Planetary Radio: Space Exploration, Astronomy and Science - MESSENGER Spacecraft Heads for Mercury

Episode Date: August 2, 2004

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Winged Messenger is off to Mercury on Planetary Radio. Hi, and welcome back to Public Radio's weekly adventure in the solar system and beyond. I'm Matt Kaplan. Sean Solomon is Principal Investigator for the Messenger mission, headed this week to hot little Mercury. I'm Matt Kaplan. and the blue sky. And you can't ask for much more than that. Stay with us. Hi, I'm Emily Lakdawalla with questions and answers. A listener asked, why does the moon appear gray in Apollo pictures when it appears yellow from Earth to the naked eye and through a telescope? Asking why the moon looks yellow from the Earth is just another way of asking why the sky is blue.

If the Earth had no atmosphere, our sky would look black even during the day, and the sun would be astonishingly bright. But the nitrogen and oxygen in our atmosphere scatters some incoming sunlight, which has the effect of dimming the appearance of the sun and brightening the appearance of the sky. Nitrogen and oxygen scatter short-wavelength blue light much more than longer-wavelength red and yellow light, which makes the sky appear blue. What does that have to do with the moon's color? Stay tuned to Planetary Radio to find out. Mercury's surface, space environment, geochemistry, and ranging.

With a little imagination, those seven words become MESSENGER, an apropos name for a mission to the innermost planet of our solar system. Though it's not nearly as far from us as Jupiter or Saturn, we really know very little about Mercury. Much of what we do know was discovered three decades ago by Mariner 10, the only other spacecraft to visit the planet. That ignorance will end when MESSENGER passes Mercury and later goes into orbit above its crater-covered surface. Every mission presents unique challenges and opportunities. For MESSENGER, no one knows these better than

Principal Investigator Sean Solomon. He directs the Department of Terrestrial Magnetism at the Carnegie Institution in Washington, D.C. That's not far from the Applied Physics Lab at Johns Hopkins, where MESSENGER was designed and built. When I spoke with him, he was looking forward with excitement to the imminent launch of MESSENGER on its long journey. Sean Solomon, thanks very much for joining us on Planetary Radio. My pleasure, Matt. After all this time, why are we going back to Mercury? Well, Mercury

holds the answers to a long list of questions of great generality for understanding the inner planets. We need to understand that Mercury, Venus, Mars, Earth, all formed by common processes out of the inner part of the solar nebula that once surrounded our young sun. Building blocks were more or less common. The processes of planetary formation were common. The outcomes were extremely different. The four planets turned out, those siblings, to have very different characteristics and

very different histories. And Mercury ended up a rather extreme member of that family. It is the densest planet in the solar system when corrected for pressure. It's the smallest planet to have a magnetic field. It's in a very unusual spin orbit resonance where three days equal two years. It's got the highest diurnal variation in temperature at the equator, and yet we have the suspicion from Earth-based radar measurements that there's ice hidden in permanently shadowed floors of craters at high latitudes near the poles.

It's a real oddball in terms of geological history, in terms of composition. And because it is so different, it probably offers the greatest opportunity to learn which set of processes govern the different outcomes in planet formation for the Earth and our neighboring planets. So as with our exploration of Mars, it sounds like we go on Mercury at least in part to learn about ourselves. In part we do. And implied in your question about why after 30 years are we finally going back to Mercury, there's also a technological component to that answer.

Mercury was visited by only one spacecraft, Mariner 10. It flew by three times in 1974 and 1975 and imaged about half the surface. It discovered Mercury's magnetic field. It discovered several species in the atmosphere. It raised a host of very interesting questions. But in the mid-1970s, we did not know how to put a spacecraft into orbit around Mercury. It took the discovery of some very clever mission designs using multiple flybys of Venus and Mercury

to slow the spacecraft enough to get into orbit, and it took several decades of advances in material science and miniaturization and other engineering innovations that have finally given us a spacecraft that we believe can not only fly to Mercury and achieve orbit but survive a very harsh thermal environment. I admit that I was surprised to see that a craft headed to Mercury had to have such an incredibly complex trajectory. Can you talk a little bit about that?

I know you're going to be coming back to Earth in a year or so. Yes. Every launch opportunity to Mercury offers a different suite of fly-by menu items, depending on the energetics of the particular alignment of the planets at any given time. But there are only limited opportunities to make use of these multiple fly-bys. And the August 2004 window that we are using has six fly-bys before we do orbit insertion. We will be flying by the Earth-Moon system about a year after we launch. Then that will be followed by two flybys of the planet Venus.

And then in 2008 and 2009, we will have three flybys of Mercury. We will incidentally be seeing almost the entire planet sunlit. So in the course of those three flybys, and we'll be seeing terrain that Mariner 10 never saw even before we achieve orbit. And then in March 2011, at the fourth encounter, we do our orbit insertion burn and we go into orbit. And the reason that all of the trajectories to orbit Mercury involve multiple flybys is that the trick is to slow the spacecraft enough

so that whatever propulsion system the spacecraft is carrying can, with an extended burn, achieve orbit. But any time we send a spacecraft in toward the innermost solar system, by the time it flies by anything, it's moving very fast. In the gravitational pull of the sun, it speeds up as it gets closer to the sun. The natural interplanetary transfer orbit from the Earth to Mercury would be quite short, just a few months, but we'd be going by far too fast, more than 10 kilometers a second, to achieve orbit with any kind of propulsion system we could carry along. I think it's fascinating that, at least in part,

the missions to the outer planets, like Cassini, take advantage of these flybys to speed up, and you have to do it to slow down. It's a wonderful business, celestial mechanics, and it's very non-intuitive. I have great admiration for the people who design these trajectories. They just have enabled a great deal of the exploration that we do. Our guest is Sean Solomon.

He's the principal investigator for the Messenger mission on its way or about to leave, depending on whether it's gotten off at the beginning of its launch window or not, as you hear this, to the planet Mercury. If we could talk a little bit more about this amazing spacecraft. You've talked about the fact that you have to deal with the proximity of, among other things, this star, which does tend to make the environment a little challenging, doesn't it? Are there special challenges in protecting a spacecraft that's headed to Mercury? Yes, several. You've identified one. Any spacecraft as close to the Sun as Mercury is will be

experiencing a Sun 11 times brighter, up to 11 times brighter, than it is at Earth's distance from the sun. And our spacecraft protects itself from the sun by using a sun shield. We keep the shield, imagine it as a parasol for our spacecraft, pointed at the sun all the time. And the sun-facing side of the shield, which is made of a ceramic cloth and is backed by multiple layers of insulating material, the sun-facing side reaches temperatures as high as 350 degrees centigrade. Centigrade, wow. But behind that sunshade, the spacecraft side, it's room temperature. Oh, my goodness, yeah.

So it's wonderfully insulating. Of course, we must have a pretty intelligent spacecraft that can sense the direction of the Sun at all times, and if it ever finds itself pointing in the wrong direction, it has to be able to correct its orientation quickly. So it has quite a bit of autonomy and self-correcting software. That said, it's important to recognize that the sun is not the only thermal hazard for a Mercury orbiter. The planet Mercury, on the daylight side, is a substantial source of heat as well.

And the heat re-radiated solar energy that's given off as infrared heat by Mercury is a substantial source of heat to any spacecraft that gets close to the planet. We can't shield ourselves from Mercury because that's the planet we're looking at. So we've used a variety of thermal design tricks as well as specially designed orbit to absorb the heat as needed, to re-radiate that heat back into space during the higher part of the orbit to design instruments that can accommodate a certain amount of heating as we go over the planet but still operate throughout the mission and through every phase of day-night cycles.

Sean, I'll stop you there, in part because we need to take a break, but actually all this talk of heat is starting to make me perspire, so it's a good time to stop for a moment. We're going to come right back, though, to Sean Solomon, the principal investigator for the Messenger mission to Mercury, right after this message. This is Buzz Aldrin. When I walked on the moon, I knew it was just the beginning of humankind's great adventure in the solar system. That's why I'm a member of the Planetary Society, the world's largest space interest group. The Planetary Society is helping to explore Mars. We're tracking near-Earth asteroids and comets.

We sponsor the search for life on other worlds, and we're building the first-ever solar sail. You can learn about these adventures and exciting new discoveries from space exploration in The Planetary Report. The Planetary Report is the Society's full-color magazine. It's just one of many member benefits. You can learn more by calling 1-877-PLANETS. That's toll-free, 1-877-752-6387. And you can catch up on space exploration news and developments

at our exciting and informative website, PlanetarySociety.org. The Planetary Society, exploring new worlds. Depending on when you hear this, Sean Solomon is either leaving or has left for the planet Mercury. That is his stand in the Messenger spacecraft, for which he serves as principal investigator. And, Sean, again, thank you for joining us on Planetary Radio. We were talking about this spacecraft and how it protects itself,

but let's talk a little bit more about how it gets its work done. It has cameras, a couple of them, I understand, but it has much more than that in its collection of scientific instruments. It does indeed, Matt. There are seven instruments in the payload. You identify the imaging system, which has both a wide-angle and a narrow-angle camera, and the wide-angle camera can take images in color. One of our objectives is to have a global, high-resolution color image of the entire surface of Mercury, including the part that Mariner 10 never saw.

We will have several instruments devoted to unraveling the chemistry of Mercury's surface using an X-ray spectrometer, a gamma ray and neutron spectrometer, a very high-resolution visible and near-infrared spectrometer, a very high resolution visible and near infrared spectrometer that will be looking for diagnostic mineral absorption bands in the reflected light from the planetary surface. That's quite a collection of spectrometers, and these all work by looking at the wavelengths of electromagnetic radiation, light among them, that bounce off of the surface? That is substantially correct.

The gamma rays are sensitive to interactions of cosmic rays with elements near the surface. The X-rays are excited by solar X-rays and are emitted at wavelengths that are diagnostic of certain chemical elements at the surface. We're going after the full suite of what Mercury is made out of, and one of the guiding rationales is that there are very different ideas for how Mercury got put together, driven by the observation that its high density means that the planet is mostly iron metal. We think that iron metal resides in a central core that makes up at least 60% of the mass of the planet is mostly iron metal. We think that iron metal resides in a central core

that makes up at least 60% of the mass of the planet because Earth-based observations have failed to detect much iron at the surface. But whether that outcome of a mostly metal planet was achieved early in the formation of the planet or was a result of an extremely hot nebula that vaporized the outer part of a planet, leaving the metal interior protected. Or Mercury may even have been the product of a giant impact

that mechanically disrupted the planet in a way that removed most of the silicate. Those ideas are still competing for precedence in explaining the outcome, but they have different predictions for what we will see at the surface in terms of the composition. So one of the reasons we have so many instruments dedicated to unraveling the composition of Mercury's surface and its variability is because that's such key information for understanding how Mercury got put together and by implication what processes contributed to differences in composition of all the inner planets.

One of the things that I'd like you also to talk about is something you mentioned briefly in the first part of our conversation, and it's that suspicion that there is ice at the poles of this very warm place. Indeed. That suspicion came from Earth-based radar, which showed bright deposits in the floors of impact craters at high latitude. And the radar brightness and the effect on the polarization of the signal were pretty well matched by water-ice on other bodies in the

solar system. The plausibility of ice is based on the fact that Mercury's spin axis has almost zero tilt, so that topographic depression, like an impact crater near the pole, can be in permanent shadow. So no seasons on Mercury. No seasons to speak of. Nothing due to the tilt of the axis like on Earth. There's also not much of an atmosphere.

The atmosphere is extremely tenuous and dynamic, but it is not massive enough to transport heat from the equator to the poles as the Earth's atmosphere does. So a shadowed region at the poles is exposed to space, and the temperature can get as cold as minus 180 degrees centigrade, minus 300 degrees Fahrenheit, cold enough so that most volatile species, including H2O, will be solid and can be stable as solid for long periods of time, billions of years. as solid for long periods of time, billions of years. So if Mercury has outcast water, if Mercury has been impacted by one or more comets or asteroidal objects that have water in them, it's easy to understand how water molecules

could have bounced around within Mercury's gravity field, and some small fraction of them found their way to these polar craters where they are trapped in these frozen deposits. So it is paradoxical that a planet that gets so hot at the equator during the day could have water ice at the poles. There are competing ideas, I should add, for what those volatile species might be. Elemental sulfur is one of them. And the way we're going after the question of whether the water-ice hypothesis is correct is to use a variety of measurements. The neutron spectrometer is key because that is sensitive to hydrogen at the

surface. We'll be carrying an altimeter that will be measuring the shapes of all the geological landforms, including the craters. So we'll be testing the idea that the craters that show the radar bright deposits are, in fact, deep enough to be in permanent shadow. And finally, we're carrying an ultraviolet visible spectrometer. It's the same spectrometer that's looking at the surface to look at reflected light, but when we point it at the limb, we will be able to sense the species in Mercury's tenuous atmosphere, and we'll be paying particular attention to the polar regions where we might be able to see signature of OH, of oxygen, of hydrogen, of sulfur, if water ice is in fact not the material making up the polar deposits. So there are a variety of tools that we'll be applying to this question

to see whether this hypothesis that one of the hottest planets in the solar system has icy poles is correct. And clearly, the only way to find out is to go there, which hopefully we are about to do with this spacecraft. Last I checked, it was sitting atop a Delta II rocket awaiting launch at Cape Canaveral. What's the current status of the mission? Very briefly, we've only got about a half a minute left. The status as of our interview today is that everything is stacked.

The spacecraft sits on the third stage. All the tests have been carried out successfully. We're waiting for our window to open on the early morning hours of 2nd of August. But everything looks to go as of today.

Well, we will wish you great success with this first mission in decades to the innermost planet, one that presents many challenges and many mysteries. Sean Solomon, again, thank you for joining us on Planetary Radio. It has been my pleasure, Matt, and I thank you for your interest in the mission and for the interest of all your listeners. And we sure hope we'll be able to talk to you again a little bit later,

and you'll have some data to share with us. We certainly will. Every time we fly by one of those inner planets, we'll have new data, and when we have our one year in Mercury orbit, we'll have an abundance of data to share. Sean Solomon has been our guest. He is the Director of the Department of Terrestrial Magnetism at Carnegie Institution of Washington, but also the Principal Investigator for the MESSENGER mission about to principal investigator for the Messenger mission about to head out for the planet Mercury.

I'll be right back with Q&A. What does the color of the sky have to do with the color of the moon? The light from the moon that we see at night passes through our sky on the way to our eyes. Our atmosphere scatters the blue light more than longer wavelength red and yellow light. At night, there is too little light for our eyes to be able to detect the blue color of the sky. What we can see is the light of the moon minus its blue light, which means that it often appears yellow. If you were to stand on the surface of the moon, as the Apollo astronauts did, there would be no atmosphere between the sun and your eyes,

and consequently the surface would appear to be gray, the true color of the moon. In fact, the moon is one of the grayest places in the solar system. The Apollo astronauts' photographs look as though they are black and white until you see the blue and green jewel of the Earth rising over the gray lunar horizon. Got a question about the universe? Send it to us at planetaryradio at planetary.org. And now here's Matt with on Planetary Radio with Dr. Bruce Batts, the director of projects for the Planetary Society. Bruce, happy messenger liftoff.

Off to Mercury for the first time in a really long time. And we're going to come back to that because it has everything to do with our trivia contest today. But what else do you have for us? Meteor shower. Traditionally the second best meteor shower of any given average year. But the one that's positioned nicest in terms of weather, at least for the northern hemisphere, the Perseids are coming up. They will be peaking on the evening of August 11th through the morning of the 12th. If you go outside then or even a couple days before or after, you'll see increased meteors.

We're talking 40 to 60 per hour during most of the shower. If you haven't looked at meteors, basically you just stare up at the sky and you will see a little streak of light as a bit of dust or sand burns up in the upper atmosphere, sand-sized particles. Now, was the Perseids a couple of years ago that were so spectacular, right? I was out in the desert watching them. No, I think that was probably the Leonids. Oh, you know, you're right. You're absolutely right. Leonids are the freaks of the meteor shower world.

Hopefully, they won't seek retribution for that comment. Because with those, you have big peaks over a three or four or five year period. And that's what we've had in the last two or three or four or five years. But for like 30 years, they then are pretty darn awful. So the Perseids are boringly consistent. Yes, they are. They're boringly consistent. And they're always there.

There's mid-August kind of thing. So a good thing to go out there and see. As I say, August 11th. Give you a little more warning this time. Go out there. There's mid-August kind of thing. So a good thing to go out there and see. As I say, August 11th. Give you a little more warning this time. Go out there. Look at them. There's even possibly be an extra burst right around 9 o'clock Universal Time on August 11th, which doesn't help those of us in North and South America, but in Europe and Asia.

That's where the Earth passes through near the center of a trail of dust from the comet that formed it. Okay. You listeners then in Europe and Asia, drop us a line. Tell us what happens at 9 o'clock Universal or Greenwich Mean Time. What else do you have for us? I've got the planets. We've got Jupiter in the evening sky getting lower and lower, but still very bright, star-like object in the west.

And before dawn, that's right, after sunset, before dawn, look for Venus. Can't miss it. Extremely bright in the east and to the lower left of Venus, you'll find Saturn. Look for Venus. Can't miss it. Extremely bright in the east and to the lower left of Venus you will find Saturn. On to This Week in Space History. August 8, 2001, not that long ago in comparison to other things, Genesis was launched. Genesis will be coming back this September. We'll give you more information as it approaches, returning the first samples from space since Apollo, first extraterrestrial samples. And these are samples of the solar wind, right?

Yes, indeedy-do, dropping into the desert of Utah in the United States. Let's go on to Random Space Fact! Did you know that Soviet cosmonaut Valentina Tereshkova, who, of course, in 1963 became the first woman to travel in space, was later elected a member of the Supreme Soviet in 1967, and then a member of the Presidium of the Supreme Soviet in 1974. And I hope she was a better politician than she was a cosmonaut, because I'm told that

she really hadn't had much training. They just wanted to stick a woman in the can and send them up so that they could go nyeh-nyeh-nyeh to us once again. I cannot comment accurately on that. For a change, I will choose not to. Trivia. Afraid of where I might go. Okay, trivia.

A couple weeks ago, we asked you, what spacecraft was the first to use a gravity assist to get from one planet to another? Where you go by a planet and accelerate as you're going past the planet, not only change your course but increase your speed as well. And as we just heard from Sean Solomon, principal investigator on the MESSENGER mission, the first probe headed to Mercury now in decades, Mariner 10, right, was headed to Mercury years and years and years ago, and now we're finally going back.

Exactly, and it used Venus for a gravity assist. Who won, Matt? Morrison Chang of Hollis, New York, is our first winner who had to wait two weeks to find out. You know, we now are allowing people more time to enter the contest because we're up on the public radio satellite service. So Morrison got in and was randomly selected. He told us that Mariner 10 used Venus to get to Mercury,

just as you reported to us moments ago. That's stunning. That's so exciting. Let's go on to next week's trivia contest, shall we? Please. A little thematic thing, as I like to do. Perseid meteor shower. What comet's debris is associated with the Perseid meteor shower?

Often meteor showers are caused by these comets coming by, and when the Earth goes trucking on through their dusty debris that they leave lying around the solar system, slobs, we get a little bit of a peak in the meteors. And in this case, so what comet was it that basically caused the Perseid meteor showers? And you can go to planetary.org slash radio, find out how to enter, and win your glorious Planetary Radio t-shirt. Percy and Meteor Showers. And you can go to planetary.org slash radio,

find out how to enter and win your glorious Planetary Radio t-shirt. Matt, what do they need to know about how long they have to do this? Even more time than usual, because we're going to take a little break. Once a year, we play one of our golden oldies, what do they call Cadillacs that are used now? I think they call them previously owned. We're going to play one of our great planetary radios from the past. Certified great. During the week of August 16.

Why? Because I'll be away. That means that you've got even longer than usual. You've got until noon on August 18. That's noon Pacific time, August 18, a Wednesday, to get us your entry. How do they enter, Bruce? Again, I'd tell them to go to planetary.org slash radio. That's like the third time I've done that, asked you to tell people once again how to do it.

And what'll they win, Bruce? Again, I think they'd win a Planetary Radio t-shirt. But I'm not positive, Matt. What do you think? I think it's time to say goodnight. Goodnight! All right, everyone, go out there, look up in the night sky, and think about how much you want a Planetary Radio t-shirt say goodnight. Goodnight. All right, everyone, go out there, look up in the night sky,

and think about how much you want a Planetary Radio t-shirt. Thank you. Goodnight. He's Bruce Betts, the Director of Projects for the Planetary Society, and joins us each week here on Planetary Radio. Boy, do I need a rest. But not just yet. We'll be back next time with Apollo astronaut Harrison Schmidt.

I hope you'll be able to join us. Have a great week.