Editing Mars Polar Lander (section)

== Mission background ==
=== History ===
As part of the [[Mars Surveyor '98]] mission, a lander was sought as a way to gather climate data from the ground in conjunction with an orbiter. [[NASA]] suspected that a large quantity of frozen water may exist under a thin layer of dust at the south pole. In planning the Mars Polar Lander, the potential water content in the Martian south pole was the strongest determining factor for choosing a landing location.{{r|press_kit_1998}} A CD-ROM containing the names of one million children from around the world was placed on board the spacecraft as part of the "Send Your Name to Mars" program designed to encourage interest in the space program among children.{{r|send_name_1998}} <!-- needs expansion -->

The primary objectives of the mission were to:{{r|press_kit_1999}}
* Land on the layered terrain in the south polar region of Mars;
* Search for evidence related to ancient climates and more recent periodic climate change;
* Give a picture of the current climate and seasonal change at high latitudes and, in particular, the exchange of water vapor between the atmosphere and ground;
* Search for near-surface ground ice in the polar regions, and analyze the soil for physically and chemically bound carbon dioxide and water; and
* Study surface [[Geomorphology|morphology]] (forms and structures), geology, [[topography]], and weather of the landing site.

=== ''Deep Space 2'' probes ===
{{Main|Deep Space 2}}
The Mars Polar Lander carried two small, identical [[impactor (spacecraft)|impactor]] probes known as "Deep Space 2 A and B". The probes were intended to strike the surface with a high velocity at approximately {{Coord|73|S|210|W|globe:Mars|name=Deep Space 2}} to penetrate the [[Martian regolith|Martian soil]] and study the subsurface composition up to a meter in depth. However, after entering the Martian atmosphere, attempts to contact the probes failed.{{r|press_kit_1998}}

Deep Space 2 was funded by the [[New Millennium Program]], and their development costs was US$28 million.{{r|AP-19991208}}

=== Spacecraft design ===
The spacecraft measured 3.6 meters wide and 1.06 meters tall with the legs and solar arrays fully deployed. The base was primarily constructed with an aluminum [[Honeycomb structure|honeycomb]] deck, composite [[Carbon-fiber reinforced polymer|graphite-epoxy]] sheets forming the edge, and three aluminum legs. During landing, the legs were to deploy from stowed position with compression springs and absorb the force of the landing with crushable aluminum honeycomb inserts in each leg. On the deck of the lander, a small thermal [[Faraday cage]] enclosure housed the computer, power distribution electronics and batteries, telecommunication electronics, and the capillary pump [[loop heat pipe]] (LHP) components, which maintained operable temperature. Each of these components included redundant units in the event that one may fail.{{r|press_kit_1998|nssdc1|nasa1}}

==== Attitude control and propulsion ====
While traveling to Mars, the cruise stage was three-axis stabilized with four [[hydrazine]] [[monopropellant rocket|monopropellant]] reaction engine modules, each including a 22-[[Newton (unit)|newton]] trajectory correction maneuver thruster for propulsion and a 4-newton reaction control system thruster for [[Spacecraft attitude control|attitude control]] (orientation). Orientation of the spacecraft was performed using redundant [[Spacecraft attitude control|Sun sensors]], [[Spacecraft attitude control|star trackers]], and [[Spacecraft attitude control|inertial measurement units]].{{r|nssdc1}}

During descent, the lander used three clusters of pulse-modulated engines, each containing four 266-newton hydrazine monopropellant thrusters. Altitude during landing was measured by a [[Doppler radar]] system, and an attitude and articulation control subsystem (AACS) controlled the attitude to ensure the spacecraft landed at the optimal [[azimuth]] to maximize solar collection and telecommunication with the lander.{{r|press_kit_1998|nssdc1|nasa1}}

The lander was launched with two hydrazine tanks containing 64 kilograms of propellant and pressurized with [[helium]]. Each spherical tank was located at the underside of the lander and provided propellant during the cruise and descent stages.{{r|press_kit_1998|nssdc1|nasa1}}

==== Communications ====
During the cruise stage, communications with the spacecraft were conducted over the [[X band]] using a medium-gain, horn-shaped antenna and redundant solid state power amplifiers. For contingency measures, a low-gain omnidirectional antenna was also included.{{r|press_kit_1998}}

The lander was originally intended to communicate data through the failed [[Mars Climate Orbiter]] via the [[Ultra high frequency|UHF]] antenna. With the orbiter lost on September 23, 1999, the lander would still be able to communicate directly to the [[NASA Deep Space Network]] through the Direct-To-Earth (DTE) link, an X band, steerable, medium-gain, [[parabolic antenna]] located on the deck. Alternatively, [[Mars Global Surveyor]] could be used as a relay using the UHF antenna at multiple times each Martian day. However the Deep Space Network could only receive data from, and not send commands to, the lander using this method. The direct-to-Earth medium-gain antenna provided a 12.6-kbit/s [[return channel]], and the UHF relay path provided a 128-kbit/s return channel. Communications with the spacecraft would be limited to one-hour events, constrained by heat-buildup that would occur in the amplifiers. The number of communication events would also be constrained by power limitations.{{r|press_kit_1998|press_kit_1999|nssdc1|nasa1}}

==== Power ====
The cruise stage included two [[gallium arsenide]] [[Photovoltaic system|solar arrays]] to power the radio system and maintain power to the batteries in the lander, which kept certain electronics warm.{{r|press_kit_1998|nssdc1}}

After descending to the surface, the lander was to deploy two 3.6-meter-wide gallium arsenide solar arrays, located on either side of the spacecraft. Another two auxiliary solar arrays were located on the side to provide additional power for a total of an expected 200 watts and approximately eight to nine hours of operating time per day.{{r|press_kit_1998|nssdc1}}

While the Sun would not have set below the horizon during the primary mission, too little light would have reached the solar arrays to remain warm enough for certain electronics to continue functioning. To avoid this problem, a 16-ampere-hour [[nickel–hydrogen battery]] was included to be recharged during the day and to power the heater for the thermal enclosure at night. This solution also was expected to limit the life of the lander. As the Martian days would grow colder in late summer, too little power would be supplied to the heater to avoid freezing, resulting in the battery also freezing and signaling the end of the operating life for the lander.{{r|press_kit_1998|nssdc1|nasa1}}

==== Scientific instruments ====
{{Gallery|align=center
|title=Scientific instruments
|file:Mars Polar Lander - MARDI instrument photo - mardi.jpg|Mars Descent Imager
|file:Mars Polar Lander - SSI instrument photo - ssi closeup s.jpg|Stereo Surface Imager
|file:Mars Polar Lander - LIDAR instrument photo - mpl lidar.gif|LIDAR
|file:Mars Polar Lander - RA instrument photo - arm.png|Robotic Arm
|file:Mars Polar Lander - RAC instrument photo.png|Robotic Arm Camera
|file:Mars Polar Lander - MET instrument photo - mettop.png|Meteorological Package
|file:Mars Polar Lander - TEGA instrument photo.png|Thermal and Evolved Gas Analyzer
|file:Mars Polar Lander - Mars Microphone instrument photo - microphone.jpg|Mars Microphone
}}

{{Gallery|align=center
|title=Images of the spacecraft
|File:Mars Polar Lander - spacecraft diagram.png|Annotated diagram of the Mars Polar Lander spacecraft
|File:Mars Polar Lander close-up.jpg|The spacecraft in stowed position just prior to encapsulation
|File:Mars Polar Lander undergoes testing.jpg|Testing performed at the Spacecraft Assembly and Encapsulation Facility
|File:Mars Polar Lander prelaunch.jpg|The ''Mars Polar Lander'' entry capsule, just prior to being mounted to the [[Star 48]] upper stage
}}

; [[Mars Descent Imager#Mars Descent Imager (MARDI)|Mars Descent Imager]] (MARDI): Mounted to the bottom of the lander, the camera was intended to capture 30 images as the spacecraft descended to the surface. The images acquired would be used to provide geographic and geologic context to the landing area.{{r|nssdc2}}
; [[Phoenix (spacecraft)#Surface stereo imager|Surface Stereo Imager]] (SSI): Using a pair of [[charge-coupled device]]s (CCDs), the stereo [[panoramic camera]] was mounted to a one-meter-tall mast and would aid in the thermal evolved gas analyzer in determining areas of interest for the robotic arm. In addition, the camera would be used to estimate the column density of atmospheric dust, the optical depth of [[aerosol]]s, and slant column abundances of water vapor using narrow-band imaging of the Sun.{{r|nssdc3}}
; [[Lidar|Light Detection and Ranging]] (LIDAR): The laser sounding instrument was intended to detect and characterize aerosols in the atmosphere up to three kilometers above the lander. The instrument operated in two modes: ''active mode'', using an included laser diode, and ''acoustic'' mode, using the Sun as the light source for the sensor. In active mode, the laser sounder was to emit 100-nanosecond pulses at a wavelength of 0.88-micrometer into the atmosphere, and then record the duration of time to detect the light scattered by aerosols. The duration of time required for the light to return could then be used to determine the abundance of ice, dust and other aerosols in the region. In acoustic mode, the instrument measures the brightness of the sky as lit by the Sun and records the scattering of light as it passes to the sensor.{{r|nssdc4}}
; [[Robotic arm|Robotic Arm]] (RA): Located on the front of the lander, the robotic arm was a meter-long aluminum tube with an elbow joint and an articulated scoop attached to the end. The scoop was intended to be used to dig into the soil in the direct vicinity of the lander. The soil could then be analyzed in the scoop with the robotic arm camera or transferred into the thermal evolved gas analyzer.{{r|nssdc3}}
; Robotic Arm Camera (RAC): Located on the robotic arm, the charge coupled camera included two red, two green, and four blue lamps to illuminate soil samples for analysis.{{r|nssdc3}}
; Meteorological Package (MET): Several instruments related to sensing and recording weather patterns, were included in the package. Wind, temperature, pressure, and humidity sensors were located on the robotic arm and two deployable masts: a 1.2-meter ''main mast'', located on top of the lander, and a 0.9-meter secondary ''submast'' that would deploy downward to acquire measurements close to the ground.{{r|nssdc3}}
; [[Thermal and Evolved Gas Analyzer]] (TEGA): The instrument was intended to measure abundances of water, water ice, adsorbed carbon dioxide, oxygen, and volatile-bearing minerals in surface and subsurface soil samples collected and transferred by the robotic arm. Materials placed onto a grate inside one of the eight ovens, would be heated and vaporized at 1,000&nbsp;°C. The evolved gas analyzer would then record measurements using a [[spectrometer]] and an [[Electrochemistry|electrochemical]] cell. For calibration, an empty oven would also be heated during this process for differential scanning [[calorimetry]]. The difference in the energy required to heat each oven would then indicate concentrations of water ice and other minerals containing water or carbon dioxide.{{r|nssdc3}}
; Mars Microphone: The microphone was intended to be the first instrument to record sounds on another planet. Primarily composed of a microphone generally used with [[hearing aid]]s, the instrument was expected to record sounds of blowing dust, electrical discharges and the sounds of the operating spacecraft in either 2.6-second or 10.6-second, 12-bit samples.{{r|nssdc5}} The microphone was built using off-the-shelf parts including a [[Sensory, Inc.]] RSC-164 integrated circuit typically used in speech-recognition devices.{{r|tps1}}