The solar system contains 8 planets, each possessing incredible uniqueness. Of these, some stand out as being perhaps the least explored. Every planet has had at least one orbiter take incredible amounts of in situ observations for years on end if not decades, except the Ice Giants. Every planet has been visited by spacecraft multiple times, except the Ice Giants. Every planet has been widely mapped and characterized from orbit, except the Ice Giants. The two ice giant planets, Uranus and Neptune, have each been visited but once (and by the same spacecraft at that). Voyager 2, launched in August 1977, flew by Uranus in January 1986 and Neptune in August 1989. It redefined our understanding of both planets for sure, but a single flyby using mid-70’s technology can only tell us so much. Saturn was flown-by on three separate occasions between 1979 and 1981, but not long after that final flyby work began on the Cassini-Huygens orbiter mission. For true in-depth and long-term reconnaissance of the whole system of a planet, its rings, and its moons, a long-lived orbiter is required. These have been proposed many times over the decades, as well as many flyby proposals, but historically there has been little serious interest in bringing these to life. Historically, however, does not always mean now.
To understand the later and current efforts to explore the ice giants, it will be helpful to understand where we are now in our knowledge of them, which means reviewing what Voyager 2 did for our understanding of them. Around Uranus, it discovered no less than 10 totally new moons and two new rings. It provided the first detailed information about the known large moons Miranda, Ariel, Umbriel, Titania, and Oberon, with later reanalysis adding more evidence to the theory that most could potentially be either former or current ocean worlds. Regarding the planet itself, Voyager discovered winds of up to 450 miles per hour (724 kilometers per hour), a hot water ocean almost 500 miles (800 km) below the cloud tops, and a magnetic field more powerful that Saturn’s that was both offset from the planet’s center and tilted more than 55 degrees from its axis of rotation. At Neptune it discovered 6 new moons and four new rings, alongside the cryovolcanic nature of Triton. Triton was discovered to also be the coldest body in the solar system that had been studied up to that point. Neptune’s winds were measured at up to 680 miles per hour (1,100 kilometers per hour), with three large atmospheric features seen. These features, the Great Dark Spot, Dark Spot 2, and The Scooter all moved around the planet from west to east, with Scooter traveling the fastest, thus its name. The Great Dark Spot was the large dark oval most people think of when they think about the planet, though it turned out to be much less of a permanent feature than Jupiter’s Great Red Spot. When follow-up observations were made by the Hubble Space Telescope in 1994, no trace of it or Dark Spot 2 could be found aside from some persistent water clouds. A northern hemisphere spot was found (named NDS-1994, for Northern Dark Spot-1994) but between 1998 and 2000 also dissipated. Many spots of all sizes have been seen at various times up to the present day, with some even exhibiting significant movement north or south. The Scooter was just a grouping of white clouds that moved faster than the dark spots and has been discovered to neither be the fastest such group nor particularly unique in the years since.
After a lull lasting many years, the 2011 Planetary Science Decadal Survey included “Uranus Orbiter and Probe” (or UOP) as its 3rd place recommendation for a new flagship mission, behind a descoped Mars Astrobiology Explorer-Cacher (MAX-C) rover which would become Mars 2020 and a descoped Jupiter Europa Orbiter (JEO) which would become Europa Clipper. In the end, the 2011 version of UOP would not end up being pursued by NASA, but the planetary science community did not forget Uranus. In mid-2019 planning for the next planetary science decadal survey began, leading to a new round of mission concept studies. Out of these studies came four ice giant missions: “Uranus Orbiter and Probe”, “Calypso: Uranus Moon and KBO Flyby”, “Odyssey: Neptune Orbiter and Probe”, and “Triton Ocean Worlds Surveyor”. This version of UOP was in the end selected within the decadal report as the #1 new flagship recommendation (though after Mars Sample Return in overall priority) for development in the decade 2023-2032. The 2021 iteration of Uranus Orbiter and Probe (UOP) was designed around the 2021 Planetary Science Decadal Survey. The decadal defined 12 Priority Science Questions, organized into three themes: Origins, Worlds and Processes, and Life and Habitability. UOP either fully or partially addresses all but one of the questions, with most of the lack of coverage because those questions relate to earth, life detection, or solid bodies. In the context of UOP, the themes take on more specific meanings. For example, Origins means investigation of the formation and composition of the Uranian system and what migratory path it has taken throughout the solar system since its formation. Processes looks at changes over much shorter time scales, like heat and energy transfer within or between bodies, how all the various components of the system interact, what external influences are affecting the system, and especially what is responsible for Uranus’s magnetosphere. Habitability is perhaps the simplest theme in this context, focusing on whether any Uranian moons were or even still are ocean worlds today. These questions and themes have far reaching implications, especially to exoplanetary systems, in which ice giant-mass planets are abundant. UOP is also a demonstration of the resurgence of atmospheric probes as an avenue of their own and a core part of a mission’s science return as opposed to just an interesting sideshow or experimental sub-mission. Probes provide something which is very rare in spaceflight: in-situ measurements of almost impossible to explore areas. Without in-situ data, remote sensing technology would be needed to try to observe proxies for the actual target measurements, requiring a great deal of effort to try and extract what the real measurements most likely are after the fact. Probe data can anchor and validate remote sensing information, increasing certainty and applicability of that data. Even among previous entry probe-carrying missions, such as Galileo and Cassini, UOP is unique due to the probe’s innovative concept of operations. The probe is deployed after the spacecraft is already orbiting the target (Uranus in this case), allowing for backup probe deployment opportunities and the spacing out of critical events.
Just as important as science is power, and like most outer planets missions UOP needs RTGs to function. Large solar arrays can work in the outer solar system, but only out to around the distance of Jupiter before the area needed to produce usable amounts of power becomes unrealistically impractical, requiring the use of nuclear power. UOP will require three Mod 1 Next-Generation RTG (NGRTG) units. NGRTG is derived from the 80s-era GPHS-RTG. GPHS is “General-Purpose Heat Source”, which refers to the plutonium and graphite blocks that provide the heat needed to produce electricity. GPHS-RTG units powered Galileo, Ulysses, and Cassini, and a spare unit powers New Horizons today. GPHS-RTG has been out of production for decades in favor of the smaller and lower-power Multi-Mission RTG (used on Curiosity, Perseverance, and Dragonfly) in part because the thermocouples used to convert the GPHS modules’ heat into electricity are no longer made. NGRTG Mod 1 is effectively a “production restart” of GPHS-RTG, putting the thermocouple materials back into production and achieving as close to the same specs as GPHS-RTG as possible with minimal design changes. Mod 2, a future variant, aims to drastically improve the thermocouples and potentially other design elements. The primary issue with this project, apart from the normal aerospace schedule tendencies, is the low supply of Plutonium-238 to fuel the RTGs and insufficient production rate. Back in the 1960s and 1970s, plutonium to fuel RTGs was taken from the Savannah River Site, the facility tasked with creating the plutonium and tritium for nuclear weapons. After the site’s plutonium production was shut down in 1988, the only source for RTG fuel has been plutonium purchased from Russia. However, Russia also stopped production, and their supplies were being depleted as well. Beginning in 2013, Oak Ridge National Laboratory began producing Pu-238 for RTGs, with the goal of reaching 1.5kg (or 10 fuel clads) produced per year by 2025. A “clad” is a Pu-238 pellet coated in iridium shielding, two of which are needed per GPHS module. A single Mod 1 NGRTG requires 16 GPHS modules (64 clads). At the CRP (Constant Rate Production) goal of 10 clads per year, it would take over 6 years of full production to produce enough GPHS modules for only UOP. A recent estimate by NASA’s Radioisotope Power Systems program put the earliest UOP launch date in the mid-2030s, based on Pu-238 supply. UOP is not the only plutonium user either, meaning that its need for essentially the entire civil plutonium reserve for the next 6+ years pushes any future RTG missions into the next decade at the earliest.
Due to the fact that the orbiter must also serve as a relay for the probe during its descent into Uranus, the communications system is more complicated than for most missions. There are five main antennas on the orbiter, including three Low-gain Antennas (LGAs), two fanbeam antennas, and one large High-gain antenna (HGA). The HGA will be used for most communications with earth, including all science downlinks. One LGA is dedicated to the radio link with the probe during descent, while the fanbeam antennas are located to provide a low-rate link back to earth at all times during Uranus Orbit Insertion. Unlike most previous outer planet missions, UOP will use Ka-band for its primary science downlink. Ka-band is around 4 times higher frequency than X-band, the frequency band usually used for science return, allowing for much higher data rates. Ka-band has not been historically used due to it having an extremely small beamwidth (requiring very precise pointing throughout the entire mission life) and there being no incentive to not use X-band outside of the higher data rate, which many missions simply dealt with not having. In recent years, however, the Deep Space Network has been getting more and more congested, especially with high-volume missions like JWST and Artemis coming online. In addition to expanding network capacity, the DSN has requested that as many new missions as possible move to Ka-band downlink to reduce the amount of time needed on the DSN and free it up for other missions to use. X-band is still generally used across the board for command and low-rate telemetry, but Ka is preferred when possible for large volume data streams such as science archives.The switch to Ka-band is not as easy for some missions as it is for others, though. The beamwidth is small enough that DSN antennas must point the uplink beam at where the spacecraft will be when it arrives while at the same time receiving the downlink from where the spacecraft was when it transmitted it. On a spacecraft, that level of pointing precision can only usually be achieved with reaction wheels, which have generally frowned on for outer planets missions due to their lower reliability when compared to rcs thrusters and the very long mission durations that they would have to operate for. Only in recent decades have flagship missions begun to use them, and for New Frontiers missions to the outer solar system, it may not be an option to begin with.
The ever important question of launch is an interesting one. The primary launch period occupies most of June 2031 and results in an E(ΔV)EJU trajectory, or a post-launch deep space maneuver followed by Earth and Jupiter flybys.
A backup period exists in April 2032, but if launch readiness ends up being pushed to the mid-late 2030’s, a modification of the gravity assist sequence to include Venus flybys (and potential associated design modifications) may be required. Later launches also mean that Jupiter may not be in the right position to provide a gravity assist, requiring its replacement with an additional earth flyby (alongside the Venus assists that are also needed at that stage). Due to decadal study rules and available information, Falcon Heavy (in fully expendable mode) was baselined as the launch vehicle due to being the highest performing launch vehicle on the NLS II contract that is currently flying. Despite this, Falcon is a perhaps slightly iffy choice due to its lack of nuclear certification and SpaceX’s apparent lack of interest in pursuing it. That could change, of course, but as with everything about this mission only time will tell what eventually happens.
So far the UOP spacecraft and mission design has not been terribly unique. The spacecraft is largely derived from earlier or current missions with layout changes or new software being the major departures. For example, the orbiter structural and electronics layout is largely based on Europa Clipper.
Both UOP and Europa Clipper have a central cylinder as the basic structure, with a rectangular electronics and instrument vault on top and four truss-mounted thruster modules on the aft end. The primary differences other than instrument choice are that UOP has RTGs instead of solar panels and that the High-Gain Antenna is mounted to the top of the vault instead of to one side of the main cylinder.
Where things begin to get interesting is the approach to Uranus. Most missions or mission studies involving an entry probe, including Galileo and Pioneer Venus Multiprobe (Cassini-Huygens is not included because Huygens was a lander with descent instruments, not a gas planet upper atmosphere probe), released their probes before inserting into orbit around the planet. This means lower fuel usage (as the dry mass of the probe does not have to be dragged through orbit insertion and the targeting/divert maneuvers can be performed from farther away) but also places many critical events and pointing requirements very close together, driving spacecraft design as well as decreasing flexibility and increasing risk. UOP 2021 differs (even from UOP 2011) in that probe release is placed after orbit insertion. This relaxes design constraints, allows for both the probe and orbiter mission profiles to be better optimized for each spacecraft’s goals, and allows for backup probe release opportunities if required, at the cost of slightly more delta-v required. The new plan looks like this: the orbiter with the probe attached falls in towards Uranus and performs the UOI burn similar to how Cassini did it with Huygens attached. The spacecraft then sweeps out to apoapsis and shortly afterwards performs a maneuver to aim the orbit at the desired probe impact point. Around halfway between apoapsis and periapsis the orbiter will perform a divert maneuver to avoid impacting the planet itself, with probe release possible at any time between entry targeting and orbiter divert. As both spacecraft approach the planet, the orbiter orients itself to receive the probe telemetry during the hour-long descent phase. After the orbiter reaches apoapsis again, a “period-reduction maneuver” is performed, raising the periapsis and targeting the first flyby of the satellite tour. The tour presented in the study is just one example, as tours are constantly refined and iterated not just over the course of development but also in-flight as well. Additionally, due to all the options and permutations involved, billions of possible tour sequences exist even when the total number of flybys is kept to just 13. The example chosen for further study used 15 resonant flybys of Titania to “equatorialize” the orbit of the spacecraft, placing it in the orbital plane of the major moons and greatly increasing tour ease and flexibility. A resonant flyby means that a single flyby cannot provide the needed orbital change to reach the next destination and must be repeated until the change has been achieved as practiced by Lucy and BepiColumbo), whereas a non-resonant flyby means that a single flyby is sufficient to reach the next target (e.g. Juno or Voyager). After equatorialization is complete, 8 non-resonant flybys of Umbriel, Oberon, and Ariel are performed before the disposal phase begins. Disposal uses 11 resonant flybys of Ariel to progressively lower the periapsis before executing a 216 m/s deorbit burn at the final apoapsis to commit the orbiter to entry. As stated above, many changes to this sequence are possible, and one highlighted variant is periodically “pausing” equatorialization to conduct flybys of Miranda before resuming.
Despite the maturity of these plans, and even with decadal endorsement, it is time for the dreaded dose of realism. There are large obstacles between us here in 2023 and the launch of UOP in 2031. Chief among them is money. Congress has historically never been super willing to pour out tons of money for NASA missions (especially robotic ones) but doubly so at the moment due to reduced federal spending limits this year. Mars Sample Return is also ballooning in cost somewhat right now, which does not help the situation. Beyond money, there are the questions of Plutonium reserves and of course the usual development problems inherent to space projects. UOP’s use of three Next-Gen Mod 1 RTGs is taxing the US’s Pu-238 supply. Each RTG will require 9.6kg (64 “clads”) of plutonium according to a March 2023 report by the Office of the Inspector General, for a total of 28.8kg (or 192 clads), but the eventual goal for production is 1.5kg (10 clads) per year by 2026. Due to the high number of clads needed, the relatively low production rate, and the potential for other possible plutonium users such as New Frontiers-5 to use significant amounts of the stockpile, it is unknown if supply will be sufficient in the late 2020s to allow all 3 RTGs to be ready by 2031. It is tempting to say, “What’s a few years, Uranus will still be there?” but later launch dates are more likely to preclude the widespread illumination of the northern hemispheres of Uranus’s moons, which for the same reason Voyager 2 did not study very much. Like any large aerospace project, UOP is bound to run into technical issues during its development, but because it is so early on (the only official NASA work so far is a few very limited technical studies which have not even been completed yet) there has not been time for any of those issues to actually present themselves, making it hard to discuss them in depth.
Despite the challenges, the future is bright for ice giant exploration overall. Beyond UOP, there is the potential for more ice giant exploration missions in the coming decades. Triton Ocean Worlds Surveyor (TOWS) is a permitted theme for the New Frontiers-7 mission, and concepts have been studied showing how Nuclear Electric Propulsion could be used to drastically increase payload (or reduce travel time) to the outer reaches of the solar system. It will be a fight to fly UOP on time, but it is absolutely possible and could rewrite our knowledge on not just Uranus, not just its moons, but perhaps the foundational history of the solar system itself.