Countdown begins: Curiosity rover 100 days from Martian landing (images)

Launched in November 2011, NASA's Mars Science Laboratory, carrying the Curiosity rover, is just 100 days from touchdown on the red planet. Here are some illustrations to tide you over during the countdown.

The rover, NASA's largest ever, weighs more than a ton and is as big as a small car. It's on a mission to study the climate and geology of Mars, as well as prepare for future human missions. It's scheduled to roam the surface for 687 Earth days, or one Martian year.

Mars Science Laboratory will use new NASA innovations, particularly in its landing. It will descend with thrusters via a parachute. During the final seconds prior to landing, the rover will be lowered on a tether to the surface. Once on the surface, the rover's larger size and wheels give it the ability to overcome obstacles up to 75 centimeters high.

As NASA's Mars Science Laboratory reaches the Martian atmosphere, 81 miles above the red planet's surface, the spacecraft, which will have traveled for more than 8 months, will begin the entry, descent, and landing (EDL) phase. Thrusters firing during entry prepare the craft for its approach and landing at Mars' Gale crater landing zone.

Following the initial entry into the Martian atmosphere, the lower heat shield of the aeroshell protecting the Curiosity rover during its travels is jettisoned, and a parachute is deployed to slow the descent. The Curiosity rover is about five times larger than the Spirit or Opportunity Mars exploration rovers and carries more than 10 times the mass of scientific instruments, presenting NASA with new challenges.

To hold the weight of the craft, the Mars Science Laboratory mission will use the largest parachute ever built to fly on a planetary mission. The chute is about 10 percent larger in area than the one used for the Mars Exploration Rover mission, and that mission was 40 percent larger than Pathfinder's parachute. Designed to survive deployment at an astounding Mach 2.2 in the Martian atmosphere, the parachute, which has 80 suspension lines, is more than 165 feet long and opens to a diameter of almost 51 feet. In the Martian atmosphere, the descent will generate around 65,000 pounds of drag force during its descent.

After the parachute has significantly slowed the descent, thrusters will be used to decelerate and ease the craft to the surface. This is unlike earlier NASA rover missions, which used an airbag system to safely touch down on the surface. Weighing nearly one ton, the massive size of the Mars Science Laboratory rover prevents NASA from using the airbag system as a landing device.

Nearing touchdown, the spacecraft will control its own rate of descent with four of its eight throttle-controllable rocket engines. At this stage, the Curiosity rover, which has been tucked up inside the descent vehicle during travel, will unfold and be lowered via a tether. When it touches down, it will almost immediately be ready to roam.

Three nylon tethers and an umbilical cord providing a power and communication connection will then lower the rover the final 25 feet to the surface. When touchdown is detected, the bridle is cut at the rover end, and the descent stage flies off to stay clear of the landing site.

NASA says the Mars Science Laboratory represents a huge step in Mars surface science and exploration capability, demonstrating the ability to land a very large, heavy payload to the surface, more precise landing capability, and long-range mobility on the surface, all which are necessary for potential human habitation of the red planet.