A history of the Mars rovers
Perhaps they are fascinating because of the striking images they send back to earth: alien landscapes like the red deserts of Mars or the stark crater-pocked surface of the Moon. Perhaps it's their resemblance to the simple toy trucks of childhood. Or maybe it's because these robotic explorers are our closest connection to a planet outside our own.
There is also the astonishing success and longevity of the two rovers still on the planet's surface, Spirit and Opportunity. The twin rovers have acted as explorers, photographers and geologists, providing new insights into the composition and history of one of our nearest planetary neighbours, though of late the two rovers have lost some of their functionality.
Spirit landed on Mars on Jan. 3, 2004, in Gusev Crater, a possible former lake in a giant impact crater. Opportunity landed 21 days later at Meridiani Planum, where mineral deposits suggest Mars once held water.
To perform their tasks, the rovers used a mast-mounted panoramic camera to determine the mineralogy, texture and structure of the local terrain and a robotic arm containing a microscopic camera in its fist for taking close-up images. The arms were also capable of holding a number of scientific instruments, including a rock abrasion tool for chipping away at the surface, magnets for collecting magnetic dust particles and three different spectrometres to examine the rocks, soils and dust particles.
Spirit and Opportunity limping, but still kicking
Designed to run for 90 days, the two rovers are still operational after more than 1,900 days of service, but they've faced roadblocks as they continue to be exposed to the Martian environment.
Severe dust storms have limited their access to solar power and forced the rovers to take longer breaks between manoeuvres, and the rovers have suffered physical setbacks as they teeter towards retirement.
In 2006, Spirit lost function of its front right wheel. In 2007, Opportunity began to have trouble with its robotic arm. At the end of January 2009, Spirit encountered a bout of "amnesia" when it failed to follow through on an order from the command centre at the Jet Propulsion Laboratory in Pasadena, Calif.
Later in May, Spirit had problems driving because its wheels were stuck in soft soil. However, its camera and instruments are still working, and so NASA has continued to fund the rovers at a cost of about $20 million US annually.
Mars also had another visitor from Earth in recent years with the arrival in 2008 of the Phoenix lander, which looked at the Martian soil and analyzed the atmosphere of the planet. Phoenix, which wrapped up its mission in November 2008, was also scouting for future rover missions.
The next rover to arrive on Mars is scheduled to be NASA's Mars Science Laboratory, which was originally slated to launch in 2009 before testing and hardware problems delayed its liftoff until 2011 at the earliest, according to NASA.
The European Space Agency's (ESA) ExoMars mission has also faced delays, and its launch has been pushed back from 2011 to 2016, and some aspects of its mission curtailed.
But NASA's plans to build a permanent moon base in 2040 is seen as merely the first step toward putting humans on Mars, and before that begins, more rovers will be needed to further study the planet.
Here's a look at some key dates in extraterrestrial rover history:
2016: Expected year of ESA's ExoMars mission.
2011: Mars Science Laboratory rover scheduled to launch. It was originally supposed to launch in 2009.
Nov. 11, 2008: Phoenix lander runs out of power could not sustain communication, thus ending the mission. NASA continues to listen for a response until December with no luck.
May 2008: Phoenix lander exceeds its 90-day mission, extending its mission on Mars for over a year.
Aug. 4, 2007: NASA's Phoenix polar lander launched.
October 2006: Canadian robotics manufacturer MDA unveils a prototype Mars rover, featuring six metal wheels with "teeth" designed to dig into alien soil, an onboard video camera and incline-sensors for greater manoeuvrability. The federal government later declines to fund the project, and two years later MDA attempts to sell its space business to an American weapons manufacturer. Industry Canada blocks the sale, arguing the loss of the space business would not be of "net benefit" to Canada.
March 2004: The U.S. Defence Advanced Research Projects Agency runs its first Grand Challenge Race, in which robot vehicles must traverse a difficult desert route. None of the 15 finalists finished. The next year five teams finish the 212-kilometre course.
January 2004: Mars rovers Spirit and Opportunity, both designed to travel about 100 metres each Martian day, land on the Red Planet.
July 1988: Two spacecraft — Phobos 1 and 2 — are launched with plans of landing on one of Mars's moons. Both spacecraft were lost before completing their missions.
July 4, 1997: The Mars Pathfinder Sojourner rover becomes the first vehicle to traverse Mars. The rover, a six-wheeled remote controlled vehicle, was capable of operating on its own using a hazard avoidance system. It lasts three times longer than expected and photographed 550 images, along with measuring atmospheric pressure, temperature and wind and collecting rock and soil samples. The mission ended on March 10, 1998.
September 3, 1976: Viking 1 lander touches down on Mars. The Viking lander was not a rover, but did take the first soil samples and photographs from the planet's surface.
April 1972: Apollo 16 astronauts conduct performance tests on their lunar rover, getting a top speed of 18 kilometres per hour.
July 31, 1971: Astronauts from NASA's Apollo 15 mission explore the area using the first manned lunar rover, allowing them to cover a greater area.
Nov 17, 1970: The Russian unmanned rover Lunokhod 1 touches down on the moon, becoming the first remote-controlled robot to land on another world. Both it and a second Lunokhod rover (that landed in 1973) explored the surface and primarily took photographs.