Human exploration of Mars started in the mid-1960s, in the first decades of the “space race.” While Russia concentrated its efforts on exploring Venus, the United States sent a series of spacecraft to our other neighbor, Mars. The pictures returned by the first few Mariner missions showed a barren, cratered surface like the Earth’s Moon. Unfortunately, they missed all the amazing geological features on Mars! It wasn’t until Mariner 9 arrived in 1971 that scientists saw the huge volcanoes, canyons and dry riverbeds that make Mars truly fascinating. Mariner 9 was the first craft to orbit the red planet, and it managed to photograph the entire surface.

Viking 1 and 2 landed on different sites on Mars, where they studied the composition of surface rocks. They also performed experiments on the soil to answer the paramount Martian question - whether life is present. The biology experiments did not detect life, although some people argue that they were inconclusive. Viking actually showed that Mars is even dryer and more hostile than previously thought - in fact, the radiation at the surface is intense enough to kill any form of life, at least as we know it.

For several decades, there was no further exploration of Mars. Then in 1992, Mars Observer launched. However, disaster struck, and we lost contact with the spacecraft before it could go into orbit. This was the first of several serious setbacks Mars exploration has suffered in recent years. Mars Climate Observer was a weather satellite intended to study the atmosphere and climate. Contact was lost when it arrived at Mars in 1999, probably because it entered the atmosphere too fast and incinerated. The Mars Polar Lander was supposed to examine the frigid polar soil for evidence of water ice, but it too was lost on arrival.
 
On the plus side, the Pathfinder lander/rover was a huge success, and the orbiting spacecraft Mars Global Surveyor has produced enough high quality images to keep scientists busy for years - more data than all other Mars missions combined! These missions have returned evidence for a warmer, wetter ancient Mars, and even liquid water on the surface of Mars in the recent past. The MGS mission also included a laser altimeter, which measured the elevation of the entire planet, and instruments that examined the surface composition. Another orbiter in 2001, Mars Odyssey, looked for evidence of water, returned data on the radiation environment, and studied the composition of minerals in the rocks and soil.
 
The European Space Agency joined the effort to explore Mars with its Mars Express mission, which arrived late in 2003. Mars Express carried a tiny lander called Beagle 2, which is designed specifically to search for signs of past water and life. Japan’s mission to Mars, Nozomi, failed due to problems with the propulsion system. The latest missions to reach Mars are the Exploration Rovers, Spirit and Opportunity. These two robotic rovers have the capacity to travel much farther than Pathfinder. They are studying the rocks and soil and looking for evidence of water. Spirit is no longer in operation, but Opportunity continues to travel onward to study more of Mars.

Today, we have in some ways a more detailed picture of Mars than we have of Earth. From high resolution images from the Mars Reconnaissance Orbiter to soil samples tested by Mars Phoenix, we have explored Mars across many wavelengths and have sampled soil and rock. In November 2011, a new mobile science laboratory about the size of a small SUV called Curiosity was launched from Earth. This rover will arrive at Mars in August 2012 and will send back data on whether Mars could have ever supported life, the climate and geology of Mars, and help us to plan for a human mission to Mars. 

Will astronauts ever set foot on Mars? A manned mission to Mars would be very exciting, but also very dangerous and very expensive. Keeping astronauts healthy for the estimated two years of a Mars mission is challenging, and providing for their safe return in the case of a mishap is even more daunting. The lowest plausible estimate of the price tag is about $50 billion. Robotic probes are far less expensive. However, the possibility of extracting water from under the Martian surface raises some interesting possibilities. It means that human explorers could have supplies of drinking water waiting for them. Water could also be a source of oxygen to breathe and hydrogen to use as fuel for the return journey.

Author: Chris Impey
Editor/Contributor: Ingrid Daubar-Spitale
Editor/Contributor: Pamela Gay
Taken from: http://www.teachastronomy.com/astropedia/article.aspx?qid=242
最后修改: 2021年08月30日 星期一 10:31