A largely unheralded spacecraft, which has been in lunar orbit for nearly two years, is about to end its mission in dramatic fashion. This weekend, flight controllers for the European Space Agency will allow SMART 1 to crash into the Moon's surface — with enough force, they hope, to create a brief flash visible through telescopes on Earth.
SMART 1's fate has been sealed since June, when a series of thruster firings sent the craft spiraling downward on a collision course. Since then the craft has been edging about 1 kilometer (3,000 feet) closer to the lunar surface on each successive 5-hour orbit. ESA's timetable calls for the impact to occur at 5:41 Universal Time (1:41 a.m. EDT) on the 3rd. At that time the Moon will be above the horizon for North Americans west of a line roughly between Chicago and Miami, and it will also be in view from the west coast of South America.
"Depending on the impact time," notes project scientist Bernard Foing, "different parts of the world will have the best seats for the final impact show, with some seats in sunlight and others at night." For example, Foing notes that SMART 1 could strike earlier or later than planned. If it runs into an uncharted lunar mountain, then the impact will occur one orbit earlier, at 00:36 UT (8:36 p.m. EDT on September 2nd). In this case the Moon would be visible in darkness from the East Coast and all of South America.
The impact point is on the lunar near side at 46.3° west, 36.4° south, in an area called Lacus Excellentiae (Lake of Excellence) just south of Mare Humorum. SMART 1 will strike at about 2 km (1 mile) per second. Its 300-kilogram (670-pound) mass should deliver roughly the same kinetic energy delivered by a 2-kg meteoroid coming from the asteroid belt and create an elongated crater up to 10 meters (30 feet) across. Impact specialists hope the collision will generate a faint, very brief flash of light. The European Space Agency has developed detailed instructions for those who want to try to observe or record the collision.
The first of ESA's Small Missions for Advanced Research in Technology, SMART 1 was launched on September 27, 2003. Rather than head directly to the Moon, the spacecraft used a low-thrust engine powered by xenon ions to gradually travel outward along a spiraling, 100-million-km-long trajectory. SMART 1 finally "reached" its destination in November 2004, becoming captured by lunar gravity at a distance of 60,000 km. Its camera and two spectrometers studied the Moon after the spacecraft settled into a smaller orbit.
Another spacecraft, Lunar Prospector, was intentionally slammed into the Moon near its south pole on July 31, 1999. Although many professional telescopes watched for some indication of the collision, nothing was seen.