Uranus Aerocapture: New Solutions for Old Problems

For many planetary missions, atmospheres are obstacles to be dreaded or overcome, be it the need for heavy payload fairings, the stress of EDL, or the inevitability of orbital decay and mission end. NASA is planning a flagship-class mission to Uranus, the Uranus Orbiter and Probe (UOP), early in the next decade, and an ongoing study may turn atmospheres into a valuable asset for not just UOP, but many missions yet to come. Space Scout has previously reported on the mission concept in detail, but some key facts are worth repeating. Notably, the spacecraft concept is based off of the Europa Clipper spacecraft, with a rectangular vault that doubles as an instrument mount, a cylindrical body, and pod-mounted main thrusters. It also features an atmospheric entry probe to study the interior of Uranus.

Of the obvious differences from Europa Clipper, perhaps the largest is power generation. While at Jupiter large solar panels are feasible, at Uranus the sun’s light is over 12 times weaker. Consequently, UOP must use nuclear power in the form of Radioisotope Thermoelectric Generators, or RTGs. The available model of RTG is the Next-Generation RTG Mod 1, which uses 9.6kg of plutonium to generate around 300 watts of power at launch. However, UOP requires 3 of these, meaning that plutonium stockpiles and production are insufficient to meet its planned 2031 or 2032 launch date.

The launch date for this historic mission is driven by UOP’s trajectory. It is, as of the time of writing, planned to use an Earth-Jupiter Gravity Assist (EJGA), meaning it will do one flyby of Earth and then a flyby of Jupiter. However, Jupiter assists to get to Uranus become much less abundant after 2032 and largely impossible after 2035 due to the alignment of the planets, and waiting for them to become an option again would delay the mission unacceptably.

There are two main problems facing the trajectory and mission designers. First is the system’s axial tilt, or obliquity. Because the planet is at an almost 90 degree angle to its orbital plane, the only times where almost all of the planet is illuminated at once are the equinoxes, when the sun shines directly on the equator. Because most moons orbit the equator of their parent planet, this is also the only time a visiting spacecraft can image the majority of the surfaces of each moon. Outside of equinoxes, and especially around solstices, only one hemisphere of anything in the system will be lit, with that hemisphere not changing until the next equinox. The next equinox is in 2049, and so UOP must arrive by then in order to have enough time to complete its tour before parts of the moons slip into darkness for another 42 years.

With the lighting requirements in mind, the slowness of non-Jupiter cruise options that are required after 2035 and the likelihood of RTG supply issues forcing the launch date into that region becomes much more obviously problematic. In order to meet the full science objectives, a way of shortening cruise duration without using Jupiter gravity assists may be needed.

Another one of the major problems faced by the mission planning team is shortening the cruise through higher energy trajectories presenting new challenges. The faster the cruise, the faster UOP will be when it needs to brake into Uranus’s orbit, a burn called Uranus orbit Insertion (UOI). In the current plans, UOI is approximately 1 km/s and will burn almost 1,900 kg of fuel, or almost 48% of the total fuel load.  A faster approach and thus larger UOI burn would add significant mass to the propulsion system and potentially start a cascade of diminishing returns due to the rocket equation. Additionally, such a fast trajectory may require the use of a super-heavy-lift launcher such as SLS or Starship, which comes with potential readiness and cost issues. A solution that only requires a heavy-lift launcher such as Vulcan-Centaur, New Glenn, or Falcon Heavy would be much more robust and flexible as it could easily work with larger rockets if needed.

In search for a potential solution to these issues, researchers at NASA’s Langley Research Center turned to study a long theorized but never tried new method of orbital insertion: aerocapture. Aerocapture is the use of a planet or moon’s atmosphere to slow a spacecraft down to either land or enter orbit. This is similar to how many modern Mars landers go directly from interplanetary speeds to entry and landing, though in this case the target is orbit and not the surface. Despite the much smaller amount of braking, it is intense enough that a heat shield of some kind is still required. A related but distinct maneuver is aerobraking, successfully used at Mars beginning in 1996. Aerobraking is the use of very shallow passes through the upper atmosphere of a planet to lower an already-established orbit and does not necessarily require special heat shielding, as multiple passes can be made and much more time allotted, enabling the spacecraft to graze the atmosphere instead of plunging in.

Aerocapture, by using the atmosphere to bleed off almost all required speed, greatly lessens the fuel requirements for orbital insertion. The single large UOI burn would be replaced by a burn at apoapsis to raise the periapsis out of Uranus’s atmosphere and one or more small “cleanup” burns to place the spacecraft in the right place to begin its orbital tour. These burns are larger than their non-aerocapture equivalents but hundreds of meters per second are still saved overall due to the elimination of the main braking burn. The orbiter can then be much smaller and lighter, as it only needs the fuel for those cleanup burns and the tour, while being able to be sent to Uranus much faster than before.

The way aerocapture would be implemented, if chosen as part of the UOP mission, is still being developed. The initial idea was to use an aeroshell similar to that used for the Perseverance rover, though a backshell would preclude instrument checkouts during cruise and may not be required. For the TPS material making up the heat shield, the current frontrunner is Conformal Phenolic Impregnated Carbon Ablator (PICA-C), which is similar to the TPS material used on Mars landers, asteroid and comet sample returns, and commercial LEO capsules, but much thinner and lighter for the same thermal protection. Despite requiring a heat shield, an aerocapture entry is much softer than a direct entry mission and so radiative heating is predicted to be largely insignificant, potentially meaning that a backshell will not be required.

The immaturity of the aeroshell design at present has drawbacks. UOP’s required form factor is very unlike anything that has yet performed an entry interface, so using an “off the shelf” aeroshell may not be the best solution. Due to its reliance on an inherently chaotic atmosphere, aerocapture can have large error bars, especially when considering how poorly characterized Uranus’s atmosphere is. This is a major reason for the clean up maneuvers, but the uncertainty may require high levels of conservatism and thus potentially high levels of margin to ensure that the orbit can be salvaged even in worst case scenarios. 

Due to the relatively well studied and defined scenario of a planetary entry, guidance can be based on Mars entries and Apollo entry guidance. After entry interface, the vehicle will modulate its bank angle to successfully steer through the corridor and skip out of the atmosphere.

The next steps for aerocapture are to refine the concept heatshield and backshell (if required) design and gain a more robust understanding of the mass savings and margins. It will be featured in two special sessions at AIAA’s SciTech conference in January 2024, but those papers will likely not be released for a few months afterwards. With UOP itself due to enter Phase A in fiscal year 2025, Aerocapture will no doubt find itself as one of the key concepts being discussed as part of the overall mission architecture. If it proves out well and is implemented successfully on UOP, many solar system missions could benefit from smaller spacecraft on faster trajectories with more flexible opportunities, helping speed along our exploration of our celestial backyard.

NGRTG – Next Generation RTG

PICA – Phenolic Impregnated Carbon Ablator

PICA-C (or C-PICA) – Conformal PICA

RTG – Radioisotope Thermoelectric Generator

UOI – Uranus Orbit Insertion

UOP – Uranus Orbiter and Probe