Mars is the only other planet in the solar system that is even vaguely Earth-like, and therefore the only other planet where life might have evolved. Another possibility is Jupiter's moon Europa, but that is not a planet.

Mars is the only planet that humans have any hope of landing on, walking on and exploring in a traditional sense any time soon. It is the only planet that might be terraformed and turned into an Earth-like planet.

For these reasons, humans have sent over 30 different spacecraft to explore Mars. The current mission to Mars involves a pair of robotic rovers that are known as the Mars Exploration Rovers.

We aren't at the point yet where we can send human beings to Mars. The first and most important reason is that different nations have sent more than 30 probes toward Mars, but fewer than one-third of those probes have survived the trip. It's not a very good track record, and certainly not one that would encourage us to replace those robotic probes with human beings.

The second reason is cost. It is currently costing about a million dollars per pound to design and deliver a robot to Mars, and robots don't have to worry about complicated things like life support systems. Robots don't have to worry about coming home, something that adds a great deal of weight to a mission. And robots do not require a soft landing on the surface of Mars. It would take a minimum of 100,000 pounds of vehicle, equipment, food and water to get a small team of people to Mars.

Each person will require 900 pounds or more of dehydrated food. At a million dollars a pound, that's $100 billion right there. And chances are that a manned mission would cost more per pound than a robotic mission because of the significant safety margins needed for human passengers.

The third reason is the engineering challenges. To make a manned mission possible, it would be necessary to produce fuel for the return flight from the Martian atmosphere. However, nothing like this has ever been attempted, and it would take a number of test missions to prove out the concept.

Another big consideration is the cosmic radiation that astronauts would absorb during such a long mission, and how to block it. Much of this radiation is blocked on Earth by the Earth's magnetic field, but Mars has no magnetic field. That leaves robots as the safest and cheapest option.

One of the trickiest parts of the Mars exploration missions is actually getting the Rovers to Mars in working condition. When NASA sent the twin Viking landers to Mars in the 1970s, they had the three basic components of any interplanetary robot, they could produce the power they needed for their missions, they could gather information with their sensors, and they could send the sensor information back to Earth.

The one thing the Viking landers could not do is move, although they did have robotic arms that could reach out and scoop up soil. NASA first solved the movement problem with the Pathfinder mission in 1997. A tiny rover could leave the lander and travel up to 5 meters away from it to look at rocks.

The new Rover robots are the largest robots to ever successfully land on another planet. On this mission, NASA has designed the rovers to act as robotic geologists. The instruments and equipment packed into the rovers are designed to look at rocks.

The rovers can take color, 3-D images of the landscape with a pair of cameras mounted on the top.

Scientists can also choose a point on the landscape and the rover can drive over to it. Three pairs of black-and-white cameras on the front, back and top of the rover let the robot see its surroundings and navigate around obstacles. The rovers have six wheels, with a motor in each wheel, to move around.

The rovers can use a drill, mounted on a small arm, to bore into a rock. The rovers have a magnifying camera, mounted on the same arm as the drill, that scientists can use to carefully look at the structure of a rock. The rovers have a mass spectrometer that is able to determine the composition of minerals in rocks. This spectrometer is mounted on the arm, as well.

The rovers can send all of this data back to Earth using one of three different radio antennas. The cost of the two rovers was about $820 million total.

A computer helps with power management, image processing, motor control, and instrument management. It also handles navigation. The rover has six navigation cameras arranged in three pairs. The computer processes stereo images from the camera pairs.

Over the course of several months, the solar panels will get dusty and the seasons will change. Daily power production from the panels will drop and eventually, there will be so little power available that the rover will be unable to move, and then it will be unable to transmit. At that point, it will be "dead."