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.
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.
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.
Giant planet formation is essentially a race between accretion of gas onto an embryonic rock-ice core, and the dissipation of that gas.
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.
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.
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.
This article originally appeared in the November 2021 issue of BBC Sky at Night Magazine.