The entry, descent and landing (EDL) process for Mars Pathfinder will begin days before landing when controllers at JPL will send commands to the spacecraft to tell it precisely when and how to begin the complex autonomous series of steps necessary to safely land on the surface of Mars. These commands are sent periodically right up to a few hours before landing, when controllers on the Earth will have the most precise knowledge of where the spacecraft is relative to Mars (the effect of Mars' gravity well is not felt until the spacecraft is less than 48 hours away).
Landing occurs at about 3:00 am local time on Mars, which will be about 10:00 am PDT on Friday, July 4, 1997. From an hour and a half before landing until about 3 and a half hours later, the spacecraft is under control of autonomous on-board software that precisely controls the many events that must occur.
The fast-paced approach of Pathfinder at Mars begins with venting of the heat rejection system's cooling fluid about 90 minutes prior to landing. This fluid is circulated around the cruise stage perimeter and into the lander to keep the lander and rover cool during the 7 month cruise phase of the mission.
Its mission fulfilled, the cruise stage is then jettisoned from the entry vehicle about one-half hour prior to landing at a distance of 8500 km from the surface of Mars.
Several minutes before landing, the spacecraft begins to enter the outer fringes of the atmosphere about 125 km. (80 mi.) above the surface. Spin stabilized at 2 rpm, and traveling at 7.5 km/sec, the vehicle enters the atmosphere at a shallow 14.8 deg angle. A shallower entry angle would result in the vehicle skipping off the atmosphere, while a steeper entry would not provide sufficient time to accomplish all of the entry, descent and landing tasks. A Viking-derived aeroshell (including the heatshield) protects the lander from the intense heat of entry. At the point of peak heating the heatshield absorbs more than 100 megawatts of thermal energy. The martian atmosphere slows the vehicle from 7.5 km/sec to only 400 m/sec (900 mph).
Then entry deceleration of up to 20 gees, detected by on-board accelerometers, sets in motion a sequence of preprogrammed events that are completed in relatively quick succession.
Deployment of the single, 24-ft. diameter parachute occurs 2-3 min. after atmospheric entry at an altitude of 5-11 km. (3-7 mi.) above the surface, eventually slowing the vehicle down to 65 meters/sec. The parachute is similar in design to those used for the Viking program but has a wider band around the perimeter which helps minimize swinging.
The heatshield is pyrotechnically separated from the lander 20 sec. later and drops away at an altitude of 2-9 km. (3-6 mi.). The lander soon begins to separate from the backshell and "rappels" down a metal tape on a centrifugal braking system built into one of the lander petals.
The slow descent down the metal tape places the lander into position at the end of a braided Kevlar tether, or bridle, without off-loading the parachute or placing excessive loads on the backshell. The 20 m bridle provides space for airbag deployment, distance from the solid rocket motor exhaust stream and increased stability. Once the lander has been lowered into position at the end of the bridle, the radar altimeter is activated and aids in the timing sequence for airbag inflation, backshell rocket firing and the cutting of the Kevlar bridle.
The lander's Honeywell radar altimeter is expected to acquire the surface about 32 sec. prior to landing at an altitude of about 1.5 km. The airbags are inflated about 8 sec. before landing at an altitude of 300 meters above the surface.
The airbags have two pyro firings, the first of which cuts the tie cords and loosens the bags. The second, 0.25 sec. later, and 4 sec. before the rockets fire, ignites three gas generators that inflate the three 5.2 m (17-ft) dia. bags to a little less than 1 psi. in less than 0.3 sec.
The conical backshell above the lander contains three solid rocket motors each providing about a ton of force for over 2 seconds. They are activated by the computer in the lander. Electrical wires that run up the bridle close relays in the backshell which ignite the three rockets at the same instant.
The brief firing of the solid rocket motors at an altitude of 80-100 meters is intended to essentially bring the downward movement of the lander to a halt some 12 meters (±10 m) above the surface. The bridle separating the lander and heatshield is then cut in the lander, resulting in the backshell driving up and into the parachute under the residual impulse of the rockets, while the lander, encased in airbags, falls to the surface.
Because it is possible that the backshell could be at a small angle at the moment that the rockets fire, the rocket impulse may impart a large lateral velocity to the lander/airbag combination. In fact the impact could be as high as 25 m/sec (56 mph) at a 30 deg grazing angle with the terrain.
It is expected that the lander may bounce at least 12 m about the ground and soar 100-200 m between bounces. (Tests of the airbag system verified that it was capable of much higher impacts and longer bounces.)
Once the lander has settled on the surface, pyrotechnic devices in the lander petal latches are blown to allow the petals to be opened. The latches locking the sturdy side petals in place are necessary because of the pulling forces exerted on the lander petals by the deployed airbag system.
In parallel with the petal latch release, a retraction system will begin slowly dragging the airbags toward the lander, breaching vent ports on the side of each bag, in the process deflating the bags through a cloth filter. The airbags are drawn toward the petals by internal lines extending between attachments within the airbags and small winches on each of the lander sides. It takes about 64 minutes to deflate and fully retract the bags.
There is one high-torque motor on each of the three petal hinges. If the lander comes to rest on its side, it will be righted by opening a side petal with a motor drive to place the lander in an upright position. Once upright, the other two petals are opened.
About 3 hours is allotted to retract the airbags and deploy the lander petals. In the meantime, the lander's X-band radio transmitter will be turned off for the first time since before launch on December 4, 1996. This saves battery power and will allow the transmitter electronics to cool down from being warmed up during entry without the cooling system. It also allows time for the Earth to rise well above the local horizon and be in a better position for communications with the lander's low-gain antenna later in the morning.
Normal digital data transmissions will cease near the time of cruise stage separation due to the dynamics of EDL. Instead, the transmitter's carrier signal and sidebands will be recorded by the Deep Space Network's Madrid station so that the effects of the many events on the signal may be discerned. The digital data downlink will automatically resume 3.5 hours after landing, long after the airbags have been retracted and the petals opened.