The Chinese Lunar Exploration Program (CLEP;
Chinese: 中国探月; pinyin: Zhōngguó
Tànyuè), also known as the Chang'e program
after the Chinese moon goddess Chang'e, is
an ongoing series of robotic Moon missions
by the China National Space Administration
(CNSA).
The program incorporates lunar orbiters, landers,
rovers and sample return spacecraft, launched
using Long March rockets.
Launches and flights are monitored by a Telemetry,
Tracking, and Command (TT&C) system, which
uses 50-metre (160-foot) radio antennas in
Beijing and 40-metre (130-foot) antennas in
Kunming, Shanghai, and Ürümqi to form a
3,000-kilometre (1,900-mile) VLBI antenna.
A proprietary ground application system is
responsible for downlink data reception.
Ouyang Ziyuan, a geologist and chemical cosmologist,
was among the first to advocate the exploitation
not only of known lunar reserves of metals
such as titanium, but also of helium-3, an
ideal fuel for future nuclear fusion power
plants.
He currently serves as the chief scientist
of the Chinese Lunar Exploration Program.
Another scientist, Sun Jiadong, was assigned
as the general designer, while scientist Sun
Zezhou was assigned as the deputy general
designer.
The leading program manager is Luan Enjie.
The first spacecraft of the program, the Chang'e
1 lunar orbiter, was launched from Xichang
Satellite Launch Center on 24 October 2007,
having been delayed from the initial planned
date of 17–19 April 2007.
A second orbiter, Chang'e 2, was launched
on 1 October 2010.
Chang'e 3, which includes a lander and rover,
was launched on 1 December 2013 and successfully
soft-landed on the Moon on 14 December 2013.
It will be followed by a sample return mission,
Chang'e 5, scheduled for 2019.As indicated
by the official insignia, the shape of a calligraphic
nascent lunar crescent with two human footprints
at its center reminiscent of the Chinese character
月 for ″moon″, the ultimate objective
of the program is to pave the way for a manned
mission to the Moon.
Such a mission may occur in 2025–2030.
== Program structure ==
The Chinese Lunar Exploration Program is divided
into three main operational phases, with each
mission serving as a technology demonstrator
in preparation for future missions.
=== Phase I: Orbital missions ===
The first phase entailed the launch of two
lunar orbiters, and is now effectively complete.
Chang'e 1, launched on 24 October 2007 aboard
a Long March 3A rocket, scanned the entire
Moon in unprecedented detail, generating a
high definition 3D map that would provide
a reference for future soft landings.
The probe also mapped the abundance and distribution
of various chemical elements on the lunar
surface as part of an evaluation of potentially
useful resources.
Chang'e 2, launched on 1 October 2010 aboard
a Long March 3C rocket, reached the Moon in
under 5 days, compared to 12 days for Chang'e
1, and mapped the Moon in even greater detail.
It then left lunar orbit and headed for the
Earth–Sun L2 Lagrangian point in order to
test the TT&C network.
Having done that it completed a flyby of asteroid
4179 Toutatis on 13 December 2012, before
heading into deep space to further test the
TT&C network.
=== Phase II: Soft landers/rovers ===
The second phase is ongoing, and incorporates
spacecraft capable of soft-landing on the
Moon and deploying lunar rovers.
Chang'e 3, launched on 2 December 2013 aboard
a Long March 3B rocket, landed on the Moon
on 14 December 2013.
It carried with it a 140 kg (310 lb) lunar
rover named Yutu, which was designed to explore
an area of 3 square kilometres (1.2 sq mi)
during a 3-month mission.
It was also supposed to conduct ultra-violet
observations of galaxies, active galactic
nuclei, variable stars, binaries, novae, quasars,
and blazars, as well as the structure and
dynamics of the Earth's plasmasphere.
Chang'e 4, originally scheduled for 2015,
was a back-up for Chang'e 3.
However, as a result of the success of that
mission, the configuration of Chang'e 4 was
adjusted to test equipment in advance of the
next mission.
=== Phase III: Sample return ===
The final phase will entail a lunar sample
return mission.
Chang'e 5-T1 was launched on 23 October 2014.
It was designed to test the lunar return spacecraft.
Chang'e 5, expected to launch in 2019 aboard
a Long March 5 rocket, will build on the success
of the previous missions, with a lander capable
of collecting up to 2 kilograms (4.4 lb) of
lunar samples and returning them to the Earth.
Chang'e 6, expected to launch in 2020 aboard
a Long March 5 rocket, will build on the success
of the Chang'e 5 mission.
=== Manned mission ===
As of 2017, China was making preliminary preparations
for a manned lunar landing mission around
2036.
== Key technologies ==
=== Long-range TT&C ===
The biggest challenge in Phase I of the program
was the operation of the TT&C system, because
its transmission capability needed sufficient
range to communicate with the probes in lunar
orbit.
China's standard satellite telemetry had a
range of 80,000 km (50,000 mi), but the distance
between the Moon and the Earth can exceed
400,000 km (250,000 mi) when the Moon is at
apogee.
In addition, the Chang'e probes had to carry
out many attitude maneuvers during their flights
to the Moon and during operations in lunar
orbit.
The distance across China from east to west
is 5,000 km (3,100 mi), forming another challenge
to TT&C continuity.
At present, the combination of the TT&C system
and the Chinese astronomical observation network
has met the needs of the Chang'e program,
but only by a small margin.
=== Environmental adaptability ===
The complexity of the space environment encountered
during the Chang'e missions imposed strict
requirements for environmental adaptability
and reliability of the probes and their instruments.
The high-radiation environment in Earth-Moon
space required hardened electronics to prevent
electromagnetic damage to spacecraft instruments.
The extreme temperature range, from 130 °C
(266 °F) on the side of the spacecraft facing
the Sun to −170 °C (−274 °F) on the
side facing away from the Sun, imposed strict
requirements for temperature control in the
design of the detectors.
=== Orbit design and flight sequence control
===
Given the conditions of the three-body system
of the Earth, Moon and a space probe, the
orbit design of lunar orbiters is more complicated
than that of Earth-orbiting satellites, which
only deal with a two-body system.
The Chang'e 1 and Chang'e 2 probes were first
sent into highly elliptical Earth orbits.
After separating from their launch vehicles,
they entered an Earth-Moon transfer orbit
through three accelerations in the phase-modulated
orbit.
These accelerations were conducted 16, 24,
and 48 hours into the missions, during which
several orbit adjustments and attitude maneuvers
were carried out so as to ensure the probes'
capture by lunar gravity.
After operating in the Earth-Moon orbit for
4–5 days, each probe entered a lunar acquisition
orbit.
After entering their target orbits, conducting
three braking maneuvers and experiencing three
different orbit phases, Chang'e 1 and Chang'e
2 carried out their missions.
=== Attitude control ===
Lunar orbiters have to remain properly oriented
with respect to the Earth, Moon and Sun.
All onboard detectors must be kept facing
the lunar surface in order to complete their
scientific missions, communication antennas
have to face the Earth in order to receive
commands and transfer scientific data, and
solar panels must be oriented toward the Sun
in order to acquire power.
During lunar orbit, the Earth, the Moon and
the Sun also move, so attitude control is
a complex three-vector control process.
The Chang'e satellites need to adjust their
attitude very carefully to maintain an optimal
angle towards all three bodies.
=== Hazard avoidance ===
During the second phase of the program, in
which the spacecraft were required to soft-land
on the lunar surface, it was necessary to
devise a system of automatic hazard avoidance
in order that the landers would not attempt
to touch down on unsuitable terrain.
Chang'e 3 utilized a computer vision system
in which the data from a down-facing camera,
as well as 2 ranging devices, were processed
using specialized software.
The software controlled the final stages of
descent, adjusting the attitude of the spacecraft
and the throttle of its main engine.
The spacecraft hovered first at 100 metres
(330 ft), then at 30 metres (98 ft), as it
searched for a suitable spot to set down.
The Yutu rover is also equipped with front-facing
stereo cameras and hazard avoidance technology.
== Potential cooperation with Russia ==
Anatoly Perminov, head of the Russian Federal
Space Agency, revealed in September 2006 in
RIA Novosti that Russia and China were working
on lunar exploration as partners, and that
the Russian-Chinese Space Sub-Commission's
priority was to conclude a joint lunar exploration
agreement by the end of that year.
== See also ==
Chinese space program
List of current and future lunar missions
List of proposed missions to the Moon
List of missions to the Moon
Robotic exploration of the Moon
== 
References ==
== External links ==
CLEP official website
Encyclopedia Astronautica
The Scientific Objectives of Chinese Lunar
Exploration Project by Ouyang Ziyuan
我国发射首颗探月卫星专题
嫦娥探月专题
