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Planetary News: SMART-1 (2004)

"I Think I Can, I Think I Can:" SMART-1 Beginning to Feel the Tug of Lunar Gravity

Click to enlarge > Little SMART-1 fires its engine to achieve capture into lunar orbit Credit: ESA

Since her launch, the European Space Agency's SMART-1 spacecraft has circled the Earth nearly 320 times on a spiraling orbit that is slowly, inexorably, taking her from the Earth to the Moon. The 'Little Solar Electric Engine That Could,' SMART-1 is now approaching the top of the hill, and after a few more months of orbiting the Earth, she will dive down the other side of the gravitational hill, to be captured into lunar orbit.

SMART-1 is arriving ahead of schedule. By conserving fuel, mission planners have been able to skip doing two risky lunar flybys. According to ESA Chief Scientist and SMART-1 Project Scientist Bernard Foing, "We now have a new trajectory design, where instead of capture in January 2005, we will capture on the 15th of November 2004. That depends of course on everything being nominal from now to then, so there will be a bit of suspense on the exact time, but it is likely to be the 15th of November."

In preparation for lunar capture, mission planners have lined up SMART-1's elliptical orbit so that on August 15, she will be at her orbital apocenter (the farthest point on her orbit from the center of the Earth) at a position from which she will feel a stronger pull from the lunar gravity than she has felt before, getting a sizeable boost on her journey from the Earth to the Moon. SMART-1 will experience two more of these lunar resonances, around September 13 and October 13, before finally being captured into lunar orbit. The first resonance maneuver will also mark the first time that SMART-1 has been closer to the moon than she is to Earth.

According to Foing, it has taken over 2700 hours of thrust from SMART-1's ion engine to deliver her to this point. Now, though, the interplay between electric thrust and Newtonian mechanics will make the navigation more like surfing than a powered drive. "Resonances amplify the effect of SMART-1's tiny thrust. As soon as we get the perturbation from the Moon, we can make use of natural elements -- the gravity of the Moon, the Earth, and a little bit of the Sun -- to get into capture. We then use the thrust only at good moments to optimize the position and conditions of the capture."

Escaping the Earth

SMART-1 is the first of the European Space Agency's Small Missions for Advanced Research in Technology, a program to develop a new breed of spacecraft that will demonstrate and test innovative technologies for future deep space missions. SMART-1 was intended to test cheaper, faster spacecraft development and operation; solar electric ion propulsion; and miniaturized science instrument technology. With her flawless launch on September 27, 2003, and the successful commissioning of her payload of seven science instruments in February, SMART-1 has passed all of the tests that she has faced so far. Now she needs only to reach the Moon.

The trip has not been completely smooth. SMART-1's ion engine is highly efficient, squeezing five to ten times more impulse out of the same amount of propellant than chemical rockets can. But the engine produces a miniscule amount of thrust, only 0.07 Newtons (0.016 pounds). Thus the engine must fire continuously for long periods of time in order to affect the spacecraft's orbit. The problem for SMART-1 was that her initial orbit left her spending a great deal of time within the hazardous environment of the Earth's radiation belt.

"We had a strategy from the first three months to thrust continuously to spend as little time as possible in the radiation belt. It was not a nice place to be -- it is like you are on the sea, and the sea is rough; you would want to leave as fast as possible," Foing says. It took until January 7 for SMART-1 to raise her orbit pericenter (the closest point on the orbit to the center of the Earth) out of the radiation belt.

Through sheer bad luck, the time that SMART-1 spent inside the radiation belt was also a time of unusually powerful solar storms. "We expected that the solar panels would be degraded as a result of being bombarded with radiation. What we did not expect was the huge solar storms that took place around Halloween 2003. Of course we lost some time to recover but some other spacecraft were completely knocked out," Foing says. The effect of the solar storms on SMART-1 was to cause "flame-outs" of the ion engine, in which the engine unexpectedly shut down; other events caused a loss of the proper spacecraft attitude. But SMART-1 had been built to recover from such mishaps, and went into a safe mode. The spacecraft suffered no lasting harm from these events, but the stormy cruise period caused considerable hair-pulling back on Earth.

Foing explains. "When we go into safe mode, it is a bit more complicated for SMART-1 [than for other Earth-orbiting spacecraft] to recover, because if you go into safe mode while you are supposed to thrust, then the orbit is not the one which was planned. Every time we don't do the plan, then we have to recalculate the navigation scheme. This was time consuming. Initially we were planning, to save money, to spend only eight hours, twice per week, in contact with the spacecraft." The repeated recalculations of the orbits and frequent communications with the spacecraft were "a bit demanding on the operations group, and in the beginning some of them could have hated us for that. After all the preparations for the launch activities we were expecting, 'okay, now only twice per week we will look at the spacecraft, now we can go back to our families,' but in fact they have spent much more time with us. So by December, things were quite tense with the team. But after we left the radiation belt, it is now much smoother."

Having reached a more benign radiation environment, SMART-1 could then use her engine more efficiently, firing it only a third of the time around pericenter. This new thrust strategy had the effect of rapidly raising her orbital apocenter (the farthest point on her orbit from the center of the Earth), bringing her closer to the Moon. It has also permitted her to conserve on fuel.

By the time that the new thrust strategy was adopted, SMART-1 could be trusted to fire when she was told to. Mission planners had learned how to drive the solar electric spacecraft -- or how to tell the spacecraft to drive herself. "Before, the ion engine sometimes had a little instability in the ion engine, and it would stop, causing 'flame-outs,' and then when we tried to contact the spacecraft, it was not at the place that we were expecting it. In February we uploaded some software indicating to the engine to restart automatically, and since this time it has been working fine. It now works in a more autonomous mode. It is very important to have onboard autonomy so that we can diminish the control from the ground because it is costly in terms of manpower and time."

Getting the science started

The increasing autonomy of SMART-1, and her successful escape from the radiation belt, permitted the commissioning of her instruments to take place in early 2004. SMART-1 carries a payload of seven science instruments within a paltry 19 kilograms (42 pounds) of payload:

• AMIE (High-resolution micro-camera)
• D-CIXS (Compact imaging X-ray spectrometer);
• SIR (Infrared spectrometer);
• SPEDE (Spacecraft potential electron dust experiment);
• EPDP (Electric propulsion diagnostics package);
• KATE (Deep space X- and Ka-band communications); and
• RSIS (Radio science)

All of the instruments have been commissioned and are working properly. AMIE has been returning images since January 18, and now conducts regular observations of both the Earth and the Moon. Foing is looking forward to the science part of the mission: "Most of the science will be done while we are around the Moon. For now we are trying to optimize the planning for when we will be in lunar orbit."

Originally, SMART-1 planners explored the idea of sending the spacecraft onward after lunar capture, perhaps to visit an asteroid. "But looking at all the possibilities," Foing says, "we decided to do a good mission to the moon, and orbit lower, rather than go to an asteroid. There was not really a good asteroid; you would have needed years to get to the object, and then it would have been only a flyby. There was not the possibility to do a rendezvous of the object. So finally we decided to do a good mission, we would stay, and get the possibility to go to a lower altitude at the Moon."

After lunar capture, SMART-1 will use her engine to accomplish the reverse of what she has done so far: she will slow her orbital speed, spiraling in toward the lunar surface. With the fuel that she has left over from her cruise, she will be able to bring her orbit into a perilune (closest approach to the lunar surface) of 3000 kilometers. At that distance, the AMIE camera will be able to capture color images with a pixel scale of 40 meters (130 feet), four times better than Clementine did. In addition to the highest resolution imaging, "We have the possibility to have resolution that is twice better than Clementine for most of the Moon, up to 50 degrees north latitude. On the other hand, we are not going to be able, in the first 6 months, to cover globally. So we will select some scenes to study, and from those scenes try to understand the origin of the moon, volcanism, cratering, and so forth. Also we could observe some scenes with SMART-1 that will help us select the a site for future South Pole Aitken Basin Sample return." NASA has recently selected such a mission, called Moonrise, as one of two possibilities for the next New Frontiers funding opportunity, with a launch date no later than June 2009.

Foing expects SMART-1 to last considerably longer than this 6 months of her nominal mission at the Moon. He hopes, in fact, that SMART-1 will be able to shake hands with several future lunar visitors that have not even launched yet. "We will try to stay there as long as possible so that we can do joint operations with the Japanese spacecraft [Lunar-A and Selene] when they are there in 2006, and we hope we will still be there when the Indians arrive in 2007 [with Chandrayaan-1], and maybe when NASA's Lunar Reconnaissance Orbiter does in 2008. That would be great. And also, then, we could decide to have a glorious end-of-life, in a crash on the lunar surface, and ask our partners to observe our final crash. So this would be a well-coordinated international program. SMART-1 is the only one in the fleet to the Moon at the moment -- but we are looking for company!"