mars proc

Process: Processes can berepetitive events occurring innature or in human society. Youcan treat future events asprocesses, such as the plan for theMars Polar Lander. Notice the verycareful phase-by-phase discussionof the mission plan from launch tofirst signal.

In-sentence list: The overview ispresented in a four item in-sentence list in the introduction.Notice that both opening andclosing parentheses are used. (Formore on lists, see Chapter 8.)

By the time you read this, we mayknow what happened to the MarsPolar Lander. It may even beworking! Contact with it was lostDecember 3, 1999 as it wasentering the Martian atmosphere.

Organization by steps and phases.Notice that the writer has dividedthe mission into phases,beginning with the launch andending with the first signal. Eachof these phases is systematicallydiscussed in its own separatesection, under its own heading.

56 PART I Project Tools for Technical Writers

G / HCP/McMurrey / Power Tools for Technical Writing Projects 2363 Ch02 p. 56 12/2000

Mars Polar Lander: Mission Overview1

The Mars Polar Lander will settle onto the surface of the red planet,

much as the Mars Pathfinder did in 1997. But instead of inflating airbags

to bounce on the surface as it lands, the Mars Polar Lander will use

retro-rockets to slow its descent, like the Viking landers of the 1970s.

The following summarizes activities planned for the (1) launch, (2) entry,

descent and landing, (3) post-landing, and (4) first signal. This summary

will conclude with a brief overview of activities that are planned for the

Mars Polar Lander.

Launch

The Mars Polar Lander is scheduled to launch January 3, 1999, at about

3:20 P.M. Eastern Standard Time on a Delta II rocket from Space Launch

Complex 17B at Cape Canaveral Air Station, FL. The Delta II is a model

7425 with two liquid-fuel stages augmented by four strap-on solid-fuel

boosters, and a third-stage Thiokol Star 48B solid-fuel booster. At the

time of launch, the lander will be encased within an aeroshell attached to

a round platform called the cruise stage. Because the landers solar

panels are folded up within the aeroshell, a second set of solar panels is

located on the cruise stage to power the spacecraft during its inter-

planetary cruise. Shortly after launch, these hinged solar panels will

unfold, and the spacecraft will fire its thrusters to orient the solar panels

toward the sun. Fifty-eight minutes after launch, the 112-foot-diameter

(34-meter) antenna at the Deep Space Network complex in Canberra,

Australia, should acquire the Polar Landers signal.

Entry, Descent, and Landing

By the time it reaches Mars on December 3, 1999, the Polar Lander will

have spent 11 months in cruise. Throughout the cruise, the spacecraft

will be communicating with Earth using its X-band transmitter and the

medium-gain horn antenna on the cruise stage.

Preparations for the landers entry into the Martian atmosphere will

begin 14 hours in advance, when the final tracking coverage of the cruise

1Source: Adapted from NASAs Mars Polar Lander/Deep Space 2: Press Kit December 1999 withpermission. Original document at: www.jpl.nasa.gov/marsnews/mplds2hq.pdf

Headings: In this relatively shortdocument, second-level headingsare used to mark off the mainphases of the Mars Polar Landermission. (For more on headings,see Chapter 7.)

Audience: To promote the spaceprogram, NASA makes lots ofinformation available to thepublic. Even though the audienceis the taxpaying public, plenty ofdetails here need some extraexplaining for that audience. Forexample, whats the deal aboutthe aeroshell? What is an X-bandtransmitter or a medium-gainhorn antenna?

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Future tense: Future tense isoften abusedthat is, usedunnecessarilyin technical prose.However, in this context, there isno other choice. When thisdocument was written, theseevents were still in the future.

Illustration: This illustration was inthe original PDF file made avail-able on the Web by NASA. To getit into another document, justtake a screen capture of the pageon which it occurs and then cropit to the desired size. (For more onscreen captures, cropping, andgraphics in general, see Chapter 11.)

Cross-references to illustrations:Notice that throughout this docu-ment direct cross-reference aremade to figures, even if the figureoccurs on the same page. This isstandard good practice: draw thereaders attention to illustrations,tables, and charts and give them aclue as to what they contain andhow are they are related.

Transitions: Notice how manywords and phrases throughoutthis document alert us to where weare in this process: 14 hours inadvance, when, About twominutes before landing, Startingat about, before entry,then, and so on. This documentvery carefully guides us throughthe events, alerting us to theirinterrelationships, sequencing,and timing. (For more ontransitions, see Chapter 20.)

CHAPTER 2 Processes: Instructions, Policies, and Procedures 57


period begins. This is the final opportunity for ground controllers to

gather navigation data before entry. About 18 hours before entry,

software that normally puts the spacecraft in safe mode in reaction to

unexpected events will be disabled for the remainder of the spacecrafts

flight and its descent to the surface. Traveling at about 15,400 miles per

hour (6.9 kilometers per second), the spacecraft will enter the upper

fringe of Mars atmosphere, as shown in Figure 1. Onboard

accelerometers, sensitive enough to detect G forces as little as 3/100ths

of Earths gravity, will sense when friction from the atmosphere causes

the lander to slow slightly. At this point, the lander will begin using its

thrusters to keep the entry capsule aligned with its direction of travel.

The spacecrafts descent from the time it hits the upper atmosphere until

it lands will take about 5 minutes and 30 seconds to accomplish. As it

descends, the spacecraft will experience G forces up to 12 times Earths

gravity, while the temperature of its heat shield will rise to 3000 F

(1650 C).

About two minutes before landing, the landers parachute will be fired

from a mortar (or small cannon) when the spacecraft is moving at about

960 miles per hour (430 meters per second) some 4.5 miles (7.3 kilo-

meters) above the surface. Ten seconds after the parachute opens, the

Figure 1. Mars Polar Lander Entry, descent and landing phases.

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Dual measurements: Many tech-nical documents, like this one,are written for internationalaudiences. That means offeringboth British or American versionsas well as international metricversions of the measurements.

Passive voice: Notice how muchpassive voice this document uses.In this context, its the mostefficient and effective way towrite. For example, descentengines will be turned off is apassive-voice sentence. Littlewould be gained by rephrasingthis sentence to read NASAground crew will turn descentengines off. In every instance ofthe passive voice in this docu-ment, we know full well who theagent of these activities ispeopleback on planet Earth.



Mars Descent Imager will power on and the spacecrafts heat shield will

jettison. The first descent image will be taken 0.3 seconds before heat-

shield separation. The imager will take a total of about 30 pictures

during the spacecrafts descent to the surface. About 70 to 100 seconds

before landing, the lander legs will deploy; 1.5 seconds after that, the

landing radar will activate. The radar will be able to gauge the space-

crafts altitude about 44 seconds after it is turned on, at an altitude of

about 1.5 miles (2.5 kilometers) above the surface.

Shortly after radar ground acquisition, when the spacecraft is traveling at

about 170 miles per hour (75 meters per second) some 4,600 feet (1.4

kilometers) above the surface, the thrusters that the spacecraft has used

for maneuvers throughout its cruise will be turned off, and the backshell

will separate from the lander. The descent engines will turn on one-half

second later, turning the lander so that its flight path gradually becomes

vertical. The pulse-modulated descent engines will maintain the space-

crafts orientation as it descends. The engines will fire to roll the lander

to its proper orientation so that it lands with the solar panels in the best

orientation to generate power as the Sun moves across the sky. The radar

will turn off at an altitude of about 130 feet (40 meters) above the

surface, and the spacecraft will continue using its gyros and

accelerometers for inertial guidance as it lands.

Once the spacecraft reaches either an altitude of 40 feet (12 meters) or a

velocity of 5.4 miles per hour (2.4 meters per second), the lander will

drop straight down at a constant speed. The descent engines will turn off

when touchdown is detected by sensors in the footpads. The engines will

have been on for a total of about 40 seconds during final descent to the

surface.

Post-Landing

The lander is expected to touch down at 12:01 P.M. Pacific Standard

Time at the Mars landing site. (Because radio signals take 14 minutes to

travel from Mars to Earth, during the landing the mission team will be

watching events in Earth-received time, with landing noted at 12:15 P.M.

To avoid confusion, all subsequent times of mission events discussed

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Placement of illustrations: Placeillustrations just after the pointwhere they are relevant, as isdone here. If an illustration justwont fit, bump it to the top ofthe next page, fill in the remain-ing white space with text, and seta cross-reference to the illustration(but do not include pagereference).

CHAPTER 2 Processes: Instructions, Policies, and Procedures 59


below are stated in Earth-received time, when a signal would be

received on Earth. Actual events will have taken place on the spacecraft

about 14 minutes earlier in each case.) The descent imager will be turned

off 60 seconds after landing.

After waiting five minutes to allow for dust kicked up by the landing to

settle, the landers solar arrays will be unfolded. Eight minutes after

landing, while the medium-gain antenna is being turned to point at

Earth, the spacecrafts gyros will be used like compasses to determine

which way is north. The spacecrafts inertial measurement units will then

be powered off. (These components are shown in Figure 2.)

After gyrocompassing is completed, the medium-gain antenna will turn

toward Earth. This antenna slew may take up to 16 minutes. A vertical

scan will then be taken by the surface stereo imager before its boom is

deployed. Both the meteorological and imager masts will then be raised.

Quotation marks: Quotationmarks are used around Earth-received time because it is anunusual phrase and because it isdefined at that point. The writercould have used italics instead(but not both quotation marksand italics) to highlight thisphrase at its point of definition.Notice that later in this samedocument sleeps is alsoenclosed in quotation marksanunusual usage indeed!

Figure 2. Mars Polar Lander spacecraft.

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Introductory-element commas:Appendix B is adamant aboutpunctuating any introductoryelement, no matter how short,with a comma. For example, At 1:46 p.m. PST is anintroductory element. So is To avoid confusion

Numbers: As you would expect,this document contains plenty ofnumbers. For numbers that areboth exact and essential, usedigits. However if a numberbegins a sentence, spell it out.(For more on numbers versusdigits, see Appendix A.)

Hyphens: Throughout thisdocument, hyphens are used oncompound modifiers beforenouns: for example, dish-shaped,medium-gain, low-resolution,and black-and-white arehyphenated. A good test forhyphens is to see if you can mis-read the phrase. For example, is itan antenna shaped like a dishor a shaped antenna of the dishtype? Obviously its the firstinterpretation. Not hyphenatingthe two words causes momentaryhesitation; you want your tech-nical writing to be as immediatelyunderstandable as possible. (Formore on hyphens, see Appendix B.)

Final section: Although theprimary focus of this document isgetting there, you can imaginereaders feeling a little cheated bynot getting some idea of what willhappen while on Mars. Thisconclusion probably gives themenough of an idea.



First signal

The first opportunity to hear from the lander will take place when it

begins transmitting directly to Earth using its dish-shaped medium-gain

antenna about 23 minutes after landing. This signal is expected to be

received at 12:39 P.M. PST. The transmission session will end 45 minutes

later, or 1:24 P.M. PST, and will include engineering data on the landers

entry, descent and landing, as well as a possible low-resolution black-

and-white pictures from the undeployed camera on the landers deck. At

1:46 P.M. PST, the lander will shut down and sleep for 4 hours and 40

minutes while its solar panels recharge its onboard battery.

Assuming that all is normal with the spacecraft, it will power up again at

6:26 P.M. PST and turn on its receiver. At this time, mission controllers

expect that they will send the lander commands such as what data rate

to use for later radio transmissions. The receiver will continue listening

for commands from Earth until 7:41 P.M. PST. At 8:09 P.M., the lander

will begin transmitting to Earth until 10:45 P.M. After that session

concludes, the lander will run through a sequence that takes about half

an hour as it prepares to shut down for the night. At 11:24 P.M. PST the

lander will power down.

Operations

Instead of a rover, the Mars Polar Lander is equipped with a robotic arm

that will dig into the soil near the planets south pole in search of

subsurface water and fine-scale layering that may physically record past

changes in climate. The lander will also conduct experiments on soil

samples acquired by the robotic arm and dumped into small ovens,

where the samples will be heated to drive off water and carbon dioxide.

Surface temperatures, winds, pressure, and the amount of dust in the

atmosphere will be measured on a daily basis, while a small microphone

records the sounds of wind gusts and mechanical operations onboard the

spacecraft.

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