Plotting the Course, a Billion Miles Away

When the next flagship planetary science mission arrives at Uranus after years of interplanetary travel, it’s going to need to know exactly where it should fly near this poorly mapped, distant world. In a recent study, astronomers attempt to strike a balance between risk and reward and present several possible trajectories.

Visiting a Lonely Planet

Setting our home planet Earth aside for obvious reasons, you might be forgiven for assuming that we’re equally familiar with the remaining seven planets in our solar system. But, even after decades of robotic exploration, this is not the case: though we’ve mapped every inch of Mars and spent years circling Saturn, humanity has only visited Uranus and Neptune once apiece. These visits, which happened more than 30 years ago and lasted only a few hours each, answered some questions about the structure and formation of these planets but left many more unresolved.

To remedy this lopsided accrual of knowledge on each planet, in 2022 the National Academy of Sciences recommended that NASA’s next flagship planetary science mission should aim not for one of the inner rocky planets, but speed all the way on to Uranus. From orbit around this distant world, a properly designed mission should be able to tell us if the planet’s core is liquid or solid, measure the structure and speed of the winds seen previously whipping around the planet, and assess the composition of the interior.

Before such a mission can begin its investigation in ~2050, however, scientists and engineers need to painstakingly plan out exactly where the spacecraft needs to go and what it needs to carry if it’s going to answer the questions we create it to resolve. A recent study led by Marzia Parisi, Jet Propulsion Laboratory, adds to this effort by considering which orbital trajectory would yield the most useful scientific measurements.

Planning Ahead

Scientists can measure things like the density structure of a planet by tracking how a probe’s speed changes as it travels along its orbit. The closer the spacecraft can get to the planet, the better, since that’s where the subtle effects will be most pronounced.

But, mission design is always a game of tradeoffs, and here is no exception. The closer a spacecraft gets to the planet, the greater its risk for accidentally dipping into the upper atmosphere and careening inwards. Complicating matters further, Uranus also has a series of rings that are relatively poorly mapped. No one wants the mission to end after decades of work in a collision with a bit of ice, so engineers have another motivator to keep the spacecraft in a wide orbit.

It would be ideal if mission designers could avoid the dangerous inner region altogether and always remain outside the rings. As Parisi and collaborators demonstrate, however, if the probe stays in this safe region throughout the planned 90-day mission, it won’t be able to conclusively differentiate between solid and liquid core models. If they instead accept the risk and “plunge” between the rings and the surface once per orbit, they’d be able to achieve the mission objectives with only eight laps around the planet.

While we’re still years away from deals with contractors to actually build the mission, studies like this are essential to design the future flagship. The final trajectory decisions won’t be made for a while, but if we ever see photos taken by a robot streaking between Uranus’s cloud tops and rings, we’ll be able to trace its journey to that moment to articles like this.

Citation

“Uranus Orbiter and Probe: A Radio Science Investigation to Determine the Planet’s Gravity Field, Depth of the Winds, and Tidal Deformations,” Marzia Parisi et al 2024 Planet Sci. J. 5 116. doi:10.3847/PSJ/ad4034