Chang’e-4 lu­nar probe satel­lite en­ters Halo or­bit

Shanghai Daily - - NATION - (Xin­hua)

THE re­lay satel­lite for the Chang’e-4 lu­nar probe, which is ex­pected to land on the far side of the Moon at the end of the year, has en­tered the planned or­bit, the China Na­tional Space Ad­min­is­tra­tion said yes­ter­day.

The satel­lite, named Que­qiao (Mag­pie Bridge) and launched on May 21, en­tered the Halo or­bit around the sec­ond La­grangian (L2) point of the Earth-Moon sys­tem, about 65,000 kilo­me­ters from the Moon, at 11:06am yes­ter­day af­ter a jour­ney of more than 20 days.

“The satel­lite is the world’s first com­mu­ni­ca­tion satel­lite op­er­at­ing in that or­bit, and will lay the foun­da­tion for the Chang’e-4, which is ex­pected to be­come the world’s first soft­land­ing, rov­ing probe on the far side of the Moon,” said Zhang Hong­tai, pres­i­dent of the China Academy of Space Tech­nol­ogy.

While in or­bit, the re­lay satel­lite can see both the Earth and the far side of the Moon. The satel­lite can stay in the Halo or­bit for a long time due to its rel­a­tively low use of fuel, since the Earth’s and Moon’s grav­ity bal­ances the or­bital mo­tion of the satel­lite.

“From Earth, the or­bit looks like a halo of the Moon, which is where it got its name,” said Zhang Li­hua, project man­ager of the re­lay satel­lite.

He said the Halo or­bit was a three-di­men­sional ir­reg­u­lar curve. It is ex­tremely dif­fi­cult and com­plex to main­tain the satel­lite in or­bit.

“If there is a tiny dis­tur­bance, such as grav­i­ta­tional dis­tur­bance from other plan­ets or the Sun, the satel­lite will leave the or­bit. The or­bit pe­riod is about 14 days. Ac­cord­ing to our cur­rent plan, we will con­duct or­bit main­te­nance every seven days.” Zhang Li­hua added.

In or­der to set up a com­mu­ni­ca­tion link be­tween Earth and the planned Chang’e-4 lu­nar probe, space en­gi­neers must keep the satel­lite sta­ble and con­trol its al­ti­tude, an­gle and speed with high pre­ci­sion.

Next, the team will test the com­mu­ni­ca­tion func­tion of the re­lay satel­lite.

Weigh­ing about 400 kilo­grams and with a de­signed life of three years, the satel­lite car­ries sev­eral an­ten­nas. One, shaped like an um­brella with a di­am­e­ter of 4.2 me­ters, is the largest com­mu­ni­ca­tion an­tenna ever used in deep space ex­plo­ration, ac­cord­ing to Chen Lan, deputy chief en­gi­neer of the Xi’an branch of CAST.

Tidal lock­ing

Tidal forces of the Earth have slowed the Moon’s ro­ta­tion to the point where the same side al­ways faces the Earth, a phe­nom­e­non called tidal lock­ing. The other face, most of which is never vis­i­ble from Earth, is the far side or dark side of the Moon, not be­cause it’s dark, but be­cause most of it re­mains un­known.

With its spe­cial en­vi­ron­ment and com­plex ge­o­log­i­cal his­tory, the far side is a hot spot for sci­en­tific and space ex­plo­ration.

The Aitken Basin of the lu­nar south pole re­gion on the far side has been cho­sen as the land­ing site for Chang’e-4. The re­gion is be­lieved to have great re­search po­ten­tial. How­ever, land­ing and rov­ing re­quire a re­lay satel­lite to trans­mit sig­nals. Es­tab­lish­ing a com­mu­ni­ca­tion link is es­sen­tial for the suc­cess of the Chang’e-4 mis­sion.

A lu­nar op­ti­cal imag­ing de­tec­tor de­vel­oped by Saudi Ara­bia is in­stalled on a mi­cro satel­lite launched to­gether with Que­qiao.

The cam­era, which be­gan to work on May 28, has con­ducted ob­ser­va­tions of the Moon and ac­quired a se­ries of clear lu­nar images and data. Three of the images was un­veiled yes­ter­day.

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