On Chasing Shadows: A New Exploration of Uranus

At first glance one may think a shadow holds no value, that it is just the absence of light. However, shadows may turn out to be key factors in NASA’s future solar system explorations by enabling ambitious new mission strategies. At the December 2024 annual meeting of the American Geophysical Union (AGU24), NASA provided updates on the status of NASA’s upcoming Uranus Flagship mission and the precursor projects that will enable it. Space Scout has reported previously on the mission, which was studied as part of the 2021 Planetary Science Decadal Survey and adopted by NASA as its next new planetary flagship. The mission is due to enter formulation and thus begin full development in 2027, and launch not before the mid-2030s. One of these further trade studies and analyses, and one that much attention continues to turn towards, is the aerocapture proposal, first covered by Space Scout in January 2024.

There are several factors that make aerocapture appealing to scientists and mission planners. All current and previous planetary orbiters have used long chemical thruster burns to capture into orbit around their destinations, requiring a ton or more of fuel and limiting the transfer speed to minimize the fuel load. Aerocapture instead places a much smaller orbiter inside an aeroshell, so it can dip into and skim through the upper atmosphere of its target planet. The drag of the atmosphere slows the orbiter into orbit, with a small orbit-raising burn needed after entry to stabilize it. This can save up to 7-9 years of cruise time and several million dollars of operating costs.

Despite the advantages, aerocapture is not simple or easy. While many missions, including Mars landers and Artemis missions, have performed similar pure-drag slowdowns, both Earth and Mars have very well-characterized atmospheric structures and profiles. At Uranus, the only close range data was taken by Voyager 2 in 1986, and there have been zero ground observations since 1996. Significant uncertainties in the atmospheric density across the planet will require the entry system to be much more robust to compensate, adding mass and risk. Studies to assess the overall sensitivity to uncertainty have not yet been performed, and this represents the most significant risk to the concept. Aerocapture has also never been used to place a probe into a very specific initial orbit. The system tour can depend on the exact capture orbit, so being able to precisely control the capture is crucial. In order to cope with the large uncertainties of an Ice Giant atmosphere, more advanced actuators and control algorithms may also be required, requiring further testing and development.

The only way to address the atmospheric uncertainties is with further observations of Uranus, but the technological risks and barriers can be significantly reduced with a near-Earth demonstration mission. Such a mission is being initiated by NASA’s Space Technology Mission Directorate (STMD), with work expected to begin in 2025. This demonstrator will likely launch as a rideshare to either Geostationary Transfer Orbit (GTO) or to Lunar Transfer Orbit. It will fall back to Earth, hitting the atmosphere at approximately 10 kilometers per second and reducing its speed by 1-2 kilometers per second, placing it in Low Earth Orbit within one atmospheric pass. Detailed prior knowledge of Earth’s atmosphere or intake tracking data from external assets will not be used, to replicate the conditions of the real mission as closely as possible. This will prove out improved guidance and control technology and leave atmospheric model improvements as the primary obstacle to implementation. Few concrete details about this demonstration are known, but the team is established and work should begin in 2025.

While the atmospheric uncertainty at Uranus is large, the planetary and exoplanet science communities have devised various helpful strategies. The primary tool for improving our knowledge of far-away atmospheres is stellar occultations. A stellar occultation is when a relatively bright background star passes behind a planet. As the light filters through the atmosphere, the brightness over time and across multiple wavelengths reveals crucial information about the atmospheric composition and density. With multiple telescopes observing the event, it is also possible to identify regional differences at the target. Occultations are most common when the target is passing in front of the galactic equator, but are surprisingly rare. Uranus is currently above the plane of the Galaxy, and there are major occultations in 2024, 2025, 2031, and 2032. NASA held a global observation campaign for the November 12, 2024 occultation, and is planning to observe the 2025 event as well. It is hoped that these observations, together with the Aerocapture Demonstration Mission, will reduce uncertainty enough to make Aerocapture viable for Uranus missions.

Even with multiple observatories around the world, atmospheric noise provides a fundamental limit to both what occultations are observable and how accurate measurements can be. To get around these issues, researchers at NASA’s Langley Research Center are beginning work on a smallsat mission to observe occultations in the early 2030s. As presented at the AGU24 conference, the Shadow Chaser mission will consist of a 20 cm telescope in a roughly 600 km sun-synchronous orbit roughly aligned with the popular “A-train” orbit for Earth observation satellites.

Due to its orbital nature, it will actually dip in and out of Uranus’s shadow multiple times for each occultation, effectively turning one event into multiple and further increasing the data throughput. Over the next 10 years, 20 occultations are expected to be visible from the ground, but perhaps 56 will be visible from space, with the brightest (and the event the mission is being designed around) occurring in February 2031.

At the moment Uranus Flagship is not anything in particular. Its architecture will only be determined once the mission begins formulation in 2027, with the current studies and analyses hoping to inform the subsequent direction of that development. The first real determination will be the formation of the core science team in late 2025, which will shape the science requirements and therefore the instrument set and tour needs. The fact that the best occultations are in 2031 and 2032 is a mark against Aerocapture, given that Uranus Flagship will have begun concept development as many as 4 years earlier. A normal Phase A time for a flagship is 2-3 years, so Aerocapture will likely be selected or discarded by the time these major occultations occur. NASA’s immediate willingness to fund immediate occultation campaigns (2024 and 2025), the Shadow Chaser mission, and an Aerocapture Demo Mission shows that the benefits are being taken seriously and there is institutional desire to infuse it into UOP if possible. The primary question is simply if the uncertainties will be controlled enough by the late 2020s to allow it to become the architecture of choice.