Around 6:45 pm PST, the mission team will be preparing the spacecraft for communications during entry, descent, and landing. Spirit's cruise stage switches from its medium-gain antenna (which requires pointing toward Earth), to its low-gain antenna, which does not need to be pointed as precisely for the Deep Space Network antennas to pick up the signal. This switch slows the rate of data transmission, but is necessary to allow communications to continue when the spacecraft changes its orientation to point its heat shield forward.
Step-by-Step Guide to Entry, Descent, and Landing
On the evening of January 3, 2004, Spirit is scheduled to land on Mars. Here's a step-by-step guide to what will happen. To watch mission controllers live during entry, descent and landing, please tune into NASA TV at 6:45 p.m. Pacific Standard Time.
Step One: Communications Prep Begins
Step Two: Spacecraft Rotates
A few minutes after 7 pm PST, the spacecraft carrying Spirit will rotate to face its heat shield forward for its final approach.
Step Three: Transmission of Tones Begins
About a quarter after 8 pm PST, Spirit will begin transmitting tones back to Earth that indicate the spacecraft's status. A low-gain antenna on the spacecraft's backshell begins transmitting simple "tones" (sustained radio frequencies coded to report the spacecraft's status). These tones give Spirit a way to keep communicating after the cruise stage is jettisoned. A dictionary of about 100 possible tones can provide information such as whether the cruise stage has separated, whether the parachute opens, and whether the deceleration rate is within the expected range.
Step Four: Cruise Stage Separates
"Six Minutes of Terror"About 8:29 pm PST, one of the most challenging aspects of the mission begins. In only six minutes, the spacecraft will slow down from 12,000 to 0 miles per hour. [Video]
Step Five: Spirit enters the martian atmosphere
The lander should come streaking in through the martian atmosphere, going about 12,000 miles per hour. Given atmospheric friction, the outside surface of the heat shield will be as hot as the surface of the sun (1,447 degrees Celsius, or 2,637 degrees Fahrenheit), but the rover will be protected by the heat shield and will stay at about room temperature inside the lander. The heat shield also aerodynamically acts as the first "brake" for the spacecraft, slowing Spirit down by thousands of miles per hour.
Step Six: Parachute Deploys
About four minutes later, the spacecraft slows to about 1,000 miles per hour and is only 30,000 feet above the martian surface. At this point, a supersonic parachute is deployed. The parachute is based on the designs and experience of those used in the Viking and Pathfinder missions. The parachute for this mission is about 40% larger than Pathfinder's, and is made of two durable lightweight fabrics (polyester and nylon).
Step Seven: Heat Shield Jettisons
Twenty seconds after the chute deploys, the heat shield's work is complete, and it separates from the lander.
Step Eight: Lander Separates
Ten seconds after the heat shield jettisons, the lander separates from the back shell and descends to the end of a "bridle", or tether. Spirit's altitude is about 20,000 feet at this point. The lander "rappels" down a Zylon tape on a centrifugal braking system built into one of the lander petals. The slow descent down the Zylon tape places the lander in position at the end of another bridle, which is made of a nearly 20-meter-long (65-foot-long) braided Zylon, an advanced fiber material similar to Kevlar. The Zylon bridle provides space for airbag deployment, distance from the solid rocket motor exhaust stream, and increased stability. The bridle incorporates an electrical harness that allows the firing of the solid rockets from the backshell, as well as provides data from the backshell inertial measurement unit (which measures rate and tilt of the spacecraft) to the flight computer in the rover.
Step Nine: Radar Ground Acquisition Begins
Around 8:34 pm PST, when the lander is about 8,000 feet above the surface, radar systems on the lander determine its altitude and vertical velocity relative to the martian surface. These measurements will help the landing system decide how long to fire the retro rockets to bring the lander's verticle speed close to zero.
Step Ten: Descent imager takes pictures of the surface
While radar measurements are acquired, a descent imager will take three pictures of the surface and compare high-contrast features (for example, a crater) to determine the horizontal velocity at which the lander is moving across the surface. This measurement helps determine which transverse rockets should be fired in the retro rocket system.
Step Eleven: Data transmission to the Mars Global Surveyor orbiter begins
The descent UHF (ultra-high frequency) antenna, mounted on the lander, begins transmitting once the lander descends from the backshell, which stays attached to the parachute. From the lander's position at the bottom of a tether (aka bridle) connected to the backshell, this antenna can transmit to the Mars Global Surveyor orbiter that will be passing overhead at this time. The link allows Spirit to supplement the tone-coded information with additional status reports that can be forwarded to Earth by Mars Global Surveyor almost immediately. The window for relaying information to Mars Global Surveyor closes by about 8:42 p.m., when the orbiter sets below the landing site's horizon, about seven minutes after Spirit hits the ground. Although this communication link has been extensively planned and simulated, there is no guarantee that Mars Global Surveyor will successfully relay information from Spirit. When the lander descends from the backshell, a low-gain X-band antenna mounted on the rover itself takes over from the backshell antenna the job of transmitting tones to Earth. It transmits tones until retro rockets fire, six seconds before impact. Then, during the spacecraft's impact, bouncing, and rolling, it transmits a carrier signal only.
Step Twelve: Airbags inflate
Airbags inflate to protect the lander for a soft lading over the hard rocks on Mars. The airbags used in the Mars Exploration Rover mission are the same type that Mars Pathfinder used in 1997. Airbags must be strong enough to cushion the spacecraft if it lands on rocks or rough terrain and allow it to bounce across Mars' surface at freeway speeds after landing.
Step Thirteen: Retro rockets fire
At this point in entry, descent, and landing, the lander is only a football-field length off the ground. Three rockets fire, bringing the airbag-cocooned lander to zero vertical velocity nearly 40 feet off the ground.
Step Fourteen: Bridle is cut and first impact occurs
At about the height of a four-story building and three seconds before landing, the bridle is cut and the vehicle freefalls to the surface. The mass of Spirit and its lander is about 544 kilograms (1,200 pounds).
Final Landing Stage
Step Fifteen: Lander rolls to complete stop
The rover, protected by its lander structure and airbags, could bounce up to four or five stories high and roll as far as 1 kilometer (0.6 miles) across the martian surface before it comes to a complete stop around 8:45 pm PST.
Step Sixteen: Communication attempt begins
About 14 minutes after Spirit hits the ground and four minutes after it stops bouncing and rolling, transmission of tones resumes from the rover's low-gain X-band antenna. If the rover lands with its base petal down, this antenna will be near the top of the bundle and in a position that may be favorable to sending a signal to Earth. Its transmission of tones ends 150 seconds later. The lander, however, may not be in this orientation. Therefore, beginning about three minutes later, another low-gain X-band antenna, this one mounted on the lander's base petal, transmits tones for 150 seconds as well.
Step Seventeen: Critical deployments begin
Immediately after landing, the rover will go through a series of critical deployments for 80 minutes or longer, depending on which base petal it lands. The lander will retract its airbags, deploy its lander petals and solar arrays, and raise its panoramic camera mast.
Step Eighteen: Data transmission to the Mars Odyssey orbiter begins
The Mars Odyssey orbiter passes across the sky above the landing site for about 15 minutes. If Spirit gets through its critical deployments in time, it will use the rover's UHF antenna to send information, perhaps including images, to Odyssey. Odyssey forwards that information to Earth between 11 p.m. and midnight PST. However, Spirit might not be ready to communicate with Odyssey by the time the orbiter flies overhead, especially if Spirit ends its roll with a side petal instead of its base petal down. Also, as with the communications attempt with Mars Global Surveyor (step eleven), there has never been a chance to test communications between a transmitter on the surface of Mars and an orbiter.