Planetary Radio: Space Exploration, Astronomy and Science - Sean Solomon on the Climactic End of the MESSENGER Mission to Mercury

Episode Date: May 5, 2015

We open with the countdown to destruction—the MESSENGER spacecraft’s impact on Mercury ended its spectacularly successful mission. Principal Investigator Sean Solomon joins us immediately after this big finish.Learn more about your ad choices. Visit megaphone.fm/adchoicesSee omnystudio.com/listener for privacy information.See omnystudio.com/listener for privacy information.

A new crater on Mercury, thanks to you and me, this week on Planetary Radio. That was the countdown to the climactic end of the MESSENGER mission, as it was heard in the Mission Operations end of the MESSENGER mission, as it was heard in the Mission Operations Center at the Applied Physics Laboratory. Standing there with his team was Principal Investigator Sean Solomon. Sean will join us to recap the brilliantly successful voyage of MESSENGER and to look forward toward the next steps in the exploration of our solar system's innermost planet. to look forward toward the next steps in the exploration of our solar system's innermost planet. Bill Nye has the week off, but Senior Editor Emily Lakdawalla is here for our regular conversation, and Bruce

Betts will make his usual visit, bringing planets, comets, and a prize for the winner of this week's space trivia contest. Emily, with the Messenger missions, Sean Solomon just moments away, I wanted to get your reaction and also some of the reactions that you summarized in a blog post right after the impact of the Messenger spacecraft. Well, it's always really hard to say goodbye to a spacecraft, especially one that's done so well as Messenger has. They really got every last possible bit of science they could out of this tiny little mission.

They mapped all of Mercury. They did much more than they promised they could do. It's been absolutely fantastic. I have no further wishes for Messenger, but I was still very sad to see it go. I loved seeing the folks that you found on Twitter who found quotations from Shakespeare that did seem to be quite appropriate. Yeah, it sprang off the fact that sites on Mercury are named for people who have contributed to the arts. And it just so happens that the site where Messenger was just about to crash was near a crater named for Shakespeare. And so, let's move on to New Horizons, because you had a blog post, and there was a lot of attention last week, for the latest images from that spacecraft. Yeah, it's really exciting. The very latest images that they have processed from New Horizons are actually showing features on the surface of Pluto. Now, what those features are,

we cannot tell you. They're bright and dark smudges. And they're going to stay bright and dark smudges for a long time. In fact, the fact that Pluto's surface is so contrasty, has bright spots and dark spots, may make it very difficult for us to figure out what we're actually looking at until we're right on top of it. But it is cool to see those features rotating around Pluto as it spins, and it's just going to get better. And they also announced they have started releasing all their raw images to the internet. They're not much to look at yet. They're just little dots, and they're going to stay little dots through June. But it's still cool to be able to go to the New Horizons website and see fresh images every couple of days. Fun stuff for image processors like you, isn't it? And there is a lot of information for

those folks, some of it fairly technical, but still fascinating. In Emily's blog about New Horizons from the 29th, that's April 29th of 2015. Emily, thanks so much. I look forward to talking to you again, as always, next week.

See you then, Matt. She is the senior editor for the Planetary Society, our planetary evangelist, and a contributing editor to Sky and Telescope magazine. That's Emily Lakdawalla. After nearly 4,000 days in space and more than 4,000 orbits of Mercury, the MESSENGER spacecraft impacted our solar system's innermost world on Thursday, April 30th.

MESSENGER, that's the Mercury Surface Space Environment Geochemistry and Ranging Mission. And by any reasonable measure, it was a tremendous success. As principal investigator, Sean Solomon has overseen every facet of the voyage since well before launch. We've been getting updates from him ever since that launch back in late 2004. Just hours after impact, Sean was on Skype with me once again. There's much, much more of our conversation than we have time for, including his in-depth exploration of Messenger's five most important and surprising discoveries.

You'll only hear number one here, so I highly recommend checking out the others on this week's show page, reached from planetary.org slash radio. Sean, thanks for coming back on Planetary Radio and congratulations on completion at least of the active portion of this spectacularly successful mission, the Messenger Mission to Mercury. Thank you, Matt. It's been a terrific mission, both in terms of the science that we've been able to do, but also the people who worked on it. I had the good fortune on Thursday of being at the Mission Operations Center at the Johns Hopkins University Applied Physics Laboratory, along with many, many current and past members of the Messenger team. I think you might have recorded

a few moments from the actual event when we declared that the mission had ended. But what was terrific for me was just being there with the MESSENGER team, the MESSENGER family, and some of the folks had moved on to other missions, some had left to go to other institutions, but it was a nice gathering of everybody who had contributed to the success of that spacecraft. Indeed, I do have that recording from a telephone, not the best quality, but we opened today's show with whoever it was that counted down to impact. Tell us about that moment, standing there at the APL Mission Operations Center with so many of these people who've been part of this team for over a decade.

Well, the end of the mission wasn't quite as we would have designed it. Had we had a little bit more propellant, we might have extended the mission a little bit longer and brought the closest approach back into view from the Earth. But we had to go behind the planet. We'd long since run out of actual propellant. We'd even run out of the helium that we were ejecting from the propulsion system to impart a small amount of thrust

and raise the closest approach altitude a little bit with the last three maneuvers. And as you know, the helium wasn't intended as a propellant. It was there to move the hydrazine and oxidizer around from the fuel tanks into the combustion chamber. So it was quite a novel use of helium to adjust the orbit of a spacecraft like that. The impact, of course, was on the far side of the planet, out of view of NASA's Deep Space Network antennas. All of us at the Mission Operations Center collected the last

scientific data. The spacecraft went into what was called beacon mode, which was a simpler kind of transmission, still useful for radio science, and then that that signal ended when, as expected, the spacecraft went behind the planet. At high probability we predicted that that would be the last orbit, that the periapsis or closest approach altitude would be so low that the spacecraft, more or less at closest approach, would impact a topographic feature that stood above the average for the surface. If by some chance our calculations had been off and the spacecraft just skimmed the top of the mountain instead of slamming into it and emerged for one more orbit,

then we would have heard the radio transmission when the spacecraft would have come out from behind the planet. We did listen. Another radio science team at one of the deep space network antennas listened as well. There was no signal. So after a suitable length of time, the part of both of those teams, we declared that the impact had occurred. I love the fact that you were literally running on fumes at the end there. Just one of many innovations by this spacecraft. I mean, we talked in the past how it actually became a solar sail as it was arriving at Mercury, but we'll leave that

discussion for one of those previous shows. I've had a number of people ask me, and I bet people in the audience are wondering, how close to Mercury did the spacecraft get while you were still collecting useful data? You know, I haven't seen the final orbit determination. That's still being worked out. But I believe that the closest approach on the orbit before the last partial orbit, where the spacecraft ended operations, was just a couple of kilometers off the surface. Wow. We did send back to Earth the last image that we acquired.

That's posted on the MESSENGER website. That's one of our highest resolution images. One of the challenges to getting that close to the planet is that many of our instruments weren't designed to take data from such a close range. So the images, for instance, have to be corrected for the smearing that occurs while the spacecraft is moving during the acquisition of the image. Our laser altimeter was being used not only to measure topographic heights, but to feed that information back into

the orbit determination, into the navigation team solutions to validate and update the estimates of where we were in our orbit. When we get that close to the planet, we are susceptible to shorter wavelength components of Mercury's gravity field than we had measured from higher above the surface. And so we didn't know what those shorter components would be. So there was the opportunity that there would be perturbations to our orbit from these short wavelength features in the gravity field. And so having the altimetry was very important to help us understand where we were relative to the surface. But the altimeter was not designed to be taking data so close to the planet. And our folks had to operate in a non-standard mode

in order to get those data. Science and innovation essentially right to the end. I'll be right back with more from Sean Solomon, principal investigator for the just-completed MESSENGER mission to Mercury. This is Planetary Radio. Hey, hey, Bill Nye here. I'd like to introduce you to is Planetary TV's not on TV? That's the best thing about it. They're all going to be online. You can watch them anytime you want.

Where do I watch Planetary TV then, Merc? Well, you can watch it all at planetary.org slash TV. Random Space Fact! Nothing new about that for you, Planetary Radio fans, right? Wrong! Random Space Fact is now a video series, too. And it's brilliant, isn't it, Matt? I hate to say it, folks, but it really is. And hilarious. See? Matt would never lie to you, would he?

I really wouldn't. A new Random Space Fact video is released each Friday at youtube.com slash planetarysociety. You can subscribe to join our growing community and you'll never miss a fact. Can I go back to my radio now? Welcome back to Planetary Radio. I'm Matt Kaplan. It is my honor to welcome Sean Solomon back to our show. This is the ninth time he has provided an update on his Messenger mission to Mercury. That mission ended with the fully expected impact of Messenger on April 30th.

Sean and several of his colleagues recently provided a top ten list of discoveries and accomplishments. He's about to tell us about his number one. You can hear numbers two through five, including the deep science behind them, on the show page at planetary.org slash radio. To me, the most fundamental was the measurement for the first time of the composition of Mercury's surface materials. That had not been done by Mariner 10. It can't be done remotely. We have no samples of Mercury in our meteorite collection. So it all had to be done with remote sensing instruments from orbit. And we took several instruments because we went to Mercury with the notion of deciding among the hypotheses that were then current for how Mercury was assembled.

And what we know and have known for half a century is unusual about Mercury is it's extraordinarily dense relative to the other planets for its size. And what that means is that it must be mostly iron metal in order to have the requisite density, given that iron is the only common element in the sun and in meteorites that is sufficiently dense. And we know that iron makes up something like a third of the mass of Earth and Venus, a little bit less than that of Mars. So the hypothesis was that Mercury ended up very metal rich. And the question is how? How did that happen for one planet out of the four inner planets? They were competing ideas.

We've talked about these before on earlier shows. It might have been a giant impact that stripped off the rocky outer parts. It might have been that Mercury was bathed in a very high temperature nebula, the disk of dust and gas that surrounded the early sun. It might have been that Mercury was bathed in a very high-temperature nebula, the disk of dust and gas that surrounded the early sun. It might have been that Mercury assembled only from those materials that were solid at the hot temperatures of the inner part of the nebula when the building blocks of planets started forming. But all of those hypotheses had a common prediction, and that

common prediction was that Mercury, like the Earth's moon, would be depleted in what are known as volatile elements, those elements that are easily removed by moderate to high temperatures. And the surprise from MESSENGER was that as we went through the inventory of elements that we could measure with our spectrometers, one by one, the volatile species showed up in very high abundance. We saw a lot of sulfur, a sulfur abundance at the surface of Mercury 10 times that at the surface of the Earth. We saw a high abundance of the alkaline metals, potassium and sodium, which are also easily removed by high temperatures. Most recently, we extracted the signature of the surface abundance of chlorine, a halogen, and that's extremely volatile. Mercury

has an abundance of chlorine relative to less volatile elements, similar to that of Mars, which is thought to be a volatile, rich planet, at least for the inner solar system. And so none of the ideas for how Mercury was assembled matched what we saw in the composition. And so it means that we really didn't understand how the inner planets formed. We didn't know how to make a planet that was metal rich because all of the ideas made predictions that were falsified by the observations. And so the theory for how the inner planets formed and how they acquired their budget of volatile elements is still being worked. We don't have a complete theory.

With apologies to all the planetary scientists out there who must be very frustrated by that, I think that's just a wonderful result. Well, so do we. It's always nice to tear up the textbooks and have to rethink some of our basic assumptions. But, you know, in the longer term, Matt, after BepiColombo, we're still going to have many, many questions about planet Mercury. I think some space agency is going to take it upon themselves to send a lander. is going to take it upon themselves to send a lander. That the next phase in the exploration,

after sending three spacecraft that orbit the planet, is going to be to try to land. That's not going to be easy. There's no atmosphere to help in landing, as there is, say, on Mars. The day-night cycle is extraordinarily long. The nights are three months long, and the temperature varies enormously between day and night. At the equator, the variation in temperature between day and night is 600 degrees centigrade, 1100 degrees Fahrenheit. If you've got a lander and it's exposed to that

kind of variation, you've got to find some way to survive. Of course, during the nighttime, that you've got to find some way to survive. Of course, during the nighttime, if you want to take some kind of power source to keep your spacecraft warm, but then you've got to survive the punishing heat of the day. Where do you go with a lander? What's the most interesting target? My vote would be explore those polar deposits.

We think there's water ice. We think there's water ice in some places exposed right at the surface, but in all of the deposits exposed close to the surface. But there's this mysterious coating of dark material that we've suggested is organic rich material dig beneath the dark material and find the water ice that's supposed to be there? Can we see any evidence of chemistry going on within those deposits? What's going on on timescales that we can sample with the spacecraft? I think that would be great fun to go to one of those places. How much longer, how many more years, I would bet, are the data from MESSENGER going to keep scientists like yourself busy?

Oh, I would say decades. You know, we're still looking at the Mariner 10 data. With all the data we collected with MESSENGER, there are still questions that cause us to go back to the Mariner 10 data. How did that look compared to what MESSENGER saw? Were there differences in the magnetic field and the environment of the planet 40 years ago compared to what weESSENGER saw, were there differences in the magnetic field and the environment of the planet 40 years ago compared to what we're seeing today. I'm sure that the BepiColombo team, with whom the MESSENGER team has been communicating regularly for a decade and a half, and are already very conversant with the MESSENGER

data set, are going to be eagerly looking for changes in the surface of the planet from the Messenger time to the BepiColombo time nine years from now. By the way, one of the important changes is that the Messenger spacecraft, of course, made a new crater on Mercury. It's not a big one, but we know rather precisely where it should be. And it's one of the youngest features on Mercury, and it will be one of the youngest features on Mercury even nine years from now. And so we're giving directions to the BepiColombo team, find MESSENGER's last resting spot, and use it to learn something about how optically fresh material is brought to the surface by a small impact crater and how it darkens over time on timescales of years or decades.

So that's MESSENGER, delivering science even beyond its own lifetime at Mercury. Sean, I don't want to give this too much of an air of finality because I certainly hope we will talk again, but I do want to tell you what a tremendous honor it has been to check in with you periodically on this program, beginning from just a week or so before launch back in 2004, right up to today when we can look back at the really magnificent record and achievements left by this mission. Matt, it's always been a pleasure talking with you, and I know you have a terrific, well-educated audience, and so I've really been delighted to be part of your show.

Thank you, and I know that they appreciate hearing conversations like this one, and I'm sure they're going to love this conversation. Sean, I'll let you go. I think you're headed to Europe, right? Yes, I'm headed to a meeting of the GRAIL science team. We've got the tough assignment of meeting in Paris this coming week. Well, bon voyage.

And that is an indication of how busy Sean Solomon is. His spacecraft that he has been principal investigator for from the start, the Messenger mission to Mercury, of course, now is part of that planet. But Sean has many other things to do. He directs Columbia University's Lamont-Doherty Earth Observatory. And you heard that he's on the science team for GRAIL, but he's also served for Magellan, Mars Global Surveyor.

When we first met him, he was director of the Department of Terrestrial Magnetism at the Carnegie Institution of Washington. Sean is a member of the National Academy of Sciences and a past president of the American Geophysical Union. And of course, the MESSENGER mission controlled and the spacecraft built at the Johns Hopkins Applied Physics Laboratory in Maryland, which is where we picked up that word of its impact on the planet Mercury just a day or two ago. I will be back to talk about what's up in the night sky with Bruce Betts in just a moment.

Time for What's Up on Planetary Radio. Bruce Betts is the Director of Science and Technology for the Planetary Society. He is ready to tell us about the night sky and probably give some stuff away and reveal a surprise about near-Earth comets, all of that coming up. How do we start? Welcome back. That sounds like a good episode. It does. Be sure to tune in. I will. You know what else sure to tune in. I will.

You know what else you can tune in for, Matt? What's that? Planets in the night sky. Oh, yeah. I do like that segue. What's that? Is that Venus that's up there incredibly bright, pretty high? Well, you'd have to give me more directional information because we've got two really bright planets. The brighter one is Venus, which is over in the west after sunset, but staying up for a couple hours after sunset. And above that,

higher in the sky in the early evening is Jupiter, which is also brighter than any star in the sky, but not as bright as Venus. It's one of those. Yep, yep, it definitely was. one of those. Yep, yep, it definitely was. And you can actually for the next week or two possibly pick out Mercury. Not too hard, but you have to catch it as soon as it gets dark, low

in the west. And you can actually if you want, prove that the solar system and the planets are mostly in the same plane because you can draw a line from Jupiter through Venus and then it'll come down not quite exactly into Mercury but in the same plane because you can draw a line from Jupiter through Venus, and then it'll come down, not quite exactly into Mercury, but in the general vicinity of Mercury. And if you follow that line all the way to the other side of the sky in the early evening, Saturn will be rising low in the east, looking kind of yellowish.

And we're in there too, right? We are. If you very carefully take and point your head and look down, you will see the Earth, I'm pretty sure. Or at least something on the Earth. Science facts. I know. It's amazing. All right. We move on to this week in space history.

It was 1961 that Alan Shepard took a suborbital flight, becoming the first American in space in the Mercury program. And now on to random space facts. You know, once again, I forgot. I should have had Sean Solomon do it this time. It's a recurring theme. I know, I know. That messenger, by the way, what a mission. What a cool mission.

What a mission. What a cool mission. What a mission. But unrelated to that, related to another much, much smaller, cool mission that hasn't flown yet, but will soon. You may have heard of this LightSail, solar sail spacecraft. Yeah, dimly, yeah. The Planetary Society is going to have a test flight in two, three weeks, depending on when the launch actually occurs. in two, three weeks, depending on when the launch actually occurs. Well, the thickness of the solar sails,

mylar in this case, for that solar sail spacecraft, light sail, are about a quarter the thickness of an average trash bag. There's no tie string on the light sail sails. Well there's uh well there are burn wires to relieve but not tie strings okay well thank you for that you're welcome on to the trivia contest uh we asked you when we were off in italy at the planetary defense conference as of april 11th 2015 about how many near-eararth comets are known to exist. That would be comets bridging within 1.3 AU, 1.3 times the Earth's sun distance. How'd we do?

Very interesting. You specified that this had to be a number as of April 11 of this year, and apparently there were some last-minute discoveries of new near-Earth comets. So a lot of people came up with 94, 96, but we also got, and probably had a plurality of people who said 101 objects, 101 of these comets that come pretty close to home base. Which of those numbers is correct? Based upon the JPL NEO site, 101. based upon the JPL NEO site, 101. And it turns out all of these comets are white with little black spots.

Well, that is going to make Arlene Lopez very happy, because Random.org, actually Random.org chose a couple of those people who said 96, but we're going to go with the 101, because Bruce was very specific. Arlene is in my hometown, Long Beach, California. I don't know. Who knows? Maybe she lives a block over. I have no idea. But she is one. The copy, the last one we've got, of the interstellar age,

inside the 40-year Voyager mission by our friend, the president of the Planetary Society, Jim Bell. So congratulations, Arlene. We will get that out to you soon. Look, one more comment. A whole bunch of people came up with this huge number, and it became apparent that they had confused what we wanted, just the comets, with the entire population of near-Earth objects. Near-Earth comets, a tiny percentage, whereas there are 11,000, 12,000 of those near-Earth objects in total. There's 101 or so discovered near-Earth comets. We get other comets that we don't know are coming that come in, the long-period comets, but in terms of the ones we've got their orbits down, it's 101-ish.

All right, we are ready for next time. In microns, more formally called micrometers, how thick are the light sail spacecraft's solar sails? How thick are they? Go to planetary.org slash radio contest. I suppose you could just, you know, check the box for your trash bags and divide by four. But however you want to go about it, you'll need to get the answer to us by the 12th. That's May 12, 2015 at 8 a.m. Pacific time. And there's a Planetary Radio t-shirt in it for you if you win. Hey, Matt. Yeah.

May the 4th be with you. Why am I getting that from so many people? Apparently Bill Nye also tweeted about that. Someone at some point decided that they could make a play on words for star wars may the force be with you and so every may the fourth people like me and and other star wars uh geeks uh get all excited okay all right i like star wars a lot i like star wars a lot trek rules let finish with this. Mark Little said if it wasn't for the superpower of Dr. Bruce

Betts, the planet juggler, to protect us, as demonstrated in the random space fact videos, I'd be hyperventilating at this point with all those comets coming close to home. Fear not. I will juggle them as needed. Say goodnight, Bruce.

Alright, everybody, go out there, look up at the night sky, and think about juggling comets. Thank you, and goodnight. And who else to turn to for that important job than Bruce Betts, the Director of Science and Technology for the Planetary Society, who will join us again

next week, I guarantee it, here for What's Up. As I finish recording this week's show, the light sail launch is still set for Wednesday, May 20th. Planetary Radio is produced by the Planetary Society in Pasadena, California, and is made possible by the winged members of the Society. Our theme music is from Josh Doyle. Daniel Gunn is our associate producer.

I'm Matt Kaplan. Clear skies.