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.

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