NASA Probe Goes Into Mercury's Orbit NASA's Messenger Probe has been taking measurements of the planet Mercury since 2004. On Thursday it slowed itself down enough to enter an elliptical orbit around Mercury to look at the planet's atmosphere and geology. Principal investigator for the mission Sean Solomon discusses what the team hopes to find.

NASA Probe Goes Into Mercury's Orbit

NASA Probe Goes Into Mercury's Orbit

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NASA's Messenger Probe has been taking measurements of the planet Mercury since 2004. On Thursday it slowed itself down enough to enter an elliptical orbit around Mercury to look at the planet's atmosphere and geology. Principal investigator for the mission Sean Solomon discusses what the team hopes to find.


Moving inward a little from Titan - well, actually, we're going to move all the way in and go to that first planet, the first planet in our solar system, which is - Mercury, of course. You got those planets right, of the eight, where the temperatures are not frozen like on Titan. They can be skyrocketing to 1,400 degrees Fahrenheit on its sunny side. Of course it is a very dry heat. So (unintelligible)...

Yesterday marked the first time in history that a spacecraft has actually entered into orbit around Mercury. NASA's Messenger probe is taking the highest-resolution photos yet. It's also looking at things like Mercury's magnetic fields and what its atmosphere is made out of.

And did you know that Mercury had a magnetic field? Did you know that Mercury had a comet-like tail? There are other tales about Mercury that may surprise you, and here to talk about is the leader of the Messenger Mission and director of the Department of Terrestrial Magnetism at the Carnegie Institution in Washington, D.C. - Sean Solomon. Hi, welcome to SCIENCE FRIDAY.

Mr. SEAN SOLOMON (Carnegie Institution): Thank you, Ira, glad to be with you.

FLATOW: So you got your Messenger going, working?

Mr. SOLOMON: We did indeed. As you probably know, the spacecraft has been flying through the inner solar system for more than six and a half years, but last night we inserted the first spacecraft ever into orbit around the innermost planet.

FLATOW: So you've been popping the champagne?

Mr. SOLOMON: Well, we've been checking out the characteristics of the orbit that we achieved. All looks terrific so far. It looks like we are in an orbit very close to the one that we planned.

FLATOW: And what is the shape of this orbit?

Mr. SOLOMON: It's highly elliptical. For reasons of thermal design, because the surface of Mercury is so hot, we need an elliptical orbit to radiate back into space a lot of the heat that we absorb when we're close to the planet on the day side.

It's a 12-hour orbit. The closest approach to the planet is 200 kilometers, 120 miles. The most distant part of the orbit is more than 15,000 kilometers, nearly 10,000 miles off the surface.

FLATOW: So you have to get it out there to cool off, is what you're saying, at some point.

Mr. SOLOMON: We need the altitude to cool off from when we are going over the hot day side of the planet.

FLATOW: And what makes Mercury so interesting to scientists to study?

Mr. SOLOMON: Well, it is one of the family of inner planets that includes our own Earth, Mars and Venus. It is the smallest. It is the closest to the sun. And it's extreme in many respects.

And what makes it interesting is that we can't claim to understand the processes that led to the formation and evolution of our own planet unless we can generalize those ideas to explain the outcome of the family of inner planets, the most extreme of which is Mercury.

So Mercury's made out of the densest stuff. It's got the most dynamic atmosphere and magnetosphere. As you mentioned, it's got a magnetic field, when larger planets, like Mars and particularly Venus, do not.

And despite the range of temperature between day and night, 600 degrees centigrade or 1,100 degrees Fahrenheit, it appears, on the basis of Earth-based radar observations, that Mercury has ice at its poles, locked and permanent cold traps on the floors of shadowed impact craters.

FLATOW: Wait, wait, wait a minute. My head is hurt - my hair is hurting on this. You mean that if you have a - you have a crater, and one side of the crater could be in the sunlight, but in the shadow there's ice, even though it could be 1,000 degrees Fahrenheit in the sun?

Mr. SOLOMON: Yes. I don't mean to hurt your hair at all, Ira.

(Soundbite of laughter)

Mr. SOLOMON: But that is a possibility. The reason is as follows: Mercury's spin axis is nearly perpendicular to its orbital plane. So an impact crater is, of course, a topographic depression that's surrounded by a mountainous ring of high terrain.

And so at the very poles of the planet, an impact crater, the entire floor is in permanent shadow. And as you move off the poles, a portion of the floor is in permanent shadow.

And the atmosphere of Mercury is so thin, so tenuous, that it does not, as does the Earth's atmosphere, transport heat from the equator to the poles. And so a permanently shadowed region on the poles of Mercury would be looking out into black space and would be extraordinarily cold, 90 degrees Kelvin, nearly minus-200 degrees Fahrenheit, cold enough to trap, as ice, water and other volatile materials.

So despite the fact that the day side is roasting, particularly near the equator, you could have permanent cold traps, deep freezes, at both the North and South Pole.

FLATOW: Wow. So what is the main instrument on this probe that is most important to you?

Mr. SOLOMON: Well, we're carrying seven. They're all equally important. But...

FLATOW: Why am I not surprised would you say that like your children, right? What's my favorite child?

Mr. SOLOMON: Exactly.

(Soundbite of laughter)

Mr. SOLOMON: The question is a kind of "Sophie's Choice" kind of - type of question...

FLATOW: Right.

Mr. SOLOMON: ...that I don't like to answer. And because this mission has been 15 years in the making - it was approved 12 years ago and confirmed 10 years ago, and it's been flying for six-and-a-half years. And I've been involved since the beginning.


Mr. SOLOMON: They're almost like grandchildren.

FLATOW: I believe it.

(Soundbite of laughter)

Mr. SOLOMON: But the other thing we've learned from flying by Mercury three times on a route to yesterday's orbit insertion, is that the planet is quite complex. And the surface and its geological history, the interior and its dynamics, the atmosphere, most of which is derived from the surface, the magnetosphere, the envelop of space that surrounds the planet and is dominated by the magnetic field generated inside the planet and the interaction of that magnetosphere experiences with the solar wind and the interplanetary environment, all of these aspects of Mercury are interconnected in very important ways. And they really test our ideas about how planets work, to understand these interactions.

So it - not only are these instruments like my children, but we are going to need the full complement of seven instruments with multiple sensors to sort out the diverse phenomena going on at Mercury and watch how these processes are all strongly interacting.

FLATOW: You got any Kodachrome? I mean, are we going to get any color pictures?

Mr. SOLOMON: We are, indeed.


Mr. SOLOMON: We are, indeed. Mercury, if you were to view it as a tourist out the window of a spacecraft, would look rather lunar-like, with a lot of gray tones. But there are subtle variations in color that we can enhance with image-processing techniques. We're going to image the entire surface for the first time in color. And we've got cameras that can take high-resolution monochrome images globally and really high-resolution images down to the 10 to 20-meter scale of targets. We've got a list of more than 2,000 targets that we've picked out from the flybys, and even from Mariner 10 images from more than 30 years ago.

FLATOW: That's - yeah, going way back.

Mr. SOLOMON: Going way back.

FLATOW: Way back. And what is the most surprising thing that my listeners don't know about Mercury that you know and you want to share with us?

Mr. SOLOMON: That I knew? Well, we're learning things every time we get close to the planet. We're learning that little Mercury had an extraordinary volcanic history that resurfaced the planet to an extent spatially and over a time period much longer than we appreciate it. We're still trying to understand why little Mercury should have a magnetic field or have ices at its poles. The dynamics of the atmosphere, magnetosphere interaction are surprising us in their magnitude and in their speed.

We see processes with which we are familiar in the Earth's magnetosphere, having to do with space weather and the effect of solar storms on processes high off the Earth's surface. We're seeing analogues at Mercury. But unlike the situation on Earth, where time scales for these processes can be measured in hours, at Mercury they're measured in minutes, and the magnitude of these effects are one or more orders of magnitude greater than that at Earth.

So it's going to be a wonderful laboratory for studying planetary processes that are much extreme than any that we see here on Earth, but that are common to those that we do experience, and therefore will deepen our understanding.

FLATOW: Talking about Mercury this hour on SCIENCE FRIDAY from NPR, talking with Sean Solomon.

So Mercury's not just this hot rock, is what you're saying in great detail.

(Soundbite of laughter)

Mr. SOLOMON: Not at all. Not at all.

FLATOW: Not just a hot rock sitting there. It's complex. It's got magnetic fields and even has a tail, a magnetic field tail that it's dragging.

Mr. SOLOMON: It has a magnetic field tail, but it also has a tail of atoms that are streaming away from Mercury's atmosphere. And that tail is produced by the pressure of sunlight on the atom's - in Mercury's atmosphere that lofts high enough to be subjected to enough sunlight to be pushed away.

Earth-based astronomers have shown that that tail extends more than a thousand planetary radii, more than two million kilometers in the anti-sunward direction of Mercury. And so to call it comet-like is a very good analogue.

FLATOW: Let's go to the phones. Fred in St. Joseph, Missouri. Hi, Fred.

FRED (Caller): Hey, you guys. Hey, I'm a rocket scientist myself. I've got a degree in astronautic engineering from the Air Force Academy. And I just want to say, hey, congratulations. Job well done. I understand it was a 15-minute burn to get that thing inserted into orbit to close it up.

Mr. SOLOMON: That is correct, Fred. We changed the velocity of the spacecraft by nearly one kilometer per second, 863 meters per second. And we basically turned the largest thruster in the spacecraft in the direction that we were heading and turned on the burn for 15 minutes. We actually turned the spacecraft during the burn to make maximum use of the thrust.

FRED: Okay. Does it require a plane change at all, or was it just used -you just were be able to negotiate from the orbit you already had?

Mr. SOLOMON: We stayed within the plane of the trajectory, if that's what you're asking.

FRED: Wow.

Mr. SOLOMON: And...

FLATOW: So you just turned the spacecraft around and put the brakes on?

Mr. SOLOMON: Exactly. Exactly. And because the brakes were on and we were in the gravitational field of the planet, there was a natural curvature to the trajectory. And we turned the spacecraft so that the thrust vector slowed our craft to the maximum extent possible during that burn.

FLATOW: And in the few seconds I have left, Sean, how long are you going to be there?

Mr. SOLOMON: Well, we're - we've got a year's worth of orbital operations ahead of us so far that will begin after a spacecraft commissioning phase on the fourth of April, when we'll be taking data nearly continuously - all kinds of data, imaging, magnetic field, you name it. And if there's propellant left over and NASA's willing, we may stay longer.

FLATOW: Good luck to you. It's been almost two decades' worth of work.

Mr. SOLOMON: Thank you very much, Ira. We're very much looking forward to the scientific phase of this mission.

FLATOW: Yeah, and we want to keep track. And we'll - you know, keep us in mind when you get some data coming back. We'll be back talking to you. Good luck.

Mr. SOLOMON: Thank you very much. Just give us a call anytime.

(Soundbite of laughter)

FLATOW: I will. Sean Solomon, leader of the Messenger Mission, director of the Department of Terrestrial Magnetism at Carnegie Institute of Washington, D.C.

When we come back, don't go away. We're going to have an orchid extravaganza. Yeah. And you'll be able to watch it, too. Flora will be here with our Video Pick of the Week, including orchids. So stay with us. We'll be right back after this break.

(Soundbite of music)

FLATOW: I'm Ira Flatow. This is SCIENCE FRIDAY, from NPR.

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