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Planetary News: Galileo (2004)

A Conversation With Bruce Murray On Mariner 10 and Mercury's New MESSENGER

Thirty-one years ago, Bruce Murray was heading the imaging team on Mariner 10, the world's first mission to Mercury. During that $100 million magical mercurial tour, the team took thousands of pictures to map 45% of the planet's surface, while the exploratory team discovered it had a magnetic field and a very thin atmosphere. After Mariner, Murray went on to serve as the only geologist on the team planning the Grand Tour, which led to Voyager in 1972. He later became the Director of the Jet Propulsion Laboratory (JPL), a position he held from 1976 to 1982. While leading JPL, he co-founded The Planetary Society with Carl Sagan and Louis Friedman in 1980, and currently serves as Chairman of the Board of Directors. Murray is professor emeritus of geology and planetary sciences at the California Institute of Technology.

Q: In looking back on Mariner 10, I'm sure you remember a number of high points, but what was the first 'wow' moment?

A: It was when we were still a large distance away, but we could finally resolve the fact that were large craters dominating the face of Mercury that we were looking at. One of the craters was particularly bright, like Tycho is in the moon - a bright ray crater [craters that seem in images to sprout systems of bright radial lines or rays].

It attracted one's eye. As we got closer and the images got better, we could see it looked very much like the lunar highlands or uplands as they are sometimes called -- in the old days, they were called the Luna Terra. That was a big discovery, because Mercury is not at all like the Moon on the inside. It is very, very dense. In fact, it has proportionately more iron in it and a bigger core than does the Earth. Yet here from the outside, it appeared to look like the Moon. So that was the first real 'wow!'

Q: How did the team respond to that moment?

A: Well, for one thing, we were glad we could see it at all. It was a hard journey. But it really was a sense of 'wow.' We came in from a long distance away with pretty good illumination for those first images, and then we flew by very quickly up close where we couldn't see anything, and then photographed the planet again fairly close in as the spacecraft was going out on the receding trajectory -- and there was a big 'wow' there.

Unlike the heavily cratered terrain that we had just seen coming in that looked a lot like the lunar highlands, there was this huge basin that looked like it was filled with lava -- bigger than Mare Imbrium [on the Moon], so we were seeing the equivalent of a mare on Mercury. Then it was really getting exciting. Caloris Basin -- that's what we named it -- was the first thing we saw on going out. So there were two episodes of discovery, going in and then coming out. And they each have very different faces, both with a lot of similarities to the Moon. That kind of drove home the lunar analogy.

Mercury also recorded a similar bombardment history as the Moon in the sense that the Moon had a period of very heavy bombardment at the end of which the maria formed -- the basins -- which then later filled up with lava. Mercury seemed to mimic that to an amazing extent, which means it had seen the same kind of impact flux, the same history of bombardment, and that meant the same set of objects bombarding it probably came from pretty far out in the solar system. It turns out Mars also shows that. If we see that in three different places - at 1/3 of an Astronomical Unit [Mercury], 1 AU [Moon], and 1.5 AU [Mars], it means that there were objects forming that were not local but rather distant objects, maybe from out beyond the giant planets.

Q: This was also one of the first missions to make use of gravity assist -- Mariner 10 followed a trajectory that utilized the gravity of Venus to slingshot it onto Mercury -- right?

A: In the sense of time, it's a close race, a close match. Pioneer 10, in December 1973, rendezvoused with Jupiter, traveling at 6 miles [9.8 kilometers] per second. Following its passage through Jupiter's gravitational field, the spacecraft sped off into deep space, escaping the solar system at 14 miles [22.4 kilometers] a second. Then on Feb. 5, 1974, Mariner 10 became the first spacecraft to use the gravitational pull of one planet to reach another planet. It accelerated as it entered the gravitational influence of Venus, then was flung outward by the planet's gravity onto a slightly different course to reach Mercury. We took some 4,000 photos of Venus on that flyby and found it to be a nearly round planet enveloped in smooth cloud layers.

Q: Was there any concern then about using gravity assist?

A: No, it was based on very sound physics. It had been first anticipated in the 1930s by the British Interplanetary Society. In 1936, Mr. Philip Kleator published a book called Rockets Through Space -- and what he said was: "Providing the time of departure is carefully arranged, Mars and Mercury will be seen at close range while a near approach to Venus will be made at least once perhaps twice . . ." That's talking not necessarily about the same trajectory, but a similar one. Nevertheless, the idea was there early on and the reason is that we have worried about comets for a long time.

Comets naturally pass close to Jupiter. If they pass on the correct side of Jupiter they get slowed down and perturbed so as to end up as short-period comets in the solar system. If they pass on the other side, where they get accelerated, they get expelled from the solar system altogether. From July 16 through July 22, 1994, Comet P/Shoemaker-Levy 9 passed close enough that it got broken up by the gravity force of Jupiter, which Galileo captured in pictures.

The important thing was the physics was clear, very clear, and the mathematics was solved, from JPL's point of view, just by massive computers. Planning for this trip to Mercury took place in the late 1960s. At that point, Mariner 2 had been to Venus, and Mariner 4 had been to Mars, so the capability of calculating trajectories very accurately, taking into account the effects of the gravity of other planets was already pretty refined. Now you had to know when to look. The planets have eccentric orbits and they have different periods so the pendulums don't line up properly very often. Studies determined this particular launch in 1973, with a swing-by of Venus in 1974 and then arrival later in 1974, was the most favorable one of all for a long time.

As an aside, there was a CIA briefing I heard once -- they were concerned about the Russians doing this before we. This fellow had come up with one trajectory that was earlier than that and I looked at carefully because we hadn't found it. I realized he had it passing right through Venus. He'd used a point for Venus and not the actual planet. The point of that is -- it has to be at a critical point, an exact target just outside the surface of course, but exactly the right distance and the right time so that the pull, by Venus in this case, would be exactly the right amount to accelerate Mariner 10 to get to Mercury. Otherwise, it would just go around in an Earth-Venus orbit and just go back and forth.

Q: What about other high points on this mission?

A: The discovery of Mercury's magnetic field was certainly a high point. I for one was convinced there would be none, and was opposed to giving it a high priority, because we had to give up favorable lighting for photography in order to fly by the night side to look for the effect of the magnetic field. My rationale was this -- if Mars spins at the same speed as the Earth and has no magnetic field, and Venus has the same mass but spins slowly and has no magnetic field, then magnetic fields depend on spin and size I suppose and if Mars or Venus have none, then the chances for Mercury are remote. Well, the exploratory people were right. It was there and that was a very big discovery.

This also feeds into the MESSENGER mission, because while we could pin down the field to some extent, we couldn't pin it down that well, and it's a relatively small field. That means it's affected by the solar wind -- the strong solar winds shift the shape of it and so forth. So everybody agreed from the beginning that the next step has to be an orbiter. With an orbiter you can go in and out of the field repeatedly and then map it and determine precisely the structure of it and from that determine where it really comes from.

Q: And that of course is one of MESSENGER's prime objectives, but before we talk about that, for the record why is it that Mariner 10 only mapped 45% of the planet -- with three flybys -- I've read that it was because Mercury's slow rotation always left that side in the dark -- is that true?

A: Not quite. We only deserved one pass and that was pass number 1, but a brilliant Italian celestial mechanician named Bepi Colombo for whom the future ESA mission to Mercury is named, informed us otherwise. He's an extraordinary guy -- the person who first conceived of a space tether. He taught physics at the University of Padua, which is where Galileo started. In fact, I think he had the chair that was once in principle Galileo's. He came to a conference I arranged here at Caltech in February 1970, when we were first conceiving of the mission and talking about the different kinds of science that could be done. He had to leave early and he asked to see me on the way out. He said: "Dr. Murray, the spacecraft will come back."

I said: "What do you mean?"

He said: "It will come back and return to Mercury."

I looked kind of blank, and he said: "Why don't you check?"

So we did. He was right. That was the first time anybody realized the geometry and the subtle physics of that kind of arrangement meant that the spacecraft ended up in a period which is twice Mercury's period. Bepi Colombo was also the first person who recognized that -- when Mercury was discovered by radar not to be in synchronous rotation about the Sun -- it was actually spinning a little faster than that -- 59 days spinning and an 88-day revolution around the Sun. He recognized that immediately and published the fact that that's in a ratio of 2 to 3 -- what's called spin resonance, a natural resonance set up between the tidal forces of the Sun and the slightly non-spherical shape of Mercury. He was a brilliant guy.

Q: Mariner 10 also discovered several species of Mercury's atmosphere that had not been known before -- what can you tell me about that?

A: Mercury has an atmosphere similar to the Moon's, which is by most peoples' standards not an atmosphere. It's a vacuum. There are particles interacting with the surface and the magnetic field and plasma around there, but there is not an atmosphere in the sense of what we're looking, at least the way most people would think about it. Mariner 10 found ionized particles in the vicinity of Mercury and more have been found telescopically. It's kind of strange. You see calcium ions and some other things that may be due to the fact that it's so close to the Sun that when the hard solar radiation hits it, it actually blows little atoms off the surface.

I think Mariner 10's main contributions were the magnetic field, the 45% of the mapping, and the lunar-like surface -- not just lunar in appearance, but lunar in the history that it recorded, with evidence of an earlier phase when the thrust faults developed, which is, presumably, associated somehow with a large core.

Q: Mercury proved pretty quickly to be an intriguing planet. Why has it been 31 years since we've been there?

A: Because Mercury resembled the Moon -- it didn't have an atmosphere and no prospects for life -- and was really hard to get there. What we needed was an orbiter and that's a big step. This was now 1975, Viking was heading for Mars and hoping to find life there in July 1976. The outer planets were getting ticked off with first Pioneer and then the Voyager mission. The momentum moved outward.

Mariner 10 was an exploratory mission and it did that job, but it didn't connect with a theme that would permit sustained exploration. In many ways it was the same thing that happened to the Moon after Apollo. Remember, it was a long, long time from the end of Apollo [1972] to Clementine in 1994 and then Prospector in 1998 -- more than 20 years, a long period even though the Moon's an easy target to get to. Mars, of course, and then the outer planets provided an enormous array of discoveries. In the case of Mars, it's always had a long-term draw, except that after Viking failed to find any evidence of life and in fact found a surface that was hostile to organic compounds, an even worse scenario than the pessimists had thought. At that point, the interest in Mars dropped to zero, and you couldn't get another Mars mission going for quite a while.

The primary reason we took our time going back to Mercury, though, is that the planet didn't connect to a theme that demanded follow-up. The follow-up was more of a scientific follow-up -- what's the inside like? The price of doing that was an orbiter and an orbiter is harder to do. That gets us into what MESSENGER is trying to do. It's a lot more difficult than it looks.

Q: We send orbiters out to Mars all the time and we're now putting rovers down there, why is an orbiter to Mercury so difficult?

A: It's difficult at first because the gravity situation is very difficult. It takes a lot more cleverness of the brain or rocket force to get into orbit around Mercury than it does to go to Mars. The actual going into Mercury's orbit is somewhat similar to going into Mars orbit in terms of the retro rockets you have to fire to get in. The two planets both have the same surface gravity. Even though Mercury is smaller, it's much more dense, so its surface gravity is similar to that of Mars and that's one measure of the requirements it takes to go into orbit.

Even worse than that, the solar flux on the spacecraft when you get to the orbit of Mercury is 10 times what it is on Earth. That's a pretty big deal. It's even worse in an orbiter. If the orbiter is obviously going to want to go close to the sunny side, but the sunny side is actually reflecting only about 10% of the light, with 90% of the light being absorbed and re-radiated as infrared heat. That means you have the Sun bearing down on you enormously on one side of the spacecraft as you go around in orbit and you have the planet coming back up on the other side and it's a very, very difficult design problem. It's what you get into in a sense in Death Valley where you have high mountains and the Sun is very hot so you have the radiation directly from the Sun but also from the mountains radiating back. When you're in a sunny ravine in the desert, it's bad. It's like being in a solar furnace. So that's another big challenge.

Actually getting there is also a big challenge, because there are only a few times that you can get there easily with a normal rocket approach. MESSENGER, however, is using conventional rocket launch.

Q: Yes, how risky is that?

A: It is high risk. They will fire off with a Delta II and make use of gravity assists, and because of various delays now, their flight, once it's launched, gets into orbit in 7.5 years, in 2011. That's a long flight. Now it goes by Mercury during that time and we'll get some data every time it goes by. That's the good news. But the bad news is that it has to live 7 years before it even fires a retro rocket to get into orbit. That's a terrible requirement. It's got to be looked at by everybody sympathetically, but it is a high risk mission hopefully with a high payoff. All you can do is the best you can.

The Mercury orbiter has always been a tough job. That's why we didn't do one for 31 years -- and it gets tougher when you look at it more closely. Still, it needs to be looked at. This is a case where NASA and the scientific community have chosen to take a risk, a significant risk of failure in order to get a significant achievement at moderate cost. Sometimes that's warranted. In a mixed strategy of a lot of different missions, in fact, you ought to do something like that.

Q: Everybody I've spoken with to this point has said that Mariner 10's visit to Mercury raised more questions than it answered -- what for you are the big questions you would like to see MESSENGER answer?

A: The first thing to do is to get a pretty good understanding of what Mercury looks like all the way around, so one big thing is to photograph the 55% of the planet that wasn't imaged by Mariner 10. The flybys that MESSENGER is scheduled to do will do that. They're designed so they get there at a time when it'll be a different phase than it was when Mariner 10 saw it -- and that will be a big payoff early. That 55% of the surface that hasn't been explored may well hold some real surprises. That has very much in the case of Mars, and, certainly, Venus didn't turn out to be the way we thought it was going to be when we imaged it with radar, and the Moon's backside doesn't look like the front side and that was a big shock -- so finding what the other hemisphere of Mercury looks like is really important.

The second one is to map that magnetic field with enough precision to determine how it really formed. The dilemma is that Mercury's surface -- the one we have seen that looks like the Moon -- has some features in it that indicate deformation from within, but they're very old. The planet has thrust faults and things like that, but they're 4 billion years old or something like that. Nothing that we've seen so far indicates any deformation from within the last billions of years.