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Solar System Exploration at 50
Exploration Stories: Favorite Historical Moments

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Real Time
David Grinspoon
Curator of Astrobiology, Denver Museum of Nature and Science
Mars from Viking
This spectacular picture of the Martian landscape by the Viking 1 Lander shows the rock "Big Joe" and a dune field with features remarkably similar to many seen of the deserts on Earth.

"Although this is not the image from my 'Viking Memory' it is one of my favorites from the Viking mission." --David Grinspoon

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

Viking: The first one I have to mention is the landing of Viking 1 on Mars and the TV broadcast in real-time of the first surface image. I happened to be 16 that summer -- a sort of budding space geek -- and I was actually living at the time in a tent with my cousin in Vermont. My cousin and I knew that this event was happening and so we drove into town and went into this good old country store. I just remember watching that old black and white TV the moment when that first picture was coming down live. (The pictures used to come down in these vertical strips, from left to right, one strip and then another strip and another.) At first it looked like -- well, we couldn't tell what it was and then gradually enough of it appeared so that we could tell that there were some rocks sitting on the surface. I remember Carl Sagan doing live commentary on the TV while it was happening, which was actually very gutsy -- we didn't know what we were going to see or if it was going to work or not.

There was something about that moment that really furthered my romance with the planets, because once you are there on the surface of another planet, or have a machine there photographing landscapes, it makes it much easier to imagine yourself looking across the Martian landscape with the dust swirling around your ankles. There was just something very evocative about that moment that I will never forget it.

Jupiter by Voyager 2
This Voyager 2 image shows the region of Jupiter extending from the equator to the southern polar latitudes in the neighborhood of the Great Red Spot.

Voyager: The next one I have to mention is the Voyager 2 flyby of Jupiter, in the summer of 1979. I was working as an undergraduate intern to the Voyager Imaging team at the Jet Propulsion Laboratory (JPL) then. Being there in the imaging team area with famous planetary scientists -- real planetary scientists, people that I had heard about, who I revered -- was just amazing. There was something about the Voyager encounters, which were flybys, that was just so exciting and so moving. We were anticipating this for years and people were working on it for years, and then the actual timescale of the flyby was just so short. The spacecraft just flew through the system, snapping pictures and taking measurements. The mission tried to do everything it could, as quickly as possible. However, inertia inexorably kept it going. We could not stop and say: "Wait a minute! Let's take another look at that!" It is kind of like in the movie "The Wizard of Oz" when Dorothy says: "Wait! Come back!" to the Wizard. The Wizard responds: "I can't, I don't know how it works!"

The latest image from the Voyager spacecraft was always up on all of the screens in the imaging room. As the spacecraft approached, the image would get larger and larger of whatever we were looking at; be it Jupiter or one of the specific moons. Slowly at first and then in an accelerating way because as the spacecraft approaches the planet or one of the moons it accelerates due to the Jovian gravity, so it actually sort of speeds up. After being there for awhile and sort of immersing yourself in those views, it felt like we were on the spacecraft, and the monitor there was our view out the window of our approaching destination and then so quickly our receding destination. It really felt like that we as a team were riding along, which in a sense we were.

Europa by Voyager 1
This picture of Europa, the smallest Galilean satellite, was taken from a distance of about 2 million km (1.2 million miles) by Voyager 1.

There was one moment in particular when the first close up images from Europa came down. It was just one of those fantastic moments of surprise and revelation. When in an instant your ideas, and in this case I think humanity's ideas, about some very important things changed from those first images. There was Europa, not at all this sort of bland, ancient, dead, icy world, it should have been according to what we were taught in school. Instead it was crisscrossed with all of these sort of strange markings. I remember Carl Sagan was in the room quipping: "You know Percival Lowell was right, only the canals are on Europa." The markings looked like canals: something new, something strange, something that seemed important, and of course it turned out to be important. So that was an exciting moment.

Triton by Voyager
This color photo of Neptune's large satellite Triton was obtained by Voyager 2 on 24 Aug. 1989 at a range of 530,000 km (330,000 miles).

I was at some of the other Voyager encounters, and the final encounter with Voyager 2 at Neptune was exciting and poignant in a lot of ways. For one thing the last solar system object that Voyager visited up close was Neptune's moon Triton, which of course is one of the coolest objects. Voyager had already been on this epic multi-year journey and then it saved one of the juiciest nuggets for last, which was the strangeness and beauty of Triton.

There was also just that sense of an ending. It had been almost nine years from the first Voyager encounter to the last. People grew up, and aged, and died, and got married and divorced. It was like a family reunion traveling through the solar system. Then there was also the realization that this was the last one. "You mean we don't get to do any more of these?" It was sad, but it was also delightful because who would have thought that Voyager would be so successful or that that the outer solar system would be so strange and wonderful and different from anything that we expected.

Cassini-Huygens: The Huygens probe entry into Titan was really in a certain way the last "first." The Huygens probe was the last time that we would drop a probe for the first time into a completely unknown atmosphere with a solid surface beneath it in our solar system. I remember being nervous about it because it seemed to be such a hard place to get a machine to work, as well as being excited about it.

The circumstances surrounding the landing on Titan were kind of personal again. I had agreed to be a part of a public event in New York City at the Hayden Planetarium at the Rose Center for the probe entry. The timing worked out so that the probe entry and the receipt of the first information to Earth was going to be at 5 a.m. New York time. So we had this public event that started at 4:30 a.m. in New York and it was the middle of winter and there was a massive blizzard going on. There we were opening the doors for this thing that we called "Breakfast on Titan" (we had bagels and coffee) and I remember thinking: "It is 4:30 in the morning in the middle of a snowstorm -- is anybody going to show up?" Well, so many people showed up, that it was standing room only (we had over 400 people).

I kind felt like my hero Carl Sagan then and how he had to, in 1976, commentate on images in real-time, only now fast forward to me as a mid-career scientist. I was there with Neil Tyson and Laura Danly (who worked at Hayden at that time) and we were doing the live commentary in front of all these people. What was remarkable about this event, in addition to the 400 people that showed up at five in the morning in the middle of the snowstorm, was that the images from that last "first"could not have been more exciting and enticing and evocative if we had to make them up. In our wildest dreams we had speculated that maybe there were rivers on Titan, because we thought that there was liquid methane. We knew conditions were right and there seemed to be clouds, so there could be rainfall. Maybe, just maybe with Huygens we would get lucky enough to see something that looked like a river.

River Channels on Titan by Huygens
This mosaic of 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. Credits: ESA/NASA/JPL/University of Arizona

When those first images came down not only did we see a river, but a fantastically branching river that looked like a textbook image of what we would possibly hope to find on Titan. There was something that looked like a shoreline with some ground fog too. Then the probe landed and it kept on taking pictures. It was "story-book" in how successful the probe was and how wonderful Titan was as -- is as a place. It was so exciting. Of course, I had been up all night and was up all the next day with my colleagues who were giving talks and leading public events. You just get into this hallucinatory state of sleep deprivation and excitement -- it is just too important to sleep. That is what I remember about the Voyager encounters too: Since they were so compressed in a day or two of discovery, you didn't worry about not sleeping for a few days. Being cooped up with colleagues, being excited, running on adrenalin, and being sleep deprived just adds to the surreal/hallucinatory state -- as if it wasn't surreal enough to be sending cameras out to Titan and Mars and Jupiter. All these events felt like rites of passage for me, for us.

Magellan: I have to mention the Magellan mission at Venus. I remember when I first laid my eyes on a press release image from early on in the Magellan mission of an area informally called the "Crater Farm." I could see in that one image that there was something really strange about Venus. It still seems really strange now, even though this image is from the early 90s. The Crater Farm image shows these very pristine looking craters, super-imposed on a volcanic background that are not lapping up at all onto the edge of the craters.

Venus' "Crater Farm"
Three impact craters are displayed in this three-dimensional perspective view of the surface of Venus.

At least in this one image there was all this luminous volcanic activity that somehow stopped while the planet got covered up with craters. Of course, that shouldn't be the history of a reasonable planet; that the volcanism should stop and the craters should keep going -- that is unless that planet has a weird history. I remember looking at that picture and thinking that there is something weird going on here and that it has important implications for the whole history of the planet, its climate, its atmosphere, its geology. And I still think that that is true. Venus sort of became a focus of a big part of my career -- figuring out what is weird about Venus as represented in that picture.

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?

In a broad sense, though our exploration of the rest of the solar system we transform the Earth into a place that we can now understand in a comparative sense. We don't have an operating manual for the Earth, but at least we are now looking around and seeing how the other planets work, which seems a necessary step in retrofitting an operating manual for our planet. I would say, and in particular to my field of comparative climates, seeing how climate and climate change is playing out and has played out on other planets has given us some needed ground truth for our Earth's climate and climate evolution -- and not a moment too soon. I think that there is some sort of strange irony in the fact that we are pushing our climate's system to the point that we need this knowledge in order to not do something worse than we have already done, just at the time when some of the same technology is giving us the gift of this broader perspective, which can help us to hopefully understand enough to make the right choices as far as how we manage our planet in the future.

In regards to a more recent happening: I enjoy looking at and thinking about the MESSENGER image of Mercury showing the pits on the edges of the craters. This is an image that shows us that there is still a lot to understand about Mercury.

Mercury Hollows
Hollows on Mercury.

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