As you've probably heard by now, the Curiosity rover landed on Mars yesterday morning (or the night of the day before yesterday, if you live in the Americas). This is the next big step in the exploration of Mars that has been going on for millennia. “Millennia?” You may ask. Yes, millennia, because the exploration of Mars didn't simply start with the first spacecraft sent there, but much before that. With its discovery, to be precise.
It's unknown who first discovered Mars, but what is known is that he or she lived thousands of years ago. The ancient Egyptians, Babylonians, and Chinese all knew of the existence of Mars and the other four planets that can be seen with the naked eye a thousand years before Rome was founded. You see, as they observed the stars, they realised all stars moved a little each night, rising and setting four minutes earlier than the night before. But always by that amount; it was as if the sky was a giant sphere with light on it that rotated around the Earth very slowly. But seven things in the sky didn't obey the rules: there were the Sun and the Moon, but aside from them there were five bright stars that moved a little among the stars each night. The Romans called them Mercury, Venus, Mars, Jupiter, and Saturn, after several important gods.
It was thought that these five, the Sun, and the Moon, all moved in circles around the Earth below the stars. The Earth was in the centre of the universe, then you had the Moon, then Mercury, Venus, the Sun, Mars, Jupiter, Saturn, and beyond Saturn were the stars. There were some oddities that were difficult to explain like this, however. Mercury and Venus always stayed close to the Sun, for example, never reaching the opposite side of the sky. Mars, Jupiter, and Saturn did reach the opposite side of the sky, where they would be said to be in opposition, but once there they did a very strange loop in the sky over the course of a few weeks.
Mars was the most troublesome of the planets for this geocentric model: not only was the loop it made at opposition biggest of all three planets that did one, but it did many other things that perplexed ancient astronomers: it would stray off the path the other planets followed across the sky a little, and it would vary in brightness. One year, it would be brighter than anything in the sky except for the Sun, the Moon, and Venus. The next, it wouldn't be all that impressive, and all the other planets would get brighter, as would a couple of stars. It would also vary in brightness a lot during a year: all planets were brighter at opposition than when they were close to the Sun, but Mars showed much bigger variation than any of the others.
The big loop at opposition and the difference in brightness over the year were explained in 1543 when Nicolaus Copernicus published a book in which he claimed the Earth wasn't at the centre of the universe, but the Sun. The Earth was the third planet moving in a circle around it, and Mars the fourth. This would bring them very close together at opposition, much closer than Earth and Jupiter, for example, so by comparison they would be much further away from each other when Mars was on the other side of the Sun. The loops Mars, Jupiter, and Saturn made in the sky were a perspective effect caused by Earth passing those planets, and Mars made the biggest because it was closest.
However, Mars' deviation from the path of the other planets and its differing brightness across the years wasn't explained until 1615, when Johannes Kepler realised the planets didn't move in circles around the Sun, but in ellipses. Sometimes they got closer to the Sun, and sometimes further away. Most planets have an orbit very similar to a circle, but Mars' orbit is more elliptic than all others (except Mercury), getting about ten percent closer to the Sun at its nearest (which is called perihelion) than at its farthest (which is called aphelion). The difference in brightness was because in some years, Mars reached opposition while at its closest the Sun (and therefore the Earth), while in others it was at its furthest from the Sun.
During the 17th century, as telescopes were being pointed at the sky for the first time, Mars was found to look bigger at times when it was brighter and smaller when it was dimmer. This made perfect sense, of course, since the planet was sometimes closer to the Earth than at other times. The first details on Mars were being seen in this century and the next as well: white polar caps and dark spots on its reddish surface. It was found to rotate in 24 hours and 39 minutes, just a bit slower than Earth, and it had an atmosphere. Mars seemed a world that was a lot like Earth: the dark spots could be oceans, or perhaps forests.
In the 19th century, this view was confirmed even more as Giovanni Schiaparelli saw long, straight lines on Mars. Some thought these were canals built by the Martians to irrigate the dry areas of the planet. At the time, canals were a huge new product of technology, the Suez canal having just been completed while work had begun on the Panama canal, so canals were a big part of public consciousness. There were even ideas to help the Martians see us as we saw them, by bigging trenches thousands of kilometres long in the Sahara, filling them with oil, and lighting them on fire! A Mars-craze erupted, as it was considered very likely there was life on Mars by now. H. G. Wells wrote a great book, The War of the Worlds, about a Martian invasion of Earth, which was among the first science-fiction books.
However, around 1900 telescopes became better, and it was getting clear the dark spots on Mars were no seas, as they showed mountains and canyons in them. The canals turned out to be an optical illusion caused by smaller spots and craters lying roughly in lines. The concept of Martians remained a big part of culture and speculation for much of the 20th century, but for a few decades Venus was considered a more likely planet to harbour life by science. When the first interplanetary probes were launched in the 1960s, they were therefore going to Venus, but when they discovered that planet to be far more hellish than expected, Mars once again gained the spotlight. It was highly unlikely by now that little green men lived there, but there could be plant life or microbes on the red planet.
The Russians were the first to send probes to Mars. The first six, from 1960 to 1964, all failed; four of them before even reaching Earth orbit, and the other two while on their way to Mars. The Americans built two probes to Mars; Mariner 3 and 4, but Mariner 3 failed as well. It was almost getting suspicious, seven probes all failing before they got to Mars. Maybe the Martians existed after all and were protecting their secrets? Fortunately Mariner 4 launched perfectly, three weeks after Mariner 3's failure. It became the first successful probe to fly by Mars, and for the first time showed photographs taken of another planet from close by, showing a rough landscape full of craters and mountains. It also showed Mars to be harsher than expected: the atmosphere was very thin, providing only 1/150th of Earth's pressure. This also means the planet's heat easily escapes into space at night, bringing temperatures down below a hundred degrees below zero, though it also warms back up quickly when in daylight, sometimes reaching temperatures similar to a summer's day on Earth: 27 degrees. For the most part, its daytime temperatures stay quite a bit under freezing, though.
It made multicellular Martian life unlikely, as very few Earth organisms could survive long in conditions like that. But a subsequent set of probes launched in 1969, and managed to get into orbit of Mars, observing it for a while. They found structures that looked like dried-up riverbeds, and discovered the tallest mountain in the entire Solar System on Mars. It's called Olympus Mons, and is nearly 22 km high. With the discovery that water may have once flowed on Mars, it was clear the only way to find out more about the planet was to land on it.
In 1975, this was finally accomplished by the Viking 1 and Viking 2 landers, which each landed on the surface after making orbital photographs for a while. They each carried three experiments intended to detect microbial life if it existed on Mars. Two of them turned out negative, but a third was positive. The most common explanation of this is that life wasn't detected, but the third experiment set off by other factors, however, this is far from certain. For the most part, the Viking experiments proved inconclusive. Yet attention turned away from Mars, and this coupled with big budget cuts to both the American and Russian space programs meant Mars was mostly ignored for twenty years.
By the 1990s, it became possible not just to land on Mars, but to ride around there, so experiments could be done on many more places than just the single location the Vikings had been able to. The first such rover, Sojourner, was a small, cheap thing with few instruments on board, mainly intended to prove the rover concept was doable. In the next few years, three probes were sent to Mars: the American Mars Global Surveyor and Mars Odyssey, and the first European mission to Mars, called Mars Express. These three probes researched and photographed Mars in greater detail than ever before, and found many traces of water on Mars: dried-up riverbeds and lakes, gullies, and even signs of ancient rainfall. Global Surveyor failed in 2006, but the other two are still in orbit and returning data every day.
Along with Mars Express, the European Space Agency had tried to put a rover named Beagle 2 on Mars, intended to find signs of life, but contact was lost with the rover during landing, and it has presumably crashed or otherwise failed. The Beagle 2 was intended to land in a rather strange way: by surrounding it with inflated balloons and simply letting it bounce on Mars' surface until it would finally stop, at which point the ballons would be deflated and the rover released. The next year, however, NASA landed two rovers, Spirit and Opportunity, on Mars using the same method. These two mainly researched the geological history of Mars, including the period, long ago, in which there was liquid water there. Opportunity has done so for eight years by now, but Spirit became stuck in soft soil in 2009 and was eventually terminated the next year after many unsuccessful attempts to get it loose.
And now, finally, we get to Curiosity. Like always happens in science, the answers Spirit and Opportunity provided led to even more questions. And so the largest and most advanced rover yet was launched. Curiosity was lowered from a floating platform by a crane yesterday (no, I'm not making this up, that's really how it was landed. The balloon method was simply too dangerous to use with the car-sized Curiosity), and returned its first pictures not much later. It's intended to work for at least two Earth years (one Martian year), but will most likely continue to work for several years afterwards. One of the things it's going to start looking for is the building blocks of life. Its data will also serve to prepare a future manned Mars landing. Curiosity landed in the Gale crater, where there is a mountain called Aeolis Mons, which it'll be slowly climbing over the course of its mission, while examining many of the things it will come across.
What happens next? Well, it partially depends on Curiosity's findings. It is likely Curiosity will be followed up with an even more advanced rover around 2020, which might finally settle the question of life on Mars. And by the 2030s, a manned mission will land on Mars, most likely after earlier manned missions to an asteroid and to Mars' moon Phobos. Several countries have plans for Mars missions: America and Russia, of course, but Europe, Japan, China and India as well. The most likely way for a manned mission to Mars to happen, however, is as an international mission. The International Space Station has proved big, expensive space missions are best performed through international cooperation, sharing both the expenses and the knowledge and expertise required. A manned mission to Mars will likely require such international cooperation to succeed. Once on Mars, there is the possibility they would stay for several months and conduct research rovers can't do; but there is also a different idea floating around: it could be a one-way trip. The astronauts could be sent with everything required to set up the first Mars base and stay on Mars, greatly reducing both the cost and difficulty of the mission, while immediately beginning the colonisation of Mars.
Well, that turned out a little longer than expected. I hope you enjoyed reading it, though, or otherwise stopped reading it when you realised you didn't enjoy it.