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Deep Space 2

The Deep Space 2 project is sending two identical microprobes along on the Mars Polar Lander spacecraft. Released shortly before the lander enters the planet’s atmosphere, the probes will dive toward the surface and bury themselves up to about three feet (1 meter) underground.

As a project under NASA’s New Millennium Program, the main purpose of Deep Space 2 is to flight-test new technologies to enable future science missions – demonstrating innovative approaches to entering a planet’s atmosphere, surviving a crash-impact and penetrating below a planet’s surface. As a secondary goal, the probes will search for water ice under Mars’ surface.

Mission Overview

At the time of launch, the two Deep Space 2 probes were attached to the cruise stage on the Mars Polar Lander spacecraft. To simplify hardware and operations, there are no electrical interfaces between the probes and the lander’s cruise stage. The probes are powered off during cruise, so there is no communication with them from installation on the launch pad until after impact on the Martian surface.

Five minutes before the lander enters Mars’ upper atmosphere on December 3, 1999, the lander will jettison the cruise stage. The force of separation will initiate mechanical pyro devices, which in turn will separate the microprobes from the cruise stage about 18 seconds later. Each Deep Space 2 entry system consists of a basketball-size aeroshell containing a probe somewhat larger than a softball.

Upon release from the lander’s cruise stage, the probes switch on power from their lithium batteries, and an onboard computer microcontroller powers up. The microcontroller performs a series of measurements of onboard subsystems to verify their health after the 11-month cruise to Mars.

About four minutes after power-up, the probes will enter Mars’ atmosphere. A descent accelerometer is turned on and samples “G” forces 20 times a second until impact. Four minutes after entering the atmosphere, an impact accelerometer begins sampling “G” forces 25,000 times a second. When impact is detected, data from the event is stored in computer memory, and the impact accelerometer is turned off.

The two probe systems will hit the Martian surface about the same time as Mars Polar Lander’s landing some 35 miles (60 kilometers) away. Upon impact, the acorn-shaped aeroshell will shatter, and the probe inside will separate into two parts. The bullet-shaped forebody will penetrate as far as 3 feet (1 meter) below the surface, depending on the hardness of the soil. The aftbody will remain on the surface to relay data back to Earth via the Mars Global Surveyor spacecraft, which has been orbiting Mars since September 1997. The forebody and aftbody communicate with each other via a flexible cable.

Unlike any spacecraft before, the Deep Space 2 probes smash into the planet at speeds of up to 400 miles per hour (200 meters per second). The probe’s electrical and mechanical systems must withstand this crushing impact. This is achieved with a combination of advanced materials, mechanical designs and microelectronic packaging techniques developed based on extensive testing. After impact, the systems must withstand extreme temperatures. The forebody buried in the Martian soil must withstand temperatures as low as -184 F (-120 C), while the aftbody that remains on the surface is exposed to an environment as low as -112 F (-80 C).

Landed mission

Following impact, the probes collect data to flight-validate their microelectronic and micromechanical technologies. Minimum data to validate most of these technologies will be collected within the first 30 minutes after impact, while minimum data from the sample/water experiment will be collected within about 10 hours after impact. Data collection will continue until the probe batteries are depleted in about one to three days. Each probe will transmit data to the orbiting Mars Global Surveyor using a radio in the UHF band (at frequencies near the upper channels of a conventional TV set) at a rate of 7,000 bits per second. The first such communications session is expected within eight hours after impact. Normally each probe will be in a low-power listening mode until it receives a signal from Mars Global Surveyor telling it to transmit data. The orbiter may either transmit the data to Earth immediately, or store the data temporarily and transmit them as soon as possible.

The first opportunity for a communication pass between Global Surveyor and the microprobes will take place at about 7:27 p.m. PST Friday, December 3. During a pass about 15 minutes, 16 seconds long, Global Surveyor will switch back and forth between communicating with each of the microprobes for about two minutes apiece, relaying the data to Earth immediately. It is possible that contact may not be made on the first pass due to the orientation of the microprobes in the Martian soil. If this is the case, contact may be established during any of a series of communications passes carried out every two hours for the first day or two after arrival.

The microprobe data are buffered onboard Global Surveyor using the memory of its camera. The data therefore are first sent to Malin Space Science Systems in San Diego, CA, which is responsible for the Global Surveyor camera. The Malin team may be able to determine about an hour after the communication pass if any data from the microprobes are present. The data will then be forwarded to JPL, where scientists may get their first look at the contents perhaps another hour later.

Science mission

Deep Space 2 has a secondary goal of collecting science data.

Accelerometer data from the descent and impact will provide an estimate of the density of the atmosphere and hardness of the soil. After impact, the probes will measure the thermal conductivity and potential water content of the subsurface soil adjacent to the bullet-like probe forebody.

Following the first successful transmission to Global Surveyor, a micromotor will drive a small drill bit out the side of the probe’s forebody. Bits of soil engaged by the drill bit will fall into a small heater cup, which is sealed by firing a pyro which closes a door. The soil is then heated, driving any water vapor into the analysis chamber. If water is present, it will be detected by measuring the difference in light intensity of a laser shining through the vapor. The tunable diode laser is set so that its light is at the point in the spectrum where water absorbs light.

Soil conductivity is determined by measuring the rate at which the forebody cools after plunging into the ground. Temperature readings are taken throughout the landed mission by two sensors mounted at opposite ends of the probe’s forebody.

End of prime mission

The prime mission ends when the probes transmit to Mars Global Surveyor one set of data evaluating the project’s engineering technologies. This transmission is expected within 10 hours after impact, but may take place up to 36 hours after impact. At the end of the prime mission, the probes will continue to collect and transmit data on soil thermal conductivity as the probes gradually cool, as well as soil temperature variations, until their batteries are depleted.

Deep Space 2 flight system

Technologies

Deep Space 2 is the second mission of NASA’s New Millennium Program, whose goal is to greatly increase the efficiency and lower costs of space science missions through new technologies. Each New Millennium mission is designed to test specific technologies never before used in space missions.

Deep Space 2 will test technologies that could pave the way for future missions featuring multiple landers released from a single spacecraft, possibly distributed around an entire planet or other body. Such networks of probes offer a unique window on global processes such as weather or seismic activity of a planet.

To meet this goal, the mission was challenged to develop an entry and landing system that is very small, lightweight and capable of conducting experiments on both the surface and subsurface of a planet or similar body while surviving environmental extremes. Deep Space 2 will validate the following new technologies:

• Entry system. Unlike other probes, the Deep Space 2 aeroshell is not required to be pointed or spin-stabilized when it enters Mars’ atmosphere. Its design uses the same principle as a shuttlecock, or birdie, in badminton; most of the weight is placed well ahead of the aeroshell’s center of pressure to insure that the heat shield passively aligns itself even if it is tumbling when it enters Mars’ atmosphere. In addition, the entry system is “single-stage” from atmospheric entry until impact – there are no parachutes, retro rockets or airbags to slow the probes down. In fact, the aeroshell is not even jettisoned by the probe, but accompanies it to the surface of the planet where it shatters on impact. This very simple system greatly reduces the number of tests required to demonstrate that the design works, and thus greatly reduces the costs to the mission. The entry system was designed at JPL. Aerodynamic analysis was performed at NASA’s Langley Research Center, Hampton, VA.
• 
• The aeroshell’s heat shield is made of an advanced thermal protection system known as SIRCA-SPLIT (silicon-impregnated, reusable ceramic ablator secondary polymer layer-impregnated technique). This material is capable of maintaining the probe’s internal temperature to within a few degrees of -40 F (-40 C) while the heat shield surface experiences temperatures of up to 3,500 F (2,000 C). This material was developed and tested by NASA’s Ames Research Center, Moffet Field, CA. The silicon carbide aeroshell structure was developed by Poco Specialty Materials, Decatur, TX.
• Testing of the entry system design went through many phases. Early tests included dropping test articles made of clay pots, Styrofoam or Pyrex from airplanes two miles (3 kilometers) high. Silicon carbide, the same material used in sandpaper, was selected as the material for the aeroshells, each of which weighs less than 2.6 pounds (1.2 kilogram). Final tests of a prototype aeroshell with a probe model were conducted using an airgun at Eglin Air Force Base, Fort Walton Beach, FL, where the probe system was shot into the ground at speeds up to 400 miles per hour (200 meters per second).
• Penetrator system. Deep Space 2 is the first penetrator sent by NASA to another planet. Development of the penetrator system required an aggressive test program with a continuous design/develop/test/fix approach. The probe’s bullet-like forebody is designed with a halfcircle nose to ensure penetration over a wide range of entry conditions. The aftbody features a wide frontal area to limit penetration and a “lawn dart” face which helps the aftbody anchor to Mars’ surface. Tests were performed using an airgun in Socorro, New Mexico, in partnership with Sandia National Laboratories and the New Mexico Institute of Mining and Technology’s Energetic Materials Research and Test Center.
• High-G packaging techniques. Crashing into a planet at 400 miles per hour (200 meters per second) presents a unique challenge in the design of electrical and mechanical systems. Decelerations could reach levels up to 30,000 G’s in the forebody and 60,000 G’s in the aftbody (one “G” is equivalent to the force of gravity on Earth’s surface). This is the same as requiring a desktop computer to operate after being hit by a truck at 400 miles per hour. In comparison, Mars Pathfinder experienced forces of about 17 G’s during its landing. There are two standard approaches for insuring high-G survival. One is to cushion the object, and the other is to provide a very rigid structure that allows the shock wave to pass through the object without deflecting it enough to break any of its components. For Deep Space 2, cushioning is impractical because of the extreme decelerations and the small size of the probes; engineers thus chose a rigid structure approach.
• The mechanical design features a “prism” electronics assembly, a science “block” and selective use of materials to maximize structural rigidity. The electrical design features chipon-board and three-dimensional high-density-interconnect packaging, encapsulated wire bonds and extensive use of flexible interconnects instead of wires. Assemblies are also typically bonded together to minimize potential loose parts and to distribute loads evenly. Micro-telecommunications system. Each probe features a miniaturized radio transmitter and receiver system weighing less than 1-3/4 ounces (50 grams), 9.9 square inches (64 square centimeters) in size, and consuming less than 500 milliwatts in receive mode and 2 watts in transmit mode. The system was developed at JPL.
• Ultra-low-temperature lithium battery. The probe’s batteries must be able to provide 600 milliamp-hours of power at temperatures as low as -112 F (-80 C). To meet those extreme needs, the Deep Space 2 project developed a new non-rechargeable lithium-thionyl chloride battery. The cells use a lithium tetrachlorogallate salt instead of the more conventional lithium aluminum chloride salt to improve low-temperature performance and reduce voltage delays.
• Each probe uses two batteries composed of four “D-sized” cells weighing less than 1.4 ounce (40 grams) each. The batteries operate within a range of 6 to 14 volts and have a shelf life of three years. The batteries were developed by Yardney Technical Products, Pawcatuck, CT. Power microelectronics. Power conditioning, regulation and switching for electronics in the bullet-shaped forebody are controlled by a power microelectronics unit making use of application-specific integrated circuits (ASICs) in which both digital and analog components are incorporated onto a single chip. The unit weighs less than 1/5th of an ounce (5 grams), has a volume of one-third cubic inch (5.6 cubic centimeters) and requires 5/100ths of 1 milliwatt to operate. The unit was developed by Boeing Missiles & Space, Kent, WA.
• Advanced microcontroller. The spartan computer system on the probes centers around an 80C51 microprocessor, a low-power chip used in products ranging from microwave ovens and videocassette recorders to cars and computer peripherals. The 8-bit system includes 128K of random access memory (RAM), 128K of permanent memory, and 32 digital-to-analog and analog-to-digital converters. The system is designed to use very low power (less than 6 milliwatts running at 10 megahertz, one-half milliwatt in sleep mode) with small volume (0.13 cubic inch (2.2 cubic centimeters)) and mass (1/10th of an ounce (3.2 grams)). Electronic circuits are embedded in plastic to ensure survival during the 30,000-G impact event. The microcontroller was developed by a consortium led by the U.S. Air Force’s Phillips Laboratory and including Mission Research Corp., the Boeing Co., NASA’s Langley Research Center, Technology Associates, General Electric and the University of Tennessee.
• Flexible interconnects. Normal wire cabling could easily break under the extreme “G” forces that the probes will endure, so a different approach was required. The flexible interconnect are strips made of alternating layers of copper traces and polyimide. The latter, a type of thin polymer or plastic film, is also used in thermal blankets on spacecraft, while the copper traces are similar to the thin copper paths on a computer or radio circuit board. Flexible interconnects are much lighter, more compact and more flexible than standard wire cables. On the Deep Space 2 probes, they are used between all electronic subsystems, and for the umbilical which connects the forebody (penetrator) to the aftbody (ground station). During flight, the umbilical is folded in a canister like a fire hose; at impact, the umbilical unfolds as the penetrator pulls away. The flexible interconnect system was developed by JPL in partnership with Lockheed Martin Astronautics, Denver, CO. The units were fabricated at Electrofilm Manufacturing Co., Valencia, CA, and Pioneer Circuits Inc., Santa Ana, CA.
• Sample collection/water detection experiment. Each probe will obtain a sample of subsurface soil using a small, ruggedized drill run by an electric motor. When the motor is powered on, a latch is released and the drill shaft extends sideways from the forebody (penetrator), pulling less than 1/250th of an ounce (100 milligrams) of soil into a small cup which is then sealed. The sample is then heated, turning any water ice in the soil into water vapor. A small tunable diode laser emits a beam of light through the vapor to a detector; if water vapor is present, it will absorb some of the light. The laser assembly is similar to tunable diode lasers flown on meteorology experiments on Mars Polar Lander, but is much smaller (about the size of a thumbnail) and thus has lower sensitivity. During operation, the water detection experiment requires a peak power of 1.5 watts. The sampling collection system is about 1 cubic inch (11 cubic centimeters) in size and weighs less than 1.6 ounce (50 grams). The instrument electronics is about one-third cubic inch (4.8 cubic centimeters) in size and weighs less than onethird ounce (10 grams). The tunable diode laser is about 1/50th of a cubic inch (0.3 cubic centimeter) in size and weighs less than 1/30th of an ounce (1 gram). The sample collection/water detection experiment was developed by JPL.
• Soil thermal conductivity experiment. The probes will use temperature sensors to measure how fast the forebody or penetrator cools down after impact, revealing how quickly heat dissipates in the soil. This approach requires far less energy than similar previous experiments on planetary missions, which have used onboard heaters to test the soil. On the Deep Space 2 probes, two platinum-resistor temperature sensors are mounted in the forebody.
• Design, development and testing. Because of the many challenges associated with developing NASA’s first planetary penetrator system, Deep Space 2 embarked on a rigorous design and test program. This started in spring 1995 to evaluate early design concepts before the project was formally approved in the fall of that year. Early tests included releasing prototype probes from airplanes and helicopters.
• As test articles became more sophisticated and expensive, the need for a more controlled test environment became necessary. To accomplish this, the project teamed with Sandia National Laboratories and the New Mexico Institute of Mining and Technology’s Energetic Materials Research and Test Center in Socorro, New Mexico, to use a Sandia airgun. This massive airgun has a 18-foot-long (5.5-meter), 6-inch-diameter (15-centimeter) barrel and rests on an 18-wheeler truck. After mounting the test article in the barrel, pressurized air is used to hurl the probe into the desert floor at speeds of up to 400 miles per hour (200 meters per second). More than 70 airgun tests over a period of two years were performed to validate the probe design under worst-case entry conditions. The last test of impact survivability was performed in September 1998.
• A variety of tests were performed to validate the aeroshell design. Tests were performed at Eglin Air Force Base in Fort Walton Beach, FL, to verify that the aeroshell shatters on impact, leaving the probe to penetrate the surface. Eglin provided a large airgun 20 feet (6 meters) long and 15 inches (38 centimeters) in diameter and led the test operations. Tests of the aeroshell’s aerodynamic properties were performed initially at Eglin’s Wright Laboratory Ballistic Range and later at a transonic wind tunnel in Kalingrad, Russia, capable of simulating Martian atmospheric pressures. The aeroshell’s heat shield material was tested at an arcjet facility at NASA’s Ames Research Center, Moffet Field, CA.

Science Objectives

As a mission under NASA’s New Millennium Program, the main focus of Deep Space 2 is testing new technologies on behalf of future science missions. In the process, however, the probes collect data of interest not only to engineers developing technologies but also to scien tists studying the environment of Mars. NASA thus organized a team of scientists to work with the data that the probe’s instruments will deliver.

The objectives for Deep Space 2’s science measurements dovetail with those of Mars Polar Lander, which is focused on understanding the climate of Mars. Deep Space 2 will attempt to:

• Determine if ice is present in the subsurface soil;
• Estimate the thermal conductivity of the soil at depth;
• Determine the atmospheric density throughout the probes’ entire descent;
• Characterize the hardness of the soil and possibly the presence of any layering on a scale of many inches to a few feet (tens of centimeters).

The layered terrain around Mars’ south pole is believed to consist of alternating layers of wind-deposited dust and water and/or carbon dioxide ice condensed out of the atmosphere. These deposits are thought to record the evidence of climate variations on Mars, much like the growth rings of a tree. Deep Space 2 will help give clues about where water ice is located today on Mars and how materials are deposited in the polar layered terrains. Since the two Deep Space 2 probes and Mars Polar Lander will touch down at different locations up to about 35 miles (60 kilometers) apart, data from each of them will tell scientists how much the polar terrain varies from one site to another.

Science activities on Deep Space 2 are organized as four investigations:

• Sample collection/water detection experiment. This experiment will obtain a tiny soil sample and heat it to detect any water that may be present. The presence or absence of water ice at a given depth will be compared to analysis of soils excavated by the robotic arm on Mars Polar Lander. One hypothesis is that much of the water that once flowed on Mars’ surface is now frozen underground; this experiment will help to refine theories of the fate of Martian water. Science team members selected for this experiment are Dr. Bruce Murray, California Institute of Technology, and Dr. Aaron Zent, NASA Ames Research Center.
• Soil thermal conductivity experiment. Temperature sensors in the forebody or penetrator will show how quickly the probe’s heat dissipates into the surrounding soil. This will provide information about Mars’ polar layered deposits. A very low conductivity would indicate very fine-grain material, likely to have been wind-deposited. On the other hand, a very high conductivity would indicate large amounts of ice in the soil. Soil conductivity has a strong influence on the subsurface temperature, and thus the depth at which ice is predicted to be stable over many annual cycles. Dr. Paul Morgan, Northern Arizona University, and Dr. Marsha Presley, Arizona State University, were selected to analyze data from this experiment.
• Atmospheric descent accelerometer. The aftbody houses a descent accelerometer that will measure the drag on the probes as they descend through the Martian atmosphere. This single piece of information can allow scientists to develop profiles of many meteorological factors in Mars’ atmosphere, including density, temperature and pressure at various altitudes. Science team members for atmospheric science are Dr. David C. Catling and Dr. Julio A. Magalhaes of NASA Ames Research Center.
• Impact accelerometer. The impact accelerometer will provide an estimate of the hardness of the soil, and possibly the presence of small-scale layers that can be compared with the materials encountered by the robotic arm on Mars Polar Lander. Scientists can interpret these terrain layers in terms of the geologic materials they are probably made of, such as ice layers, wind-blown dust and sediments. Data on the small-scale strata of Mars’ polar layered terrains could yield important information on climate evolution. Dr. Ralph D. Lorenz, University of Arizona, and Dr. Jeffrey E. Moersch, NASA Ames Research Center were selected for this experiment.