India's Chandrayaan spacecraft is set to eject a television-sized probe that will crash onto the lunar surface on Friday. The probe will make observations as it descends, testing systems needed to land future robotic spacecraft on the Moon.
Chandrayaan, which launched on 22 October, has been travelling on an elongated path around the Moon since it went into orbit there on Saturday. But engine firings on Tuesday and Wednesday should move the craft into a circular orbit that brings it just 100 km from the surface.
After letting the spacecraft circle the Moon for two days, mission planners will release the 29-kilogram Moon Impact Probe (MIP) on Friday evening India time. Scheduled to hit the surface near the south pole, it is not designed to survive the impact.
The probe, which was built by the Indian Space Research Organisation (ISRO), will take 20 minutes to descend to the surface.
During that time, it will film the surface with an onboard video camera, says T K Alex, director of ISRO's Satellite Centre (ISAC) in Bangalore.
Soft landings
An altimeter will measure the probe's height as it nosedives, helping to test technologies needed to make 'soft' landings on the Moon. India's next lunar mission, scheduled for 2010, involves not only an orbiter but also a Russian-built lander that will study samples of lunar soil.
"I think the main purpose [of the Moon Impact Probe] is preparations for future soft landings," says Christian Erd, mission project manager at the European Space Agency in Noordwijk, The Netherlands. "It is a demonstration of the separation mechanism and getting to the surface."
The probe will also carry a mass spectrometer that will measure traces of gas and dust floating above the Moon. Signals from all three onboard instruments will be relayed back to the spacecraft and from there to ground stations on Earth.
Dust plume
"It is a sort of mini-satellite, equipped with its own instruments, batteries, and telemetry," says Rajagopal Sreedharan, head of the space physics laboratory at the Vikram Sarabhai Space Centre (VSSC), which built the six Indian instruments onboard Chandrayaan.
MIP will strike the surface near the lunar south pole, a region that shows hints of water ice in the soil. Parts of the terrain there also receive near-constant sunlight, providing prime spots to set up future solar power stations.
"We are impacting the Moon's south pole because of the huge amount of interest in the region," he told New Scientist.
When the probe hits the surface, it is expected to kick up a huge amount of dust that will be analysed by instruments on the Chandrayaan orbiter.
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At 8:34 pm Indian time Friday night (1504 UTC), India became the fourth country to land its flag on the Moon.
The unmanned lunar orbiter Chandrayaan-1 ejected its Moon Impact Probe (MIP), which hurtled across the surface of the Moon at 1.5 kilometres per second (3000 miles per hour), and successfully crash landed near the Moon's south pole.
Besides carrying three important scientific instruments, the lunar probe also carried the image of the Indian national flag, painted on all sides.
India's tricolour is marked on all sides of the Moon Impact Probe
Chandrayaan-1 (meaning 'Moon craft' in Sanskrit) reached its target lunar orbit on Wednesday.  The orbiter will remain in a circular orbit 102 kilometres above the Moon's surface for two years.  Its instruments will be gradually commissioned over the next few days.

With this landing, India became both the fourth country to place a flag on the Moon and the fifth group to send a spacecraft to the Moon. The other countries which have sent spacecraft to the Moon are the United States, the former Soviet Union, Japan, and China, along with the European Space Agency (ESA), a consortium of 17 countries.  
Japan and China currently each have scientific satellites orbiting the Moon, though China has not yet put a spacecraft on the moon's surface.

The MIP has a mass of 29 kg, is about the size of a microwave oven, and was designed and assembled in India.
After the orbiter ejected it, the probe took about 25 minutes to reach the Moon's surface. 
On-board digital cameras made a high resolution movie of the surface during descent, and scientists also conducted measurements with the probe's mass spectrometer and radar altimeter.
Data was beamed back to India via the orbiter, and it is currently being processed and analysed.
The Moon's south pole 
Data from the altimeter experiments will be used to refine the instrument in order to control the soft landing of a future probe.  
Plans are already being prepared for the Chandrayaan-2 to be launched by 2012.

India's first lunar mission was launched from the Satish Dhawan Space Centre on the Andhra Pradesh coast on October 22.
The launch vehicle was an Indian designed and built rocket that had been previously proven by carrying scientific and commercial payloads to earth orbit, including weather and communications satellites.
The cost of this mission is estimated at 340 crore (3.4 billion) rupees (US$78 million). 
The mission carries five scientific instruments built by India's technology sector, and six developed cooperatively with foreign nations.

Goals for the orbiter include making a detailed map of the Moon's chemical make-up and mineral resources, as well as a three-dimensional digital map of the entire surface. 
The mission will examine the surface for sources of water, and take comparison photos of the light side and dark sides of the surface.