Why we need to return to Uranus and Neptune

As Neptune and Uranus could solve the mysteries of planet formation, we need to go back and study them in depth.

Uranus (left) and Neptune (right), as seen by the Voyager 2 spacecraft. Credit: NASA/JPL-Caltech; NASA

During the 1980s, the Voyager 2 spacecraft transformed our view of the ice giants Uranus and Neptune from mere wandering points of light to complex and beautiful planetary systems.

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The Voyager data remains a treasure trove of information on the planets’ interiors, atmospheres and diverse satellites.

But with only a single encounter recorded for each planet, we haven’t even scratched the surface of these worlds. Out there on the ‘frozen frontier’, discoveries await the next robotic explorers.

This leads many to ask why we haven’t sent any more spacecraft or orbiters to visit Uranus and Neptune.

Today, the international community is lobbying space agencies to mount a mission to these ice giants, the only major class of planet yet to have a dedicated orbital explorer.

The scale of the challenge is matched by the scientific importance of these two worlds.

An image of Uranus taken by Voyager 2. Image Credit: NASA/JPL
An image of Uranus taken by Voyager 2. Image Credit: NASA/JPL

Why explore Uranus and Neptune?

As we gaze out to exoplanets, we’ve realised that worlds of a similar size to our ice giants are commonplace.

By contrast, the larger gas giants like Jupiter and Saturn seem rare. And yet we still struggle to understand how Uranus and Neptune formed.

Giant planet formation is essentially a race between accretion of gas onto an embryonic rock-ice core, and the dissipation of that gas.

Cloud systems seen in Neptune's southern hemisphere, photographed by the Voyager 2 spacecraft. Credit: NASA
Cloud systems seen in Neptune’s southern hemisphere, photographed by the Voyager 2 spacecraft. Credit: NASA

To achieve worlds similar in size to Uranus and Neptune requires fine-tuning of this relationship and we need to know much more:

  • How much ice and rock is present in these ice giants?
  • How is material distributed?
  • How have these worlds been cooling since their formation?

To understand their evolution, we need a mission capable of measuring gravitational and magnetic fields, directly sampling the planetary composition, and studying the atmospheres and magnetospheres.

The ice giants are the missing link, the piece of the puzzle to help us unlock the mysteries of planet formation.

A shot of crescent Uranus captured by Voyager 2 on 25 January 1986 from a range of 600,000 miles. Credit: NASA
A shot of crescent Uranus captured by Voyager 2 on 25 January 1986 from a range of 600,000 miles. Credit: NASA

Uranus presents extreme seasons and magnetic fields, a consequence of the cataclysmic impact that tilted it onto its side, whereas Neptune’s powerful meteorology is driven by heat from within that’s absent (or locked away) on Uranus.

Uranus has a classical system of satellites and delicate rings. Neptune, conversely, possesses an interloper from the Kuiper Belt: the massive moon Triton (incidentally, the target of a future planned mission called Trident).

These worlds can’t be effectively studied remotely: we have to visit them with sophisticated new spacecraft in order to reveal terrains that were in total darkness for Voyager 2, 30 years ago.

Neptune's famous Great Dark Spot feature, as seen by the Voyager 2 spacecraft. Credit: NASA
Neptune’s famous Great Dark Spot feature, as seen by the Voyager 2 spacecraft. Credit: NASA

Why haven’t we been back to Uranus or Neptune?

With such tantalising potential for discoveries, why haven’t we returned to the ice giants since the 1980s?

Reaching 20 or 30 AU requires an international effort and some good timing. With today’s rockets and chemical propulsion, we need to slingshot around Jupiter, to get as much mass as possible into orbit around Uranus or Neptune.

That means Jupiter must be in just the right place, which happens once every 12–13 years. The next window is in the early 2030s, so we have no time to lose.

ESA and NASA are currently developing plans for planetary exploration over the next decade, and if they see the ice giants as the highest priority, then we can be ready.

Because the discoveries could be astonishing and they’re out there, waiting for us.

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This article originally appeared in the November 2021 issue of BBC Sky at Night Magazine.