-scale Mission,” 51st Lunar and Planetary Science Conference (2020). Full text.

{ 27 comments… add one }
  • James Early June 8, 2021, 12:43

    For real exploration of the ice giants and their moons we need a nuclear electric power supply. There is simply not enough Pu235 to supply multiple missions nor high power missions. A 10kw power supply would also provide adequate ion propulsion similar to the Dawn spacecraft which could allow orbit modifications to explore the moons.
    The Spacex Starship will likely be operational before NASA deploys this power supply. The Starship would be low cost and would have the payload capacity for a significant upper stage booster to shorten the mission time.

    • George King June 10, 2021, 16:15

      Are you referring to Plutonium-238 rather than Plutonium-235 for RTGs? Or to something different from RTGs? Pu235 perhaps has a prohibitively short half-life at least for that application.


      Over in Europe, they seem to be making progress with Americium-241 RTGs and RHUs, with Am241 having less power per unit of mass than Pu238 but potentially for a longer duration, subject to how long the thermocouples last for the RTGs.

      Progress at least as per this December 2019 article, which stated hopefully not too optimistically then that “a series of projects have advanced the development of heat sources and heat-to-electricity conversion technologies that could enable a range of ESA missions in the latter half of the 2020s and into the 2030s.”


      We’ll see anyway, and I have a fairly hard time, as a layperson in this field, finding current updates on the ESA’s specific progress on this. The Wikipedia article on RTGs is a bit out of date in regard to Am241.

      In all events, it would be nice to have an alternative — albeit one vis-a-vis Am241 with different power level and duration parameters, for worse and better — to the limitations imposed by Pu238 availability and access for deep space missions.

  • Andy Prada June 8, 2021, 13:47

    What a wonderful legacy for those despised tech multi billionaires if they could see to sponsoring such missions as proposed. What else is all that money going to do?

    • Patient Observer June 8, 2021, 20:56

      Being narcissists, tech billionaires use their money to make more money and with mass adulation a goal for some . I do not wish to divert from the topic but Musk is far less of a visionary than his legions of fan boys want to believe. He is a sociopath, a liar and a thief of ideas. Supporting information is easy enough to find. Don’t count on altruism from that bunch.

      Back to topic. I hold hope for the “TEM” nuclear rocked under development in Russia. It’s purpose includes solar system exploration. If I did the math correctly and assuming a 50% fuel mass, it has a ∆V of 125,000 mph. I have no idea what would be left for payload.

      To put this speed in perspective, New Horizons had a total ∆V of 37,000 mph. The TEM could accelerate to 60,000 mph, coast for while an then decelerate for orbital insertion at with one of the outer planets. Or, conceivably it could travel to several different planets analogous to Dawn’s multi asteroid sojourn . The electrical output is reputed to be 1 megawatt. After reaching its destination, a somewhat lower power output would be ample for instrumentation and communications.

      Or, the rocked could just head off to interstellar space. Assuming its 55 kms is reduced by 16 kms to achieve solar system escape velocity, it would cover about 8 AU/year. Not too shabby.

      A lot of potential for the TEM not to happen but if it does, deep space exploration will never be the same.

      • tesh June 9, 2021, 6:56

        Are non-tech billionaires nicer?

        Back on-topic, it would be great if such a rocket were to be developed. Sadly, for me, I am unlikely to see it become a reality…

        • Patient Observer June 9, 2021, 15:51

          Tech billionaires may possibly be less experienced in life, more narcissistic and more likely to develop a messianic personality (looking at Musk).

          TEM is within Russia’s technical capability but it may not be highly placed on the priority list. Hoping for the best.

      • Andrei June 9, 2021, 19:47

        The TEM? Oh it’s under construction, images can be seen on Russian space web. http://www.russianspaceweb.com/tem.html
        TEM is supposedly civilian, but there’s a twin project named Ekipazh (waggon) which is supposed to have quite the same role, with the Russian military as main contractor. The latter suggest that the military intend to once again use nuclear powered surveillance satellites. No details are of course known of the latter project.

        • Patient Observer June 10, 2021, 7:16

          All big government technology projects have a dual role it seems. The Space Shuttle had an important military role and, of course, all early satellite launchers started life as ICBMs. This raises the likelihood of TEM becoming real. If so, it and its successors could revolution interplanetary transportation/exploration.

          • Gary Wilson June 10, 2021, 12:42

            There are some interesting web sites which give some details about the Transport and Energy Module out there. Assuming the technical problems are able to be overcome it looks like a very capable method of travelling in space. I wonder if there is a black budget already assigned to it in the US? We may see a competition arise very soon because as others have pointed out there are definitely military applications sad to say.

      • AlexTru June 11, 2021, 3:45

        I am sure we will not see anything new dedicated to space travel developed in Russia, Space ages are passed for this country, almost no knowledge left from Gagarin Era. So TEM – it is Sci-Fi tales…

  • torque_xtr June 8, 2021, 18:45

    The selection of Venus missions made me musing about supra-generational endeavors in the world of today. The new data from the surface of Venus, our closest neighbour planet, is almost one-in-a-lifetime event. All the people who actively worked on the previous batch of data from Venus landers would be at best in their 70s by the time next data arrives. Even their children probably will not resume these studies because they already have started different careers long before another mission flies. Even harder with the ice giants…

    Yet, still we go!

    Meanwhile I wonder what can be done with radar observations of outer Solar System, using newly installed radar transmitters and planet-wide interferometers as recievers. It was said that this allows for unlimited signal round-trip times, and for observations as far as Neptune. And the demo radar image of Apollo 15 landing site is really fascinating. It’s resolution is ten times better than Hubble’s and translates to some tens of km at the distance of the ice giants. But I wonder, could the resolution of radar astronomy go all the way up to the Event Horizon Telescope capabilities if a millimeter wave transmitter is used? The resolution of the image of M87* shadow is equivalent to a single kilometer in the far reaches of Kuiper Belt. This means almost all known bodies of Solar system in great detail. In addition, it may be possible that probing of ice and gas giant interiors is achievable to some good depth. I guess it would still cost less than a flagship-class space mission, a dedicated transmitter that sends unlimited radiowave “probes” which fly at lightspeed, more than ten thousand times faster than all these slingshot-boosted spacecraft!

    *Not counting the possibility that some private company would say “we do not want to wait”, as it’s already mentioned in the comments :-) And launch some ridiculously big rocket with their own orbiter, maybe not so well equipped and tested than a NASA one, but still much better than nothing.

  • Michael Fidler June 8, 2021, 22:17

    We should be considering launch rockets capability when this missions would fly, specifically Starship. Since the military is seriously looking at this rocket system for quick transport of large cargoes around the earth. Could this cheap rocket be used both for a far faster transport to the outer planets without the need for multi planet flybys? The weight of the spacecraft could also be much higher so the high tech and expensive miniaturization would not be required plus large laser communication could be used. The use of a larger nuclear fuel supply could be increased for power supply.
    One interesting possibility is the spacecraft becoming a long term outpost like the Voyagers. With large scopes not just to monitor Neptune and its satellites but also the Centaur’s and Kepler belt asteroid/comets. Could open up a lot of possibilities.

    • Patient Observer June 9, 2021, 15:55

      At the risk of gaining a reputation for a naysayer, Starship is essentially a hoax and NASA’s selection of a lunar version for Artemis is an abomination driven by politics and grandstanding. Plenty available on the internet to this effect.

      • Gary Wilson June 11, 2021, 14:41

        Why do you think it’s a hoax Patient Observer? The test flights of the Starship seem real enough. It’s a big step to mount it on the Ultra Heavy booster and then start by putting it in orbit, but they seem to be moving toward that goal as can be seen by numerous overflights of the launch complex using drones.

        • Gary Wilson June 15, 2021, 14:21

          For anyone wanting to see the progress of Starship there are good resources readily available. Marcus House does a very good job of describing what is happening with Spacex as well as other private space development companies (Blue Horizon for example). I’m not thrilled about the massive income inequality now so widespread either just to clarify that. It took a lot of corruption by various groups including politicians to “modify” the tax systems to favour the wealthy. That will only change with appropriate legislation in various countries and that will require the public to become aware of it and press for change by using the ballot box and right to assembly and protest.

  • Martin June 8, 2021, 22:46

    “When will we see an Ice Giant orbiter?”
    Sadly, not in my lifetime.
    Although you could say that about about a probe to Alpha Centauri, but for very different reasons.
    That’s what hurts.

  • P June 9, 2021, 0:31

    Thanks for the link, but I’ve seen so many of these proposals that have come to naught over the years I’m starting to think I won’t see this in my lifetime. The only thing that gives me a bit of hope is the recent Venus mission announcements, but look how long that took!

    Either way, Uranus first please, although a twin mission would be wonderful.


  • Jeff Wright June 9, 2021, 4:51

    This is why I still think SLS is worth its cost. You can’t beat hydrogen…Cassini rode harsh solids too…and the new Terra V…I mean Terran V won’t be enough. ACES should go with SLS for Vulcan is a joke.
    With ice everywhere..it is hydrolox and NTR infrastructure for NASA and the outer planets and Musk for Mars. Both/And!

  • ljk June 9, 2021, 12:20

    I know a method of rocket propulsion that has been available since the 1950s that would work today and get both instrumented probes and human crews into the outer Sol system within one year after launch…


    Guess which nation has the best chance of using Orion for just such purposes?

  • Ian Kubath June 9, 2021, 14:37

    If we wanted to get some tech billionaires interested in an ice giant orbiter, maybe Charles Stross idea can open up their purse strings!


  • Wojciech Jacyk June 9, 2021, 17:29

    It is worth mentioning that China is considering a interstellar heliosphere probe that would carry out a flyby of Neptune in 2038 and possibly carry a small atmospheric probe.


    • Paul Gilster June 9, 2021, 19:46

      Worth mentioning indeed. I didn’t know this.

    • NS June 10, 2021, 3:23

      Unfortunately according to link the Chinese missions (2 probes are planned) haven’t been approved yet, and are proposed to launch in 2024, which seems like an awfully tight deadline. Speculating but they sound similar to the Innovative Interstellar Explorer mission, which IIRC would be constrained to launch windows 12 years apart because of the need for a Jupiter fly-by. 2036 seems a lot more reasonable.

      • Andrei June 11, 2021, 11:21

        Yes, unless some people are bolting the instruments together right now – awaiting approval. It seem unlikely they will be able to meet that date. Then again, the Chinese have had an unusual way of doing things, calling in scientists from all faculties and universities to solve certain problems in the development of the hardware. This have accelerated development in a way which would make both ESA and NASA green in envy.
        And with an excellent education system, they have a lot of people to call on, so what’s holding the Chinese space program back is not the know-how or capability to build the hardware, but having to do things on a comparably small budget.
        The limited budget is part of the reason the Chinese put all eggs in the same basket with Tianwen / Zhurong.
        If this success will open the wallets from the party leadership is of course unknown, my own estimate is that it seem likely – for the propaganda value – meaning that we’ll indeed see more first forays by China to destinations that so far only been visited by USA in the decades to come.

  • Gary Wilson June 10, 2021, 13:45

    I was wondering if you would be interested in doing an article on nuclear electric propulsion Paul or has one been done previously? It seems like a very realistic intermediate propulsion technology for the near future to get fairly large probes to the outer solar system.

    • Paul Gilster June 10, 2021, 19:49

      Yes, planned for the near future.

  • Gary Wilson June 11, 2021, 14:56

    As another aside it seems that SLS may be subject to yet more delays as Congress is seeking a main propulsion test article for the system. This seems to be a way to keep work happening at Stennis where the main stage core engines are tested. These tests are now over but the Congress person for that district wants further tests to be done even though NASA says it has more than enough main engine test data now. And NASA wonders we people suspect that SLS is mainly a jobs program for various states.

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