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Who Needs Jupiter?

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A photograph of Uranus against a black background.
An image of Uranus recently captured by JWST that reveals the planet's complex ring system. [NASA, ESA, CSA, STScI; Image Processing: Joseph DePasquale (STScI)]

A planned flagship spacecraft to Uranus was aiming to use Jupiter for a gravitational boost to shorten its journey to the outer solar system. Now that the mission’s launch date has been pushed back, it’ll need to find another way to its target.

Redesigning the Route

Uranus and Neptune, the ice giants of our solar system, have been comparatively neglected by humanity’s fleet of interplanetary probes. While all of the other major bodies have been visited by at least an orbiter or even a lander, the most detailed exploration we’ve done of these two planets happened when the Voyager 2 spacecraft went screaming past on its way toward interstellar space.

A two-panel top down view of the solar system with different trajectories overplotted.

Schematics of different trajectories within the solar system. Left: The path the Galileo spacecraft took to Jupiter. This path arrived nearly parallel to the planet’s motion. Right: One of the proposed paths to Uranus. This path will intersect the planet’s orbit at a much steeper angle to shorten the flight time. Click to enlarge. [Simon et al. 2026]

So it was no surprise when a panel of scientists and engineers charged with setting scientific exploration priorities between 2023 and 2032 recommended finally sending a spacecraft to Uranus, depositing it into orbit around the planet, and keeping it there for a while to collect data. The study itself suggested a basic design for the mission: a pair of spacecraft, one orbiter and one sacrificial atmospheric probe, collectively weighing about 4,000 kg.

Getting these spacecraft to their destination will be a challenge The thriftiest path to get to Uranus, a trajectory called a Hohmann transfer that’s been used for other missions aimed elsewhere in the solar system, would take 16–17 years. Rather than accept the long wait, the original authors suggested that the mission instead fly toward Jupiter, using the giant planet’s massive gravitational tug to slingshot the spacecraft toward Uranus.

However, this plan relied on Jupiter being in just the right place when the spacecraft approached. This in turn relied on the mission launching sometime in 2031 or 2032: any earlier or later, the slingshot would be aimed in the wrong direction. Now that timelines and budgets have slipped for this flagship mission, hitting this launch window seems unlikely. And, without the boost from Jupiter, the mission will have to cross 20 au of open interplanetary space on its own.

Solar Electric Propulsion to the Rescue

A 3-row figure showing the same interplanetary probe flanked by every-growing sets of solar panels.

Several proof-of-concept designs for the mission’s SEP components. How these designs would be packaged into a rocket fairing for launch is shown at right. Click to enlarge. [Simon et al. 2026]

Recently, a team led by Amy Simon, NASA Goddard Space Flight Center, took on this challenge and tried to chart a new path through the solar system. The team realized that solar electric propulsion (SEP), a technique that relies on using solar panels to create electricity and accelerate ions to create thrust, might offer a path forward. By using SEP, the mission could leave Earth, unfurl wings of solar panels, then gradually accelerate for as long as there was enough sunlight to power the engine.

The power levels would likely get too low just a few astronomical units into their journey, but that would be enough: after discarding the SEP stage, the mission could coast to Uranus on a journey that would take only 13 years. Even better, this technology has a proven flight history and would require little maturation or development. The BepiColombo mission to Mercury is currently using SEP, and similar techniques are used on satellites in Earth orbit today.

Simon and collaborators also took on a number of other challenges that have cropped up since the mission was proposed, including fewer available plutonium power sources and a re-assessment of Uranus’s rings that revealed hazards in the original flight path. Their work brings the mission closer to the launch pad and illustrates how much iteration goes into the spacecraft that finally takes to the stars.

Citation

“Uranus Orbiter and Probe: Mission Challenges and Concept Updates Since the Origins, Worlds, and Life Decadal Survey,” Amy A. Simon et al 2026 Planet Sci. J. 7 143. doi:10.3847/PSJ/ae680c