Mars, Mars, Mars:
Exploring Mars

The time is well within living memory when the idea of humans sending a spacecraft to Mars was considered idle fancy--science-fictional nonsense--perhaps something for the far future. That far future turned out to be not so far off after all: the first attempts were made as early as 1960, and by 1965 we had successfully placed a craft in orbit around Mars. By 1971, we had hardware actually arrive on the surface of Mars, though that year's two probes only crash-landed. By 1976, we were reliably getting good-quality photographs and other data from the surface of Mars. What an incredible journey!

Overview

Our explorations of Mars were once limited to what the unaided human eye could see in the night skies; in that era, which lasted till the invention of the telescope about four hundred years ago, our knowledge of Mars didn't go far beyond that it was a bright red point in the sky that--like only a very few others--didn't behave like a star. Only toward the end of that era was did we even start to realize that those few "wanderers", and our own Earth, revolved around the Sun. With the coming of the optical telescope, and--in later years--other remote-sensing devices, we discovered much about the universe in general and planets and Mars in particular. But not till we were able to actually send our instruments out to Mars, first to its planetary neighborhood, then actually down onto its surface, were we able to discover the answers to many age-old questions--as well as a whole new stack of questions to ask.

Mars Exploration

The rocket scientist and popular author Willy Ley, writing over half a century ago in The Conquest of Space (an excellent book even today, made all the more wonderful owing to the many plates of beautiful illustrations by Chesley Bonestell), observed that the history of astronomy could be divided into three eras: naked-eye observation; observation with instruments (including but not limited to telescopes); and direct observation, meaning "going there"--which last, in Ley's time, was still wishful thinking. Let us use that concept to lay out our look at the human exploration of Mars.

Naked-Eye "Exploration"

From the dawn of humanity, folk have been looking at the night skies. Every civilization has quickly noted that of the points of light in those skies, the vast majority--what we know as "stars"--were fixed, as if the sky were a sphere with us at its center and the stars painted onto the inside of that sphere, with the sphere rotating about our position. (Many early views of the construction of the universe reflected that view--the page of this site on Mars in history and culture has much more on that topic.) But they also noticed that several of those points seemed to wander about the fixed stars. Those "wanderers" were the planets. Two of them--what we today know as Mercury and Venus--could, for various reasons, only be seen in early evening and early dawn, and thus were each usually wrongly thought of as two distinct astral bodies. The three others--what we today know as Mars, Jupiter, and Saturn--were always understood each to be a single celestial body.

Ideas of what the planets were varied from culture to culture, but none bore any remotest relation to reality (there is considerably more information on this aspect of the human understandings of Mars in this site's page on Mars in history and culture

In this "first era" of the "exploration" of Mars, the only real knowledge about the planet concerned its apparent movements in the sky relative to the background of "fixed" stars. The movements of the planets deeply puzzled the ancients, because they seemed quite erratic, each planet seeming first to speed up in its movement, then slow down and fall back. The crux of their problem in understanding was that they all thought that the Earth was necessarily the center of the universe, a belief amply summed up in the Ptolemaic system (after Claudius Ptolemaeus, who documented it in his work The Almagest), which for a thousand years or more dominated Western thought about cosmology. (See illustration at left.) But the impossibility of reconciling what was before their eyes to that simplistic scheme led to ever-more ludicrous "fixes" that made an already intolerably complicated system yet worse. (There is a famous remark made by Alfonso X, king of Castile in 1175 A.D., known to history as "Alfonso the Wise", on the Ptolemaic system--for which Alfonso later commissioned improved tables--that "If the Lord Almighty had consulted me before embarking on creation, I should have recommended something simpler.")

The one thing the ancients did know was the historical track of all the chief bodies visible in the skies, and they compiled immense tables of data on those movements, from which often they could make predictions of astronomical events. But these tables were just mechanics, not understanding--the planets were just bright dots in the sky.

Toward the end of this first era of astronomy came a development destined to be of significance to later understandings of Mars and the cosmos in general. In 1543, the noted Polish astronomer Nicolaus Copernicus published a work many decades in the making, De Revolutionibus Orbium Coelestium, http://omniknow.com/common/wiki.php?in=en&term=Nicolaus_Copernicus in which he set forth what we today call the Copernican or "heliocentric" model of the cosmos. It put the Sun, not the Earth, at the center of the universe, and correspondingly held that the Earth and all other heavenly bodies rotated around the Sun. His work, though not the first to set forth such a position, was the first to set it forth in a detailed, useful, coherent form. This blast at the old systems, which had insisted--and long continued to insist--that our Earth was the centrum of all the cosmos, though not much celebrated when first proposed, essentially marked the beginning of the modern era of science.

Instrumented "Exploration"

The second age of astronomy arrived with the discovery of the telescope, and it almost immediately brought great leaps forward in human understanding of the heavens, most notably an eventual shift away from the classic Earth-centered view of the universe to a much more realistic Sun-centered view. The shift was not accomplished without major clashes, but we will reserve a fuller discussion of those to the page on Mars in history and culture.

This era, which took our view of Mars from that of a bright point of light in the sky to that of another world far off in space, lasted only a relatively brief three and a half centuries. Moreover, progress occurred mainly in the early decades of that era, real additions to knowledge (as opposed to continuing small refinements of what was already known) becoming progressively fewer and fewer over time. The reason for that is the inherent limitation of trying to view a far-off astronomical body from beneath a thick atmosphere; the character of Earth's atmosphere places significant limits on the ability of any ground-based telescope to acquire data. In the early years of this era, the telescope opened up new vistas, and each increase in the power of available telescopes augmented those vistas; but eventually--and relatively early on at that--the magnifying power of telescopes reached the limits that atmospheric distortion necessarily imposes. (The smaller the detail one is trying to pick out, the more the effects of distortion--from heat-generated ripples in the air, as exemplified by stars' "twinkling", to simple haze--will interfere with viewing.)

The first known use of a telescope for astronomical purposes was in 1609 (the instrument was only invented the year before, 1608, in the Netherlands, or perhaps even earlier), when that genius Galileo turned his augmented gaze on the heavens soon discovered many new things; most immediately, he looked at the Moon, and observed mountains and valleys there, making it plain that the Moon is a "world" at least somewhat comparable to our own. (At right: Galileo's telescope.)

Another important discovery that Galileo made was that the planet Venus exhibited phases just as the Moon does; that was critical information, because it was something impossible under the then-extant Earth-centered view of the universe. At that time, though Copernicus had published his Sun-centered theory of the universe two-thirds of a century earlier, that theory was widely considered to be just an interesting mathematical exercise, rather than an actual description of reality; but evidences such as Galileo's--the Venusian phases, and the fact that a planet could have objects going round it (In 1610, Galileo had discovered the four major moons of Jupiter)--began to give plausibility to the system being literally true.

Meanwhile, in the same year Galileo began his epochal telescopic survey of the heavens, the German astronomer and mathematician Johannes Kepler had made a relatively small yet highly significant correction to the Copernican system, a correction that vastly simplified the problem of relating actual observation of the skies with the predictions of theory. Till then, the Copernican system had been largely ignored, for several reasons, perhaps the chiefest of which was that it was no less complicated (and perhaps even a bit more so) than the old Tychonian system of the Danish astronomer Tycho Brahe, who had finally updated old Ptolemy by adding to his system even more complexities--chiefly to try to account for the then-incomprehensible movements of the planets.

Kepler (using, ironically, copious tables of data compiled by Tycho) was able to educe what came to be known as "Kepler's Laws of Planetary Motion", the first of which stated that The orbit of a planet about the Sun is an ellipse with the Sun at one focus of the ellipse. By realizing that the orbits of astronomical bodies are not the perfectly circular paths that Copernicus (like all before him) had assumed, but rather were elliptical, Kepler at one stroke made the Sun-centered system relatively simple and in excellent accordance with what could actually be seen going on in the skies. (Kepler's work was later to be invaluable to Isaac Newton in his elucidation of the law of gravity and the laws of motion.)

In 1609 Galileo was also discovering significant things about Mars. The first and foremost, and perhaps most exciting, was that the telescope revealed that Mars was not simply a bright point, but was clearly a substantial sphere--in other words, that it was a world. Galileo also, by careful observation and record-keeping, established that Mars varied in apparent brightness and size on a regular, periodic basis; those data provided yet further substantial support to the Copernican Sun-centered theory.

Now came great clashes between the believers in the no-longer-tenable old Earth-centered system and the new Sun-centered system, most famously in the story of Galileo's persecution by the Church. But, as we noted above, our discussion of that history is elsewhere on this site. Here, we will simply recount the "second era's" major milestones in the advance of our real knowledge of Mars.

In 1659, the Dutch astronomer Christiaan Huygens, using a newer, more powerful telescope (which he designed), observed Mars and--by noting the periodic recurrence of a particular spot on Mars's surface (now thought to have been Syrtis Major)--correctly concluded that Mars has a 24-hour day (it is exactly 24.623 hours long: Huygens, almost 350 years ago, got it right to with better than 97% accuracy). And that already remarkable accuracy was bettered in 1666, when the Italian astronomer Giovanni Cassini calculated, from observations, the Martian day to be 24 hours and 40 minutes, almost perfect accuracy. In that same year, Cassini also became the first to note that Mars has a polar cap (this is the north polar ice cap).

Cassini later (by then working in France) made extensive observations and calculations that enabled him, in 1672, to calculate the distance from the Earth to Mars, and got it substantially correct. He made his measurement at a time when the Earth and Mars were "in opposition", meaning that they lay in a straight line out from the Sun (see diagram at left)--or, to oversimplify, when Mars was at about its closest approach to Earth (Mars is not always at the exact same distance when it is in opposition, but the distances from one opposition to another don't vary that much). The figure Cassini got--about 87 million miles, only about 6% to 7% low--was something of a shocker, for it made plain for the first time how truly huge "astronomical" distances are, and was thus another major step forward in the transition from the old, cozy view of the universe to the modern one.

In that same year of 1672, Huygens became the first observer to detect the south polar ice cap on Mars.

In 1704, the astronomer Giancomo Miraldi, using the new and--for its day--large and powerful Campani Telescope in the Paris Observatory, verified the existence of the polar caps of Mars, further noting that the south cap does not quite lie on the true south pole of Mars. Later, in 1719, Miraldi speculated (correctly, as it turns out) that the caps were constituted of ice (Mars was in that year at an especially close opposition to Earth, enabling correspondingly good telescopic viewing). Poor Miraldi--he seems almost forgotten today, though there is a lunar crater named after him--also reconfirmed the earlier measurements of the length of the Martian day.

Sir William Herschel, best remembered for his discovery of the planet Uranus in 1781, was shortly thereafter (1782) appointed the (British) "King's Astronomer" (something different from "Astronomer Royal", a title then held by Nevil Maskelyne, with whom Herschel was acquainted). As part of his wide-ranging astronomical studies, Herschel extensively examined Mars, using excellent telescopes that he built himself, including a reflector, a type much harder to make than the then-more-usual refractor; indeed, Herschel substantially advanced the art of telescope-making.

(Herschel's sister Caroline, for years his secretary and assistant, went on after his marriage to become an astronomer of some note in her own right.

In 1784 Herschel (portrait at right) published the verbosely titled paper On the remarkable appearances at the polar regions on the planet Mars, the inclination of its axis, the position of its poles, and its spheroidal figure; with a few hints relating to its real diameter and atmosphere; in it, Herschel (perhaps to justify the length of that title) set forth much data--some refining known points, some more or less novel. Among his pronouncements:

· the Martian axial tilt is 30° (not bad--we now know it to be 25.19°);

· the length of the Martian day is 24 hours, 39 minutes, 22 seconds (only 14 seconds off);

· Mars has seasons like the Earth's save about twice as long--since the Martian year itself is about twice Earth's--and the periodic growth and shrinkage the polar ice caps are a consequence of the changes in the Martian seasons (which is so);

· the patterns of light and dark areas on the planet correspond to seas and dry land (very wrong);

· Mars has (he concluded from star-occultation observations) an atmosphere, though a thin one (correct); and,

· Mars is inhabited by intelligent creatures (Herschel believed that all the planets were so inhabited, even the Sun) who, he remarked, "probably enjoy a situation similar to our own" (quite wrong).

Though (as we will see below) diligent astronomers after Herschel continued to add bits and pieces to our knowledge of Mars--mostly by further refining already known data--there were few if any "great leaps forward" throughout the remainder of this era of astronomy. There was (as we document on another page of this site) a great deal of wild speculation and furious argument in that time, largely based on what each "expert" thought he had seen through his telescope. (Recall that there could be such wide disagreement because the telescope had by now largely reached the limits of its ability to discern detail, owing to unavoidable distortions from Earth's atmosphere.)

Here is a summary of some of the salient developments in actual knowledge of Mars during that period:

And so went the second astronomical era of "exploring" the planet Mars.

True Exploration: Going There

It is not even a half a century yet since humankind entered the third era of astronomical exploration, "going there". But by now we have already sent many spacecraft toward Mars, and they have returned us a treasure-house of data, some of which have dramatically revised our older views of Mars and conditions there.

Spacecraft sent toward Mars can be of four basic kinds: a "flyby", in which the craft passes by the planet, gathering data as it goes; an "orbiter", which settles into orbit around the planet to gather data on a long-term basis; a "lander", which makes a soft touchdown on the planet's surface and gathers data from where it lies; or a "rover", which makes a soft touchdown, then moves about the surface, gathering and reporting data. Sometimes those can be combined: a mission could comprise a craft that enters orbit but also sends down a lander; or a surface mission could comprise both a fixed "base station" and a separated rover. Sometimes mission problems can convert one sort into another, as with an intended orbiter that ends up only doing a flyby. The Table below lists all Mars-bound spacecraft past, present, and--so far as is known now--planned.

At our present level of rocket capability, and for the foreseeable future, it is not possible to launch a mission to Mars at just any old time we might have the hardware ready. That is because both Mars and the Earth orbit the Sun, but not at the same speed (the farther out an orbiting object is from its "primary", the slower it moves on that orbit--that's just a law of nature). Thus, it is only every so often that their relative motions are such that they are--as planetary distances go--close to one another; one does not want to try to send a spacecraft out that would not only have to travel the distance from the Earth's orbit out to Mars's orbit, but additionally travel some great extra length to catch up with where Mars is.

That is easier to follow if you look at an orrery, a mechanical device that physically embodies the relative motions of the planets. There is a very nice online graphical orrery available at the "Solar System Live" page of John Walker's Fourmilab web site; here are two views taken from that orrery, showing (not, of course, to scale) the inner planets as they were aligned on 5 November 2004 (at left) and 5 November 2005 (at right). Notice how far Mars is from the Earth in 2004, and how close it is in 2005. The times when an astronomical target is at or near its optimum placement relative to Earth determine what are called target "launch windows"--periods of time during which it is feasible to launch a spacecraft toward that target.

Note that the launch window is not the time when the target (here, Mars) is actually closest to us--because the spacecraft takes quite some time to get to its target; thus, the launch window is the near-approach period minus the spacecraft travel time. A spacecraft is not launched toward where its target is: it's launched toward where its target will be by the time the craft gets there.

Here is the record (omitting planned missions later cancelled and thus never attempted) of all Mars-bound spacecraft:

Here are links to news on presently active missions:

• NASA's Mars Global Surveyor Orbiter

• NASA's Mars Odyssey Orbiter

• ESA's Mars Express Orbiter

• NASA's MER-A "Spirit" and MER-B "Opportunity" Mars Rovers

• ESA's Rosetta Space Probe (en route--flyby due February 2007)

• NASA's Mars Reconnaissance Orbiter (en route--arrival due March 2006)

The treasure trove of information the many missions have brought us (including the valuable experience with those that wholly or partially failed) is inestimable: scientists will be laboring for years yet just to complete analyses of the data we already have, and more pours in hourly. We will not here attempt to set forth all that data, or the conclusions being drawn from it--that we do chiefly on the pages of this site dedicated to discussing planets and the possibility of life on Mars.

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