Going out with a bang
Lunar orbiter to impact
BY STEPHEN CLARK
Posted: July 24, 2006
A pioneering space probe has set a course for a dramatic end to its three-year mission in September, when it will collide with the Moon in a spectacular send-off for scientists to learn new information about the lunar surface.
The end will come with a fiery crash into the Moon's nearside at about 0541 GMT (1:41 a.m. EDT) on September 3, according to the best data now available. This time is only certain within seven hours.
The exact timing of the impact will slightly change as the probe's orbit evolves in several upcoming trim maneuvers designed to fine-tune the craft's approach. Five such burns are planned for July 27, July 28, August 25, and on the final two days before SMART-1 hits the surface.
Scientists' "best guess" on the impact site puts it in the Moon's mid-southern latitudes in a region known as the Lake of Excellence. The location is believed to be of mostly volcanic origin and areas of highlands and hills are also nearby. The area is also known as Lacus Excellentiae in Latin.
At the time of impact, the Lake of Excellence will be shrouded in darkness - only slightly illuminated by light from the Earth in a phenomenon called "earthshine." The boundary between bright sunshine and nighttime on the Moon will be located not far away, possibly allowing material blown above the surface to reach altitudes high enough to be lit by sunlight.
Ground controllers in Germany had to command a two-week series of thruster firings to raise the low point of SMART-1's trek around the Moon to push back the inevitable crash from about August 17 to the current date of September 3. The plan also brought the forecasted impact site into the direct view of scientists on the ground. If left undisturbed, the 630-pound probe would have smacked into the far side of the Moon facing away from Earth.
The set of orbit-raising maneuvers began June 19 and ended on July 2 - about five days ahead of schedule.
Astronomers at observatories across the globe plan to try to see the plume of debris ejected high above the lunar surface as SMART-1 drives into the Moon. Organized groups at the European Southern Observatory in Chile, Kitt Peak in Arizona, telescopes in Hawaii, and other locations will attempt to spot the impact and its aftermath
The most favorable position for observing an on-time impact will be in North and South America and Hawaii. There, the quarter Moon will be high in the sky for prime viewing.
Larger telescopes could detect a flash at the moment of impact due to vaporized hydrazine fuel. A few minutes later, dust and rocks thrown high above the Moon might be seen. If the debris reaches an altitude of over 12 miles, it could be lit by sunlight. If so, amateur astronomers with smaller backyard telescopes could see the dust cloud backdropped by the darker lunar surface.
"We are calling upon astronomical observatories and amateurs worldwide to participate in a coordinated observation effort with SMART-1, including the final orbits until impact," said European Space Agency SMART-1 project scientist Bernard Foing.
Instruments to be operating during SMART-1's final hours include an infrared spectrometer, an X-ray spectrometer, and a tiny camera to take pictures of the Moon as the spacecraft passes near the surface.
The material propelled into sunlight will be closely analyzed to determine details such as its mineral composition and physical properties. Some of the debris could be excavated from underground during SMART-1's crash, so scientists are especially interested in learning about the sub-surface of the Moon.
During SMART-1's final orbits, the spacecraft should be speeding just a few miles above the lunar surface. Accurate predictions of the expected impact time and location are hard to produce because of unknown variations in topography along the probe's trajectory over the Moon. Officials say an unexpected mountain or cliff could cause the craft to crash earlier than anticipated.
By early September, SMART-1 will complete one orbit of the Moon about every five hours. With each orbit's closest approach to the surface, the probe will slowly descend further.
Two low passes before the most likely impact time are also being closely watched in case SMART-1 hits the Moon early. If the spacecraft strikes the Moon at 0037 GMT, observers in South America and the Canary Islands will garner the best view of the event. One orbit earlier - at 1933 GMT on September 2 - astronomers based in Europe and Africa will have the best chance to view the crash.
At first contact, engineers believe SMART-1 will gently glide down at a vertical speed of just under 45 miles per hour, assuming the impact is on a relatively flat surface. However, the craft will be traveling at a horizontal clip of well over 4,000 miles per hour. The low-angle impact could carve a crater up to three feet deep and a couple dozen feet wide.
The testbed carries an efficient ion engine that is fueled by a relatively small amount of xenon fuel and electricity. Strides have also been made in autonomous navigation through ground software that can remotely track the craft's position and velocity through images of stars taken by an on-board camera. Several communications tests through lasers and higher radio frequencies were also carried out.
On the scientific front, SMART-1 has captured and returned up to 1,000 images per week during the past 15 months. Its miniature visible camera is fitted with several color filters, so scientists were also able to take several black-and-white pictures of the same area and artificially create a color image.
The spacecraft's instruments have also been working on studies of lunar composition and the search for ice hidden in the bottoms of polar craters. SMART-1 has also collected evidence on the Moon's evolution and origin. Extensive mapping operations have also been a priority for planning in advance of future lunar missions.
SMART-1 has been circling the Moon since November 2004, almost 14 months after launching aboard an Ariane 5 rocket in September 2003. The probe took a circuitous route to the Moon, completing 332 orbits around Earth while using its electric propulsion system to gently nudge it higher before finally slipping into lunar orbit.
While transitioning to a stable science orbit a few months later, SMART-1 was quickly granted a mission extension in February 2005 that allowed the mission to continue operations through August of this year.
The mission is the first member of the European Space Agency's Small Missions for Advanced Research in Technology program. SMART missions are designed to test new technologies before employing them on more expensive projects.