Mro-launch qxp

Spacecraft

The Mars Reconnaissance Orbiter spacecraft consists of the payload and the systems

that enable the payload to do its job and send home the results. 

The spacecraft has been assembled around a main structure made of strong, light-

weight materials including titanium, carbon composites and aluminum honeycomb. 

Propulsion Subsystem

The spacecraft's propulsion system will perform maneuvers such as trajectory correc-

tions during the cruise to Mars, braking to insert the spacecraft into orbit the day it

arrives, and orbit adjustments during aerobraking.

The orbiter carries a total of 20 small rocket engines, or thrusters, of three sizes. The

six largest ones each produce about 170 newtons (38 pounds) of thrust when they are

in use. The main function for these six is to slow the spacecraft's velocity as it first flies

close to Mars so that it will be captured into orbit by Mars' gravity. All six will fire at the

same time for this mission-critical event, called Mars orbit insertion. By using relatively

small engines rather than a single larger one, the mission is less susceptible to an

orbit-insertion failure due to an engine malfunction. The six 170-newton thrusters will

also be used for the spacecraft's first trajectory correction maneuver, scheduled for 15

days after launch.

Six intermediate thrusters can each produce 22 newtons (5 pounds) of thrust. They will

be used for other trajectory correction maneuvers during the trip to Mars and for the

maneuvers to adjust the altitude of aerobraking dips into the upper atmosphere. The

smallest eight thrusters each produce 0.9 newtons (0.2 pounds) of thrust. They can

control the orientation of the spacecraft as an alternate system to attitude-control reac-

tion wheels or when the momentum built up in the wheels needs to be reduced. They

will also be used to control rolling of the spacecraft during orbit insertion and trajectory

correction maneuvers.

All of the thrusters consume hydrazine, a propellant that does not require an oxygen

source. Hydrazine is a corrosive liquid compound of nitrogen and hydrogen that

decomposes explosively into expanding gases when exposed to a catalyst in the

thrusters. The spacecraft's propellant tank can hold the 1,220 kilograms (2,690

pounds) of hydrazine fuel, about 70 percent of which will be used for orbit insertion. To

push the hydrazine to the thrusters, the spacecraft uses pressurized helium stored in a

separate tank. 

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Power Subsystem 

The spacecraft's electrical power comes from two solar arrays, with a pair of nickel-

hydrogen batteries to fill in when the spacecraft is in the shadow of a planet or the

arrays are not facing the Sun. 

Each array is 5.35 meters (17.56 feet) long by 2.53 meters (8.30 feet) wide. On the

front side, 9.5 square meters (102 square feet) of the surface is covered with 3,744

individual photovoltaic cells. The cells can convert about 26 percent of the solar energy

that hits them into electricity. At Earth, the two panels combined can generate about

6,000 watts. But at Mars, sunshine is weaker and the output will be 2,000 watts. The

arrays are mounted on opposite sides of the orbiter, with a gimbaled connection that

allows them to be turned at any angle to face the Sun. 

Communications Subsystem

To communicate with Earth, the spacecraft has three antennas, three amplifiers and

two transponders. It has the capacity to transmit data at up to 6 megabits per second,

but in practical use transmitting from Mars to Earth it's peak rate is expected to be

about 3.5 megabits per second, about 10 times higher than any previous Mars mission

has used. 

The main antenna is the biggest thing on the spacecraft other than the solar panels. It

is a parabolic dish antenna 3 meters (10 feet) in diameter, mounted with a gimbal that

allows it to be pointed without needing to change the orientation of the whole space-

craft. This is the high-gain antenna, meaning it is highly effective in pulling in or send-

ing out a signal. It must be pointed at Earth to be effective. The spacecraft's other two

antennas are smaller, low-gain antennas, which cannot communicate at such high data

rates, but do not need to be pointed at Earth to work. They serve as backups for emer-

gencies and special events, such as launch and orbit insertion. The low-gain antennas

are mounted on the high-gain antenna, one facing forward and the other backward so

that communication with Earth will be possible regardless of the spacecraft's orienta-

tion.

Two of the three amplifiers boost signals in the X-band of radio frequencies, planned

as the primary channel for the mission. One of these is a backup in case the first fails.

Each can send signals at 100 watts of power. The third amplifier supports the mission's

demonstration of using the shorter-wavelength Ka band as an alternate channel. It can

send signals at 35 watts. 

The orbiter's transponders translate between digital data (the ones and zeros used by

computers) and radio signals, modulating the outgoing signals to put data into them

and demodulating signals received from the ground. They also have two specialized

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High-gain__antenna__Solar_panels__Shallow'>Mars Color Imager

Context

Camera

High-gain

antenna

Solar panels

Shallow 

Subsurface

Radar

Compact Reconnaissance 

Imaging Spectrometer for Mars

Mars Climate Sounder

Electra

High-

Resolution

Imaging Science

Experiment

Optical

Navigation

Camera

Orbit

insertion

thrusters

Thrusters

Low-gain

antennas

Spacecraft

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