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Sunday, November 27, 2011
...the communications time lag with MARS and the Rovers...!
MARS communication "lag" and the Solar conjunction interference !
Space Communications with Mars
Opportunity in quest of life on Mars. Document Lombry.
Response time and interferences (III)
At the perihelion opposition, at 56 millions km from the Earth, a distance as short as 0.37 AU (1 AU = 149.56x108 km), it takes 3 minutes and 7 seconds for a signal emitted by the DSN to reach Mars. But when Earth and Mars are the farthest apart at 2.52 AU, it takes 20 minutes and 57 seconds to transmit the same radio signal. The communication lasts seven times longer !
Due to these time delays it is impossible to communicate with and control the rover in real time.
When Earth and Mars are in conjunction (opposite sides of the Sun) at a distance of 2.49 AU another problem arises. This distance is not as much of a problem as having the Sun in the way, for it produces a lot of radio interference making communication almost impossible. Indeed, for distances of less than 10 solar radii around the Sun, the thermal noise contribution is quite severe and the use of amplifier at reception still increases this difficulty. Therefore it is very important than the spacecraft flying to Mars reaches the Red planet far before the conjunction so that engineers and scientists can gather data during a few months before be handicapped by communications problems.
Comments by George:
The minimum communications time lag of ~3
minutes with Mars and the new giant Rover
"Curiosity" means, that all has to be near
perfectly "pre-programned" to auto-pilot.
That the sensors on the Space craft and Rover
are functioning well, as to location in orbit,
"after acquiring orbit around Mars", and then
as to when to initiate "de-orbit" maneuvers of
the Rover from SPace craft, etc. Really quite a
feat that nobody else but the all-powerful, all
perfect USA has accomplished ! Hence people,
all welcome to another successful mission to
If other nations had, orbiting space craft to
MARS, with plenty of "cameras" and other
sensor instruments by now, we could do a
surveillance of what goes on with the ROVERS.
AND AS EARTH SATELLITE military
surveillance cameras, we could have quite
enhanced close ups of all that is there, even the
ROVERS ...! As todays satellite surveillance
capabilities, is claimed to be quite enhanced to be
able to picture anything as small or smaller than
a ROVER ! SO WHY DO WE NOT HAVE
SATELLITE SURVEILLANCE TO PICTURE
THE APOLLO'S ON THE MOON, WHICH IS
MUCH EASIER ??? WE COULD ALL INFER
THE REASONS WHY NOT ! CAN YOU...???
SO ARE SCIENTIST AND GOVERNMENTS
AND ACADEMIA MORE INTELLIGENT
THAN "PURE LOGIC" ? Generally not so !
Welcome all to "a_bite_in_the_chunk"...!
The intrigue is amazing !!!
Rover Descent Phases.
Final Minutes of Curiosity's Arrival at Mars
This graphic portrays the sequence of key events in August 2012 from the time the NASA's Mars Science Laboratory spacecraft, with its rover Curiosity, enters the Martian atmosphere to a moment after it touches down on the surface.
The Mars Descent Imager (MARDI) camera on the rover will provide high-definition video of the descent, looking downward, beginning at the time of heat shield separation. An engineering experiment, the Mars Science Laboratory Entry, Descent and Landing Instrument (MEDLI), will measure atmospheric conditions and the performance of the heat shield on the way down.
The values indicated in this graphic are an example case. The actual timing and altitudes for these events may differ due to differences among the candidate landing sites and unpredictable factors in atmospheric conditions on landing day. For example, the touchdown is indicated in the chart as about 392 seconds after atmospheric entry, but it could be as long as about 480 seconds after entry, depending on which landing site is selected. Also, even for a given site, times for the opening of the parachute could vary by 10 to 15 seconds for a successful landing.
Enclosed inside the capsule formed by the back shell and heat shield, the craft enters the atmosphere at an altitude of about 125 kilometers (78 miles) and a velocity of about 5,800 meters per second (about 13,000 miles per hour).
The parachute deploys about 240 seconds later at an altitude of about 10 kilometers (about 6 miles) and a velocity of about 470 meters per second (about 1,050 miles per hour). After about 28 more seconds, the heat shield separates and drops away at an altitude of about 7 kilometers (about 4 miles) and a velocity of about 160 meters per second (about 360 miles per hour). The rover, with its descent-stage "rocket backpack," is still attached to the back shell on the parachute. The radar on the descent stage begins collecting data about velocity and altitude.
The back shell, with parachute attached, separates from the descent stage and rover about 77 seconds after heat shield separation, about 1.8 kilometers (1.1 miles) above the ground and still rushing toward the ground at about 100 meters per second (about 225 miles per hour). All eight throttleable retrorockets on the descent stage, called Mars landing engines (MLE's), begin firing for the powered descent phase.
Four of the eight engines shut off just before nylon cords begin to spool out to lower the rover from the descent stage for the "sky crane" landing. The rover separates from the descent stage, though still attached by the sky crane bridle, at an altitude of about 20 meters (about 66 feet), with about 12 seconds to go before touchdown. The rover's wheels and suspension system, which double as the landing gear, pop into place just before touchdown at about 0.75 meters per second (about 1.7 miles per hour). When the spacecraft senses touchdown, the connecting cords are severed and the descent stage flies out of the way.
Image Credit: NASA/JPL-Caltech
All seems right and possible to me ! SO WHY IS
THERE SO MUCH SKEPTICISM ?
SIMPLY THE ANSWERS ARE THAT THE PARAMETERS ARE
ASSUMED AND THAT CO2 MARS ATMOSPHERE SIMULATIONS
WERE ONLY DONE BY COMPUTER MODELLING, AND OF
COURSE COULD NOT BE DONE BY REAL MODELLING IN A
CO2 GIANT CHAMBER !
The MSL spacecraft will enter the Martian atmosphere at approximately 6.1 km/s, making it the second fastest NASA entry to Mars (Pathfinder entered at 7.3 km/s in 1997). However, the MSL aeroshell is much larger than Pathfinder’s (4.5 vs 2.65 meters), and MSL is also much heavier. As a consequence, the flow around the MSL spacecraft is expected to become turbulent early during the entry, and the resulting heat flux and shear stress on the heatshield will be the highest ever encountered at Mars. In addition, MSL is flying a guided lifting trajectory, a first for Mars entry. The design of the entry system to withstand such environments relies primarily on simulation tools, such as computational fluid dynamics (CFD). Because the Martian atmosphere is mainly composed of CO2 (as opposed to air), it is very difficult to conduct experiments on Earth that simulate all of the aspects of a Martian entry. As a consequence, the uncertainties in the engineering models for the heating encountered and the aerodynamic performance of the spacecraft are high, with the result that the spacecraft was designed with large margins (which come at the cost of mass). Including these margins, the heatshield is designed to withstand 216 W/cm2 of heating, 540 Pa of shear, and 0.37 atmosphere of pressure.
The only way to reduce these margins on future Mars missions is to obtain data on the performance of the system during the entry. These data can be compared with the pre-flight predictions to evaluate the assumed level of uncertainty and to identify places where the current models require improvement. The MEDLI suite will provide these data, and will in fact return the largest EDL dataset ever obtained during a non-Earth entry. The data collected from the MEADS sensors will be combined with data from the Inertial Measurement Unit (IMU) to provide data on surface pressure distribution, vehicle orientation, dynamic pressure, Mach number, and the atmospheric density and winds as a function of altitude. The data collected from the MISP sensors will be used to evaluate the peak heat flux, distribution of heating on the vehicle, map transition to turbulence, and evaluate the thermal protection system (TPS) surface and in-depth material performance.
Also, will everything "fire" and function when
needed ! What fail-safe systems are there ?
The landing distances and times in the phases/
stages seem to be sufficient.
So "past experiences" are to illuminate this
FINALLY : Transparent wheel hubs might be to
be able to see what is going on in there ! As to
getting something lodged in there, could be that
how hard a landing do these wheels take ???
Or will it get stuck or overturned on rough
terrain ??? Sorry the inside hubs.......! ! !
Taaaa DAAAAAAhhhhhhhh !