* Originally By: Harald Franzrahe
* Originally To: Alle
* Originally Re: Mars Pathfinder Frequently Asked Questions
* Original Area: NASA News And Press Releases
* Forwarded by : Blue Wave v2.12
Mars Pathfinder Questions and Answers
Updated 27 November, 1996
Some of the more frequently asked questions and their answers have been
posted here for your convenience, and more will be added as they come in.
General Launch & Entry, Descent Surface Sojourner
Topics Cruise and Landing Operations Rover
If you'd like to have a question answered by a member of the Mars Pathfinder
team, please contact:
David Dubov
ddubov@pop.jpl.nasa.gov
Mars Pathfinder Webmaster
and it will be passed along to the appropriate person for an answer. Keep
'em coming!
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General Topics
Why was this mission chosen for the Could this mission stand alone,
Discovery Program? and how does it complement other
missions?
Describe how Mars Pathfinder is unique, What is being done to make sure
and how it follows the "better, faster, it doesn't go the way of Mars
cheaper" philosphy. Observer?
What planetary protection measures are
being used on Mars Pathfinder and Mars
Global Surveyor?
Why (in your opinion) was this mission chosen for the Discovery program
versus other proposed missions?
Mars Pathfinder and the Near Earth Asteroid Rendezvous (NEAR) mission were
not chosen by the same process as the later Discovery missions. Pathfinder
was originally designedto demonstration technology for inexpensive entry and
landing on Mars, as a precursor to a network of landers called Mars
Environmental Survey, or MESUR. Because Pathfinder was the first mission it
fell into what was then considered a "Discovery" mission class - that is it
had to be done for less than $150M in 1992 dollars. The MESUR Network
missions were never funded, but Pathfinder is now a technology demonstration
for landers in the Mars Surveyor program. Later Discovery missions are being
chosen through Announcements of Opportunity and are being developed by teams
headed by scientists.
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This mission seems to be part of a series starting with the Mars Global
Surveyor, but those other missions are not part of the Discovery program.
Could this mission stand alone if necessary? How does it complement the
other (US and internationally) planned missions?
Pathfinder is a "Discovery" mission. The Mars Surveyor Program is separate
from Discovery and will launch one or two missions to Mars at every
opportunity (every 26 months). Mars Global Surveyor launches in November
1996, Mars Surveyor 98 will launch an orbiter and a lander in December 1998
and January 1999. The 98 Surveyor missions are both using Pathfinder
components, especially the computer and software. The Mars Surveyor 98
lander is using much of the entry and descent technology demonstrated by
Pathfinder, including the aeroshell and the parachute. The science from
Pathfinder will be completely complementary with the Surveyor Program
science. In fact, the Mars Surveyor 98 lander is using the same camera and
weather station technology that Pathfinder is using.
In addition to the U.S. missions, Russia will launch Mars 96 in November
1996. It comprises an orbiter, two small landers, and two penetrators. There
is a U.S. experiment on the landers and the landers and penetrators will
relay data through the Mars Global Surveyor orbiter. Pathfinder carries
instruments provided by several different countries. The Mars Surveyor 98
missions also have international payloads, including Russian contributions
to the U.S. Infrared Radiometer on the orbiter, and to the lander payload.
The Japanese will fly an aeronomy orbiter in 1998 to study the upper
atmosphere of Mars. A U.S. instrument is scheduled to be on board this
Japanese mission.
We are currently studying the feasibility of a joint U.S.-Russian mission
called Mars Together in 2001. One option for this mission is for the U.S. to
launch an orbiter, and to provide a "carrier" spacecraft to be launched with
a Russian lander on a Russian launch vehicle. We are also studying a sample
return mission which could be made affordable by partnering with other
countries.
--Donna Shirley, Mars Exploration Program Manager
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Describe your spacecraft and instruments including what is unique, versus
ones in the past, that allowed it to be built quickly and cheaply?
The Mars Pathfinder spacecraft is quite different from other missions built
at JPL. First of all, as a lander mission, the prime focus is on getting the
lander/rover instrument package safely to the surface of Mars. This means
that this spacecraft must be able to electro-mechanically transform itself
autononmously from a "cruise" configuration much like a Galileo (without the
cruise science observations of course) into a stable science platform on the
surface of Mars. All of this must be done on a budget quite small compared
with previous planetary missions. This adds considerably to the technical
challenge.
These challenges were met by first taking maximal advantage of past work: we
"inherited" hardware from the Cassini mission to Saturn; we utilized designs
of equipment flown to Mars on the Viking missions of the 70's; and we have
an improved understanding of the environmental uncertainties from science
observations obtained over the last 2 decades. Secondly, improvements in
computer technology have allowed us to model, design and test aspects of our
system that were impossible 20 years ago. Finally, we have built a small
"Skunkworks-like" team that has accomplished only that work necessary to do
the mission, with little red tape nor redundancy in effort.
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What is being done to make sure it doesn't disappear like the Mars Observer?
No one can build a complex spacecraft that is absolutely guranteed to work.
Embarking on unique, first-of-a-kind enterprises, by their very nature,
invoke risk taking. However we can go to great lengths within the limits of
our budget to minimize technical risk. Much like the design process leading
to a passenger jet, spacecraft designers must ensure that design margins
conservatively exceed the uncertainty in the expected environment and that
the spacecraft is tested to those environments. Much work then must be
placed in understanding the environment, followed by as much testing as
money and time will allow. We feel quite certain (and many independent
reviewers have agreed) that although Mars Pathfinder is about 1/10th total
mission cost of Mars Observer, that we have struck an appropriate balance
between cost and risk.
--Rob Manning, Mars Pathfinder Flight System Chief Engineer
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In what way do planetary protection provisions affect Mars lander missions
(such as Pathfinder) and Mars orbiter missions (such as Global Surveyor)?
The major impact of the planetary protection requirements on Pathfinder is
that we must carefully clean the spacecraft before launch in order to keep
from contaminating Mars. Although scientists now believe that it would be
difficult to sustain and cultivate life on Mars, they would prefer that we
not take any chances. They have developed a system for rating different
missions by the potential impact that they could have. The most stringent
missions are those which will be returning samples from Mars or are
performing life detection experiments there. In both cases, complete
sterilization is required. In our case (and MGS), we are allocated a
specific number of biologic spores which are deemed acceptable. We have to
clean the spacecraft (or perform mission design tricks - I write about
below) to reduce the number of spores below this level (I am glad to say
that we are well below the acceptable number). The only parts of the
spacecraft which we actually need to clean is the part that will come in
contact with the Martian atmosphere and surface. The other parts (meaning
the third stage and cruise stage) do not have to be cleaned because they
will either not hit Mars (we specifically bias the aim point of the Delta
away from Mars so that upper stage does not hit the planet) or will burn up
during entry (we had to perform a break-up re-entry analysis of the cruise
stage to prove this will occur). MGS did not have to do any planetary
protection related cleaning because they can guarantee that the spacecraft
will not enter the Martian atmosphere for a long time with high probability.
--Richard Cook
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Pre-Launch, Launch and Cruise
Why does Mars Pathfinder How many pieces in a Why does the second stage
launch at 2:09 am? fairing? shut down and then start
up again later?
When does it enter Earth's When is the first How many miles
shadow? two-way commiuncation? (kilometers) will
Pathfinder fly to Mars?
While watching pre-launch
pictures, technicians
appear dressed in isolation
suits. Why?
Why it is that 2:09 a.m. on 2 December (and earlier times on dates
thereafter) is the precise time Pathfinder must launch? What is is about the
Earth's position that makes this important?
When the spacecraft leaves the Earth to go to Mars, it must be going in a
particular direction. Since the Earth rotates, the launch site is only lined
up with this direction twice per day (for an instant in each case). Since
the two opportunities are about 12 hours apart, the launch vehicle people
make us choose one or the other. It is okay to launch at a time slightly
different from the ideal time because the spacecraft can use it's propulsion
system to correct for the error. The spacecraft has a limited amount of
fuel, however, so we can't accept a very big error (up to approximately 1
minute is okay).
You can see all of Mars Pathfinder's launch opportunities at the Launch
Windows Page.
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Does the fairing of the Delta II rocket fall away in two pieces or more?
Two.
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Very briefly, can you explain why the second stage temporarily shuts down at
9 minutes 20 seconds after launch and starts up again about an hour later?
The first burn of the second stage is used to place the spacecraft in a low
parking orbit around the Earth. It then coasts until it gets to the right
point in its orbit to do the burn to go to Mars. The second stage then
ignites again to begin pushing the spacecraft towards Mars. The third stage
finishes off the job because the second stage fuel tanks are nearly empty.
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What is the exact time after launch that the spacecraft enters Earth's
shadow?
It depends on launch date. For December 2, we go into shadow at 3:15 am and
exit at 3:45 am.
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After emerging from the shadow, the first two-way communication between
Earth and the spacecraft is from "flight managers at JPL" -- is this
accurate?
This is not quite correct. The spacecraft begins transmitting when it
separates from the third stage (at about 3:25 am). The Deep Space Network
station in Goldstone, California should detect this signal about five
minutes later, and we should begin to get engineering data from the
spacecraft. We don't actually try and send a command to the spacecraft for
several more hours (about 4-5 hours after launch). All of these operations
are conducted by engineers at JPL.
--Richard Cook, Mars Pathfinder Mission Operations Manager
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How many miles (kilometers) will the spacecraft fly to Mars, and how many
miles (kilometers) will the Earth be from Mars on arrival day (4 July,
1997)?
Because the path which the spacecraft takes to get to Mars, essentially
"catching up" to the planet, it will travel approximately 312 million miles
(500 million kilometers) in its seven month journey. However, when
Pathfinder actually arrives at the planet, the Earth and Mars will be
separated by approximately 120 million miles (200 million kilometers).
--Dave Spencer, Mars Pathfinder Trajectory and Navigation Team Member
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While watching pre-launch pictures, technicians appear dressed in isolation
suits. Why?
Technicians and engineers that work in the vicinity of the lander must wear
what we call "bunny" suits (it was a joke name originally, but many years
ago the name caught on). These are clean head-to-toe garments that prevent
dirt and biological contamination of the lander by the workers.
At other times when the hydrazine fuel was being loaded into Pathfinder's
fuel tanks, some workers had to wear "SCAPE" suits. These suits are also
head-to-toe, but they also provide self-contained breathing equipment which
is strapped to their backs. In fact they look a lot like space suits.
Hydrazine, in addition to being highly flammable, is an extremely caustic
and dangerous liquid. These suits are designed to protect the workers in the
unlikely event of a hydrazine leak.
--Rob Manning
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Entry, Descent and Landing
How can you use such a What has been done to What is stopping the lander
small parachute on ensure successful from landing on a large
Mars? operation? rock?
What stops the Why is Pathfinder not Why does Pathfinder use
parachute from falling landing near the petal/airbags vs.
on the lander? so-called "face"? traditional landing gear?
Why is Pathfinder
landing at night (Mars
local time)?
If the Mars atmosphere is less than 1% than that of Earth, how can a
parachute of the size you are using be sufficient? It would seem to me that
you would need a parachute close to 1000' wide to achieve the same effect.
Would you please explain the dynamics of placing a lander on Mars, and why a
small parachute would work on Mars as it does on Earth?
You ask a very insightful question. The bottom line is you're right,
parachutes this small aren't sufficient on Mars! On Mars Pathfinder, as on
Viking, we use a "small" 40.5 ft (12.5 m) chute. It was scaled so that, with
our lighter lander, it does about as much for the our descent speed as does
Viking's. Our terminal velocity seconds before getting to the ground (where
the atmosphere is "thickest") is still about 65 m/s (146 mph)!!
You are correct, it would indeed take a a larger chute to get slower
"normal" Earth-like terminal velocities. Our chute on Mars is about the
equivalent of a chute 38 times smaller in area on Earth (6.5 ft across!),
and this includes the effect of Mars' lower gravity! A chute that could
lower our lander to the Martian ground at a gentle 10 m/s (22 mph) would
have to have an area about 42 times larger than our "little" chute (or a
diameter of 263 ft)! That's 42 times the mass (and volume) of our 10 kg
chute, or 420 kg, more than the mass of our entire lander! It wouldn't fit!
We would need to have a "gossamer" (ultra-light weight material) parachute
and then figure out how to get it open at high speeds!
This is why we turned to solid rockets to stop our lander just before we hit
the ground. Viking, too, used liquid rockets to slow the terminal decent.
Also Pathfinder's airbags protect the lander from the local terrain
variations (bumps, craters, rocks, hills, etc.) after the rockets do their
thing.
So why do we do we use a chute at all? Well, parachutes might not be all
that good a laying a lander gently down on the Martian surface, but they do
a spectacular job of braking something moving very fast. Remember, the drag
FORCE a chute generates (therefore its deceleration), is proportional to the
square of the velocity and only linearly proportional to the atmospheric
density; so even a thin atmosphere and a "small" chute will do much to slow
our entry vehicle down once the heatshield's aerobraking has been mostly
achieved.
This is also true of heatshields, our entry vehicle (like Viking's) enters
the upper atmosphere at 7 km/s (or more than 15,000 mph!). Most of this is
reduced by the friction with the heathsield. But even 2 minutes later, our
vehicle is still screaming in at nearly 400 m/s (900 mph) when the parachute
opens before slowing down to 65 m/s near the ground. I'd say that reducing
our velocity by a factor of 6 (a factor of 36 in kinetic energy), isn't all
that bad for only 10 kg of extra payload mass, wouldn't you?
So, the short answer is, you're right, parachutes don't work on Mars like
they do on Earth (neither do airbags, but that is another story), but they
do a great job when you need to slow down something that is whipping through
the Martian atmosphere FAST!
--Rob Manning
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As with other missions, the ability to deploy the panels for solar use, high
gain communications, or any other use for that matter, has not been as good
as could be. What has been done in ensuring successful operation now or in
the future? It seems without them opening up all the way it can cause major
problems. Like Galileo's high gain antenna and Mars Surveyor's 20 degree
shortfall in deployment of its solar panels.
The problems associated with moving parts are difficult ones to solve. Since
the cost to send one kilogram of material into space is so high, spacecraft
designers must be very stingy in allocating mass to the engineers who make
the mechanisms. You might be surprised that the typical spacecraft mechanism
can be destroyed with your bare hands!
The other part of this equation is that MOST of the time, mechanisms must
only need do their jobs under rather benign weightless conditions in space,
BUT they must also be able to handle the much rougher conditions that
precede getting there: ground handling and launch. It is these phases of the
mechanism's life that are the most traumatic. They are the most difficult to
quantify as well. I don't think the designers of the Galileo high gain
antenna mechanism would have expected that the antenna would be closed for
so long before finally opened in flight and that it would have had to
survive three cross-country road trips in a van! (Both of these events were
a direct result of the Challenger disaster.)
There is no magic formula for making mechanisms work in all situations, but
we have been learning just how subtle these problems can be. The trick is to
learn from your (and other people's) mistakes. Mars Pathfinder has more than
its share of moving parts. We knew that going in, so we went out of our way
to be a bit paranoid about it. We hired the very best spacecraft mechanical
engineers we could find. Going to Mars made the job a bit more difficult in
some cases because of our need to have the mechanisms work under very harsh
environmental conditions (harsher even than in deep space). For example, the
Rover, the IMP camera and the high gain antenna actuators must all work
under very cold conditions (as low as -90 deg C). Most lubricants do not
lubricate at those temperatures. We had to make sure that the actuators were
either warmed before they were used or had adequate torque margins for the
motor to overcome the sticky lubricant before it warmed up with use. In some
cases we "overkilled" the problem (e.g. the lander petal actuators) and
provided much more torque than we thought we really needed - just in case.
(I could go on and on.) It is safe to say that the mechanisms on Mars
Pathfinder were a LOT of work. But we tested and tested them (even beating
them up!) under many rough conditions until we were finally satisfied that
they will work fine when we need them to.
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What is stopping the lander from "landing" on a large rock, and making it
impossible to open up the craft to do its thing on Mars?
We all wondered about this at the beginning. So we tried it! First of all,
it turns out that we had a hard time getting the inflated, 17 ft beachball
of a lander close to a big rock! As long as those airbags stayed inflated,
it wanted to roll away from anything big and pointy. Secondly, even when we
did manage to coax it right next to a wicked boulder, the petals opened
right up even if it meant having the whole lander do a backflip! It took
some work, but we actually made that happen once in our Mars Yard at JPL and
without damage! It really helped that the petal actuators (a motor and a
gear train mounted on each of the three petal hinge lines) had the torque
margin to actually LIFT the lander off of the ground (they can even
indefinitely support the lander in a sort of "iron cross" once open). And
these tests were done under Earth's gravity, which gave the rocks a distinct
advantage. With many many tests behind us, in not one case would it have got
stuck.
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What stops the parachute from falling onto the craft and gumming things up
that way?
Lots of people asked us that question at the beginning. If you don't give
this some serious thought, there is a real risk that the lander could get
covered by the chute (a bit more than embarrassing). Fortunately we designed
the timing and sizing of the solid rocket firing (remember there are three
mounted inside the backshell) such that when the lander inside its inflated
airbags comes to a stop some 12 m above the Martian surface, the software
activates a cutter that cuts away the bridle thereby freeing the backshell
from the lander. The rockets, with still a quarter second of impulse left
over, launch the backshell up and away taking the parachute with it (at an
angle, tumbling as it goes). Meanwhile the lander and airbag go bouncing
away in the other direction! (Don't you wish you could be there to watch it
all happen?). You can click here to see an artist's rendition of this
procedure...
--Rob Manning
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To satisfy the curious, Pathfinder should have landed near the so-called
Face/Pyramid area. Or is that a hoax?
That there is a rock formation on Mars that looks somewhat like a face is
certainly true, but it is also true there are many similar naturally
occuring structures on the Earth, Moon and Mars that resemble faces, animals
and even man-made designs. The best way to see what large geologic stuctures
exist on Mars is to use the high resolution cameras on board the
recently-launched Mars Global Surveyor. Mars Pathfinder can not be
accurately aimed to any site smaller than a typical US county. We are
specifically targeting the ancient Ares Vallis outflow channel. Fortunately
the channel is big so we will not miss it! This site is ideally suited to
Mars Pathfinder's geologic mission. We believe a huge flood carved that
channel and deposited a large number and variety of rocks from the highland
water source into the flood basin where we intend to land. Our miniature
robotic geologist, the Sojourner Rover, will be able to analyse these
various rock types and give us an idea of how they were formed and about
Mars' early history.
In addition, all of the power for the spacecraft is collected from solar
panels. In order to get maximum power, one of the landing site requirements
was to have the sun be high in the sky. This restricted the landing site to
+/- 20 degrees from the equator of Mars. Cydonia (where the so-called "face"
is located) is at too high of a latitude for the lander to receive adequate
power.
--Rob Manning