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The Mars Exploration Program (MEP) is a long-term effort to explore the planet Mars, funded and led by the U.S. space agency, National Aeronautics and Space Administration (NASA). Formed in 1993, MEP has made use of orbital spacecraft, landers, and rovers to explore the possibilities of life on Mars, as well as the the planet's climate and natural resources. The program is managed by NASA's Science Mission Directorate by Doug McCuistion of the Planetary Science Division. As a result of 40% cuts to NASA's budget for fiscal year 2013, the Mars Program Planning Group (MPPG) was formed to help reformulate the MEP, bringing together leaders of NASA's technology, science, human operations, and science missions.

Background
While the majority of efforts to explore Mars have taken place in the late 20th century, it was observed as early as the 16th century by the Babylonians, Egyptians, Greeks, and Romans. It was not until the invention of the telescope in the 17th century that Mars was studied in depth. The first attempt at sending a probe to the surface of Mars, nicknamed "Marsnik 1," was by the USSR in 1960. The probe failed to reach Earth orbit, and the mission was ultimately unsuccessful. Failure to complete mission objectives has been common in missions designed to explore Mars; roughly two-thirds of all spacecraft destined for mars have failed before any observation could begin.

The Mars Exploration Program itself was formed officially in the wake of the failed Mars Observer in September 1992, which had been NASA's first Mars mission since the Viking 1 and Viking 2 projects in 1975. The spacecraft, which was based off of a modified Earth-orbiting commercial satellite, carried a payload of instruments designed to study the geology, geophysics, and climate of mars from orbit. The mission ended in August 1993 when communications were lost three days before the spacecraft had been scheduled to enter orbit.

Goals/strategy
According to NASA, there are four broad goals of the MEP, all having to do with understanding the potential for life on Mars.

Goal 1: Determine if life ever arose on Mars
In order to understand Mars' potential for life, it must be determined whether or not there ever was life on Mars, which begins with assessing the planet's suitability for life. The main strategy regarding the MEP, nicknamed "Follow the Water," is the general idea that where life is present, there is water (at least in instances on Earth). It is likely that if life ever did arise on Mars, there would need to be a supply of water that was present for a substantial amount of time. Therefore, a prominent goal of the MEP is to look for places where water is, was, or could possibly be, such as dried up riverbeds, under the planetary surface, and in Mars' polar ice caps.

Aside from water, life also needs sources of energy to survive. The abundance of superoxides makes life on the surface of Mars very unlikely, which essentially rules out sunlight as a possible source of energy for life. Therefore, alternative sources of energy must be searched for, such as geothermal and chemical energy. These sources, which are both used by life forms on Earth, could be used by microscopic life forms living under the Mars' surface.

Life on Mars can also be searched for by finding signatures of past and present life. Relative carbon abundance and the location and forms that it can be found in can inform where and how life may have developed. Also, the presence of carbonate minerals, along with the fact that Mars' atmosphere is made up largely of carbon dioxide, would tell scientists that water may have been on the planet long enough to foster the development of life.

Goal 2: Characterize the climate of Mars
Another goal of the MEP is to characterize Mars' climate, with regards to its current and past climate, as well as factors that influence climate change on Mars. Currently what is known is that the climate is regulated by seasonal changes of Mars' ice caps, movement of dust by the atmosphere, and the exchange of water vapor between the surface and the atmosphere. To understand these climatic phenomena means helping scientists more effectively model Mars' past climate, which brings a higher degree of understanding of the dynamics of Mars to NASA scientists.

Goal 3: Characterize the geology of Mars
The geology of Mars is differentiable from that of Earth by, among other things, its extremely large volcanoes and lack of crust movement. A goal of the MEP is to understand these differences from Earth along with the way that wind, water, volcanoes, tectonics, cratering and other processes have shaped the surface of Mars. Rocks can help scientists describe the sequence of events in Mars' history, tell whether there was an abundance of water on the planet through identifying minerals that are formed only in water, and tell if Mars once had a magnetic field (which would point toward Mars at one point being a dynamic Earth-like planet).

Goal 4: Prepare for the human exploration of Mars
The human exploration of Mars presents a massive engineering challenge. With Mars' surface containing superoxides and lacking an ozone layer to protect from ultraviolet radiation from the sun, scientists would have to thoroughly understand as much of Mars' dynamics as possible before any action can be taken toward the goal of putting men on Mars.

Challenges
Mars exploration missions have historically had some of the highest fail rates for NASA missions, which can be attributed to the immense engineering challenges of these missions as well as some bad luck. With many of the goals of the MEP involving entry, descent, and landing of spacecraft (EDL) on the surface of Mars, factors like the planet's atmosphere, uneven surface terrain, and high cost of replicating Mars-like environments for testing come into play.

Atmospheric challenges
Compared to the earth, the atmosphere of Mars is about 100 times thinner. As a result, if a landing craft were to descend into Mars' atmosphere, it would decelerate at a much lower altitude, and depending on the object's mass, may not have enough time to reach their terminal velocity. In order to deploy super- or subsonic decelerators, velocity must be below a threshold or they won't be effective. Therefore, technologies must be developed so that a landing craft can be decelerated enough to allow adequate time for other necessary landing processes to be carried out before landing.

Mars' atmosphere varies significantly over the course of a Mars year, which prevents engineers from being able to develop a system for EDL common among all missions. Frequently-occurring dust storms increase lower atmospheric temperature and lessen atmospheric density, which, coupled with the extremely variable elevations on Mars' surface, forces a conservative selection of a landing site in order to allow for sufficient craft deceleration.

Topographical challenges
The surface of Mars is extremely uneven, containing rocks, mountainous terrain, and craters. For a landing craft, the ideal landing area would be flat and debris-free. Since this terrain is almost impossible to find on Mars, landing gear must be incredibly stable and have enough ground clearance to prevent problems with tipping over and instability upon landing. In addition, the deceleration systems of these landers would need to include thrusters that are pointed at the ground. These thrusters must be designed so that they only need to be active for an extremely short amount of time; if they are active and pointed at rocky ground for more than a few milliseconds, they start to dig trenches, launch small rocks up into the landing gear, and cause destabilizing backpressure to be forced upon the lander.

Finding an adequate landing site means being able to estimate rock size from orbit. The technology to accurately determine rock size under 0.5 meters in diameter from orbit has not yet been developed, so instead rock size distribution is inferred from its relationship to thermal inertia, based on thermal response of the landing site measured by satellites currently orbiting Mars. The Mars Reconnaissance Orbiter also helps this cause in the sense that its cameras can see rocks larger than 0.5 m in diameter.

Along with the possibility of the lander tipping over on sloped surfaces, large topographical features like hills, mesas, craters and trenches pose the problem of interference with ground sensors. Radar and Doppler radar can falsely measure altitude during descent and the algorithms that target the touchdown point of the lander can be "tricked" into releasing the lander too early if the craft passes over mesas or trenches while descending.

Mars-like environment replication costs
With Mars EDL sequences only lasting about 5-8 minutes, the associated systems must be unquestionably reliable. Ideally, this would be verified by data obtained by carrying out large-scale tests of various components of the EDL systems on Earth-based testing. However, the costs of reproducing environments in which this data would be relevant in terms of Mars' environment are considerably high, resulting in testing being purely ground based or simulating results of tests involving technologies derived from past missions.

Program costs
Mars exploration missions, as do most NASA missions, can be fairly costly. For example, NASA's Curiosity Rover (landed on Mars in Aug 2012) has a budget exceeding $2.5 billion. NASA also has goals of collaborating with the European Space Agency (ESA) in order to conduct a mission involving returning a sample of Mars soil to Earth, which would likely cost at least $5 billion and take ten years to complete.

MEP budget cuts
In February 2012, NASA was faced with severe budget cuts to many of its programs, with a $300 million cut to the Planetary Science division for fiscal year 2013. In response to these cuts, the House Appropriations Committee’s Commerce-Justice-Science subcommittee approved a budget two months later that reinstated $150 million to the Planetary Science budget. The reinstatement had one stipulation: the money must be used toward a mission that is part of the Mars Sample Return program.

Future plans
As a result of the smaller budget for the MEP, NASA was forced to cancel plans for a Mars orbiter in 2016 to study the climate of the planet. NASA does not have any missions for the future of the MEP, as a plan is still being developed that will encompass the next few decades.