February 23, 2005
Exploring the Other Globe with the Smart One
Bernard Foing, Chief Scientist for the European Space Agency, kicks off a regular essay series exclusive to Astrobiology Magazine. In this part, he takes a tour of the novel ion propulsion employed by the current lunar orbiter, SMART-1.
Astrobiology Magazine -- The European Space Agency's SMART missions - Small Missions for Advanced Research and Technology - are designed to test new spacecraft technology while visiting various places in the solar system. SMART-1 is now at the moon, mapping the surface mineralogy. Future missions can use the technology being tested by SMART-1 to go to Mars, Venus, Mercury, comets, and the sun.SMART-1 flew to the moon using a new type of ion propulsion system. These engines work by expelling a continuous beam of charged particles --ions-- out the back, which then pushes the spacecraft forward. Because SMART-1 is also testing miniaturized instruments, it is tiny by spacecraft standards, weighing only 367 kilograms (809 pounds). It could fit into a cube just one meter (3.3 feet) across, although the solar panel wings extend out about 14 meters (46 feet).
In this article, Bernard Foing, Chief Scientist at ESA and Project Scientist for SMART-1, explains what finding water on the moon could mean for future exploration.
This is the first in a series of exclusive articles by Bernard Foing for Astrobiology Magazine.
Developing a space program is always a financial headache, but our SMART-1 mission had to be developed from scratch in just 3 years, within a cost envelope of 100 million Euros - a low cost for planetary missions. We got around some of the costs of space exploration by sharing the launch with two commercial satellites, so we ended up paying only 10 percent of the bill for access to space.
From Earth orbit, we used a novel, "Star Trek" ion engine to get to the moon. The engine on SMART-1 is ten times more efficient than a conventional chemical propulsion engine. Instead of producing a chemical reaction and using the exhaust gases for propulsion, we use solar power to produce electricity, and use that electricity to charge our propellant, xenon gas. This allows us to reduce the overall mass of the spacecraft, allotting only 25 percent of its mass to fuel rather than the 50 to 60 percent typical with chemical-reaction thrusters.
After some acrobatics in the navigating, SMART-1 was captured into a lunar orbit on November 15, 2004. The spacecraft has almost completed its first lunar map, imaging the moon's surface in medium resolution. After we established a polar orbit, we just imaged one strip of the surface after another as the moon rotated below the spacecraft. The moon makes one full rotation over the course of a month, so SMART-1 will be able to capture images of the entire surface.
At present, we know the composition of only 10 percent of the moon. That's not enough information to derive its global composition. This is further complicated by the fact that the surface of the moon is jumbled up. Over its history, a number of 10-kilometer-wide bodies have slammed into the moon and tossed things around. That's what created all of the giant impact basins we see on the moon's surface.
The moon's impact history is a theme we want to explore. The same local solar system conditions that affected the young moon also affected the Earth during its first 500 million years. So we want to use the moon as a history book to understand the bombardment record on early Earth. And that is the period of time when we believe life first emerged on the Earth, about 3.8 billion years ago. We know that there were giant impacts that may have sterilized any life forming on the Earth. We can use the moon to develop a better sense of the sterilization events that could have taken place here.
SMART-1 will also search for water ice on the moon. There are some areas near the moon's poles where the bottoms of craters are almost always in shadow; they get almost no direct light from the sun. These are some of the coldest places in the solar system, about 50 Kelvin (minus 223 Celsius, or minus 370 Fahrenheit). They're even colder than Titan. If, for instance, comets or water-rich asteroids delivered water to the moon, by ending up in such cold locations that water would be trapped there, possibly forever.
Some inert deposits of hydrogen have been detected on the moon's poles. We are not sure if this hydrogen is coming from the solar wind or if it is due to water ice from comets or asteroids. But our infrared spectrometer will be able to take spectra even in the dim light of these permanently shadowed areas. They are very dark, but they're illuminated by a tiny bit of light that reflects off the rims of the craters. Some spectral fingerprint should be measurable if this is water ice.
If it is water ice, then on a future expedition we can go to the moon and take a core of these different water ice layers. If the water ice came from asteroids or comets, we could then reconstruct the bombardment history of volatile materials that were wandering around in the region of the solar system where the Earth and the moon formed. This could help us learn where Earth got its water.
Even if the hydrogen isn't water, if it was deposited from the solar wind, we can still extract it and combine it with oxygen, which is very abundant in the soil, to make artificial water. This could be valuable to future human explorers on the moon. Personally, I'd rather drink this artificial water than water from cometary melts. Comet water also contains a lot of organic compounds, and my doctor has recommended against drinking it.
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