Space Exploration page
Spacecrafts are all using reference stars during their manouvers or for
keeping their orientation, so astronomy is fundamental for robotic and human space exploration.
Like past centuries sea explorators, spacecrafts need to have a precise clock and a reference
fixed in space to track their trajectories, since classical RF based localisation
methods fail in accuracy while the target distance is increasing.
Some example shall demonstrate the above thesis:
- The Apollo missions to the Moon were conducted with limited informatic
help, so many analogic methods were developed; LEM and CM had optical
periscopes for reference star aligning, but Apollo 13 mission with a
crippled guiding computer and being surrounded by bright debris (false-stars) after the SM explosion could not use those astronomical methods.
An astronomical shortcut was found anyway, the astronauts did trajectory correction manouvers by keeping the Sun and the Earth disks collimated on the LEM windows engraved with reticles. That successfully leaded them home.
- The fixed probes and the rovers on Mars have antennas for data
transmission that must be pointed toward Earth receivers; our planet
as seen from Mars has a know elongation from Sun, so the latter is pointed
first and the Earth is found afterwards.
A key asset is the capacity to determine the Sun position during the Mars day, or sol, or even the red planet's tiny moons (Phobos and Deimos) in the sky for rovers pointing purpose.
- Satellites star trackers (basically optical sensors pointing the celestial sphere) are used to know nominal or contingency navigation situations, i.e. for keeping a fixed orientation, or for detecting the rotation rate during planned and/or uncontrolled spin.
A key asset for satellites and deep-space probes is the Attitude and Orbital Control Subsystem (AOCS), mainly based on star tracker sensors.
The Giotto ESA spacecraft which first closely encountered a cometary nucleus (Halley comet in 1986) temporarily lost its orientation after a bump of a debris travelling at 68 km/s; its AOCS restored the correct antenna
orientation in half an hour by the use of the star tracker.
- The modern method conceived is using the quasars (QUAsi-StellAr Radio Sources) for radio telescopes reference points while they track a distant spacecraft; without astronomy achievements, that tracking method couldn't be even imagined.
Grange Obs. competence is not limited to astronomy, ranging from geological and planetary sciences to space technical, logistic and operational engineering, and from optical sensoring patronizing (20+ years experience in CCD systems) to Crew training, medical and psychological expertise.
For example, a rover on Moon or Mars shall experience a different gravity from that available on Earth in terrain simulators: the gravity on the Moon is 1/6 and on Mars is about 1/3 the one we feel on ground (1 g).
What is the effect of gravity on rovers locomotion?
A 300 kg rover launched to Mars will weight 114 kg only once landed on its surface; consequently, its wheels shall sink in the terrain less than the same rover in a terrestrial terrain simulator (the same rover shall weight 50 kg on the Moon).
So the apparent friction coefficient shall change, since it is a percentage of the weight acting on the vertical axis; reduced gravity would actually decrease the local friction at the wheel/terrain interaction level.
The phenomenon is like that experienced by a car when gets going on a dry or a on a slippery road, the difference is skid; such locomotion type is quite common on planetary rovers, Apollo missions lunar rover
footages show how much dusty terrain was spit during the travel (the wheels milled the surface regolith, sinking just millimeters on the soil).
That has also an impact on rover motors power which must be trasmitted to the ground, depending on the mechanical gear used (if any); the same skid example above can be used, a car on a slippery road could better keep going by using a top gear than using a lower one.
Try yourself, if you have got a car with manual gear selector!
Indeed, rovers have an electrical motor each wheel, and the mechanical gear is substituted by the power sent to it (and available from solar panels, frequently covered by dust and therefore less efficient in energy production); anyway the point is, rovers haven't ABS for preventing skid and too much power is not useful for keeping going.
Could the Apollo astronauts use a compass on Moon for rover localization? The answer is no, because of the lack of an intrinsic magnetic field on the Earth natural satellite; neither on Mars it does exist, so the Sun or stars orientation appears the only feasible rover localization method on the red planet, along with DEM comparison with PanCams (panoramic cameras) imaging, which is valid only when the terrain is not flat.
In fact, the Phoenix Mars polar probe could not initially localize itself since the surrounding terrain was flat for kilometers; the first sunrise and sunset could help to determine its longitude, and the heigth of the Sun at noon (reaching the highest point on the sky) made clear its latitude.
Earth has the magnetic field triggered Van Allen radiation belts, which protect humans from the lethal solar plasma; our planet has as well an ozone layer for blocking UV rays dangerous for our skin.
On the Moon or Mars no such natural protections exist and spacesuits cannot completely do, so human exploration medical risks increase; for that reason, field exploration time should be reduced, as it was done during the Apollo lunar landings.
Moreover, a reduced gravity shall increase the human osteoporosis risk; to keep that as a minimum, the ISS Crew rotation is limited to 6 months (and they look quite battered after their return to Earth).
Along with medical problems (the main one being the impossibility to perform emergency surgery), Mars human explorators shall have year-long missions, then psychological problems would arise for the long journey duration (i.e. isolation) and the delay in live Earth transmissions (i.e. scarce worth considering).
We all watched
JPL footages showing rovers being tested on ground terrain simulators, to do a particular manouver rehearsal, like climbing an hill or searching a viable procedure for for getting out of a sand stuck situation.
Why do tecnicians wear masks during tests? Because soil simulants could be dangerous while inhaled.
The problem to replicate the martian soil on simulation terrains on Earth is challenging; apart from gravity effect on rovers locomotion as reported above, particular planetary soil physical conditions cannot easily be implemented on ground, in order to have a locomotion test high confidence level.
Terrains have key parameters such as the friction angle or the soil cohesion having great influence on the rover wheel/terrain interaction; those parameters vary depending on soil granulometry and geological type.
In addition to that, the un-shielded planetary radiation makes the soil surface even more singular, in fact the Moon has a layer of regolith suspended due to a static electrical phenomenon, and Mars soils superficial layer is like vitrified due to UV rays; therefore, simulants need to be doped and compacted (since cohesion is greater).
To conclude, it is possible to use real-time Mars rover steering or payload operations? Not quite, because the planet could be so distant from Earth to have up to 20 minute delay in radio transmission; all surface operations must be planned in advance and sent to the rover via telecommands the Mars sol before.
Rovers on Mars normally proceed very slowly, to prevent unseen obstacles encounter: in fact a crevass is imaged by NavCams (navigation cameras) when it could be too late to intervene from Earth control room.
Let's assume a telecommand has been sent to the stopped rover to go forward: the procedure was to activate PanCams and NavCams and send the picture to control center, then to apply a given eletrical power to the wheels for a given time; finally, another shot of PanCams and NavCams was ordered, sent to Earth afterwards.
How to realise the rover has done the planned traverse distance? Well, the two images are compared, and their disparity (the difference in perspective) is measured; if the observed shift is less than the planned distance, wheels skid has occurred.
Next time the motor power ordered shall be less; indeed an idea of what had happened upon commands execution could had flashed throught engineers' mind, if only Mars gravity had been simulated in a terrestrial equivalent soil test range.
How could be possible that? It depends also from rover attitude (the road slope). Please ask for Grange Obs support if you are planning to go to Mars!
Grange Obs. can offer consultancy for space exploration studies; we have been on Moon or Mars many times (in our thoughts) by observing them throught the telescopes, and we definitively figure out what it could happen there since we performed multi-disciplinary physical studies.
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