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Interplanetary Cruise and Approach to Mars
No matter which day in its launch period Rover A leaves Earth, it will reach Mars on
January 4, 2004, so the journey may last anywhere from 194 to 210 days. Similarly,
Rover B has a fixed appointment for arriving on January 25, 2004, so the duration of
its journey to Mars will be from 194 days to 214 days, depending on its launch date.
Engineers refer to the first few months of each trip as the cruise phase, while the final
45 days before arrival are known as the approach phase. During both phases, each
spacecraft is connected to a cruise stage that will be jettisoned in the final minutes of
the flight. Solar panels on the cruise stage will provide electricity for the spacecraft in
Thrusters on the cruise stage will be fired to adjust the spacecraft's flight path three
times during the cruise phase and up to three more times during the final eight days of
the approach phase. The first one or two of these maneuvers will commit the space-
craft to a specific target area on Mars. An additional thruster firing may be added dur-
ing Rover A's cruise stage to allow ground controllers to retarget the landing site later if
this is deemed necessary. Later maneuvers for both missions will refine the targeting
based on calculations using frequently updated determinations of the spacecraft's posi-
tion and course. The final trajectory correction maneuver, which is optional, is sched-
uled just six hours before landing.
Like NASA's Mars Odyssey orbital mission, the Mars Exploration Rover project will
combine two traditional tracking schemes with a relatively new triangulation method to
improve navigational precision. One of the traditional methods is ranging, which mea-
sures the distance to the spacecraft by timing precisely how long it takes for a radio
signal to travel to the spacecraft and back. The other is Doppler, which measures the
from the craft.
The newer method, called delta differential one-way range measurement, adds infor-
mation about the location of the spacecraft in directions perpendicular to the line of
sight. Pairs of antennas at Deep Space Network sites on two different continents
simultaneously receive signals from the spacecraft, then use the same antennas to
Successful use of this triangulation method is expected to shave several kilometers or
miles off the amount of uncertainty in delivering the rovers to their targeted landing
The months in which the rovers travel from Earth to Mars will also provide time for test-
ing critical procedures, equipment and software in preparation for arrival.
Entry, Descent and Landing
The Mars Exploration Rovers will use the same airbag-cushioned landing scheme that
successfully delivered Mars Pathfinder to the Red Planet in 1997.
About 70 minutes before entering Mars' atmosphere, each rover spacecraft will turn to
orient its heat shield forward. From that point until the rover deploys its own solar pan-
els after landing, five batteries mounted on the lander will power the spacecraft.
The planned sequence of events for entering the atmosphere, descending and landing
• Entry Turn Starts: L - 76 min. Turn completed by L - 56 min
• Cruise Stage Separation: L - 21 min
• Parachute Deployment: L - 102 sec, altitude 8.6 km (5.3 mi), speed 472 km/hr (293 mph)
• Heatshield Separation: L - 82 sec
• Lander Separation: L - 72 sec
• Radar Ground Acquisition: L - 35 sec, 2.4 km (1.5 mi) above ground
• Descent Images Acquired: L - 30 sec, 2.0 km above ground
L - 26 sec, 1.7 km above ground
L - 22 sec, 1.4 km above ground
• Start Airbag Inflation: L - 8 sec, 284m above ground
• Retro-Rocket Firing: L - 6 sec, 134m, 82 km/hr (51 mph)
Numbers approximate, • Bridle Cut: L - 3 sec, 10 m above ground
for Rover A landing • Landing: Entry + 343 sec
• Bounces, Rolls Up to 1 km
• Roll Stop: Landing + 10 min
• Airbags Retracted: L + 66 min
• Petals Opened: L + 96 min
to L + 187 min
Entry, descent and landing
is essentially the same for each of the two rover missions. Fifteen minutes before
atmospheric entry, the protective aeroshell encasing the lander and rover will separate
from the cruise stage, whose role will at that point be finished. Each cruise stage will
ultimately impact Mars.
Each spacecraft will hit the top of the atmosphere, about 128 kilometers (80 miles)
above Mars' surface, at a flight path angle of about 11.5 degrees and a velocity of
about 5.4 kilometers per second (12,000 miles per hour). Although Mars has a much
thinner atmosphere than Earth does, the friction of traveling through it will heat and
slow the spacecraft dramatically. The surface of the heat shield is expected to reach a
temperature of 1,447 C (2,637 F). By 4 minutes after atmospheric entry, speed will
have decreased to about 430 meters per second (960 miles per hour). At that point,
about 8.5 kilometers (5.3 miles) above the ground, the spacecraft will deploy its para-
Within 2 minutes, the spacecraft will be bouncing on the surface, but those minutes will
be packed with challenging events crucial to the mission's success.
Twenty seconds after parachute deployment, the spacecraft will jettison the bottom half
of its protective shell, the heat shield, exposing the lander inside. Ten seconds later,
the backshell, still attached to the parachute, will begin lowering the lander on a tether-
like bridle about 20 meters (66 feet) long. Spooling out the bridle to full length will take
10 seconds. Almost immediately, a radar system on the lander will begin sending puls-
es toward the ground to measure its altitude. Radar will detect the ground when the
craft is about 2.4 kilometers (1.5 miles) above the surface, approximately 35 seconds
before landing.
The Mars Exploration Rover design has two new tools, absent on Mars Pathfinder, to
avoid excessive horizontal speed during ground impact in case of strong winds near
the surface. One is a downward-looking camera mounted on the lander. Once the
radar has sensed the surface, this camera will take three pictures of the ground about
4 seconds apart and automatically analyze them to estimate the spacecraft's horizontal
velocity. The other innovation is a set of three small transverse rockets mounted on the
backshell that can be fired in any combination to reduce horizontal velocity or counter-
act effects of side-to-side swinging under the parachute and bridle.
Eight seconds before touchdown, gas generators will inflate the lander's airbags. Two
seconds later, the three main deceleration rockets on the backshell -- and, if needed,
one or two of the transverse rockets -- will ignite. After 3 more seconds, when the lan-
der should be about 15 meters (50 feet) above ground and have zero vertical velocity,
its bridle will be cut, releasing it from the backshell and parachute. The airbag-protect-
ed lander will then be in free fall for a few seconds as it drops toward the ground.
The first bounce may take the airbag-protected lander back up to 15 meters (49 feet)
or more above the ground. Bouncing and rolling could last several minutes. By com-
parison, the airbag-cushioned Mars Pathfinder bounced about 15 times, as high as 15
meters (49 feet), before coming to a rest 2-1/2 minutes later about a kilometer (0.6
mile) from its point of initial impact.
Twelve minutes after landing, motors will begin retracting the airbags, a process likely

Interplanetary Cruise and Approach to Mars

No matter which day in its launch period Rover A leaves Earth, it will reach Mars on

January 4, 2004, so the journey may last anywhere from 194 to 210 days. Similarly,

Rover B has a fixed appointment for arriving on January 25, 2004, so the duration of

its journey to Mars will be from 194 days to 214 days, depending on its launch date.

Engineers refer to the first few months of each trip as the cruise phase, while the final

45 days before arrival are known as the approach phase. During both phases, each

spacecraft is connected to a cruise stage that will be jettisoned in the final minutes of

the flight. Solar panels on the cruise stage will provide electricity for the spacecraft in

Thrusters on the cruise stage will be fired to adjust the spacecraft's flight path three

times during the cruise phase and up to three more times during the final eight days of

the approach phase. The first one or two of these maneuvers will commit the space-

craft to a specific target area on Mars. An additional thruster firing may be added dur-

ing Rover A's cruise stage to allow ground controllers to retarget the landing site later if

this is deemed necessary. Later maneuvers for both missions will refine the targeting

based on calculations using frequently updated determinations of the spacecraft's posi-

tion and course. The final trajectory correction maneuver, which is optional, is sched-

uled just six hours before landing.

Like NASA's Mars Odyssey orbital mission, the Mars Exploration Rover project will

combine two traditional tracking schemes with a relatively new triangulation method to

improve navigational precision. One of the traditional methods is ranging, which mea-

sures the distance to the spacecraft by timing precisely how long it takes for a radio

signal to travel to the spacecraft and back. The other is Doppler, which measures the

from the craft.

The newer method, called delta differential one-way range measurement, adds infor-

mation about the location of the spacecraft in directions perpendicular to the line of

sight. Pairs of antennas at Deep Space Network sites on two different continents

simultaneously receive signals from the spacecraft, then use the same antennas to

Successful use of this triangulation method is expected to shave several kilometers or

miles off the amount of uncertainty in delivering the rovers to their targeted landing

The months in which the rovers travel from Earth to Mars will also provide time for test-

ing critical procedures, equipment and software in preparation for arrival.

Entry, Descent and Landing

The Mars Exploration Rovers will use the same airbag-cushioned landing scheme that

successfully delivered Mars Pathfinder to the Red Planet in 1997.

About 70 minutes before entering Mars' atmosphere, each rover spacecraft will turn to

orient its heat shield forward. From that point until the rover deploys its own solar pan-

els after landing, five batteries mounted on the lander will power the spacecraft.

The planned sequence of events for entering the atmosphere, descending and landing

• Entry Turn Starts: L - 76 min. Turn completed by L - 56 min

• Cruise Stage Separation: L - 21 min

• Parachute Deployment: L - 102 sec, altitude 8.6 km (5.3 mi), speed 472 km/hr (293 mph)

• Heatshield Separation: L - 82 sec

• Lander Separation: L - 72 sec

• Radar Ground Acquisition: L - 35 sec, 2.4 km (1.5 mi) above ground

• Descent Images Acquired: L - 30 sec, 2.0 km above ground

L - 26 sec, 1.7 km above ground

L - 22 sec, 1.4 km above ground

• Start Airbag Inflation: L - 8 sec, 284m above ground

• Retro-Rocket Firing: L - 6 sec, 134m, 82 km/hr (51 mph)

Numbers approximate, • Bridle Cut: L - 3 sec, 10 m above ground

for Rover A landing • Landing: Entry + 343 sec

• Bounces, Rolls Up to 1 km

• Roll Stop: Landing + 10 min

• Airbags Retracted: L + 66 min

• Petals Opened: L + 96 min

to L + 187 min

Entry, descent and landing

is essentially the same for each of the two rover missions. Fifteen minutes before

atmospheric entry, the protective aeroshell encasing the lander and rover will separate

from the cruise stage, whose role will at that point be finished. Each cruise stage will

ultimately impact Mars.

Each spacecraft will hit the top of the atmosphere, about 128 kilometers (80 miles)

above Mars' surface, at a flight path angle of about 11.5 degrees and a velocity of

about 5.4 kilometers per second (12,000 miles per hour). Although Mars has a much

thinner atmosphere than Earth does, the friction of traveling through it will heat and

slow the spacecraft dramatically. The surface of the heat shield is expected to reach a

temperature of 1,447 C (2,637 F). By 4 minutes after atmospheric entry, speed will

have decreased to about 430 meters per second (960 miles per hour). At that point,

about 8.5 kilometers (5.3 miles) above the ground, the spacecraft will deploy its para-

Within 2 minutes, the spacecraft will be bouncing on the surface, but those minutes will

be packed with challenging events crucial to the mission's success.

Twenty seconds after parachute deployment, the spacecraft will jettison the bottom half

of its protective shell, the heat shield, exposing the lander inside. Ten seconds later,

the backshell, still attached to the parachute, will begin lowering the lander on a tether-

like bridle about 20 meters (66 feet) long. Spooling out the bridle to full length will take

10 seconds. Almost immediately, a radar system on the lander will begin sending puls-

es toward the ground to measure its altitude. Radar will detect the ground when the

craft is about 2.4 kilometers (1.5 miles) above the surface, approximately 35 seconds

before landing.

The Mars Exploration Rover design has two new tools, absent on Mars Pathfinder, to

avoid excessive horizontal speed during ground impact in case of strong winds near

the surface. One is a downward-looking camera mounted on the lander. Once the

radar has sensed the surface, this camera will take three pictures of the ground about

4 seconds apart and automatically analyze them to estimate the spacecraft's horizontal

velocity. The other innovation is a set of three small transverse rockets mounted on the

backshell that can be fired in any combination to reduce horizontal velocity or counter-

act effects of side-to-side swinging under the parachute and bridle.

Eight seconds before touchdown, gas generators will inflate the lander's airbags. Two

seconds later, the three main deceleration rockets on the backshell -- and, if needed,

one or two of the transverse rockets -- will ignite. After 3 more seconds, when the lan-

der should be about 15 meters (50 feet) above ground and have zero vertical velocity,

its bridle will be cut, releasing it from the backshell and parachute. The airbag-protect-

ed lander will then be in free fall for a few seconds as it drops toward the ground.

The first bounce may take the airbag-protected lander back up to 15 meters (49 feet)

or more above the ground. Bouncing and rolling could last several minutes. By com-

parison, the airbag-cushioned Mars Pathfinder bounced about 15 times, as high as 15

meters (49 feet), before coming to a rest 2-1/2 minutes later about a kilometer (0.6

mile) from its point of initial impact.

Twelve minutes after landing, motors will begin retracting the airbags, a process likely