The 65-meter radio telescope at the foot of Sheshan Hill, the most precise radio telescope in Asia Photo: CFP

 

"Beijing, Tianma (a winged horse in Chinese folklore) is operating normally." The voice is cool and controlled. It echoes through the Very Long Baseline Interferometry (VLBI) Center Hall at the Shanghai Astronomical Observatory (SHAO) in the Sheshan Science and Technology Park, about 40 kilometers from the city center. These days, even when most people in Shanghai have gone home, scores of people are still at work here.

They concentrate on the many different batches of information displayed on the large screen at the center of the room, watching the real-time operation of the Chang'e-3 lunar probe. Live images produced from radio telescopes are being shown on a smaller screen on the left and voices from the Beijing Aerospace Control Center broadcast intermittently from loudspeakers.

At 1:30 am on December 2, Long March 3B rocket blasted off from the Xichang Satellite Launch Center, carrying the Chang'e-3 on a lunar voyage that some Chinese have been dreaming of for thousands of years. The Chang'e-3 lunar probe comprises a lander and a rover named "Yutu" (Jade Rabbit), after the pet of the moon goddess Chang'e who, in Chinese folklore, flew to the moon with her and has lived there ever since.

On Saturday December 14, after a 12-day journey, Chang'e-3 is scheduled to land in the Sinus Iridum region of the moon and the rover will start to explore the moon's surface, sending 3D images back to Earth. This is China's first attempt at landing a spacecraft beyond the Earth's surface and man's first lunar landing since 1972.

An important part

Hong Xiaoyu is SHAO director, and commander in chief of the Chang'e-3 VLBI Orbital Measuring Subsystem, a key part of the project. He told the Global Times, "the VLBI technique plays an important part in the Chang'e-3 mission. Without VLBI a soft landing would be impossible."

In 2004 China officially established its lunar exploration program which involves spacecraft orbiting, landing and returning to Earth. Following the success of Chang'e-1 and Chang'e-2 missions in 2007 and 2010, the Chang'e-3 is now beginning the second phase of the program, the landing. VLBI has been a part of the Chang'e program since Chang'e-1 and has kept developing.

VLBI originated from radio astronomy - by employing radio interferometry, multiple radio telescopes can produce the effect of single huge conventional telescope. Hong said, the further the distance between two radio telescopes the definition is accordingly higher. On Earth the VLBI network and subsequently the definition is limited because of the size of the Earth but scientists have been expanding this by sending radio telescopes into space.

Hong said the Chinese VLBI network currently comprises observation stations at Beijing, Shanghai, Kunming, Urumqi and the SHAO VLBI center.

The new VLBI center in the Sheshan Science and Technology Park just came into service this year. It is also the first time Shanghai's newly completed 65-meter radio telescope, the Tianma, regarded as the most precise radio telescope in Asia and the fourth in the world, has been involved in the Chang'e missions. "The resolution achieved by this VLBI network is equivalent to a super-telescope with an aperture of more that 3,000 kilometers," Hong said.

The Shanghai Astronomical Observatory on West Sheshan Hill, on the site of a French Jesuit observatory founded in 1900 Photo: CFP

The soft landing

The average distance between the Earth and the moon is 380,000 kilometers and it took the Chang'e-3 112 hours to complete its Earth-to-moon journey before it moved into a 100-kilometer lunar circular orbit on December 6, and then on December 10 going into an elliptical orbit which placed it at its nearest point just 15 kilometers from the moon's surface. On Saturday it will make its soft landing.

On its journey while Chang'e was within 20,000 kilometers of Earth it was being followed by ocean survey vessels and land tracking stations. When it passed that mark it was tracked by the VLBI subsystem.

Wang Guangli, the deputy chief designer of Chang'e-3 VLBI Orbital Measuring Subsystem explained, "The antennas of radio telescopes are very lengthy so they turn very slowly. They do not do well with close quarters tracking but long distance measurement is their specialty."

Zheng Weimin, the chief designer of the Chang'e VLBI subsystem, explained: "The four key components of the VLBI subsystem are real-time VLBI data processing which demands rapid measurements, Delta-DOR measurements which need precision measurement, lunar surface target positioning which needs accuracy to guide the lunar lander, and same beam high-precision positioning technique which sets the relative positions of the lander and the rover."

Zheng told the Global Times, the VLBI subsystem was mostly responsible for the exact positioning of the lunar probe and rover. The team was working on a 1 kilometer accuracy range - not so easy when the team is 380,000 kilometers away. But with current testing Zheng said the actual accuracy Saturday get may well below the 1 kilometer criteria.

After Chang'e-3 makes its soft landing, the rover will separate from the probe and the two units will photograph each other. "A safe soft landing of the probe and the completion of these photographs will mean the Chang'e-3 mission will have succeeded. Later the rover will carry out scientific exploration work and the VLBI subsystem will keep tracking it," Zheng said.

VLBI is generally used internationally to measure astronomical bodies. But in adapting it for deep space exploration Chinese scientists had to overcome several hurdles including modifying the basic formula to fit the shorter distances.

Zheng explained that in astronomy the time frame did not have to be so precise - one week or one month made no difference in measuring stars, quasars or other heavenly bodies. However real-time data reception, processing and transmission are some of the most important requirements for space exploration. The time gap for the Shanghai VLBI center to provide the data it receives to the Beijing center has been slashed from 10 minutes for Chang'e-2 to under 1 minute now.

"Astronomy can tolerate mistakes but space exploration cannot," Zheng said. "Everyone on the team wants the mission to succeed."

This stone sculpture at the Shanghai Astronomical Observatory features an antique telescope. Photo: CFP

Graduate students

For the Chang'e-3 mission, there are scores of people working in the observation stations. About 60 technicians work round the clock at the Shanghai VLBI center and almost a third are graduate students.

The technicians at the center work in 16 sectors each with different responsibilities. No.1 sector handles task scheduling, No.4 is responsible for data acquisition, No.6 deals with data transmission. Other sectors handle orbit determination, position and operation management. Each module is set by dual posts of A and B, prepared for all contingencies.

Guo Li is a SHAO technician and she has worked as a team member since Chang'e-1 when she was still a graduate student. She works in the position determination sector which provides a precise location of the lander and rover at all times.

She said three or four people work in each sector. "At key moments like the launch, braking and the soft landing, the members work together, otherwise, we work in shifts."

From December 1 to late December, Guo and her colleagues are living at SHAO in the Sheshan Science and Technology Park. They arrive for work at the center at 9 am every day and return to their dormitories at 10 pm.

"I was very excited when I took part in the Chang'e-1 mission but now I am more relaxed. If the mission runs smoothly, I will be really happy," Guo said.

Taking responsibility

Zhang Jinwei, a graduate student who studies astrogeodesy and astromechanics and works for the position determination sector told the Global Times, that the most important thing he has learned is that everyone involved in the Chang'e-3 VLBI subsystem took responsibility. "If there is just a tiny problem in any of the sectors everyone will try to help."

Bi Huichun, another graduate student who studies electronics and communications engineering and works for the operation management sector said, "As a fresher involved in this mission for the very first time, I know I am just a small cog in the team but I will treasure this experience for a long time."

Pan Hongjian is the deputy administration manager at the subsystem center and said staff had practiced for the big event with a series of drills in November. The kitchens have even provided specially designed meals, Lunar Exploration Dinners, for the staff who have to work long hours in air conditioned rooms.

It is not just the kitchens that are helping the mission. Many support staff and departments help keep the subsystem center on track - like the teams ensuring that power remains unaffected no matter what.

 "The State Grid Shanghai Songjiang Power Supply Company set up seven teams to carry out the work and maintenance," Pan said. He said that several years ago, before the new Sheshan Science and Technology Park had opened, the technical support team worked from old building in Xujiahui but a power cut seriously interfered with their work during one mission.