Methods of power.

Even though solar power is the method of choice, we had to settle for something else because the area we will explore is very dark. Nuclear power of any sort would be too heavy, and the fuel cells used by the shuttle would not be practical. Batteries would have to be used.

What type of battery would we use? We would use something that could last long enough, of course. It would need to last about as long as the MER robots will, 90 days. It would also take 100 W approx. to power the robot. Multiply by 24, for a day, and then multiply it by 90 to get 216,000 Watts. So we just put in a 216,000 watt battery. To avoid exposure to the elements, the battery is hidden in the main body of our robots.

Communications

The rover will talk to Mars Odyssey and Mars Global Surveyor, which are constantly orbiting the red planet. In the 16 minutes it takes each orbiter to go from horizon to horizon, there is an opportunity for the rover and orbiters to "converse" for about ten minutes.

The vast majority of science data will be returned to the mission team on Earth through the rovers' UHF antenna, which communicates with the orbiters. Mars Odyssey is expected to relay most of the information. Odyssey's sister spacecraft, Mars Global Surveyor is expected to return much less information. We expect to send information through the X-band link direct to Earth rarely.

The orbiters, with their more capable X-band communications systems, can transmit data back to Earth at a faster rate. This high performance is critical, as time on the large antennas here on Earth that were built to receive the data (the Deep Space Network) is limited given all of the spacecraft that rely on them.

Cruise Stage Communications

The cruise mode of the MER, expected to be like ours.

The cruise stage has two antennas that are used to communicate with the Earth. The low-gain antenna is omni-directional and is used when the spacecraft is near the Earth. Because it radiates in all directions, the low-gain antenna does not need to be pointed at the Earth to enable a communications link. The medium-gain antenna is a directional antenna that must point toward the Earth for communications, but has more power to communicate when the spacecraft is farther away from the Earth.

The medium-gain antenna acts like a floodlight and can direct the energy into a tighter beam to reach Earth. Just like a floodlight directs more light into a focused area than a normal light bulb does out of a lamp, the medium-gain antenna can direct the data from the spacecraft into a tighter beam than the low-gain antenna.