Orbiters on future missions will examine the planet's surface even more precisely. They will also search for evidence of water's existence both now and in the past, on the surface and underground. Scientists are looking for signs of water since they believe it must be present to sustain life, even one-celled organisms they think may have lived on Mars at one time. Orbiters will also play a key role as communications relays for the assets such as rovers, landers and aerial vehicles on the surface or in the atmosphere of Mars. Orbiters may also play a vital role in bringing back samples from the surface of Mars.

Orbiter Missions

The Mars Global Surveyor mission has already achieved all of its science goals, and continues to provide a global characterization of the Martian topography, gravity, magnetic fields, thermal properties, surface composition, internal structure, atmospheric structure, and atmospheric dynamics.

In April, 2001, the 2001 Mars Odyssey orbiter will launch on a journey to Mars to map the terrain's makeup of minerals and elements and look for hydrogen, whose presence could be an indication that water was once present.

Mars Odyssey will also study the variations in the Martian surface and take pictures that will provide a map of the entire planet. Instead of getting close-up visible images of a very small part of Mars like those taken by the Mars Global Surveyor, which started orbiting the planet in March, 1998, the '01 orbiter will take visible and infrared images of lower resolution but cover the entire planet.

The Mars Express Orbiter, a European mission launching in 2003, will carry an instrument whose radar will penetrate many miles into the Martian surface to search for reflections from pools of subsurface water. This radar sounder instrument, called MARSIS (Mars Subsurface and Ionospheric Sounder), is a joint project of NASA and the Italian Space Agency (ASI).

The Mars Reconnaissance Orbiter, a NASA mission launching in 2005, will continue searching for water's signature by studying the planet's minerals. The '05 orbiter will also study the climate to see if water is released into the atmosphere during different seasons of the Martian year. This orbiter may be equipped with an ultra high-resolution camera capable of imaging surface features like large rocks that would be hazardous to landers and interfere with mobility of rovers.

Data Relays

Beyond 2005, orbiters will provide more powerful relay communications capabilities for vehicles on the Martian surface. It won't be practical for rovers, busy navigating hazardous terrain, to point their small antennas towards faraway Earth and transmit information. Instead, it will be easier for the rover to send data to a Mars orbiter, which in turn will precisely point a large antenna at Earth and communicate large volumes of images and other data.

Orbiters will also be used to bring samples back from Mars. Just as it is not efficient to communicate large volumes of science data directly from the Martian surface to Earth, it is not practical to launch a rocket containing samples from the Martian surface and expect it to navigate itself back to Earth and land safely. Instead, the preferred solution is to launch a small canister containing samples into orbit around Mars, have an orbiter rendezvous with and capture the canister and then eject the orbiter from Mars orbit back towards Earth.

Entering and Leaving Orbit

In addition to the technologies for orbital imaging, telecommunications and rendezvous and capture, the technologies for entering and leaving Mars orbit are vitally important for the development of low cost missions. Chemical propulsion has been used on every orbital mission since Mariner 9, the first Mars orbiter in 1971. In 1997, a new capability called aerobraking was used not to enter or leave orbit, but to gradually modify the shape of the orbit by skimming through the high atmosphere.

NASA and CNES, the French space exploration program, are currently collaborating on the deveopment of aerocapture, a technology that would send a spacecraft approacing Mars deep into the atmosphere where it would decelerate by several thousand miles per hour (mph) in less than ten minutes. It would then emerge into an orbit around Mars. The aeroshell needed to protect the spacecraft and the guidance system needed to place it accurately in the right orbit are the keys to this technology.

Chemical propulsion is a relatively mature technology although new types of lightweight components will make it possible to place larger payloads into Mars orbit or to remove larger payloads and send them back to Earth. Electric propulsion, on the other hand, is an emerging technology with the prospect of providing large advantages for missions which require a spacecraft to first orbit Mars, then leave Mars orbit on return to Earth and then possibly enter orbit around Earth. Electric propulsion was demonstrated for the first time on a Deep Space mission in 1999. Future advances in the technology will open up major new opportunities for Mars exploration.