CAMBRIDGE, U.K.--The European Space Agency (ESA) on 27 September dispatched a spacecraft on a mission that will try to solve a 4-billion-year-old riddle--how the moon was formed--and look to the future by testing a next-generation type of propulsion.
The $125 million SMART-1 mission was launched on an Ariane 5 rocket from French Guiana. At 367 kilograms and fitting into a cube 1-meter across, the bantamweight spacecraft dispenses entirely with chemical propellant. Its 14-meter solar panels provide power to ionize wisps of its 82-kilogram supply of xenon gas and shoot it out the back of the craft. This generates a thrust of 70 millinewtons, or about the weight of a postcard against the hand; because that force can be applied continually so long as the sun falls on the arrays, the craft's speed will build slowly. Ion drives have been used in recent years to keep orbiting satellites in position, but few spacecraft have used them as their main source of propulsion. By expelling its ions, SMART-1, now in Earth orbit, will ever-so gradually spiral out to an altitude of 200,000 kilometers before feeling a tug from the moon; it should be captured in March 2005.
After completing its 18-month voyage (Apollo 11, for comparison, took 3 days), SMART-1 will transform into a science mission. The big science question that SMART-1 hopes to shed light on is how the moon was born. The prevailing theory is that it coalesced from the debris of a titanic collision between Earth and a large body some 4.5 billion years ago. Rocks hauled back by Apollo missions suggest that the moon has similar constituents to the Earth's mantle. But that was a limited sample, and "there is a desperate need for a global inventory of what the moon is made of," says principal investigator Manuel Grande of the Rutherford Appleton Laboratory in Oxfordshire. X-rays in sunlight impinging on the moon cause surface atoms to fluoresce; these photons, with wavelengths that are characteristic for each element, can be picked up by SMART-1's x-ray spectrometer.
SMART-1 project manager Giuseppe Racca cautions that because many of the craft's systems and instruments are experimental, they cannot be guaranteed to work as predicted. But if SMART-1 delivers, he says, "it will open a new era of lunar science."