Mro-launch qxp

functions: transmitting in ways that provide spacecraft-position information to naviga-

tors and automatically responding when they detect incoming signals. One of the two

transponders is a backup to the other. 

In addition to its subsystem for communication with Earth, Mars Reconnaissance

Orbiter carries a communication package called Electra for communicating with other

spacecraft near Mars or on Mars. Electra will use the ultra-high-frequency radio band.

Command and Data Handling Subsystem

The spacecraft will manage its data with a space flight computer and a solid-state

recorder. 

The computer features an X2000 Rad 750 microprocessor, a computer chip based on

a 133 megahertz PowerPC processor but enhanced to reliably endure the natural radi-

ation and other rigors of an outer-space environment. The Rad 750 provides process-

ing power of up to 48 million instructions per second for use by payload instruments as

well as the spacecraft subsystems. 

The computer will run flight software, which includes many applications for operating

the spacecraft, for protecting its health if problems arise and for recovering from prob-

lems. 

The solid-state recorder provides storage space for up to 160 gigabits of information. It

will be the primary location for holding data from the science instruments until it can be

transmitted to Earth. The computer also has available 128 megabytes of high-speed,

random-access memory.

Guidance, Navigation and Control Subsystem

To control which way it is pointing, the spacecraft will use several instruments to sense

its orientation and movement, and dual systems for changing its orientation, such as

for pointing of instruments.

Each of two star trackers (one of which is a backup) is a camera that takes pictures of

the sky and has computer power to compare the images with a catalog of star posi-

tions and recognize which part of the sky it is facing. This gives precise information

about the spacecraft's orientation, and the trackers can update the information several

times per second. Sixteen Sun sensors (eight of which are backups) are mounted at

various points around the spacecraft and report whether or not sunlight is hitting them.

If the spacecraft loses its bearings, these sensors enable it to locate the Sun so that it

can turn the solar arrays toward the Sun. 

An instrument called an inertial measurement unit, which also has a back-up duplicate,

includes accelerometers to measure changes in the spacecraft's velocity in any direc-

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tion and ring-laser gyroscopes to measure how fast the spacecraft's orientation is

changing. 

To change its orientation, the orbiter's primary tool is a set of three 10-kilogram (22-

pound) reaction wheels spinning at right angles to each other (plus a spare). Speeding

up the rotation of a wheel turns the spacecraft in the opposite direction from that

wheel's spin. It's the basic physics of every action having an opposite reaction.

Orientation can also be changed with the spacecraft's smallest thrusters. That method

can accomplish a turn faster, but it uses some of the limited supply of propellant and

the motion is not as smooth as a turn using reaction wheels. With repeated use, the

reaction wheels spin faster and faster. Occasionally they need to be slowed down.

While the wheels are being slowed, the thrusters are used to maintain the spacecraft's

orientation.

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Mars: The Water Trail

Forty years ago, as the world eagerly awaited results of the first spacecraft flyby of

Mars, everything we knew about the Red Planet was based on what sparse details

could be gleaned by peering at it from telescopes on Earth. Since the early 1900s,

popular culture had been enlivened by the notion of a habitable neighboring world

crisscrossed by canals and, possibly, inhabited by advanced lifeforms that might have

built them -- whether friendly or not. Astronomers were highly skeptical about the

canals, which looked more dubious the closer they looked. About the only hard infor-

mation they had on Mars was that they could see it had seasons with ice caps that

waxed and waned, along with seasonally changing surface markings. By breaking

down the light from Mars into colors, they learned that its atmosphere was thin and

dominated by an unbreathable gas, carbon dioxide.

The past four decades have completely revolutionized that view. First, hopes of a lush,

Earth-like world were deflated when Mariner 4's flyby on July 15, 1965, revealed large

impact craters, not unlike those on Earth's barren, lifeless Moon. Those holding out for

Martians were further discouraged when NASA's two Viking landers were sent to the

surface in 1976 equipped with a suite of chemistry experiments that turned up no con-

clusive sign of biological activity. Mars as we came to know it was cold, nearly airless

and bombarded by hostile radiation from both the Sun and from deep space.

But along the way since then, new possibilities of a more hospitable Martian past have

emerged. Mars is a much more complex body than Earth's Moon. Scientists scrutiniz-

ing pictures from the Viking orbiters have detected potential signs of an ancient coast-

line that may have marked the edges of a long-lost sea. The three orbiters and two

rovers active at Mars in 2005 have each advanced the story. The accumulated evi-

dence shows that the surface of Mars appears to be shaped by flowing water in hun-

dreds of places; that some Mars rocks formed in water; and that, still today, significant

amounts of water as ice or hydrated minerals make up a fraction of the top surface

layer of Mars from high latitudes to some mid-latitude regions. 

Although it appears unlikely that complex organisms similar to Earth's could have exist-

ed on Mars' comparatively hostile surface, scientists are intrigued by the possibility that

life in some form, perhaps very simple microbes, may have gained a foothold in

ancient times when Mars may have been warmer and wetter. It is not unthinkable that

life in some form could persist today in underground springs warmed by heat vents

around smoldering volcanoes, or even beneath the thick ice caps. To investigate those

possibilities, the most promising strategy is to learn more about the history of water on

Mars: How much was there? How long did it last? Where were the formerly wet envi-

ronments that make the best destinations for seeking evidence of past life? Where

might there be wet environments capable of sustaining life today? 

The consensus strategy for answering those questions uses a balance of examining

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