[FPSPACE] Chang'e-2 -- why the silence?

Geert Sassen geert at navtools.nl
Wed Jan 9 19:30:08 EST 2013

Jumping from Chang'e-2 to numbers 3 and 5 but there is a very interesting
article in China daily about these missions
http://europe.chinadaily.com.cn/china/2012-03/16/content_14845488.htm which
Emily Lakdawalla also quotes in her as usually very good analysis

Most amazing is the apparent switch to a lunar orbit rendez-vous/docking on
the return lag of the sample return missions. Previous information on the
planned sample return missions always seemed to suggest a Luna-16 type
direct return trajectory. A Luna-16 type flight seemed also to be suggested
by the impact location of Chang-e-1 which was at the time supposed to be
close to the selected landing site for the sample return mission.

The big problem with Luna-16 type missions is that they are very
constrained in their landingsite, there is only a narrow band of locations
on the moons near and far side which allow the direct (vertical) ascent to
Earth. An ascent from any other location would require at least a
dogleg manoeuvre during ascent and additional midcourse corrections on the
return trip. For most locations, a short stay in lunar orbit with a
subsequent TEI burn is the most fuel-efficient method, so the method
apparently adopted by Chinese would not be hindered by the constraints
which the Soviet missions had.

Furthermore, docking in lunar orbit prior to return to Earth avoids the
need to take heavy parts (like heatshield etc) all the way down to the
Lunar surface, and thus saves weight on fuel/engines required for landing
(also, the mothership engines can perform TEI, further saving on fuel
weight for the lander), but it adds a lot of complexity and it does seem
very ambitious for a mission which is only the fifth Lunar mission and the
third lunar lander for China!



On Tuesday, January 8, 2013, Paolo Ulivi wrote:

> there should be a total of at least 500 pics taken during the flyby. I
> hope that they will someday archive them online as they did for Moon
> images - see
> observations by other instruments (radiometer and laser altimeter) had
> been announced, but as they have common boresight as the science
> camera, which apparently was not used, they were not used either.
> I collected a few info on the monitoring cameras on CE-2 and they
> apparently cleverly choose to use the only camera with a field of view
> narrow enough (7.2 degrees, the other cameras had wide angle, fisheye
> optics)i
> hopefully we will know more when they present some result at next
> week's SBAG and at the LPSC in March.
> On Tue, Jan 8, 2013 at 11:41 AM, Geert Sassen <geert at navtools.nl<javascript:;>>
> wrote:
> > As far as I know several pictures have been taken, at least one more,
> > close-up, picture was shown on Chinese TV and probably there are more.
> >
> > Other instrumentation was not activated, as has been stated in several
> > media. In fact, the pictures were taken with one of the engineering
> camera's
> > (the one which was originally used to picture the deployment sequence of
> the
> > solar panels). The real observation camera in the science package was not
> > used as this is a push-broom type of camera (like most similar camera's)
> > which would have required the whole spacecraft to slew in order to make
> > correct images (as Mars Express did during Phobos encounters). This is a
> > more complicated manoeuvre and would have resulted in only one or two
> > pictures at the very most, and even then only if they managed to pinpoint
> > the target good enough to get it into the camera view. Using the
> engineering
> > camera was a 'quick and dirty' method of getting images, but it did
> require
> > a very close passage as this camera was obviously never intended for
> > tele-shots of faraway targets.
> >
> > As I see it, the whole encounter was primarily a engineering-test for
> > similar missions in the future, not much science results were expected.
> >
> > Regards,
> >
> > Geert Sassen.
> >
> > On Tuesday, January 8, 2013, James Oberg wrote:
> >>
> >> So, a month after a sensationally-impressive asteroid fly-by, and not
> >> another peep out of the Chinese news media? ONE photograph returned, no
> >> other instrumentation active? Even then it's worth celebrating rather
> than
> >> forgetting about completely.
> >>
> >> What are we missing?
> >>
> >
> >
> >
> > --
> > Geert Sassen
> > http://www.facebook.com/geert.sassen
> >
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Geert Sassen
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