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the relay support and would still be NASA's most capable asset for evaluating potential

landing sites for future missions even though imaging resolution would be reduced by

30 percent at the higher altitude. Assuming its atmospheric instruments were still oper-

ating, the Mars Reconnaissance Orbiter could also provide near-real-time support for

future landers, just as Mars Global Surveyor did for the Mars Exploration Rovers.

Communications 

From launch day until the end of its operations, Mars Reconnaissance Orbiter will rely

on NASA's Deep Space Network of antenna stations on Earth. The radio links between

the spacecraft and the Deep Space Network antennas will provide tracking capability

during the trip to Mars; uplinked commands to the spacecraft; and downlink of all the

engineering, science-instrument and relayed data from the orbiter.

The Deep Space Network transmits and receives radio signals through large dish

antennas at three sites spaced approximately one-third of the way around the world

from each other. This configuration ensures that spacecraft remain in view of one

antenna complex or another as Earth rotates. The antenna complexes are at

Goldstone in California's Mojave Desert; near Madrid, Spain; and near Canberra,

Australia. Each complex is equipped with one antenna 70 meters (230 feet) in diame-

ter, at least two antennas 34 meters (112 feet) in diameter, and smaller antennas. All

three complexes communicate directly with the control hub at NASA's Jet Propulsion

Laboratory, Pasadena, Calif. The network provided routine communications for 30 fly-

ing spacecraft during the second quarter of 2005.

The orbiter is expected to return 34 terabits of data during its primary science phase,

more than five times all previous Mars missions combined. Most of that data will come

through two sessions per day, each about eight hours long, using 34-meter antennas.

Plans call for using 70-meter antennas for supplemental coverage during the portions

of the primary science when Mars is farthest from Earth. 

Mars and Earth will be about 384 million kilometers (2.39 million miles) apart -- nearly

their maximum distance from each other -- right at the beginning of the orbiter's prima-

ry science phase in November 2006. At that distance, radio signals traveling at the

speed of light take about 21 minutes to make the one-way trip, and the orbiter is

expected to send data to a 34-meter antenna on Earth at a rate of about 600 kilobits

per second (about three CD-ROMs worth of data per hour). The minimum Earth-Mars

distance during the orbiter's prime mission will be in December 2007, when the two

planets will come within 88 million kilometers (55 million miles) of each other. Radio

signals travel that distance in about four minutes, and Mars Reconnaissance Orbiter is

expected to transmit to a 34-meter antenna that far away at a rate of about 2.6

megabits per second (about 14 CD-ROMs worth of data per hour). The spacecraft's

maximum data-transmission rate, with a 70-meter antenna on the receiving end, is

expected to be about 3.5 megabits per second, fast enough to fill a CD-ROM in about

three minutes.

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The mission will use X-band radio frequencies as its principal communication channel

with Earth. Counting the capacity from supplemental use of 70-meter antennas as well

as two daily sessions with 34-meter ones, the quantity of data received per day via X

band is expected to vary from about 40 to 90 gigabits (about 60 to 140 CD-ROMs

worth). The mission will also demonstrate the use of Ka band radio frequencies. After

that demonstration's goals are met, Ka band transmissions might add significantly to

the total amount of science data sent to Earth by the orbiter. The Ka band equipment

uses less power than its X band counterpart to send the same amount of data.

However, Ka band transmissions are more susceptible to being disrupted by water in

Earth's atmosphere

Planetary Protection Requirements

In the study of whether Mars has had environments conducive to life, precautions are

taken against introducing microbes from Earth. The United States is a signatory to an

international treaty that stipulates that exploration must be conducted in a manner that

avoids harmful contamination of celestial bodies. In compliance with that treaty, the

flight hardware of the Mars Reconnaissance Orbiter has been designed to meet

NASA's Planetary Protection Policy.

For a planetary orbiter mission such as this, the current policy offers two alternative

strategies: by assuring a long-term orbital lifetime (more than 50 years) or by flying a

very clean spacecraft (measured by a microbiological burden of no more than 50,000

microbial spores). The low-altitude orbit needed for the mission to achieve its science

goals makes it unfeasible to depend on impact avoidance, since a low-altitude space-

craft failure could result in an impact of Mars considerably before 50 years passes.

Consequently, the project has demonstrated compliance with the total microbiological

burden requirement. 

In the event of a premature entry into the martian atmosphere, portions of the space-

craft that will be heated to 500 degrees Celsius (932 degrees Fahrenheit) or hotter for

at least half a second during friction with Mars' atmosphere will be sterilized by that

process. For other parts of the spacecraft, a combination of dry-heat microbial reduc-

tion, manufacturing controls and cleaning are keeping the number of spores within per-

missible levels.

Another type of precaution is to be sure any hardware that doesn't meet these stan-

dards does not go to Mars accidentally. When the Atlas V's Centaur upper stage sepa-

rates from the spacecraft, the two objects are traveling on nearly identical trajectories.

To prevent the possibility of the Centaur hitting Mars, that shared course is deliberately

set so that the spacecraft would miss Mars if not for its first trajectory correction

maneuver, about 15 days later.

The NASA planetary protection officer is responsible for the establishment and

enforcement of the agency's planetary protection regulations.

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