The Case for Mars (28 page)

The use of shortwave radio for communication has an additional benefit for Mars explorers. The same system can be used for exploration via deep-ground-penetrating radar. A 3 MHz radio signal has a wavelength of 100 meters. In the dry Martian environment, such signals, if directed downward, could be expected to penetrate about 10 wavelengths, or a thousand meters, into the ground. Many leading Mars geologists believe that Mars may have a liquid water table between 500 meters and 1,000 meters below the surface. Even if this is not true globally, it is almost certainly true in some localities, as geo-thermal heat will cause pockets of subsurface ice to melt and form hot subsurface reservoirs. (Mars is geologically alive. It is estimated that some of the volcanic features in Tharsis may be less than 200 million years old. From the point of view of Mars’ 4.5-billion-year planetary age, they might as well have erupted yesterday.) A rover team driving along with a shortwave radio could send radar pings into the ground. If liquid water exists underground within a kilometer or so of the surface, its much higher electrical conductivity than the surrounding dry soil or ice will cause the radio signal to be strongly reflected back to the rover’s receiver, and the time delay between transmission and reception will tell the crew how deep the reservoir is. If they should discover a heated pool relatively close to the surface it will be time to get out the drilling rig. Water is, after all, the s
taff of life.

NAVIGATION ON MARS

In addition to maintaining communication with the home base, Mars explorers will also need to navigate. While good maps of Mars are available from orbital imaging, the essential problem for a Mars rover crew will be determining their own location. This is critical not only for documenting the location of various scientific finds, but, more importantly, to prevent getting lost. On the deserts of Mars, as in the North African desert during World War II, getting lost means dying. A radio beacon at the base could help a crew find its way home, but its range would reach at most only to the nearby horizon (just 40 kilometers away, remember). Upon approaching the limits of the base beacon’s range, a departing rover crew could station a second beacon on a hilltop, and then another, and another, to mark a return path. Such techniques are, however, quite limiting, and as in the story about the bread-crumb trail being eaten by birds, are subject to catastrophic failure if one of the beacons composing the trail should cease functioning. What other navigation techniques will be available to the rover crew?

Well, to an aerospace engineer, the first thing that comes to mind is to use navigation satellites. If a satellite is stationed in low polar orbit about Mars, its latitude at any given moment will be known. If you put a radio beacon on the satellite (the
Mars Global Surveyor
to be launched in 1996 actually has such a beacon), the rover crew can listen for it, and when they compare the time of closest approach with the satellite’s itinerary recorded in the rover’s computer, they can determine their latitude. In addition, the satellite’s rate of approach to the rover crew’s position will be fastest if the rover is sitting directly below the satellite’s ground track but much slower if they are stationed far to the side. Measuring the Doppler shift caused by the approach and recession rate of the satellite beacon can allow the rover crew to determine how far east or west they are from the satellite’s north-south ground track. Once again, comparing this information with computer records specifying the satellite’s longitude as a function of time would allow the crew to determine their
longitude.

These high-tech techniques are quite accurate. A similar approach is used on Earth in the Argos satellite system to track the movement of falcons and elk (except that in this case the beacon is on the elk and the receiver and required calculations are done on the satellite) with precision of about one kilometer. However, there are a number of problems. The satellite is in a roughly two-hour orbit with Mars turning beneath it. Therefore, an observer on the ground will encounter the satellite only once during the day and once at night, so that a single satellite will only allow position fixes to be taken once every 12 hours. This can be remedied by having more satellites orbiting in a number of north-south planes spaced around the planet, but then you start running into real money. And what if the satellite beacon, or the rover receiver, or the rover computer, should fail? What then? Are there more reliable low-tech navigation techniques that can serve as a backup?

On Earth, the magnetic compass has long served as the mariner’s key navigation tool. Unfortunately, compasses won’t work on Mars because the planet has virtually no magnetic field. However, the time-honored techniques of celestial navigation can be used on the Red Planet, with much greater facility, in fact, than has ever been possible on Earth.

If you’ve ever practiced celestial navigation, you’ll know that determining latitude is easy, while determining longitude is hard. To determine latitude all you need is a sextant to measure the angle between the celestial pole and the horizon. That angle is your latitude, period. In Earth’s Northern Hemisphere this measurement is easy to do, because the celestial pole is marked within I° accuracy by the pole star, Polaris. The direction of Polaris also tells you which way is north—with better accuracy than any compass. Does Mars have a conspicuous pole star? Not really, but its celestial pole, located at 21.18 hours right ascension, 52.89° north declination, is still pretty easy to find, as it lies almost exactly halfway between the two bright stars Deneb and Alpha Cephei. So, with a sextant and a clear night (a more common occurrence on desert Mars than on the rainy, misty old Earth) you can readily determine your latitude on Mars.

What about longitude? On Earth, with the aid of an accurate clock set on some standard time, such as Greenwich Mean Time, you can determine your longitude by measuring the time of sunrise and
comparing it to the value given in an almanac for the sunrise time for that day at the Greenwich meridian (the Prime Meridian, 0° longitude) at your latitude. For example, if the almanac says that the Sun will rise at 6
A.M
. at your latitude on the Prime Meridian on March 21, and you observe the Sun rising at 7
A.M
. on your Greenwich Mean Time clock, you would know you are at 15° west longitude, since the Earth turns at a rate of 360 degrees in 24 hours, or 15 degrees per hour.

This works reasonably well on Earth, but it works much better on Mars, because on Mars in addition to the Sun, the two rapidly moving asteroid-like moons Phobos and Deimos can also be used as longitude beacons. As seen from the surface of Mars, Phobos, the inner moon, would have a visual magnitude of -10, which makes it about 300 times brighter than Venus as seen from the Earth at its brightest, while Deimos would have a magnitude of -7, about twenty times brighter than Venus. Except during dust storms, both of these satellites should be clearly visible from the Martian surface both by day and by night. Both moons are in almost precisely equatorial orbits, so by measuring their angular distance from the zenith when they are at their highest in the sky, you can use these moons to determine your latitude, even during the middle of the day. Phobos orbits Mars every 7 hours 39 minutes, while Deimos has a period of 30 hours 18 minutes. Between the Sun, Phobos, and Deimos, the Martian navigator will have lots of sunrise and moonrise events to chose from to compare against his almanac and clock, and with each rising, longitude can be determined. In fact, using a bit of math which would be A-B-C for a traind navigator; it is possible for an observer on Mars equipped with a sextant, a clock, and an almanac to determine his latitude and longitude simultaneously whenever any two of the three objects (Sun, Phobos, and Deimos) are visible in the sky.

By the way, on Earth we define one nautical mile as one minute (l/60th of a degree) of latitude. This is about 1.82 kilometers. However, if we define a Martian nautical mile as equal to one minute of latitude on Mars, it turns out that a Martian nautical mile
equals
one kilometer almost exactly (well, 983 meters). So, on Mars, navigators and metric system buffs will finally be able to get along!

KEEPING TIME ON MARS

There has been a fair amount of discussion in the
literature of possible timekeeping systems for use on Mars. Having just discussed surface navigation, now would be a good time to address this matter.

As we’ve seen, the Martian day lasts 24 hours and 39.6 minutes of terrestrial time. Timekeeping systems proposed to date have usually preserved terrestrial clock units, with an additional partial hour thrown in just after midnight.
²⁰
Alternatively, totally novel, usually decimal-bas
ed clock
s using a completely new set of time units have sometimes been proposed.
²¹

It should be clear from the previous section’s discussion that a clock employing unequal hours would be a nightmare for those attempting navigation or astronomy on the Martian surface. On the other hand, a decimal or other novelty clock would probably be disorienting and in any case would require a complete overhaul of Mars’ existing system of surface geographical coordinates (which employs the same base-60 degrees, minutes, and seconds system that is used to map the Earth).

The practical answer is simple—just divide up the Martian day into 24 Martian hours, each composed of 60 Martian minutes, each of which in turn is composed of 60 Martian seconds. The conversion factor between Martian days, hours, minutes, and seconds and their terrestrial equivalents would thus be 1.0275 across the board. A time of day on Mars, say 6:00
A.M
., would have exactly the same physical significance with regard to the orientation of the planet toward the Sun as it does on Earth. All the equations of celestial navigation used on Earth would then remain precisely valid. That is, regardless of whether you are on Mars or Earth, one hour of time would equal 15 degrees of longitude, one minute of time would equal 15 minutes of longitude, and one second of time would equal 15 seconds of longitude.

Such a clock solves all the practical problems associated with daily timekeeping on Mars. In fact, it is in implicit use already among mission planners at Jet Propulsion Lab today who, for example, might describe a future Mars orbiter’s path as a 6:00
A.M.
—6:00
P.M
. orbit, meaning a satellite that travels tracking the dawn—dusk terminator on Mars. That is, “6:00
A.M
.” as they use it is really a Mars local time in the sense described above, and the twelve hours separating it from “6:00
P.M
.” are Martian hours. It
is unfortunate that such a clock annoys physicists who regard the terrestrial second as the sacrosanct unit of physical time. They really shouldn’t worry—Martian crystallographers and others who require a high degree of precision in quoting their measurements of frequencies will still be able to quote the measurements in terms of terrestrial seconds. The standard International System of Units can remain intact. However, for purposes of operating on Mars, the terrestrial second is no more useful a unit of timekeeping than the terrestrial day, and must yield to its Martian counterpart.

TELEROBOTICS: EXTENDING THE REACH OF THE CREW

For safety reasons, while two members of the crew (a scientist and a mechanic) are out on a rover excursion, the other two will generally remain behind at the hab base. Thus, if the rover crew should get into trouble, the reserve crew can ride to the rescue in a backup vehicle (such as one of the open rovers). In general there will always be at least two people stationed at the base, and, in between rover excursions (which might typically last one to ten days each), the entire crew of four will be there. Granted, there are many useful activities that the crew can undertake at the base—analyzing samples, conducting various scientific and engineering experiments, and engaging in construction and necessary maintenance of equipment. Nevertheless, since the primary function of the initial Mars missions will be exploration, it would be extremely beneficial if personnel stationed at the base could use part of their time to explore. This they will be able to do, provided that the expedition is supplied with a contingent of telerobots.

Martian telerobots would be small wheeled or treaded roving vehicles equipped with TV cameras, microscopes and other scientific instruments, manipulator arms, and a radio. Controlled from the Mars base via either shortwave radio or through an aereosynchronous relay satellite, these telerobots could be driven rapidly by remote control, as the radio link time delay would be insignificant (the Earth—Mars radio time delay, up to 40 minutes round trip, prevents effective telerobotic operations conducted from Earth). The telerobots could be deployed by the rover crews as they travel, allowing the base crews to explore in greater detail various sites that rover crews identify as interesting but had no
time to investigate at length themselves. The telerobots could also be used to probe into places too small or risky for humans, such as caverns or narrow crevices.

However, the base crews could also deploy some telerobots themselves, lofting them up on balloons and then bringing them down to land thousands of kilometers away. (A balloon on Mars could be expected to fly 2,000 kilometers in a single day.) The flight path of the balloons can’t be controlled of course, but provided that the wind patterns of Mars have been mapped out in advance by missions like MAP, the path the balloon-borne telerobot takes could well be predicted. As the telerobot flies, its cameras can be used to send real-time aerial images to the base crew, who, looking through the telerobot’s eyes, will be able to choose the best time and place to land the system. Upon landing, the telerobot could either release the balloon, thereby committing itself to its selected landing region for life, or, if the winds are light, it could attempt to secure the balloon’s anchor line to a rock formation. In the latter case, the telerobot could then leave the balloon and explore the area for a few hours, but then reattach itself to the balloon, release anchor, and take off to visit still another more distant site.