Sci Tues for 10/218
In the slowest trip to the Moon ever attempted, the SMART-1 robotic spacecraft has completed 50 orbits around the Earth as mission managers work on a glitch in a craft they purposely designed to frustrate.
As planned, SMART-1 is spiraling slowly out of Earth's gravitational clutches, relying on an initial rocket boost and a modest but dogged high-tech ion engine. The engine is working exactly as designed, though it will still be more than 16 months before the tortoise-like craft, the European Space Agency's first mission to the Moon, settles into its final lunar orbit.
"Yes, it is frustrating," SMART-1 project manager Giuseppe Racca told SPACE.com late last week, when asked if it's hard to have to wait so long to get to the science portion of the mission. "And actually this is not the way one would chose to travel if the objective was only the Moon."
While the robotic probe has a full slate of Moon investigations planned once it reaches lunar orbit, part of the mission's purpose is to test the ion propulsion system for a future trip to Mercury. Racca said a Mercury journey with the high-tech engine would be shorter than with conventional propulsion.
An ion thruster uses electricity generated by solar arrays to charge a stream of xenon gas. A nozzle pushes charged particles, called ions, out the back, giving the craft a gentle but continuous push. Ion engines shine best on long-distance voyages. NASA first tried one out on the Deep Space 1 craft, which tested several technologies and, limping to the finish line, took the best comet photos ever made back in 2001.
The scheme employs a rocket to initially get the craft into Earth orbit. Thereafter, the ion engine uses less fuel than conventional thrusters, so the craft is lighter and cheaper, but its a bit more plodding at first. Once in deep space, an ion engine can gradually accelerate a craft to very high speeds.
"The Moon is too close to Earth and too 'easy' to reach to exploit the benefits of ion propulsion," Racca explained.
Navigation problem
SMART-1 launched Sept. 27. The mission's first serious in-space glitch cropped up recently.
The probe's automated star-tracker, which keeps it on course and in the proper attitude, has " failed to provide good attitude information in a few cases," according to an ESA statement. The glitch is related to radiation belts that surround Earth, Racca said. "We are confident that we can solve all the problems of the star tracker by changing parameters in the software," he said.
And either way, Racca does not expect the star tracker's woes to affect the science portion of the mission.
"Most of the problems are related to the presence of the Earth and of the radiation belts," he said. "Both will not be there at the Moon, so, provided we can reach the Moon, the star tracker performance would not affect the mission."
Free ride
Meanwhile, SMART-1 has already reached its top speed and backed off. The milestone occurred during the first orbit, about 10 hours after launch, when the craft hit about 22,400 mph (10 kilometers per second). It now fluctuates dramatically between 3,800 and 19,900 mph (1.7 and 8.9 kilometers per second) depending on its distance from Earth.
As with any object in an elliptical, off-center orbit, SMART-1 travels fastest when it is close to Earth and slower on the outer reaches of its path.
The craft will spiral outward in ever-widening ellipses around Earth for another two months. Ion thrust will then carry it to a special spot in space, about 37,300 miles (60,000 kilometers) from the Moon, called Lagrangian point L1. Lagrangian points are locations where gravity from two objects essentially balances out. A craft at a Lagrangian point is free to go either way with very little thrust.
"It is kind of a gateway for the entrance into Moon orbit, almost free of charge," Racca said.
SMART-1 will enter lunar orbit in March 2005. There it will look for clues to the Moon's origin and try to confirm that there is frozen water hidden at the lunar poles.
 | An artist's impression of SMART-1 leaving Earth on its way toward the Moon. How will SMART-1 get there? Take a step-by-step trip to the Moon, the slow way. Gallery >>> |
advertisement
