Chandrayaan-2 is the 110th space mission to the moon, and the 11th this decade. A bulk of the moon missions, 90 out of the 109 so far, were sent between 1958 and 1976. There was a complete lull in moon exploration after that.
Missions to moon slowly resumed in the 1990s but have picked up steam only in the last decade. The discovery of the presence of water on the moon, by Chandrayaan-1 mission in 2008, has been one of the prime reasons for a renewed interest in moon.
Here are the different kinds of moon missions:
Flybys: These are the missions in which the spacecraft passed near the moon but did not get into an orbit around the moon. These were either designed to study the moon from a distance, or were on their way to some other planetary body or deep space exploration and happened to pass by the moon. Some early examples of flyby missions were Pioneer 3 and 4 by the United States and Luna 3 of the then USSR.
Orbiters: These were spacecraft that were designed to get into a lunar orbit and carry out prolonged studies of the moon’s surface and atmosphere. India’s Chandrayaan-1 was an Orbiter, so were 46 other moon missions from various countries. Orbiter missions are the most common way to study a planetary body. So far, landings have been possible only on moon and Mars. All other planetary bodies have been studied through Orbiter or flyby missions.
Impact Mission: These are an extension of orbiter missions. While the main spacecraft keeps going around the moon, one or more instruments onboard make an uncontrolled landing on the moon. They get destroyed after the impact, but still send some useful information about the moon while on their way. One of the instruments on Chandrayaan-1, called Moon Impact Probe, or MIP, was also made to crash land on the moon’s surface in a similar way. ISRO claims that data sent by the MIP had presented additional evidence of presence of water on moon, but these findings could not be published because of calibration errors.
Landers: These missions involve the soft-landing of the spacecraft on the moon. These are more complicated than the Orbiter missions. In fact, the first 11 attempted lander missions had all ended in failure. The first landing on the moon was accomplished on January 31, 1966, by the Luna 9 spacecraft of the then USSR. It also relayed the first picture from the moon’s surface.
Rovers: These are an extension of the lander missions. The lander spacecraft, because they are bulky and have to stand on legs, remain stationary after landing. The instruments onboard can carry out observations and collect data from close quarters but cannot come in contact with the moon’s surface or move around. Rovers are designed to overcome this difficulty. Rovers are a special wheeled payloads on the lander that can detach themselves from the spacecraft and move around on moon’s surface, collecting very useful information that instruments within the lander would not be able to obtain. The rover onboard Vikram lander in the Chandrayaan-2 mission is called Pragyaan. Earlier this year, a Chinese lander and rover mission reached the moon. These are still active.
Human missions: These involve the landing of astronauts on the moon’s surface. So far only NASA of the United States has been able to land human beings on the moon. So far, six teams of two astronauts each have landed on the moon, all between 1969 and 1972. After that, no attempt has been made to land a moon. But NASA has now announced plans to send another manned mission by the year 2024.