Документ взят из кэша поисковой машины. Адрес оригинального документа : http://www.iki.rssi.ru/conf/2009elw/presentations/presentations_pdf/session7/Lorenz_ELW.pdf Дата изменения: Mon Mar 2 19:59:34 2009 Дата индексирования: Sun Apr 5 19:09:31 2009 Кодировка: Поисковые слова: обвмадеойс нефептощи рпфплпч

Tiltmeter for Europa Lander
Ralph Lorenz JHU Applied Physics Laboratory
With contributions from Terry Hurford GSFC & Frank Sohl, DLR




Equipotential

0.000000

o

With no tide present, tiltmeter measures zero. Both the gravitational equipotentials, and the surface onto which the tiltmeter is sitting, are spherical. Satellite surface 0.000783
o

0.000000

o

With strong tide, tiltmeter pendulum responds to horizontal acceleration. If satellite is rigid (low k2) surface remains spherical and thus tiltmeter reading is high

With strong tide on a deformable satellite, tiltmeter pendulum responds as before. However, the surface (and thus the tiltmeter housing) also tilt to follow the equipotential. The tilmeter output (difference between pendulum and housing) is therefore zero.


Europa Mode ls 0 -0.0001

0.000012 Titan Oc ean 0.00001 Delta-Tilt (radians) Titan Rigid

-0.0002 -0.0003
Tilt (radians)

0.000008

-0.0004 -0.0005 -0.0006 -0.0007 -0.0008 -0.0009 0

Thic k Ic e k=0.18 Solid Body k=0.0 Thin Ic e k=0.26

0.000006

0.000004

0.000002

0
1 2
Tim e (days )

3

-4

1

6 Tim e (days )

11

16

Figure 2. Absolute tilts relative to a sphere (left) on Europa for thin ice (1 or 10km look the same on this plot), thick ice (100km) or a rigid body. The steady-state values here cannot be measured by a tiltmeter without astronomical references, but the changing component (shown again on an expanded scale in Figure 3) is easily measured without additional data. Plot on right shows the changing part only of the tilt for Titan.


Figure 3 shows the changing part of the tidal tilt signals expected (at a point at 45 deg latitude on the prime meridian of Europa where the tilt is maximum). Notice that all are 23 orders of magnitude greater than the resolution of a commercial tiltmeter and thus can be well-characterized by a spacecraft instrument. A thin-ice Europa will lie somewhere between the thick-ice case and zero. Nondetection of a cyclical tilt implies a strengthless crust.


Commercial unit (~$1000) Simple pendulum with capacitive position sensing - notionally ~ 1 nanoradian accuracy. (Pendulum mass in this instance only ~150g ) Can be sampled (capacitance sensor is limiting factor) at ~10 Hz.


Alternative sensing technology fluid vial with liquid position sensed electrolytically. Sensors of this type were flown on Huygens probe to Titan to measure surface attitude and wave motion, as well as parachute dynamics (Lorenz et al., 2006) Geophysical instruments (e.g. Applied Geomechanics Inc) can attain few nanoradian accuracy.


Likely real tilt history on Europa will be more complex - discontinuous jumps as crust slides along cracks (e.g. cycloids). These jumps will also cause seismic waves to radiate through the crust and ocean which can also be detected



Challenges for Europa Lander application
Isolate pure tidal tilt from once-per-orbit thermal changes (tide and thermal distortion have same period.) Isolate instrument from direct sunlight with insulation blankets ; array of heaters to actively balance solar input. Local thermal deformation of crust unavoidable. Star camera mechanically coupled to pendulum or pendulum mounting might augment accuracy. Accommodate uncertain landing attitude (leveling mechanism) Tolerate landing loads (need caging mechanism for pendulum type; liquid vial type likely g-tolerant) Radiation effects on parts. Measurement introduces a weak constraint on landing location - must be away from subJovian/Antijovian points, and from a great circle through poles at 90 and 270 degrees longitude.