 Welcome to this edition of NASA Images. I'm Lynn Bonderant. During this show, we're focusing on results of spacecraft examination of Comet Halley and the seventh planet from the Sun, Uranus. Let's begin with some images from Uranus acquired in early 1986. The place is the NASA Jet Propulsion Laboratory in Pasadena, California. Well, just about two minutes ago, Voyager 2 passed through its closest approach to Uranus. Next speaker is Dr. Bradford Smith, the imaging team leader. Just for the moment, let me show you a lower resolution image that we have. So what you're seeing here is where it is very red, you're seeing a lot of haze, and where it's blue, it's relatively free of haze. And the first thing you notice is that as you look toward the limb, of course, you're gonna be looking through more atmosphere and seeing more haze. And so we see this brightish, what we call limb brightening around the limb of the planet, indicating looking into the atmosphere and seeing quite a bit of the particulates of haze that are in the atmosphere. I thought it might be a good time now to give a summary of what we've learned so far. It's gonna be a very brief summary, and I'm certainly not gonna be able to go into many details, but at least just to review where we stand and what we've done. Over the last several hours, we have been seeing the playback of recorded photographs taken primarily of the satellites, the major satellites, the five satellites of Uranus at the time that we passed by those most closely. I think that I wanna run through some of those. I think we'll start out with this one of aerial. This is a part of a mosaic of one of the brightest of the satellites, and the whole mosaic consisted of this in the upper left, and then one picture down the lower left, and some of the more interesting things are such views as this, a rather large bright area which is probably associated with a crater, another crater here, bright area. And then moving on with another view of aerial, the upper right, and some of these canyons through here down around this area, much of the unusual terrain and quite strange. Another view, the lower right and this feature right here appears that some of these valleys are sunken. There's been internal activity on these moons and a lot of it and it's surprising. Move on then to, over on, this is not, does anyone here as close as the one we just saw? And Miranda, this is a somewhat distant shot of Miranda. We will get much closer, an unusual feature we saw there was called this chevron formation, but we'll get a better picture of that as we get in closer to Miranda. Because indeed we did get close. This is the one where we saw things down to a kilometer resolution and structure here in Miranda which is quite amazing. The black dropout across the middle here we believe is somewhere in the transmission in there since this is played back from a tape recorder, another playback will recover the information from that section. Here, a curious flow of material down in this direction, that's what it appears to be, but of course it waits for further analysis to see what it is, but something's been going on in Miranda, it's a very small moon, 500 kilometers across and to have this much activity in such a moon is quite different than what we saw, for example, in the moons of Saturn. Another part of it, the mosaics, still another part, again, very curious markings on the surface and right in the middle, the chevron we saw at a great distance is now this object here. What is it? Well, further analysis may tell, we hope. And then backing off with a wide-angle lens to get the complete picture. And then looking back toward Uranus, and let's now talk a little bit about all the other things we've seen besides photographing the satellites. We have discovered one more small satellite, making 10 in all that have been discovered by the Voyager and its approach and near miss or close approach to the planet Uranus. We've also discovered another ring, so that makes a total of 10 small moons discovered and 10 rings, one discovered by Voyager, the other we're known previously. This is a ring picture taken on the approach to Uranus and you can see the major, I think this is the Epsilon ring, there's a bright one and then some others inside of it. We also tried another one when we got behind the planet with the sun shining through the rings toward the Voyager spacecraft to take a picture of that. We expected to see a very bright glimmer because of the very small particles in the ring, which in this backlighting arrangement would tend to glow just as when you see dust motes in the sunlight shining through a window toward you, you can see lots more dust than you can if you actually look from the point of view of the sun. Small particles tend to scatter life directly forward, keep it going. Surprisingly enough, we saw very little from that viewpoint, which is a bit of a mystery. How come there's so little dust? We've measured the magnetic field now. We were almost two Uranus before we discovered that there was indeed a magnetic field. If it hadn't had no magnetic field, that would have been a real problem. But there is, it's about 15% weaker than the Earth's on the surface. The curious thing about it is this. Uranus has its axis pointed pretty much, its pole, south pole by committee decision, pointed toward the sun. So if the sun's over there, that's where this pole is spinning around like this on that axis. The magnetic field, however, is on a pole at an angle, 55 degrees. So as Uranus spins around on its spin axis, the magnetic field is spinning around like this, which is making, is going to make some very curious effects on the total field. We have displayed now on the screen a picture of titanium. There'll be two of these, very much the same. One is sort of an insurance for the other. This one is in and the second one should be coming in any minute. In the meantime, we've also had the best two pictures of Umbriel, which is here, very almost featureless. And by the way, these are the two pictures that came in there are very similar to each other, taken very close together. I think the thing which surprised most of us was not that Miranda necessarily had all these tectonic structures on the surface, but that there was such a variety, that it was really a very complex distribution. And I think the other thing that surprised me was the extreme relief, which seems to be present on the surface. It's not a very subtle property at all. One sees very sharp scarps, ridges. One sees a grooved terrain. One sees terrain similar to that, which we saw on Enceladus, which is somewhat twisted. It's just a remarkably complex surface. And are those cliffs up at the top? Those are very high cliffs, which are quite remarkable. They terminate a very rugged, hilly terrain, which may well be a place where there is basically lateral thrusting. And down at the lower portion of the planet, we call it a planet, it looks like one of those. That's right. So this moon, Uranus, we see again some of the same curious lines that are going through. That's right, that's right. Fractures, the surface has been heavily fractured by some sort of processing. There may well be some layering on it in the lower right-hand portion of this image where one sees these alternating dark and lighter streaks, that's right. Perhaps that's layering of material. It's really a remarkably complex surface for an object that's barely 300 miles in diameter. The rings of Uranus have provided some thrills and surprises for several of Voyager scientists, those who specialize in studying planetary rings. One of those is Dr. Jeff Cuzzy from NASA's Ames Research Center, and he's with me now. And just let's talk about these strange rings, and I'm gonna ask if somebody can to put up on one of our video ports 70, this picture we want. Jeff, the rings have been, well, we knew about that they were there before. Before Voyager got there. But we've discovered quite a bit more about them, and particularly about the detailed structures that we've been there. What can you say about them? Well, you know, the main rings of Uranus, Oliver, were known from stellar occultations. Several of us who think about rings tend to associate small objects, debris, if you like, of leftover from the formation of the moons and the planet that are associated with rings. When you have all this debris around with the impacting meteoroids that are always there, you always expect to have dust. Now, this dust is very short lived because it drags into the planet, it gets destroyed by the magnetic field, spider ray, one thing another. So the location of the dust is a very important tracer of where the material is. And so we were very eagerly anticipating the existence of the structure of the dust. Now, because of the magnetic field and probably because of the perturbing effect of the magnetic field on the charged particles that are small, most of the dust seems to have been removed from the Uranus system. And so- This is a picture we got at that point. Yeah, that is an absolutely spectacular picture and it shows basically that most of the region of the known rings is more or less replete with dust. The Voyager Uranus encounter has shed new light on this unique planet. Some of the preliminary findings include 10 new moons were discovered, one new ring was found. Two shepherding moons are located near the Epsilon ring. The rings are slightly varying in composition and color and are complex in structure. The five known moons are an ice rock mixture with evidence of ancient geological activity. Aerial was volcanic in the past. Miranda is considered the strangest body in the solar system by many JPL team members. It has scarps, cliffs, and layering. The atmosphere of Uranus is 15% helium and the rest is hydrogen, ammonia, and methane. The planet may have a rocky core of the size of Earth. Uranus is covered with an ocean of mostly water, methane, and ammonia. Our next report comes to us from the NASA Ames Research Center near San Francisco. The report was released in 1987 and summarizes some of the findings of the Pioneer Venus spacecraft which earlier examined Comet Halley as it looped around the sun. As long as man has been conscious of the sky, comets have been objects of special fascination. Our ancestors thought they were omens of ill fortune. Today, we see them much differently. Dr. Jeffrey Cousy is a NASA scientist who specializes in comets. Today we know comets are not magical apparitions, but mountain-sized dirty snow walls of ice and rocky dust. They're living fossils left over from the early days of the formation of the solar system. But there may have been some truth to the old superstition. A large comet hitting Earth may have been the cause of the death of the dinosaurs and other comets may have caused earlier episodes of mass extinction. Even today, Earth is not entirely safe from comets. In 1908 in a remote area in Russia, 200 square miles of forest were blown down by an explosion with the force of a 12-megaton bomb. It was probably caused by a large comet exploding in the atmosphere. Experts estimate that a comet strikes Earth about once every million years. There are a number of reasons for studying comets. Well, the more we learn about the material comets are made of and the way that material is put together, the better idea we'll have of the situation under which the planets of the solar system are formed. It is extremely difficult to reconstruct our planet's origins by studying the material it is composed of today. Even the oldest rocks on Earth have been changed and modified repeatedly by geological processes such as melting and solidification, wind and water erosion. In the 1970s, NASA studied a number of different missions to Halley's comet, one of the brightest and best known of the thousand or so comets that passed periodically by Earth. But budgetary constraints kept such a mission from getting off the ground. Nevertheless, the 1986 return of Halley's comet proved to be a scientific spectacular. Almost all the world's telescopes were riveted on the comet. In addition, the Soviet Union, Japan and the European Space Agency launched spacecraft to rendezvous with a famous visitor. Meanwhile at NASA's Ames Research Center in Mountain View, California, US scientists found an innovative way to participate. Richard Fimmel, NASA's pioneer project manager. In 1981, we started looking into how we could use an existing spacecraft to make measurements of Halley's comet as it came by. This spacecraft is the Pioneer Venus Orbiter which was already in orbit around the planet Venus. It was inserted in orbit in December of 1978. The surface of Venus is completely obscured by a heavy cloud cover, so we really knew very little about the surface of the planet. The Pioneer Venus mission consisted of a pair of spacecraft, the orbiter, and a multi probe, which carried four instrumented probes which were released before the spacecraft reached Venus and they plunged to the surface of Venus making their measurements with the instruments on board as the probes were descending to the planet's surface. Together, the orbiter and the multi probes provided information about the atmosphere of Venus why it is so seringly hot and dense. Also, another instrument on board the spacecraft took thousands of images of the cloud cover of the planet Venus, helping to explain the atmospheric circulation about the planet and unraveling many of the mysteries that Venus has held. Fortunately, the cosmos arranged it so that we had a ringside seat as Halley's comet came by in its path closest to the sun, which is the time when the comet is its most active. Halley's, like all comets, is made up of several parts. The nucleus is a large dirty snowball frozen as hard as marble. It is composed of roughly 40% water ice, 10% frozen gases, and 50% dust and rock. When a comet's looping orbit carries it close to the sun, the intense sunlight begins to boil dust and gas from its surface. Invisible hydrogen gas forms a large egg-shaped cloud called a coma, and the comet develops two spectacular tails, a straight tail made of electrically charged gas and a curved dust tail. At the University of Colorado Boulder, Dr. Ian Stewart, chief scientist of the Pioneer Halley's mission, determined that he could make unique and valuable measurements of the comet using one of the orbiter's instruments. In 1983, when we looked into the possibility of observing Halley from Pioneer Venus, we realized that it would come within shouting distance, celastically speaking, and that we could make really valuable measurements of the hydrogen coma. When Dr. Stewart explained his conclusions to us, we were immediately enthusiastic. We knew, of course, that it would mean a lot of extra work, but we also knew that it would provide the United States with a unique Halley mission capable of making continuous measurements through the entire perihelion when the comet was at its most active. Relative to using the ultraviolet spectrometer instrument to look at comet Halley, that was not originally planned as part of the Pioneer Venus orbiter experiments as they were laid out. Earl Montoya, pioneer program manager for NASA. We had very innovative, bright people say, what if we rotated the spacecraft so it would look out at comet Halley? Well, it turned out that the mission could be accomplished the gathering of these measurements using the ultraviolet spectrometer device at no additional cost. The operation seems to be going very, very smoothly at this point. The spacecraft instrument that Dr. Stewart used is called an ultraviolet spectrometer. Like a prism, a spectrometer divides light into different colors or wavelengths. By measuring the brightness of the light at each color, scientists can tell a great deal about its source. Dr. Stewart's ultraviolet spectrometer is fixed to the rotating orbiter. So the spacecraft had to be reoriented in order to observe the comet. Now, normally the spacecraft in orbit around Venus is orbiting with the high gain antenna pointed towards the south ecliptic pole, or we would look at it as upside down as the spacecraft is spinning. In order to observe Halley's comet, which is coming by Venus up and above, we had to carefully rotate the spacecraft 135 degrees so that the ultraviolet spectrometer would be looking up and have the comet in its field of view as it rotated once every 12 seconds. In the process of doing this, a number of things had to be paid attention to. We had to keep the solar panels properly oriented so they were receiving sunlight and to provide the power for the spacecraft. We had to make sure that our star tracker had a bright star in the field of view so that we would be able to maneuver and check the orientation of the spacecraft. And we also had constraints on the position because the high gain antenna can only move a limited amount in elevation and that has to stay pointed at Earth as it is despawned. Halley's orbit takes it from the outer reaches of the solar system, about the orbit of Uranus, to within 55 million miles of the Sun. Well, because Halley's comet is most active when it's close to the Sun, it is most important to observe it at that time. As Halley's comet made its closest approach to Venus and we were in the midst of our critical imaging sequence to get an image of the comet's coma, nature threw a number of obstacles in our way, threatening to disrupt the entire data stream force. First, we had a giant solar flare which interfered with radio communications between the spacecraft and Earth. That was followed then by rain in the Mojave Desert and snowstorm in Madrid and both of these also attenuated the radio signal causing more errors in the data. Well, in spite of all these problems, we were able to get all of the data that we needed. The centerpiece of our observations of the comet was this false color image of the hydrogen coma. The scale is immense. The image that you see is 12 million by eight million miles and briefly, during its passage past the Sun, Halley was the largest object in the solar system. The image uses color to represent brightness. The blues and reds are the darker levels of light and the yellows and greens are the brightest parts of the comet. Well, analyzing the image and other data allowed us to estimate the rate at which water was subliming away, evaporating away from the ices of the nucleus. Our measurements turned out to be unreasonable accord with European and Soviet measurements and they indicate that the comet is losing one or two hundredths of a percent of its total mass each time it passes the Sun. Before this last visit, many scientists had assumed Halley's was a relatively fresh comet so the extent of the erosion and the amount of dark crust on the nucleus came as a surprise. Pioneer Venus observations showed large daily variations as much as 25% in the rate at which water was boiling off the comet. The best explanation for this is that only portions of the comet's surface had freshly exposed ices that could act as sources of water vapor. The evaporation rate varied depending on how much of this fresh material was exposed to sunlight as the nucleus rotated. The orbiter's observations fit nicely with those of the European spacecraft Jotto which passed about 300 miles from the nucleus. Its cameras revealed a cold black object three to four times larger than the size estimated for a fresh comet. The nucleus also appeared highly irregular, almost potato shaped, suggesting it was once much larger and has eroded away over a long time. Pictures from Jotto also showed gas jets spewing from the nucleus and the orbiter measured their velocities as high as 1,100 miles an hour. While the gas escapes, most of the dust that boils off the nucleus remains in the comet's feeble gravitational grip. As the comet moves back out away from the sun, the dust rains back onto the comet's surface. Scientists have begun to realize that this dust plays a central role in the life cycle of a comet. When the layer of dust becomes about three inches thick, it'll act as an insulating blanket and insulate the ice from the heat of the sun. And once that happens, the ice no longer evaporates so the comet loses all its cometary features like tails and dust tails and ion tails. And then it looks a lot like an asteroid. Scientists estimate that comets make only a thousand or so passes around the sun before they're reduced to inactive objects like asteroids. With even a partial dust crust, a comet is slower to warm up and develop a tail. Once heated, however, the dust blanket retains the warmth and the comet keeps its tail longer. That's why Halley's was brighter in April when it was leaving the sun than in December when it was approaching. The orbiter's observations show that the nucleus remains surprisingly inactive until a few weeks before the comet's closest approach to the sun. But it stayed 30% more active on the outbound leg of its journey. It is surprising that the activity turns on so suddenly. The comet gets to within a certain distance to the sun and then it starts to pop like popcorn. The pioneer Venus orbiter made a number of other important observations. For instance, it measured the concentrations of hydrogen, oxygen, carbon, and hydroxyl ions given off by Halley's nucleus. Combined with the observations made by other spacecraft, this information is helping scientists calculate the comet's composition. While it has given scientists a much better picture of what Halley's is like, the new data is also raising questions that can only be answered by future comet missions. One such mission is CRAF, the comet rendezvous and asteroid flyby. It involves shooting a probe into the heart of a comet nucleus to get the first direct measurements of what it is made of. Scientists will study the new information on Halley's for decades. As a result, our knowledge about the origin of the solar system, about its primordial mountain-sized building blocks and how they join together to form the Earth and other planets, will continue to grow. In this fashion, humanity will gain the understanding of its ultimate environment, the cosmos. That's all we have for this edition of NASA Images. But before we go, let me remind you that you're cordially invited to see the displays at the Visitor Center here at the NASA Lewis Research Center. We're located near the Hopkins International Airport in Cleveland. Admission is free. Until next time, this is Lynn Bonderant saying goodbye.