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Last Updated: Monday, 2 August, 2004, 12:15 GMT 13:15 UK
Q&A: Mercury space probe
The US space agency is sending a spacecraft to explore the innermost planet in the Solar System: Mercury.

Sean Solomon
Solomon hopes to answer some enduring mysteries about Mercury
Although temperatures can reach 450C, scientists think there may be ice at the poles.

Principal investigator Sean Solomon told BBC News Online about some of the questions he wants to answer about this enigmatic planet.

What do we actually know about Mercury? Have we neglected it from a space exploration perspective?

Most of what we know about Mercury comes from the Mariner 10 mission. In the 1970s, Mariner 10 flew by three times in 1974 and 1975. But it mapped less than half the planet and told us nothing about the composition of the surface.

In the last two decades, ground-based astronomy has discovered other elements in Mercury's atmosphere including sodium, potassium and calcium, which have to be derived from the surface.

The launch is one of the most dangerous parts of any mission and it's the one where the spacecraft team has no control
Radar astronomy also discovered Mercury's polar deposits, which are bright at radar wavelengths. The leading hypothesis is that they consist of water ice in permanently shadowed regions at high latitudes. So to say that Mercury is neglected is not the complete story.

How can such a hot place have ice at the poles?

Well the reason that the low latitude regions are so hot is because Mercury is so close to the Sun. The reasons that the high latitude regions are capable of being so cold is that there are places deep in the floors of impact craters there that never see the Sun.

In the absence of a strong atmosphere, high latitiude regions just see outer space, where it's extremely cold. Despite the high temperatures at the equator, that heat never makes it into the floors of high latitute impact craters.

If you [release] water from the interior and impact the surface with a comet, some of that water will bounce around in Mercury's gravity field and be trapped in the cold crater floors. It can be preserved there for billions of years.

What is the aim of the mission?

One of [our goals] is to simply find out what Mercury is made of. We know it's mostly iron because the density is so high. What we don't know is how the planet ended up more iron-rich than the other planets. The clues are going to be in the chemistry of the surface - the silicate outer portion about which we know so little.

A lot of what Messenger is going to measure is the chemical remote sensing of crustal materials. We want to see the parts of the planet that Mariner 10 never saw close-up.

Mercury, Nasa
Closest planet to the Sun
Diameter: 4,800KM
Mercurian year: 88 days
Has global magnetic field
We partly want to do this to complete our understanding of Mercury's geological history. But we also want to acknowledge that the exploration history of other planets turned up surprises when we saw a global view.

The discovery of Mercury's magnetic field by Mariner 10 was a big surprise 30 years ago. We still don't know why it's there and what created it. We want to find out whether the polar deposits are really ice. We want also want to know whether the core is still partly fluid or is completely solid. This could help us answer it is the modern source for the magnetic field.

Finally, we want to look at Mercury's atmosphere, which is extremely dynamic. It consists of species captured from the solar wind and kicked off the surface by a variety of processes. Some are very exotic by comparison with what we think of as common atmospheric constituents on planets.

How do you protect Messenger from such high temperatures?

The Messenger spacecraft is going to be exposed to high temperatures in two ways. Direct illumination by the Sun is almost constant. The way Messenger is going to protect itself from the Sun is to extend a kind of parasol - a shade that stands between the spacecraft and the Sun at all times.

Mercury Messenger launch, Nasa

The other source of heat is Mercury itself. When the spacecraft goes over the side of the planet that is in daylight, it radiates a great deal of infrared energy reflected from sunlight.

We have solved the heat problem in a couple of ways. Mercury is in an elliptical orbit so it's not always the same distance from the Sun. We make sure that during the noon part of our orbit, Mercury is nearly as far from the Sun as it ever is.

What will you be thinking during the launch?

The launch is one of the most dangerous parts of any mission and it's the one where the spacecraft team has no control. We basically signed up for Nasa providing a launch vehicle. They're very reliable launch vehicles, but they don't work 100% of the time. So the anxiety level is indeed high.

After launch, we figure we are going to be in much better hands and with a working spacecraft. We've got a long cruise and multiple flybys as well as a dangerous orbital insertion manoeuvre. But getting past the launch is hurdle number one.

Space probe blasts off to Mercury
03 Aug 04  |  Science/Nature
Mercury probe launch is delayed
02 Aug 04  |  Science/Nature
Nasa set to revisit Mercury
16 Jul 04  |  Science/Nature
Mercury passes across Sun
07 May 03  |  Science/Nature
Spacecraft to go back to Mercury
11 Jun 01  |  Science/Nature


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