Pluto

by Dr. Marc Buie

Pluto just in 1988 passed perihelion, which is the point at which it's closest to the sun, and it's gonna begin its hundred... and twenty year voyage to its most distant place in its orbit. And over this time Pluto is going to receive less and less sunlight, and basically cool off, so we have, now, an opportunity to study Pluto when it is at its warmest, and if we don't take the opportunity now to... make these observations we'll have to wait another two hundred and forty years to repeat the experiment.

Pluto for young people -- and I consider myself still young, although I might not look it anymore -- Pluto... is sort of the last "astronomers' planet". We haven't yet had a close-up view with a spacecraft. We have an opportunity here to see the development of a science- and a knowledge-base about Pluto develop in our lifetimes. And certainly the past ten years have been exciting, watching what we learned about Pluto, and I am certain we are going to learn a great deal more, but this is sort of the special epoch in human history where we are learning for the first time what this planet is all about...

Why are the observations important?

The general exploration of the solar system means that we get to go out and study the planets that are around us. Each one of these is a laboratory for different conditions in and around a star, and every one of these is important. Pluto (is) one of the few sets of such planets that we have to observe, and it makes sense to learn as much as we can about our own neighborhood, in hoping and planning for -- one of these days -- that maybe we'll break out of our own solar system and go on and explore other planets. But until then, we need to make the most of the eight neighboring planets that we have... Each of these planets has its own personality, they are all unique and tell us something special about that location in the solar system, and Pluto will certainly be no exception in that regard.

...The most exciting thing that could come out of this (PASSPORT TO KNOWLEDGE: ed) observation, I think, is that we could take a picture, or say, a set of 3 pictures, get a map of the surface and compare it against the map that we did 3 years ago, and look and find a real change. That some patch on Pluto is now brighter, or darker, and this will start to tell us something very important about how fast the surface might be changing in response to its changing seasons. And I would consider that to be a very fundamental discovery...

What made me want to become an astronomer anyway?

For me, my career and what I decided to do with my life and studying Pluto, mostly, has grown out of a... childhood fascination with space ...my earliest memory is of the Mercury launch -- sitting in a dark, cold, living room, watching the TV, and I must have only been three or four years old at the time, but I can clearly remember watching the lift-off.

...and as I was growing up there was this great future that we were looking forward to. With what we were thinking, back when I was in junior high school, I figured by now we'd be on Mars. And now I would say that I am a little bit disappointed that we kind of retrenched, and we are not as far along as I think we ought to be or could be. But on the other hand I have to say that the thing that's got me... excited about science was the space program, and the exploration we were about, and doing and looking beyond the Earth, and out into the solar system. And from my earliest memories that captured my imagination and has propelled me into what I do today.

How can the Hubble Space Telescope make unique observations of these planets, and how many orbits will it take?

The most exciting thing that could come out of this observation, I think, is that we could take a picture, or say, a set of 3 pictures, get a map of the surface and compare it against the map that we did 3 years ago, and look and find a real change. That some patch on Pluto is now brighter, or darker, and this will start to tell us something very important about how fast the surface might be changing in response to its changing seasons. And I would consider that to be a very fundamental discovery.

At this point, having learned a fair amount about Pluto, we are now moving on to things about... is it changing? Pluto is in a very elliptical orbit and is moving away from the sun, and we expect it to be cooling off. The atmosphere may collapse, all of the frost that we know that's on the surface might migrate to the dark pole, and I am gearing up observations from the ground, and hopefully from Space Telescopes as well, that will continue to monitor for these changes ...one possible change we might be looking for is the change in the brightness of the surface. As the frost migrates to the dark pole it might leave behind da rker material. Where this might be important if we ever are successful in sending a spacecraft out to Pluto, we need to know how to set our exposure time, something as simple as that. If you get there and you take pictures with the exposure time too long, you'll get saturated pictures, and then you've wasted your ...you trip out there for a bunch of over-exposed pictures.

...With three orbits of space telescope time looking at Pluto, you can do a fairly good job of mapping the surface at one instant of time. Now, Pluto takes six and a half days to rotate, so that you need to have three orbits spaced equally over that six and a half day period, and each time you get a snapshot of the surface. By taking these three snapshots one could piece together a complete map from just three orbits of Space Telescope time, and that would give us something to compare against observations taken a few years ago from Space Telescope, from which we could begin to look for changes that might ... might have cropped up on the surface... observations like these, every three to five years, would be very useful in continuing to monitor for changes on the surface, and it's something that we ought to be doing as just a matter of continued scientific exploration of Pluto.

...the other thing that's happening is that Pluto's rotation axis is tipped relative to its orbit, similar to Uranus, so at times we see it "equator-on", and at times we see it "pole-on". At the time of perihelion ...not only was it closest to the sun, but we were also looking at it equator-on, seeing the entire surface. As time goes on, and Pluto moves around in its orbit, we are moving more from an equator-on view to a more pole-on view. The longer we go, the less of the surface we are going to see, because one pole is receding from us and is going to be hidden from view and be in total darkness for over a hundred and twenty years.

With a single orbit of Space Telescope time you can still get a map of the surface, but it would only be one side, and we can use one... this portion of the map to compare against previous maps, but there'd be the other side of Pluto we would be wondering about, maybe that was the side that changed, and we just got unlucky and we are looking at the boring side.

Two orbits doesn't quite give you full coverage, because if you manage to adjust the experiment so that you looked exactly at opposite hemispheres of Pluto you got all the land, so to speak, that's on the limb or the edge of Pluto, which is highly foreshortened, and you just can't get that much information about the regions that are near the limb when you take a picture. So when you take a single snapshot you are getting the information that's more in the center of the disk. But taking three is really a minimum. Four is better, but three will work, and then you'll get these pieces that are on the limb will be more closely toward the center, and that will help us do a better job putting together a complete map.

The maps we can get out from the Hubble space telescope will not be maps that you might be used to seeing, like of the Earth or even Jupiter or most any of the other planets, because it's so small, but they will be at least as good as those that you could draw of the moon, naked eye.

Pluto is actually quite small. It is very large in my mind because I work on it all the time, but I have to keep reminding myself that Pluto is a place that is actually smaller than our own moon, and it's just this tiny pinprick of light up in the sky and it takes a fairly big telescope to see it at all ...the other interesting aspect about Pluto and Charon, Charon being the satellite (moon: ed.), is that the satellite is half the size of Pluto. Before it was discovered, the Earth/moon system was considered to be the most unusual in that respect, with the satellite being quite large compared to the planet, as opposed to the Galilean satellites around Jupiter which are very very tiny compared to Jupiter. But in the case of Pluto ...the satellite is only half the size of Pluto, which makes them very nearly the same...

What's the best, and worst thing about working with the Hubble Space Telescope?

One of the best things that the Hubble Space Telescope has going for it, for doing observations on Pluto, is... that it is a robotic instrument. Commands are put together that tell it what to look at, and when to look at them, then they are done weeks and months in advance by this team of people at Space Telescope Science Institute. So I can put in observations that say, "look at Pluto now, for one orbit, forty-five minutes, then in three days do another one, and three days later, do another". OK, this is unique. Ground-based observatories are not allocated in terms of time this way. Ground-based observatories, you get a night. It's twelve hours. And in that time you have to, you know, you make your observations But if all I need is forty-five minutes, then I've got to figure out what to do with the rest of the time. If I can't, then I don't get the time.

The worst thing about the Hubble Space Telescope is the complexity, and there is a great deal of uncertainty. I've got a lot of experience, having worked with Space Telescope in the past. And in the past we've gone through a great deal of expense and effort to put together an observing plan. We've said, "OK, let's look at Saturn and we'll take these pictures and put them all together. " And then something happens on the telescope, just hours before your observation is supposed to take place ...there is some failure, some component on the spacecraft fails, and all of the sudden it goes in what's called "safe-mode", and all the observations that were in that week are now lost and they have to completely re-plan, fix the problem in the telescope, and -- it can happen -- if you are looking at a sort of unique time, that you may not get to repeat the experiment, or you may have to wait a month.

Why is the Internet a unique tool to connect students to real science and real scientists?

...I think it's great working with high school students, working with undergraduates, graduate students. I was fortunate enough as an undergraduate to be working in a laboratory rather than just simply taking classes and that was very important for me in terms of my getting into science and where I am in my career today and I am just trying to give a little bit of that back in any way I can.

The Internet is an amazing resource ...those of us that do research all the time, we have actually been in contact, and have been using the Internet for a decade already, and it has changed the way we work. Most of the people I work with are not at Lowell Observatory, they are at other observatories -- University of Hawaii, M.I.T., Cornell -- you name it. I can collaborate and work with anybody ...a student, in principle, can contact anybody.

But the other neat thing that's come out about the Internet in the past couple of years is the World Wide Web, this mechanism for browsing electronic documents that people can put together. So, for instance, any information that I have about Pluto ultimately I can put into the form of electronic Web pages, and then people, if they are working on a school project, they want to find out something about Pluto, they just go and they do a search on the Internet for information about Pluto, and they are immediately connected to state-of-the art information, by someone who is doing research on Pluto, and can find out what do we know NOW, and not have to wait for the five or ten year cycle for that information to trickle into the encyclopedias, and then from the encyclopedias to the schools, and then you are finally looking at that stuff ...and the volume of information that we can provide is, is far gre ater than you'll ever have in this type of fixed format information services.