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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
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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
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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.
58 PART I Project Tools for Technical Writers
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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
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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.
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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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