Preliminary findings on the surface gravity on Minmus

[TheCardinal]

Preliminary findings on the surface gravity on Minmus
By TheCardinal

Foreword
Although i'm technically a planetologist, the performed research and findings have more to do with Physics than with Planetology. For that reason the findings are more in it's place at the Physics Division. The research consisted of several experiments on the body named Minmus. The purpose of these experiments was to find a good approximation of the surface gravity of Minmus.


Preface
Minmus is one of the two moons of Kerbin. It is somewhat smaller than Mun and it's composition seems to be quite different from that of Mun. Minmus has several "lakes" where Mun has none. The word "lake" is used loosely here to describe 'the flat surfaces of those area's. In no way does the word imply that the material found in those area's is water. The material is with certainty not frozen water as the distance to our sun is to small to keep it frozen. Water therefor would have evaporated fairly quickly.
What material can be found in those "lakes" has yet to be determined. The surrounding hills and mountains most probably are sand, rock and boulders.
As the composition of Minmus is largely unknown the surface gravity cannot be determined from a distance, only experiments on Minmus can provide that answer.

Performed experiments

Experiment 1: A probe landed on Minmus and was ordered to fire it's landing engine for a short while. After burnout and during the ascent of the probe data was collected. Every ten seconds, altitude, velocity and mission time were registered.
The probe fairly quickly reached a maximum speed of over 106.7 m/s after which the engine was cut. Due to the gravitational pull of Minmus the upward speed slowly deminished until it fell back to Minmus. The descent was only recorded for about 30 seconds in order to prevent the destruction of the probe. In about 5 minutes the probe reached a maximum altitude of 14864 meters.

When examining the data it instantly became clear that the gravitational pull deminished while ascending. The highest values (0.49 m/s2) were found close to the surface of Minmus while the lowest (0.01) were found at the apogee of the probe. Not surprising as the diameter of Minmus is relatively small. The variation of found values for the gravity pull were such that it became clear that the instrumentation of the probe was inadequate. The altimeter and speedindicator seem to be lagging behind and aren't precise enough. No firm conclusion could be made about the gravitational pull of Minmus on the basis of this experiment. However the values at lower altitude can be used as an indicator of the value to be expected (around 0.49 m/s2)

Experiment 2: The same probe as in experiment 1 was brought to an altitude of 1008 meters en ordered to keep that altitude while reducing the horizontal speed to 0. After achieving these conditions, the engine was cut and during the descent of the probe data was collected. Due to the low altitude the shorter interval of 5 seconds between measurements was taken. Every five seconds, altitude, velocity and mission time were registered.
The probe impacted Minmus with a speed in excess of 29.8 m/s. The fall took less than a minute. The probe was destroyed as a result of this fall.

The variation of values in this experiment was slightly smaller than in experiment 1 but once again the margin of error due to the unprecise instrumentation is all to obvious. The values range between 0.42 and 0.57. As the effect observed in experiment 1 (lower gravity pull at heigher altitude) could not be found in the measurements of experiment 2. The margin of error exceeds the slight change in gravity pull due to the relative small change in altitude. As a first approximation of the surface gravity the mean was taken of the derived values. The result was 0.49 m/s2 plus or minus 0.02.

Experiment 3: A new probe landed on Minmus with a somewhat improved instrumentation. Like in experiment 2 a droptest was initiated from an alitude of about 751 meters. Due to the altitude an interval of 1 second between measurements was taken. Every second the altitude, velocity and mission time were registered.
The probe impacted Minmus with a speed in excess of 27 m/s. The fall took 55 seconds. The probe was severely damaged by this fall and can't be used again for any experimentation.

Even though the altitude and speed were registered in three decimal places the variation did not lessen. On the contrary, the values in experiment 3 ranged from 0.396 to 0.604. The resulting mean surface gravity is 0.484 m/s2 plus or minus 0.004.
Due to the large spread of values it is questionable whether or not the margin of error (0.004) is correct. The large spread is partly caused by the imprecise time measurement. It is also uncertain if the new instrumentation (like the old instrumentation) is bothered by lagging or the degree of the eventual lag.

Discussion:
All three experiments have shown that the surface gravity of minmus must be closely around 0.49 m/s2. However, due to the big variation in values it would be unwise to specify a value more precisely than 0.49 m/s2. All experiments have been hampered by imprecise instrumentation and time keeping. Using the mission clock as time keeper leeds to timevalues which could be between half a second too early to half a second too late. Therefor 0.49 m/s2 should be used as an fair approximation of the sought value.

Conclusion:
In order to achieve a more precise value of the surface gravity of Minmus better instrumentation is required, especially for time regisration. The value of 0.49 m/s2 is currently the best approximation which can be made without improved instrumentation. As the measurements in experiment 1 have shown a deminishing gravity as the altitude increases, great care should be taken as to where (read: which starting altitude) the measurements are made.

March 24, 2013
TheCardinal
Division Planetology

(Measurements and data available for peer review when required)

[BGraves]

"The material is with certainty not frozen water as the distance to our sun is to small to keep it frozen"

What is the temperature of the Minmar surface at those lakes? Rather than counting on an unsure factor (the factor of distance from the sun) it would probably be easier to rule out water if the temperature of the lakes is higher than the freezing point of water.

[TheCardinal]

"The material is with certainty not frozen water as the distance to our sun is to small to keep it frozen"

What is the temperature of the Minmar surface at those lakes? Rather than counting on an unsure factor (the factor of distance from the sun) it would probably be easier to rule out water if the temperature of the lakes is higher than the freezing point of water.

As nog temparature sensor was available in the probe, no surface temperature has been measured. However, considering the orbit, distance and rotation period of Minmus, the influx of energy is such that water couldn't stay frozen on the dayside. As no liquid water ever has been observed on the surface of Minmus, the conclusion can only be that the material in the lakes can't be water.

I do agree that it must be confirmed by temperature measurements on the lakes of Minmus.

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