Uranus

by Carolyn Porco

Well, you have to understand that I consider myself to be one of the most privileged human beings on the face of the earth because I personally participated in the Voyager (robotic spacecraft: ed) mission to the outer solar system, and I was there for the encounters with Saturn, Uranus and Neptune. And I was a member of the Voyager imaging team for the encounter with Uranus, and then Neptune, so those two planets... I have a personal relationship with them practically, so they are all very exciting to me, so just the thought of studying any of them is... is like going home, almost...

As far as what I find personally exciting about Uranus, Uranus is a very puzzling object. It's tilted relative to its orbit... its axis is tilted some ninety-eight degrees, so it's one of the two or three oddball planets in the solar system that has such an exaggerated tilt, but it's the only large, gaseous planet that falls into that category, so it obviously went through a very catastrophic event. People believe it got hit by an Earth-, Mars- or Earth-sized object, sometime while it was forming, which tilted the planet on its side and caused it to have this bizarre rotation. And all the objects in orbit around it, the rings and the satellites are all in the equatorial plane, so they formed afterwards, obviously, or else they would not find themselves in the place that they do. So that's puzzling: is that really what happened? I mean, I just told you something that we believe to be true, but is that really what happened? Did a large proto-planet actually hit Uranus and cause it to tilt on its side?

If indeed that did happen, then it also has very interesting implications for the composition of the objects that orbit around Uranus because such an event would probably have spewed out materials from the proto-Uranus object, and the material that got spewed out into this disk would have been very different from the material that existed in the outer part of the (proto-planetary: ed) disk, for example, in composition. And that ties into the fact that the Uranian rings and the Uranian satellites are very spectroscopically peculiar compared to the objects that are orbiting around Saturn and Jupiter, for instance. (see the next update for more on HST and its instrumentation: ed.)

...The material, the dark material that we see in the Jovian and the Saturnian systems, that is, in the rings and in the satellites in orbit around those two planets, is very dark but it's red, and we are coming more to the idea that the material in those systems is organic... However the material, the dark material in the Uranian system is sp ectrally gray, that means it has about the same brightness across the visible portion of the spectrum, so it's different. Now, we don't know why it's different. And it may be compositionally different, it may be different because of some process that's going on in the Uranian system and has been going on since Uranus formed, we just don't know the answer to that...

Why are the observations important? ...the chemistry of the solar system is a very interesting issue for us... We want to understand how the solar system originated, why it looks the way it does, because, ultimately, frankly, what we really want to know is how we got here -- we're all pretty self-centered about this -- we want to know how life got started on the Earth, and chemistry is a very important issue for that. So we also want to understand the chemistry of the outer solar system because we believe the material in the outer solar system, as you march further and further out into the outer solar system, you get more and more pristine, unaltered material ...so in the outer planets, or in the systems of the outer planets, the satellites and the rings, there is material there that we don't understand quite yet, and it may have bearing on the chemistry of the rest of the solar system. So, if we could figure out what we don't know now, and that is what is the composition of the material in the Uranian system, and its satellites, and in its rings, then we will have a piece of the puzzle that we didn't have before, even a piece of the puzzle that we were unable to get with Voyager.

What made me want to become an astronomer anyway?

I actually became interested in astronomy through an interest in religion and Eastern philosophy. I was at a very questioning stage in my early teen years, thirteen or fourteen, I was going to a parochial school, Catholic school, I was having a lot of religious concepts more or less shoved at me, and I was supposed, of course, to accept them unquestioningly ... I just started to think about religion in general, about philosophy, I started to read existentialism and the whole thing ...and along with this internal questioning I found myself one evening ...waiting for the bus to go home, this is in the Bronx...and I am waiting at the bus station and it's bedlam, it's rush hour, it was dark, it was a Fall or Winter evening, there are cars and people rushing everywhere and I just, you know, looked up and I saw a very bright object, I don't know if it was Jupiter or if it was Sirius, but it was a very bright object, and I just started to mull about this, and think about, you know, what was out there. And so my thinking went from being very internally-oriented to being more externally-oriented, and I started to read about planets and stars and galaxies and... and what the universe contained as a whole.

...I also had a friend who was going through a similar thing, she had got a telescope for Christmas, and she and I went to the top of her roof with the telescope and we peered through it, and I don't remember if it was Jupiter or Saturn, actually, that we first saw, but you would have thought we discovered it, it was so exciting... we hopped up and down and hooted and hollered it was just one of those wonderful moments, kind of a communion with the universe. So I came to the study of astronomy actually starting on more or less a religious quest, and then it got diverted into a real interest in what was out there and how I fit into things.

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

One of the most pressing questions that remains after Voyager visited the (outer) solar system and now is making its way to the stars, is the question of the composition of the satellites and rings that encircle Uranus. Voyager was not equipped with the proper instrumentation to investigate that issue. From what we have been able to discern about the composition of these bodies it appears they are very different than the objects, compositionally, very different than the objects that we see encircling Jupiter or Saturn, for instance and we want to know why. And Hubble is in a position to come up with that answer, and I think that we would be losing a valuable opportunity to not go for it.

The Hubble space telescope ...first of all ...is a very reasonably large telescope, it's two and a half meters, and it's above the atmosphere, so it doesn't have the obscuring or scattering effects of the atmosphere ...and it is equipped with very sophisticated instrumentation, so all of those (things) taken together allow you to have a resolving power, a spatial resolving power around planets like Uranus, that far exceeds anything you can do on the Earth, so we can see finer details around Uranus than we could before.

So you need the Hubble space telescope, for example, to get good, visible pictures of the rings, and to see some of the ten satellites that were discovered by Voyager. OK, there are ten satellites in close orbit around Uranus. We had known, of course, before that Uranus was encircled or orbited by five larger satellites, but Voyager found ten smaller satellites, and they are close enough to Uranus that in a ground-based telescope you can't really see them because of the glare and the scattering by Uranus in a ground-based telescope prevents you from seeing them. ...if you really wanted to press for very high spectral resolution then you would go to the Faint Object Spectrograph (we'll explain more about the various Hubble instruments when we decide which planet to observe, and our Planet Advocates begin to specify the details of what we might look at: ed.) and instead of an image you would be taking the light that comes from the rings or the particular satellite and spreading (it) out across the spectrum and looking, at very high spectral resolution, at that light to discern, if there are any ...specific spectral features ...one would take a spectrum of an object to figure out what its composition is, because each different element or compound leaves a different signature on the light that's reflected from an object. And so you take a spectrum to figure out if element or compound A exists on the surface of this object, or B (or) C or D, and you can distinguish different compositions by looking at a spectrum. (see the "Live from the Stratosphere" Teacher's Guide for more on the electromagnetic spectrum)

...I'm certain that if we had three orbits to devote to Uranus we could do a bang-up job in understanding the composition, or at least getting visible spectra, ultraviolet and visible spectra of the rings, and of the ten new satellites that Voyager discovered.

...Let's (consider) the premise that you ...devote only one orbit to Uranus and answer all of these fundamental questions. What else would you do with the other two orbits? I guess I would be split between sharing with Neptune, or sharing with Pluto, for different reasons. Neptune also has a set of rings. They're dynamically even more puzzling than the rings of Uranus ...Neptune has arcs, a series of arcs, they seem to have a particularly dynamical relationship with one particular satellite, Galatea, but we are not completely convinced that that is the whole story with regard to the Neptune arcs and we could answer that question if we could take images of Neptune's ring arcs and pin down their orbital period. That would be a fascinating thing to do ...However, you'd have to devote at least ...one or two orbits to Neptune in order to answer that question, because you want to see the motion of the arcs.

My other favorite, however (I guess I'm coming up with three favorites here!), the other favorite is Pluto ...Pluto is an object we haven't sent a spacecraft to yet. It's one I'm dearly hoping we do send a spacecraft to, but that spacecraft is not likely to arrive at Pluto until the year 2013, so we'd have to wait a very long time. We have the opportunity with the Hubble Space Telescope to answer some very interesting questions about Pluto: what is the nature of its surface, does the surface of Pluto look, or have anything to do at all or resemble in any way, the surface of Triton (the large moon of Neptune: ed.)? They seem to us to be kissing cousins, Triton and Pluto, but you know, we thought Neptune and Uranus were kissing cousins, and they look quite different -- they are very, very different -- so it's very likely the same will turn out to be the case for Triton and Pluto.

So, I'm hoping we'll get at least one orbit on Uranus, and if we need three then there are certainly very fundamental questions you could answer ...students would have, I think, a thrill participating in them if we got observations on Uranus. If we end up needing only one one orbit, then I guess my next favorites would be Neptune's rings, and after that Pluto.

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

...the best thing is that it is far more sophisticated than anything we've had in orbit around the Earth before. Why it's in orbit around the Earth -- which is part of the reason why it's complex and complicated to use -- circumvents a lot of the problems that have beleaguered ground-based astronomers ever since the telescope was invented. So, the good part of it is that the observations are much finer that you could obtain from the ground, and the worst part is that it will involve a lot of people, and everyone has got to get their act together...

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

This is the name of the game ...debating one scientific objective over another, passing value judgments as to what would be most important, most valuable, most informative to do. And there isn't always an answer, there isn't always a yes or no, and a right or wrong. That's a fallacy we should dispense with right from the start.

It often is the case that every scientific objective or scientific requirement that a scientist would sit down and try to evaluate is equally valuable and it's almost a matter of tossing coins and seeing if it comes up heads or tails as to which will win out and which will not. So, weighing the merits and the value of a particular observation is what scientists have to do all the time. We do it in the course of choosing what particular topic it is we want to investigate. Are we going to branch out and try a new line of investigation, which should that be, what is the area that has the most valuable, the most fundamental questions? There is often many of them and you just choose we just choose what tickles your fancy. In this case it will probably be much the same.