CHANGE 2

Originally, Chang e was to be followed several years later by a rover and eventually a sample return mission, the follow-on missions being approved by the government in October 2008. The next year, though, it was announced that a second orbiter would take more detailed pictures to assist the subsequent rover and sample return missions, and study the Moon’s chemical composition. To improve communications, it would carry an x-band antenna able to transmit high-speed data at a faster rate with smaller equipment. To conclude the mission, it would either be crashed into the Moon, return to a high orbit in the Earth-Moon system, or be moved to solar orbit.

Chang e 2 was duly launched on the evening (18:59) of National Day, 1st October 2010, using the new Long March 3C rocket out of Xi Chang. Unlike its predecessor, the more powerful rocket enabled a direct ascent, so it reached the Moon five days later, entering a 12-hr lunar orbit on 5th October. It had the tightest possible launch window, at one second, to launch at that particular moment. Two maneuvers were needed to achieve the 100-km operational orbit of 118 min: its first orbits were at an altitude of 119-8,599 km, 3.5 hr, and then the circular orbit was achieved on 9th October. It was claimed that the 50-kg engine achieved an accuracy in thrust of 1 cm/sec. Within two weeks, Chang e 2 had made the first of two dives over the lunar surface, down to 15 km on 27th October for close-look photography. This followed the type of diving maneuvers of the Soviet

Union in its lunar photography pro­gram (Luna 22).

The first images were published on 11th November – a mixture of broad view and close-ups. The first analyses were published the following spring. The most interesting were pictures of the crater Daniell in the Lacus Somniorum taken on 23rd October 2011, which, compared to American LRO images, found fresh evidence of crater wall collapses, avalanches inside the 17-20° slopes of the crater walls, debris streams up to 4 km long, and fresh soil exposed. Chang e 2 carried a new gamma-ray spectrometer made by the Purple Moun­tain Observatory with three times better performance than its predecessors, which, within 24 hr of arrival in lunar orbit, had detected potassium thorium, magnesium, silicon, aluminum, oxygen, titanium, and calcium [12].

On 23rd May 2011, Chang e 2 made its second dive, swooping to only 15 km
over the Sinus Iridum, the Bay of Rainbows, to take high-resolution images of the planned landing site for the Chang e 3 rover and also of the two poles. Sinus Iridium is a bay surrounded by high mountains and promontories in the north-west corner of the Moon, is flat (slope of 2°), has many interesting features such as high iron content and age (over 4 eons), and is covered in parts by volcanic ejecta, multilevel terraces, and magnetic anomalies. Chang e found an average regolith thickness of 3 m, but up to 15 m where the bay rises in terraces and slopes in the surrounding areas. It is a negative gravity anomaly, the adjacent mare (Imbrium) being positive. In comparison to the mare, Iridum has a thinner regolith, and higher iron but lower titanium content. Chinese scientists published a map of four distinct districts of the Sinus Iridum according to their chemical composition, impacts, and flooding of the original Iridum crater, with a history dating back from 2.62bn to 4.06bn years. A temperature map of the bay was compiled (for the record, it ranges from 253 to 300 К daytime). Cheng Shengbo of the College of Geoexploration of Jilin University in Chanchun, who analyzed the Sinus, believes that it is representative of key periods of lunar evolution and a rich site to sample [13].

With its main tasks accomplished, in June 2011, mission controllers decided to move the spacecraft to the L2 point. The L points were named after Joseph Louis Lagrange, who defined a series of points in the configuration of the Sun-Earth – Moon relationship where orbits were comparatively stable, the L2 point being 1,5m km further out than the Earth when the three bodies are in a Sun-Earth-Moon configuration, one providing some protection from solar radiation. According to mission director Wu Weiren, the purpose was to test the tracking network further out than ever before, observe the solar wind, and pave the way for unmanned missions to Mars. Chang e 2 was in excellent condition and considered in good shape for such an extended mission. It was not the first spacecraft positioned there, for it had been preceded by the American Microwave Anistropy Probe. On 10th June, Chang e 2 fired its engine to leave lunar orbit on an 85-day journey to L2, where it arrived on 25th August after 77 days. This took place in two stages, the first being a lunar orbit of 5 hr 3 min before a second bum to kick it out of lunar orbit. It was the furthest distance any Chinese spacecraft had ever reached from the Earth; 600 kg of leftover fuel were required for the maneuver. In June 2012, China announced its intention to move Chang e 2 a second time, on this occasion to intercept an asteroid at a distance of 1,000 km in 2013.

Chang e 2’s lunar map was published the following February, showing both sides of the Moon to a detail of 7 m. It was based on 746 images with a digital volume of 800 GB. Once again, the mission combined a high level of technical expertise with a substantial scientific outcome. The series is summarized in Table 9.2.

Table 9.2. Chang e series.

Both from Xi Chang.