Chandrayaan-2 (from Sanskrit: Chandra, "Moon" and yāna, "craft, vehicle") is the second lunar exploration mission developed by the Indian Space Research Organisation (ISRO) after Chandrayaan-1. It consists of a lunar orbiter, the Vikram lunar lander, and the Pragyan rover, all of which were developed in India. The main scientific objective is to map and study the variations in lunar surface composition, as well as the location and abundance of lunar water. The spacecraft was launched from the second launch pad at the Satish Dhawan Space Centre in Andhra Pradesh on 22 July 2019 at 09:13:12 UTC by a LVM3-M1 rocket. The craft reached lunar orbit on 20 August 2019. The Vikram lander attempted a lunar landing on 6 September 2019; the lander crashed due to a software error. The lunar orbiter continues to operate in orbit around the Moon. A follow-up landing mission, Chandrayaan-3, was launched in 2023 and successfully performed a lunar landing.

History

On 12 November 2007, representatives of the Roscosmos and ISRO signed an agreement for the two agencies to work together on the Chandrayaan-1's follow-up project, Chandrayaan-2. ISRO would have the prime responsibility for the orbiter, rover and the launch by GSLV, while Roscosmos was to provide the lander. The Indian government approved the mission in a meeting of the Union Cabinet, held on 18 September 2008 and chaired by Prime Minister Manmohan Singh. The design of the spacecraft was completed in August 2009, with scientists of both countries conducting a joint review. Although ISRO finalised the payload for Chandrayaan-2 on schedule, the mission was postponed in January 2013 and rescheduled to 2016 because Russia was unable to develop the lander on time. In 2012, there was a delay in the construction of the Russian lander for Chandrayaan-2 due to the failure of the Fobos-Grunt mission to Mars, since the technical issues connected with the Fobos-Grunt mission which were also used in the lunar projects including the lander for Chandrayaan-2 needed to be reviewed. The changes proposed by Roscosmos necessitated increase in lander mass and required ISRO to decrease mass of its rover and accept some reliability risk. When Russia cited its inability to provide the lander even by a revised time-frame of 2015 due to technical and financial reasons, India decided to develop the lunar mission independently. With new mission timeline for Chandrayaan-2 and an opportunity for a Mars mission arising with launch window in 2013, unused Chandrayaan-2 orbiter hardware was repurposed to be used for the Mars Orbiter Mission. Chandrayaan-2 launch had been scheduled for March 2018 initially, but was first delayed to April and then to October 2018 to conduct further tests on the vehicle. On 19 June 2018, after the program's fourth Comprehensive Technical Review meeting, a number of changes in configuration and landing sequence were planned for implementation which increased the gross lift-off mass of spacecraft from 3,250 kg to 3,850 kg. Initially an uprated GSLV Mk II was the chosen launch vehicle for Chandrayaan-2 but this increased spacecraft mass and issues with launch vehicle upratement forced the launch vehicle to be switched to more capable LVM3. Issues with engine throttling were found during testing pushing the launch to the early 2019 and later two of the lander's legs received minor damage during one of the tests in February 2019 delaying the launch even further. Chandrayaan-2 launch was scheduled for 14 July 2019, 21:21 UTC (15 July 2019 at 02:51 IST local time), with the landing expected on 6 September 2019. However, the launch was aborted due to a technical glitch and was rescheduled. The launch occurred on 22 July 2019 at 09:13:12 UTC (14:43:12 IST) on the first operational flight of a GSLV MK III M1. On 6 September 2019, the lander during its landing phase, deviated from its intended trajectory starting at 2.1 km altitude, and had lost communication when touchdown confirmation was expected. Initial reports suggesting a crash were confirmed by ISRO chairman K. Sivan, stating that "it must have been a hard landing". The Failure Analysis Committee concluded that the crash was caused by a software glitch. Unlike ISRO's previous record, the report of the Failure Analysis Committee has not been made public. Chandrayaan-2 orbiter performed a collision avoidance manoeuvre at 14:52 UTC on 18 October 2021 to avert possible conjunction with Lunar Reconnaissance Orbiter. Both spacecraft were expected to come dangerously close to each other on 20 October 2021 at 05:45 UTC over the Lunar north pole.

Objectives

The primary objectives of the Chandrayaan-2 lander were to illustrate the ability to soft-land and operate a robotic rover on the lunar surface. The scientific goals of the orbiter are

Design

The name Chandrayaan means "mooncraft" in Sanskrit and most other Indian languages. The mission was launched on a GSLV Mk III M1 with an approximate lift-off mass of 3850 kg from Satish Dhawan Space Centre on Sriharikota Island of Andhra Pradesh. , the mission has an allocated cost of ₹ 9.78 billion (approximately US$141 million which includes ₹ 6 billion for the space segment and ₹ 3.75 billion as launch costs on GSLV Mk III M1. Chandrayaan-2 stack was initially put in an Earth parking orbit of 170 km perigee and 40400 km apogee by the launch vehicle.

Orbiter

The Chandrayaan-2 orbiter is orbiting the Moon on a polar orbit at an altitude of 100 km. It carries eight scientific instruments; two of which are improved versions of those flown on Chandrayaan-1. The approximate launch mass was 2379 kg. The Orbiter High Resolution Camera (OHRC) conducted high-resolution observations of the landing site prior to separation of the lander from the orbiter. The orbiter's structure was manufactured by Hindustan Aeronautics Limited and delivered to the ISRO Satellite Centre on 22 June 2015.

Vikram lander

The mission's lander is called Vikram named after cosmic ray scientist Vikram Sarabhai (1919–1971), who is widely regarded as the founder of the Indian space programme. The Vikram lander detached from the orbiter and descended to a low lunar orbit of 30 x using its 800 N liquid main engines. After checking all of its on-board systems it attempted a soft landing that would have deployed the rover, and performed scientific activities for approximately 14 Earth days. Vikram crash-landed during this attempt. The combined mass of the lander and rover was approximately 1471 kg. The preliminary configuration study of the lander was completed in 2013 by the Space Applications Centre (SAC) in Ahmedabad. The lander's propulsion system consisted of eight 58 N thrusters for attitude control and five 800 N liquid main engines derived from ISRO's 440 N liquid apogee motor. Initially, the lander design employed four main throttle-able liquid engines, but a centrally mounted fixed-thrust engine was added to handle new requirements of having to orbit the Moon before landing. The additional engine was expected to mitigate upward draft of lunar dust during the soft landing. The four throttle-able engines of lander were capable of throttling between range of 40 to 100 percent incrementally in steps of 20%. Vikram was designed to safely land on slopes up to 12°. Some associated technologies include: Engineering models of the lander began undergoing ground and aerial tests in late October 2016, in Challakere in the Chitradurga district of Karnataka. ISRO created roughly 10 craters on the surface to help assess the ability of the lander's sensors to select a landing site.

Pragyan rover

The mission's rover was called Pragyan ) with a mass of 27 kg, and would have operated on solar power. The rover was to move on six wheels, traversing 500 m on the lunar surface at the rate of 1 cm per second, perform on-site analyses and send the data to the lander, which would have relayed it to the Mission Control on the Earth. For navigation, the rover would have used: The expected operating time of Pragyan rover was one lunar day, or ~14 Earth days, as its electronics were not designed to endure the frigid lunar night. However, its power system has a solar-powered sleep/wake-up cycle implemented, which could have resulted in longer service time than planned. Two aft wheels of the rover had the ISRO logo and the State Emblem of India embossed on them to leave behind patterned tracks on the lunar surface.

Science payload

ISRO selected eight scientific instruments for the orbiter, four for the lander, and two for the rover. While it was initially reported that NASA and European Space Agency (ESA) would participate in the mission by providing some scientific instruments for the orbiter, ISRO in 2010 had clarified that due to weight restrictions it will not be carrying foreign payloads on the mission. However, in an update a month before launch, an agreement between NASA and Indian Space Research Organisation (ISRO) was signed to include a small laser retroreflector from NASA to the lander's payload to measure the distance between the satellites above and the microreflector on the lunar surface.

Orbiter

The orbiter has several scientific payloads.

Vikram lander

The payloads on the Vikram lander were:

Pragyan rover

Pragyan rover carried two instruments to determine the abundance of elements near the landing site:

Discoveries and results

The orbiter, which is still active, did experiments on Lunar Atmospheric composition, trace elements, and more

Mission profile

Launch

The launch of Chandrayaan-2 was initially scheduled for 14 July 2019, 21:21 UTC (15 July 2019 at 02:51 IST local time). However, the launch was aborted 56 minutes and 24 seconds before launch due to a technical glitch, so it was rescheduled to 22 July 2019. Unconfirmed reports later cited a leak in the nipple joint of a helium gas bottle as the cause of cancellation. Finally Chandrayaan-2 was launched on board the LVM3 M1 launch vehicle on 22 July 2019 at 09:13:12 UTC (14:43:12 IST) with a better-than-expected apogee as a result of the cryogenic upper stage being burned to depletion, which later eliminated the need for one of the apogee-raising burns during the geocentric phase of mission. This also resulted in the saving of around 40 kg fuel on board the spacecraft. Immediately after launch, multiple observations of a slow-moving bright object over Australia were made, which could be related to upper stage venting of residual LOX / LH2 propellant after the main burn.

Geocentric phase

After being placed into a 45,475 × 169 km parking orbit by the launch vehicle, the Chandrayaan-2 spacecraft stack gradually raised its orbit using on-board propulsion over 22 days. In this phase, one perigee-raising and five apogee-raising burns were performed to reach a highly eccentric orbit of 142,975 × 276 km followed by trans-lunar injection on 13 August 2019. Such a long Earth-bound phase with multiple orbit-raising manoeuvres exploiting the Oberth effect was required because of the limited lifting capacity of the launch vehicle and thrust of the spacecraft's on-board propulsion system. A similar strategy was used for Chandrayaan-1 and the Mars Orbiter Mission during their Earth-bound phase trajectory. On 3 August 2019, the first set of Earth images were captured by the LI4 camera on the Vikram lander, showing the North American landmass.

Selenocentric phase

After 29 days from its launch, the Chandrayaan-2 spacecraft stack entered lunar orbit on 20 August 2019 after performing a lunar orbit insertion burn for 28 minutes 57 seconds. The three-spacecraft stack was placed into an elliptical orbit that passed over the polar regions of the Moon, with 18072 km aposelene and 114 km periselene. By 1 September 2019, this elliptical orbit was made nearly circular with 127 km aposelene and 119 km periselene after four orbit-lowering manoeuvres followed by separation of Vikram lander from the orbiter on 07:45 UTC, 2 September 2019.

Planned landing site

Two landing sites were selected, each with an ellipse of 32 ×. The prime landing site (PLS54) was at 70.90267°S 22.78110°E (600 km from the south pole, ) and the alternate landing site (ALS01) was at 67.87406° South 18.46947° West. The prime site was on a high plain between the craters Manzinus C and Simpelius N, on the near side of the Moon.

Failed landing attempt

Vikram began its descent at 20:08:03 UTC, 6 September 2019 and was scheduled to land on the Moon at around 20:23 UTC. The descent and soft-landing were to be performed by the on-board computers on Vikram, with mission control unable to make corrections. The initial descent was considered within mission parameters, passing critical braking procedures as expected, but the lander's trajectory began to deviate at about 2.1 km above the surface. The final telemetry readings during ISRO's live-stream show that Vikram's final vertical velocity was 58 m/s at 330 m above the surface, which a number of experts noted, would have been too fast for the lunar lander to make a successful landing. Initial reports suggesting a crash were confirmed by ISRO chairman K. Sivan, stating that "it must have been a hard landing". However, it contradicted initial claims from anonymous ISRO officials that the lander was intact and lying in a tilted position. Radio transmissions from the lander were tracked during descent by analysts using a 25 m radio telescope owned by the Netherlands Institute for Radio Astronomy. Analysis of the doppler data suggests that the loss of signal coincided with the lander impacting the lunar surface at a velocity of nearly 50 m/s (as opposed to an ideal 2 m/s touchdown velocity). The powered descent was also observed by NASA's Lunar Reconnaissance Orbiter (LRO) using its Lyman-Alpha Mapping Project instrument to study changes in the lunar exosphere due to exhaust gases from the lander's engines. K. Sivan, tasked senior scientist Prem Shanker Goel to head the Failure Analysis Committee to look into the causes of the failure. Both ISRO and NASA attempted to communicate with the lander for about two weeks before the lunar night set in, while NASA's LRO flew over on 17 September 2019 and acquired some images of the intended landing zone. However, the region was near dusk, causing poor lighting for optical imaging. NASA's LRO images, showing no sight of the lander, were released on 26 September 2019. The LRO flew over again on 14 October 2019 under more favourable lighting conditions, but was unable to locate it. The LRO performed a third flyover on 10 November 2019. On 16 November 2019, the Failure Analysis Committee released its report to the Space Commission, concluding that the crash was caused by a software glitch. Phase One of descent the Rough Braking Phase from an altitude of 30 km to 7.4 km above the Moon's surface went as intended with velocity being reduced from 1683 m/s to 146 m/s. Anomalous deviation in performance began 693.8 seconds into powered descent after the end of first phase and with the beginning of Absolute Navigation Phase (also known as Camera Coasting Phase) where lander's orientation is deliberately kept fixed. It was found that lander's main engines had slightly higher thrust of 422 N than nominal at 360 N, so during this phase lander slowed down more than it should have. The thrust control algorithm was configured to apply corrections towards the end of the phase and not instantaneously allowing large navigation errors to be accumulated. After end of camera coasting phase, rate of applying corrections was limited due to builtin safety constraints such as maximum rate at which attitude can change. Other contributing issues were, coarse throttling of main engines, polarity related software error, wrong computation of remaining time of flight by onboard algorithm and very rigid requirement to land inside the planned 500×500 meter landing site regardless of non-nominal flight status. Subsequently, Vikram lander ended up increasing its horizontal velocity (48 m/s) to reach landing site while descending at high rate (50 m/s) causing Vikram to land hard, though it managed to impact relatively near the intended landing site. The complete official report has not been made public. Vikram's impact site was located at -70.881°N, 22.784°W by the LROC team after receiving helpful input from Shanmuga Subramanian, a volunteer from Chennai, Tamil Nadu, who located debris from the spacecraft in pictures released by NASA. While initially estimated to be within 500 m of the intended landing site, best-guess estimates from satellite imagery indicate initial impact about 600 m away. The spacecraft shattered upon impact, with debris scattered over almost two dozen locations in an area spanning kilometres. The crash site was later named Tiranga Point after the Chandrayaan-3 landing. The orbiter part of the mission, with eight scientific instruments, remains operational, and will continue its seven-year mission to study the Moon.

Telemetry, tracking, and command (TT&C)

During various phases of launch and spacecraft operations of Chandrayaan-2 mission, the TT&C support was provided by ISRO Telemetry, Tracking and Command Network (ISTRAC), Indian Deep Space Network (IDSN), NASA Deep Space Network and National Institute for Space Research's (INPE) ground stations located in Alcântara and Cuiabá.

Aftermath

There was an outpouring of support for ISRO from various quarters in the aftermath of the crash landing of its lunar lander. However, prominent Indian news media also criticized ISRO's lack of transparency regarding the crash of the lander and its analysis of the crash. Indian media also noted that unlike ISRO's previous record, the report of the Failure Analysis Committee was not made public and RTI queries seeking it were denied by ISRO citing section 8(1) of the RTI Act. ISRO's lack of consistency regarding the explanation around the rover's crashing was criticized, with the organization providing no proof of its own positions until the efforts of NASA and a Chennai based volunteer located the crash site on the lunar surface. In the wake of the events surrounding Chandrayaan-2, former ISRO employees criticized unverified statements from chairman K Sivan and what they claimed is the top-down leadership and working culture of the organization. S Somanath who succeeded K Sivan as ISRO Chairman also expressed his dissatisfaction at the lack of transparency around landing failure, and misleading representation of it.

Scientists involved in the mission

Key scientists and engineers involved in the development of Chandrayaan-2 include:

Reattempt

In November 2019, ISRO officials stated that a new lunar lander mission was being studied and prepared. It was launched on 14 July 2023; with the designation Chandrayaan-3, which was a second attempt to demonstrate the landing capabilities needed for the Lunar Polar Exploration Mission proposed in partnership with Japan for 2025. The new mission was designed with a detachable propulsion module, also behaving like a communications relay satellite, a lander and a rover, but with no orbiter. S. Somanath, the VSSC director, announced that there would be more follow-up missions in the Chandrayaan programme. In December 2019, it was reported that ISRO requested the initial funding of the project, amounting to inr 750000000, of which inr 600000000 is intended for machinery, equipment and other capital expenditure, while the remaining inr 150000000 was sought under a revenue expenditure allowance. K. Sivan stated that its cost would be around inr 6150000000. It performed a soft landing on the Moon on 23 August 2023.