Why Uranus?

Over the past few weeks I've answered several media enquiries about the future of Ice Giant exploration - here was one exchange by email with Metro News.

Q: Why is Uranus called one of the most intriguing bodies in the solar system?

Uranus is known as an Ice Giant, sitting in between its larger Gas Giant cousins (Jupiter and Saturn), and the smaller terrestrial worlds.  When we look out at the ever-growing collection of extrasolar planets (worlds beyond our Solar System), we find that planets of similar sizes to Uranus and Neptune are commonplace.  We might therefore have two great examples of the most common outcome of the planetary formation process, right here in our Solar System, and yet their composition, their nature, and their origins remain a mystery.

Uranus is also a world of extremes – tipped onto its side by a catastrophic collision in the distant past, the planet’s atmosphere, magnetosphere, satellites and rings experience the strangest seasons of anywhere in our Solar System.  And because we’ve only had a robotic spacecraft visit Uranus once, there are places on those icy moons that no eyes have ever seen – who knows what might be there, waiting for us.

Q: Why wasn’t it explored, so far?

The great distance to Uranus presents an enormous challenge for planetary exploration.  Although it has been explored by a single flyby (the Voyager 2 mission in 1986), we need to be in orbit to fully explore the planet, its satellites and rings, over long periods of time.  Not only does it take a long time (8-13 years) to get there, but we also need enough fuel to slow us down to enter orbit, and then enough to conduct a comprehensive tour of the Ice Giant system.  If we use Jupiter gravity assists, slingshotting by Jupiter on the way to Uranus, we can get more mass (spacecraft, fuel, and scientific payload) into the Uranian system, but that opportunity only comes once every 12-13 years, when Jupiter is in the right place.  With the recent announcement from the US decadal survey, I hope that international agencies (NASA, ESA, and others) will be ready for the next opportunity in the early 2030s, so there’s no time to lose.

There’s also the challenge of providing power to a spacecraft at such great distance – the weak sunlight isn’t going to help, so we have to reply on the decay of radioisotope power sources to provide the electrical energy, and those fuel sources need to be robust enough to survive a multi-decade mission.

Q: How a mission to the seventh planet could change the way we see the solar system?

A mission of exploration to Uranus touches on themes of exploration that span not just our system, but planetary systems in general.  Namely, we’ll be using Uranus as a window onto the distant past, searching for clues to the puzzle of planetary origins, and how worlds of this size appear to be a common outcome of the planet formation process.  Secondly, we’ll be studying a diverse collection of ‘ocean worlds,’ icy moons of Uranus that show signs of ancient geophysical activity and might harbour subsurface oceans.  Where there’s water, we might be able to extend the reaches of the ‘habitable zone’ of a planetary system, further out into the frigid realms of the outer solar system than we ever thought possible.

Q: What discoveries do you expect if the mission to Uranus takes place?

The most exciting discoveries will be the ones we haven’t even dreamt of yet.  I’m excited to see the interior structure of an Ice Giant, with potentially unimaginable quantities of water in an exotic phase of matter (slushy hot ice) locked away at great depths, and maybe signatures of that ancient cataclysmic impact.  I’m looking forward to seeing a probe descending under parachute into the Uranian clouds, sampling the gases and aerosols as it falls.  And there are terrains on the northern hemispheres of the moons that no eyes, human or robotic, have ever seen, because they were in total winter darkness when Voyager 2 flew past in 1986 – who knows what we’ll discover there?

Q: What is needed to place Uranus at the top of the space exploration agenda?

Let’s be clear – it is already top of the scientific agenda, as a result of the prioritisation from the US decadal survey in 2022, as a flagship-class mission that engages the entire community of planetary scientists.  Its themes span the realms of heliophysics and astrophysics too, and the idea of an Ice Giant mission has broad support from the international community, most noticeably ESA through it’s Voyage 2050 strategic plan.  What’s needed now is funding (i.e., increases in existing budgets), both in the US and across the world, to support this ambitious mission on a time frame that works to meet the next launch opportunities in the early 2030s.  That’s the biggest challenge that the Uranus mission now faces.

Q: What to expect in the future regarding Uranus exploration?

The coming years will be a race to move this mission from a concept to reality, funding and building the hardware that will eventually sit atop a rocket faring, waiting to launch to Uranus.  It will be a long journey of a decade or more, just to reach the launch pad.  Then a launch around 2031, slingshot past Jupiter, to arrive in the mid-2040s.  If all goes well, the mission will then orbit Uranus for several years, returning a suite of spectacular discoveries just like the Cassini mission to Saturn and the Juno and Galileo missions to Jupiter.  

Q: What other celestial bodies have to be urgently explored and why?

In planetary science, we learn more by comparison than by single-target missions.  Our exploration and understanding of Ice Giants will remain incomplete until both Uranus and Neptune have been comprehensively explored.  Neptune is just out of reach in the coming decade, but the next opportunity for a gravity slingshot arrived at the end of the 2030s.  A voyage to Neptune, and its incredible moon Triton (a captured dwarf planet from the distant Kuiper Belt), will be an incredible next step beyond the Uranus mission.