You might think we've studied the outer solar system planets pretty well. After all, eight spacecraft have visited Jupiter and a ninth (Juno) is on its way. Four missions have visited Saturn, including the long-running Cassini spacecraft. But that's only half the story. Farther out and much different than Jupiter and Saturn, Uranus and Neptune have only ever been visited once, by Voyager 2. These fleeting flybys revealed worlds unlike any other in the solar system and, yet, we have since then never returned.
It's against this backdrop that the European Space Agency (ESA) is considering its next series of large missions. With the first of these so-called L class missions (another Jupiter mission) slated to launch in 2022, ESA has begun to solicit ideas for the decade beyond. Visiting Uranus or Neptune is one plan being put forth. Let's take a look at why a mission to one of these planets would be so exciting.
In the aftermath of Voyager 2's 1989 visit to Neptune, a picture of the formation of our solar system began to crystallize. Located farther from the Sun, the outer planets were able to take advantage of cooler temperatures to accumulate the hydrogen and helium which give them their large, fluffy builds. Even farther out, in particular, Uranus and Neptune were able to accumulate ices as well as gasses. Scorched by the heat of the Sun, the inner planets were reduced to rocky bodies.
The picture lasted all of ten years. As the discovery of exoplanets mounted, it became clear that solar systems could form in a nearly-infinite number of different ways. Jupiter-type planets have been found orbiting closer to their star than Mercury orbits to the Sun. Huge rocky planets, much larger than the Earth, seem quite common. And astronomers found lots and lots of Neptune-sized planets. Clearly, a new formation model was needed, and astronomers continue to work on one today.
One of the most important constraints on potential models is the composition of the planets. We need the right amount of stuff in the right place in the right time to make the planets that we see today. And even though we can make some measurements from here on Earth, our knowledge of Uranus and Neptune's composition lags greatly behind that of all the other planets. How can we hope to understand the thousands of planets we're finding outside our solar system if we haven't even studied all those within it?
If the chemical makeup of a planet doesn't get your blood pumping, there is plenty of other stuff that will. Did you know that Uranus orbits on its side? That's right - unlike the other planets, the north pole of Uranus points towards the Sun, not perpendicular to it! Also unlike the other planets, the magnetic fields of these icy worlds are tilted at a huge angle compared to their rotation. Since we think that rotation plays an important role in generating a magnetic field, this poses a serious problem to our theories.
Neptune is also host to what may be one of the most fascinating objects in the solar system - Triton. The sixth largest moon, Triton is likely a captured dwarf planet with a composition similar to Pluto. How do we know this? Well, for one, it orbits backwards! While every other large moon orbits counter-clockwise, Triton orbits clockwise. The coldest known moon, its surface is covered in nitrogen ice and occasionally it erupts in frozen geysers. I could go on and on, but, the point is, we've discovered all this amazing stuff from one spacecraft flyby that observed less than half of Triton's surface. Imagine what else might be out there...
So, if Uranus and Neptune are really as fascinating as I've claimed, why haven't we visited them again? The answer lies in the vast space between the planets. It took the Cassini mission seven years to reach Saturn. Uranus is twice as far; Neptune, three times. These are timescales which challenge what even public institutions can manage. We can (and would have to) get there faster, but it'll cost us. Cassini has cost more than $3.25 billion dollars. We can expect a similar price tag on visits to Uranus and Neptune. That makes ESA's L class budget of $1 billion pale in comparison, so there is clearly still work to be done. But, can we in good conscience continue to completely ignore a quarter of the solar system? I think not.
Want to read more? Check out this proposal submitted to ESA.