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Solar System Exploration at 50
Exploration Stories: Favorite Historical Moments

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Outer Solar System
Fran Bagenal
Professor and Co-Investigator, University of Colorado and the New Horizons Mission
Jupiter Swirls
This close-up of swirling clouds around Jupiter's Great Red Spot was taken by Voyager 1.

What do you think are the most significant events that have occurred in the past fifty years of robotic planetary exploration? Why?

Voyager's exploration of the outer solar system is definitely significant in my opinion. Voyager gave us the first close -- up views of Jupiter and Saturn (even though Pioneer had gone by earlier, but Pioneer hadn't taken much in the way of pictures).

Also, Voyager's views of Uranus and Neptune -- the first, and largely only close-up, view of both systems.

And then of course turning and looking back at the Earth in the classic Carl Sagan "Blue Dot" picture.

The Solar System
Earth is on the left: "Look again at that dot," Sagan wrote in his book "Pale Blue Dot." "That's here. That's home. That's us. On it everyone you love, everyone you know, everyone you ever heard of, every human being who ever was, lived out their lives. The aggregate of our joy and suffering, thousands of confident religions, ideologies, and economic doctrines, every hunter and forager, every hero and coward, every creator and destroyer of civilization, every king and peasant, every young couple in love, every mother and father, hopeful child, inventor and explorer, every teacher of morals, every corrupt politician, every 'superstar,' every 'supreme leader,' every saint and sinner in the history of our species lived there -- on a mote of dust suspended in a sunbeam." -- Carl Sagan, "Pale Blue Dot," 1994

The Voyager mission made an impact in a lot of areas of planetary science. Not to mention going on to do the heliophysics of the outer solar system -- Voyager is still grinding away.

One could also point to Mariner 9 at Mars back in the early 70s when we really got our first view of a non-Earth planet.

Olympus Mons by Mariner 9
Mariner 9's picture of Olympus Mons.

I actually, as a teenager, heard Carl Sagan talk about it and I was impressed by this mission. This was before Pioneer, this was before Viking, before everything. This mission made us realize that Mars was an object with a different kind of geology, but that there were some similarities, as well as differences. Sagan was showing images of impact craters and volcanoes and potential river-like features, though they really were not that clear at the time, and starting to do comparative geology. I was impressed by how Sagan was able to show what we could conclude from the images, as well as being quite honest about what we did not understand. It was just a glimpse compared with the Voyager pictures at Jupiter.

The thing that I think is interesting about these early missions is the mysteries -- they pose these huge questions. Why for instance were Io, Europa, Ganymede, and Callisto so very different? That wasn't expected -- that they could be so different. Tidal heating was just first being thought of, you know. And at Saturn -- "Look at all these complicated wind systems! What makes these complicated wind systems?" When we got to Uranus and Neptune there were these moons, but why were they so small and bashed up? Or Saturn's moon Mimas, "Why does it have this massive impact crater? Why did it not break apart?" When we got to Neptune the big surprise was Triton's terrain, which was covered in these strange surface features: the cantaloupe terrain, the evaporative features and the plumes -- "What were these big smoke-stack plumes?!"

Icy Triton
A global color mosaic of Triton taken in 1989 by Voyager 2 during its flyby of the Neptune system.

It was very bizarre -- it still is bizarre, I don't think we really understand it. Are we going to see the same sort of thing at Pluto, or will it be completely different? I think the big issue for New Horizons is: What will the surfaces look like on these very distant bodies? I think that the ice processes are probably very different from the geology processes that we see now.

Voyager's Titan
A Voyager view of Saturn's moon Titan.

And then Titan. With Voyager we had a couple of flybys of Titan and then the question of "What is going on with Titan?!" We couldn't see through the atmosphere at that point. We knew there was a thick atmosphere. We knew that there was nitrogen and methane, but that was about it. So you pose these great questions that another mission can go back and take another look, as did Cassini-Huygens.

Titan River Channel
This mosaic from Cassini-Huygens showing three frames provides unprecedented detail of the high ridge area including the flow down into a major river channel from different sources on Saturn's moon Titan. Image Credit: ESA/NASA/JPL/University of Arizona

The big thing with Galileo was what was going on with the probe and where is the water? That led to the whole reason for Juno. So I think that that was a major result. Of course Galileo was at the same time very frustrating -- the big frustration had to do with the problems with the antenna. We got limited amounts of information back in comparison with other missions. But at the same time the images we got back were quite good. Like Europa and the brown streaks -- what was that brown gunk? That was the big deal and that is one of the reasons that we want to go back to Europa.

Europa Features
This mosaic of a region in the northern hemisphere of Jupiter's moon, Europa, displays many of the features which are typical on the satellite's icy surface. Brown, linear (double) ridges extend prominently across the scene. They could be frozen remnants of cryovolcanic activity which occurred when water or partly molten water ice erupted on the Europan surface, freezing almost instantly in the extremely low temperatures so far from our sun. Dark spots, several kilometers in diameter, are distributed over the surface. A geologically older, smoother surface, bluish in tone, underlies the ridge system. The blue surface is composed of almost pure water ice, whereas the composition of the dark, brownish spots and ridges is not certain. One possibility is that they contain evaporites such as mineral salts in a matrix of high water content.

Detection of the magnetic field on Ganymede was another big deal and the induction signals that tell us of the possibility of liquid water underneath the ice on Europa are important. Also, important is the dynamic of the volcanism on Io and the physical changes on the surface. So just mysteries again -- Galileo didn't really resolve very much at all. It just raised some big issues, which we are now are trying to address in some ways.

In your field of work, what are some examples of the great achievements and discoveries in planetary science and robotic exploration throughout the past 50 years?

50 years is a long time. It is funny; recently I had to produce a five-minute PowerPoint presentation for the National Academy of Sciences that showed planetary exploration from the past 50 years. And in the end what I did, which is something that I rarely do, was make a presentation that was timed. That is, I hit "go" and it went through everything, because otherwise I would have never have gotten through it in five minutes. There is just too much. So I had five minutes -- the whole of the solar system to do in five minutes! I think I ended up going through 50 slides and it really did crank through.

I think a major achievement has to do with our understanding of our solar system, which is a fairly major step forward that has happened in the past 20 years or so. So just looking at the giant planets -- Jupiter, Saturn, Uranus, Neptune -- we have questions that we look to have answered though the missions we send to them. Such questions as: "How are the giant planets similar and how are they different?" "What formation process of the solar system led to them being the way they are and where they are?" We only had a little bit of spectroscopic studies of these objects before we sent out Pioneer and Voyager -- they were really mysterious. There were some hints of strange behavior from ground based and spectroscopic observations, but until you get close and fly past them you really don't know. So by searching out answers we are led to not only understanding what they are made of, and how they got to be that way, but also the whole understanding of origin and evolution of the solar system.

Then, beyond Neptune, there are the Kuiper Belt objects. Our knowledge of these objects has changed dramatically in just the past 15 years. First you start off as counting them as dots of light, then you start doing spectroscopic studies, then you do comparisons, then you can do shape, and if you are patient then maybe you can do mass if they have moons (which surprisingly many of them do) and then, eventually, we will fly by one -- Pluto in July 2016 -- (maybe another afterwards) and we will get a sense of their surface. There seems to be this systematic process that we go through with all objects, be it asteroids or the Jovian system, or other planetary systems.

The below is one of my favorite pictures.

Jupiter and Io
Voyager image of Jupiter with its moon Io in front.

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