China's first probe, Chang'e-1, after making a pretty arduous journey of more than 10 days, successfully completed its first braking at perilune and entered the moon's orbit at 11:37 a.m. on Monday or Nov. 5th, becoming China's first circumlunar satellite. So an eon Chinese fairy tale on "legendary goddess flying to the moon" has finally come true.
Chang'e-1 "joins hands" with Moon to enter its "working" orbit
Since last Wednesday, or October 31, the probe has covered another 114 hours with a distance of 436,660 kilometers, said Pei Zhaoyu, spokesman for the China National Space Administration (CNSA). It performed its first braking at perilune at 11:37 a.m. Monday and, 22 minutes later, it was captured by the lunar gravity and entered a 12-hour elliptical moon orbit with a perilune of 210 km and an apolune of 8,660 km, Pei noted.
This signifies Chang'e-1 has become China's first circumlunar satellite.
When Chang'e-1 approaches the moon, it is captured by the lunar gravity, so its flying track is a double-curve track around the moon, it has been said. So the satellite has to reach the perilune at the pre-set time and with the pre-set speed before implementing the accurate braking. Otherwise, it would likely fly away from the moon if the braking was too early, or it would crash into the mood if the braking was too late. Consequently, the first braking at perilune is a most crucial moment in the long journey of Chang'e-1.
In a bid to ensure that it travel on the pre-set orbit, Chang'e-1 successfully carried out its first orbital correction around 10:25 a.m. last Friday, or on Nov. 2. As a matter of fact, the moon probe kipped two of the three pre-set orbital corrections thanks to the precise control of orbital transfers.
Chang'e-1 carried out its second braking at perilune around 11:00 a.m. on Tuesday or Nov. 6, and its orbit was adjusted to a 3.5-hour orbit with a perilune of 200 km and an apolune of 1,700 km; it is scheduled to perform its third orbital correction around 8:00 a.m. Wednesday, or Nov. 7, and will be adjusted to the polar orbit with a circle of 127 minutes and at a height of 200 km. By then, the lunar probe will formally enter the "working" orbit for scientific exploration.
Chang'e-1 faces 3 major challenges in its "encirclement" of moon
Why should Chang'e-1 have braking at perilune?
As Chang'e-1 gradually approaches the moon, the acting force of the moon it has been subjected to intensifies correspondingly and its flying speed also increases remarkably, noted Huang Jiangchaun, deputy chief designed of the lunar probe, adding that its speed is about 2.4 km per minute on the point of braking at perilune. If its speed does not slow down, it will not possibly enter the moon orbit as it has not been captured by the lunar gravity.
"The reason for performing the pre-set braking at perilune for three times is based chiefly on the designed capacities of engines carried by the satellite," said Huang, explaining that it will not only reduce mission quotas for each braking and but will effectively save fuel carried abroad.
The first braking was designed to cope with three major hurdles or challenges, said Ma Yongping, deputy director of the Beijing Aerospace Control Center (BACC). The first challenge comes from the uniqueness of the opportune time for braking. If the braking at perilune is not carried out at the exact right time, it will then miss by a split second and can hardly enter the moon orbit, he added.
The second challenge is attributable to the complexity of the lunar space environment. As this is China's first-even moon orbiting, we do not have the relatively adequate knowledge of the lunar space environment, Ma acknowledged. And the third challenge derives from the surveying and control of Chang'e-1's "flying" orbit.
So, Chang'e-1 has to undergo the braking at perilune for three times in line with pre-scheduled plans. The probe's speed was slowed down to 2.06 km per second from 2.4 km per second with the first braking, and began to travel along a 12-hour elliptical moon orbit, and the moon probe subsequently reduced its speed to 1.8 km per second with the second braking, and travels along a 3.5-hour moon orbit, and cut further to 1.59 km per second with the third braking, and eventually enters the 127-minute polar orbit. It is very hard for Chang'e-1 to fulfill the mission if without such sophisticated remote orbit measuring and control techniques.
Satellite braking steps are complex
How was Chang'e-1's first braking at perilune carried out?
Before the braking was made, said BACC chief engineer Wang Yejun, his control center analyzed and studied the track control strategies and did ample preparations for the first braking based on data transmitted by surveying and control stations. BACC worked out parameters for the position adjustment, orbit control and engine ignition six hours and 3.5 hours respectively prior to the engine ignition and inserted twice parameters four hours and one hour respectively before the engine ignition. These data and instructions became the basis for implementing the braking.
The vehicle was initialized in a circular barking orbit and the trajectory divided into three distinct phases, namely, de-orbit, descent and braking, according to Long Jiang, deputy chief commander of China's lunar orbiter project. Despite these fairly complex steps or phases, he said "we are fully confident as the state of our (Chang'e-1) satellite and the state of our whole design and its all component systems and facilities are pretty sound. The successful braking has also given an eloquent testimony to the strength and capabilities of our team."
There remain 3 key links for success of moon probe
"It is still no time to sit back and relax ourselves however," admonished Long Jiang, as there are still three crucial links to tackle for the eventual attainment of the ultimate moon probing objective.
First of all, the satellite still needs to perform two additional brakes at perilune, which are vital for the ultimate entry of the targeted orbit, a link quite crucial for the mission. Second, when it moves into the moon orbit, the satellite has to maintain its "three pre-set positions", namely, to orient payload toward the moon for its scientific exploration purpose, to orient its sun wing toward the sun to ensure enough energy, and to orient aerials toward the earth for the sake of transmitting data from instruments aboard to the earth and, thirdly, when it moves into the moon orbit, the "working model" of some auxiliary systems, including those for heating control and electricity supply, still needs to be regulated.
"To deal with some key links in the follow-up work, we have drafted or worked out a host of ensuring measures on the ground, which have passed verifications and tests," said Long Jiang, "so our whole team fully confide in fulfilling these follow-up tasks."
By People's Daily Online and its author is PD report Liao Wengen