Here is a list of probes we have sent to Mars, as of 2007. They are listed in chronological order. For each, I give the name, country, year, mission, and whether successful.
|Mars 1||Soviet Union||1962||flyby||failed|
|Mariner 3||United States||1964||flyby||failed|
|Mariner 4||United States||1964||flyby||successful|
|Zond 2||Soviet Union||1964||flyby||failed|
|Mariner 6||United States||1969||flyby||successful|
|Mariner 7||United States||1969||orbiter and lander||successful|
|Mars 2||Soviet Union||1971||orbiter and lander||successful|
|Mars 3||Soviet Union||1971||orbiter and lander||successful|
|Mariner 9||United States||1971||orbiter||successful|
|Mars 4||Soviet Union||1973||orbiter||failed|
|Mars 5||Soviet Union||1973||orbiter||successful|
|Mars 6||Soviet Union||1973||orbiter and lander||successful|
|Mars 7||Soviet Union||1973||orbiter and lander||failed|
|Viking 1||United States||1975||orbiter and lander||successful|
|Viking 2||United States||1975||orbiter and lander||successful|
|Phobos 1||Soviet Union||1989||Phobos lander||failed|
|Phobos 2||Soviet Union||1989||Phobos lander||failed|
|Mars Observer||United States||1992||orbiter||failed|
|Mars Polar Lander||United States||1997||lander||failed|
|Mars Global Surveyor||United States||1997||orbiter||successful|
|Mars Climate Observer||United States||1999||orbiter||failed|
|Mars Odyssey||United States||2001||orbiter||successful|
|Mars Express||European Space Agency (ESA)||2003||orbiter and lander||orbiter - successful|
lander - failed
|Mars Reconnaissance Orbiter||United States||2006||orbiter||successful|
This does not include missions canceled before launch. As you can see, we've sent far more probes to Mars than any other world in our solar system, including our own Moon. For comparison, we've only sent four probes to Saturn.
|Pioneer 11||United States||1979||flyby||successful|
|Voyager 1||United States||1980 (launched 1977)||flyby||successful|
|Voyager 2||United States||1981 (launched 1977)||flyby||successful|
|Cassini||United States||2004||orbiter and Titan lander||successful|
Of course, Mars is a lot closer than Saturn but that's not the total explanation in the huge difference in number of probes sent. There's far more public interest in Mars than any other world. Mars is the most Earth-like world we know about. This is why Mars figured so prominently in early science fiction, such as by H. G. Wells, Ray Bradbury, Edgar Rice Burroughs, etc. Certainly, we have the technology for a manned flight to Mars, and the only reason we haven't done it is politics. Congress just isn't going to fund such a mission anytime soon. The Soviet Union, on the other hand, not burdened with having the justify expenditures to constituents, fully intended to send humans to Mars, and if the Cold War hadn't ended, would probably have sent humans to Mars by 2020. Also, if you look at the above list, you notice that the United States did not attempt to send any probes to Mars between 1975 and 1992. They had the same "been there, done that" mentality that ended the Apollo program.
One other thing you notice from looking at the above list is the large number of failed missions to Mars. The real reason for this is simply that if you try to send that many probes to Mars, some of them are going to be failures. Unfortunately, the UFO believers have tried to attach some sinister significance to this. They suggest that aliens don't want us interfering with their projects on Mars, as if the so-called "Face on Mars" was shooting down these probes, or that UFO's were, intentionally or unintentionally, interfering with our communications. Probably an even more popular theory within the UFO community is the claim that the probes to Mars that have been publicly classified as failures were not in fact failures but in fact found proof of UFO's, but there is a vast government conspiracy to hush this up, so the government just declares that they lost contact with the probes so they don't have to release the data. Well, this nonsense doesn't deserve a response except to say that this is nothing more than the latest in a long tradition of fiction associating Mars with aliens. Examples include Schiaparelli, Percival Lowell, H. G. Wells, Ray Bradbury, Edgar Rice Burroughs, Arthur C. Clark, "My Favorite Martian", "Mars Attacks", and Marvin the Martian on Bugs Bunny. What this does show you is the great interest that the public continues to have in Mars, and I think the public would support funding a manned flight to Mars at this time.