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The Verge's This weekend, China embarks on a historic mission to land on the far side of the Moon says (near the end):

We know that the mission is set to launch on top of one of China’s Long March 3B rockets from the country’s Xichang Satellite Launch Center. And thanks to air closure notices, takeoff time is estimated to occur around 1:30PM ET on Friday, December 7th. China may only announce that the mission was a success after the spacecraft is on its way to the Moon, though Jones says we might hear earlier than that from other sources.

“It might be that the first indication we have of launch is that some poor soul near Xichang launch center is woken up thinking there’s an earthquake and complaining about it on social media.” Jones says.

If Chang’e-4 does make it to space, it will spend less than a month traveling to the Moon, likely touching down sometime in the first week of January. If that happens, China will have officially moved into its own elite group, as the only country to visit the side of the Moon we cannot see from Earth.

Question: Launch is tomorrow, so I'm wondering why it will take a month to get to the Moon? An easy explanation might be that it will reach lunar orbit in several days, and then take several weeks before it lands on the Moon, but there are lower energy orbits that take several weeks to get there, and Chang'e-4 is over a ton, so maybe the fuel saving compared to a "direct flight" is worth something in terms of delta-v.

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    $\begingroup$ I searched for a bit but China doesn't seem very forthcoming with mission details $\endgroup$
    – Dragongeek
    Commented Dec 8, 2018 at 0:19
  • $\begingroup$ @Dragongeek planetary.org/blogs/emily-lakdawalla/2018/… and several links therein $\endgroup$
    – uhoh
    Commented Dec 8, 2018 at 1:16
  • $\begingroup$ twitter.com/Yeqzids/status/1071140675901546496 $\endgroup$
    – uhoh
    Commented Dec 8, 2018 at 1:22
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    $\begingroup$ @HopDavid thanks for the fun links! Okay I'll have a look t̶o̶d̶a̶y̶ this week. $\endgroup$
    – uhoh
    Commented Dec 16, 2018 at 5:54
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    $\begingroup$ Perhaps its a targetting heuristic - Aim straight at the moon at launch day and after a month's travel the moon will be in the same place again ? (humour) $\endgroup$
    – Criggie
    Commented Apr 15, 2019 at 2:57

2 Answers 2

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According to Spaceflight Now (emphasis mine):

Chang’e 4 is expected to enter lunar orbit Tuesday [Dec 11 2018] after a series of course-correction maneuvers, then use braking rockets to descend to the moon’s surface, targeting a landing inside the 110-mile-wide (180-kilometer) Von Karman crater in moon’s South Pole-Aitken basin region in early January.

Your suggestion is correct - Chang'e 4 will enter Lunar orbit within a few days on a traditional transfer orbit. The good completion of the trans-lunar injection was announced by CASC (source in Chinese on Weibo) - the primary contractor for CNSA.

As to what Chang'e 4 will be doing in the intervening weeks, I haven't been able to find any information. However, since the exact landing date hasn't yet been revealed, it seems likely that details won't be forthcoming.

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    $\begingroup$ Thanks! I've just asked How fast are trips to the Moon for unmanned spacecraft typically? $\endgroup$
    – uhoh
    Commented Dec 8, 2018 at 15:29
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    $\begingroup$ there is some additional information here: planetary.org/blogs/guest-blogs/2018/change-4-lunar-orbit.html $\endgroup$
    – Hobbes
    Commented Dec 16, 2018 at 12:44
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    $\begingroup$ Re As to what Chang'e 4 will be doing in the intervening weeks, I haven't been able to find any information: One possibility is that it was waiting for the target landing point to be in the its orbital plane. Plane changes are very expensive orbit maneuvers. The cheapest lunar orbit insertion maneuver on a direct transfer from low Earth orbit to the Moon is to thrust against the Moon-relative velocity vector, thus staying in the same orbital plane. Then the vehicle waits while the Moon slowly rotates (one rev per month) until the landing site lies on the orbital plane. $\endgroup$
    – David Hammen
    Commented Jul 3, 2019 at 12:34
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I don't know the exact path, but I do know there's a path to the moon that takes about a month. The reason it's used is that it takes less energy.

The Apollo missions got there in three days but your economics are different when you are consuming supplies for every day in space.

See this question for more information: Low Energy Transfer within Earth-Moon system

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    $\begingroup$ @uhoh I was basically confirming your hypothesis. For an unmanned craft there usually no reason not to take the minimum energy path even if the savings are small. $\endgroup$
    – Loren Pechtel
    Commented Dec 8, 2018 at 1:59
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    $\begingroup$ What is the month long path to the moon? A bi elliptic with a 600,000 km altitude apogee takes about a month. But saves less than .1 km/s. $\endgroup$
    – HopDavid
    Commented Dec 9, 2018 at 4:25

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