Near Asteroid Earth Rendezvous - Report Example

Near Earth Asteroid Rendezvous was renamed after the scientist Gene Shoemaker as NEAR Shoemaker. Its purpose was a study of the near earth asteroid Eros433 in a 24km near orbit spanning a period of one year. The mission holds the record as the first to land on the small area of an asteroid and conduct studies by orbiting the asteroid. Objective of the mission is distributed as primary and secondary objectives. Primary objectives involved the return information analyses for physical, geological, chemical, mineral properties of the asteroid. Secondary objectives are more focused on regolith property study, effect of solar winds and spin activity. Information collected will act as further evidence to study asteroids in comparison to meteors and comets. It was equipped with first ever electronic instruments in a spacecraft and a mission accomplished in shorter duration of 26 months much less than the estimated cost of 150 million USD. Goal of the mission was study of 433 Eros being in orbit over the period of one year. Eros is classified as a second largest near earth S-class asteroid. NEAR shoemaker orbit was maintained at 200km with a slow inward movement to reduce the orbit in the period of 3 months frequency. Starting from 200km, the orbit was gradually reduced to 50km and finally to 35 X 35 km in July 2000. This inner movement of orbit was followed by external movement by increasing the orbit back to 200km to make the study over variations of distance, time, cause and effects. The mission was accomplished with a successful inner orbit to land on Eros 433 saddle region in Feb 2001. NEAR, Near Earth Asteroid Rendezvous marked the first of the discovery series of small launch vehicles. The small launch vehicles were allocated a budget of $150 million much lower than the cost prevailing during the period. It is significant in introduction of exclusive propulsion unit to reduce the dependability on sequential development model and keep the interface simple. The Delta V requirement of NEAR drove the need for a strong yet light structure with a dual mode propulsion system. Solar panels and crucially placed antennae combined with limitation on the trajectory compelled the development of a different thruster system. It is the components based on flights, well organized and designed testing methodology and integrated approach with a small group of team members that drove the successful mission of NEAR from design, development, manufacture, testing and integration in a smaller time frame of 16 months when compared to the projects of this magnitude. APL spearheaded the effort to build the spacecraft instruments, science investigations and manage the NEAR operations. APL used the services of Aerojet to develop and manufacture and test the exclusive propulsion system required for NEAR mission. NEAR mission objective was targeted at 433 Eros on geological, mineral, physical properties and characteristics combined with secondary objectives. Program for NEAR was initiated in December 93 with a time period of 26 months for a 2 week launch between February 1996 to march 1996. It had the upper limit of 30 seconds launch window each day. Space craft design was crucial for the team. With two conditions of time and cost, the importance for a spacecraft design that needs lesser time to build but strong enough to succeed in the mission became a necessity. The time crunch was felt in the systems integration and testing phase. Each subsystem performs well when tested alone but tendency to fail as an integrated system has higher incidence of occurrence. Technical professionals rely on integration testing to overcome the functional deficiency as a complete system. Time restriction on system integration testing could be circumvented by a simple design spacecraft. The simplified design formed the basis of cost reduction, time saving and prevention of sudden failures during integration as the launch deadline neared. The priorities drove the design of the spacecraft to be a bit complex to manage with the thruster projecting externally. It was designed to be located perpendicular to the focus line of earth and the sun. Torque became synonymous during rapid maneuvers and slew due to the thruster placement. Precious fuel was used to handle the torque created. Center of gravity and accurate tracking became high concentration zones. NEAR spacecraft team saved cost through the reduction of instruments size. The camera is a best example of imaging innovation with a telescope beaming a pointed light on a charge coupled device, an electronic film with a small casing about 5 inches. Success of NEAR is attributed to the instruments design based on existing and proved flight instrument designs. The proven designs were remodeled and enhanced to include the required features thus reducing the time taken to complete the project. Testing of the hardware was easier with the flight based instruments. The instruments had already existing ground support facilities for testing to reduce the pressure on limited testing schedule time. Miniature instrument design, proven hardware had marked the success of NEAR and proven that space expeditions can be expedited with cost effectiveness. NEAR shoemaker was launched on a Delta II launch vehicle with solid rocket boosters and PAM-D III stage. Once launched with exit from orbit of the earth, it was on hibernation for 4 months. Its hibernation ended when it flew within 1200km of 253 Mathilde asteroids. It marked the start of information transmission of NEAR shoemaker. Further, the active NEAR made the two part burn through a deep space slew and rapid maneuver to shed the Newton thruster. This reduced the speed and facilitated the perihelion to reduce by 4 points. It synchronized to EROS by orbital incline alteration and decrease of the distance with all instruments recording the data. The mission was designed for a 4 schedule burn and an orbit insertion but the first burn could not be completed due to a technical glitch. The entire schedule was reorganized and NEAR flew by Eros followed by a maneuver to initiate the burn to synchronize NEAR to 433 Eros orbit speed. All the time of this execution, information collection was executed by the near Infrared images by IR spectrograph and radio tracking to capture the information of the flyby. With the orbit speed synchronized to Eros, the trajectory adjustment and alignment was executed using the hydrazine thruster burn. NEAR was fine tuned to the exact speed of EROS through a 2 minute burn to achieve a 300 kilometer per hour velocity. Eros had a heliocentric orbit and NEAR was matched to the orbit of Eros before Orbit insertion. A slew and maneuver followed to slow the spacecraft to relate to EROS was executed. Scan was conducted for presence of satellites for Eros for scientific study and probabilities of collision. The orbit reached 35 km and was maintained for ten days to study the asteroid for the primary objectives of the mission. NEAR moved closer and farther and registered a flyby within 5.3 km of EROS. Spacecraft NEAR alternated between normal and retrograde movements to study the asteroid in multi dimensions. The closest flyby recorded by NEAR is within 2 to 3 km of Eros and made number of passes serially within the 5 to 6 km range before final touchdown on Eros. After touchdown, the spacecraft continued with its contact till late February 2001 after which the contact was lost. NEAR spacecraft has a simple mechanism and designed with minimum possible development phase allocation and test time utilization. It is designed with a single moving mechanism other than the solar panels and protection systems. Design is module based for possibility of independent development and execution of the spacecraft systems. Distinct features deployed for the first time in the spacecraft includes x-band power amplifier to support the projects on comparative studies, hemispherical resonator gyroscope to capture the high frequency resonance and highly reliable accurate controller of power supply. Shape of the spacecraft is octagonal prism with a wind mill look with scientific instruments mounted on the forward and aft deck. Electronics and propulsion units are fitted in the inside of the spacecraft. The wind mill arrangement carries the solar panels made of gallium arsenide and the four windmill blades represent the exposed panels. The antenna feed has the antenna for the x-band radio along with the magnetometer. The forward deck has the solar x-ray monitor to observe the current activity. Rest of the instruments is fitted on the aft deck at the other end of the spacecraft. The spacecraft requires a delta V potential of 1450 m/s. Total delta V potential required is achieved by a main thruster with a 450 Newton (N) capacity filled with one of the bipropellant such as hydrazine or nitrogen tetroxide. Three-axis stabilization is used and the optimum thrust is provided by the four and seven 21 N and 3.5 N hydrazine induced propulsion. Hydrazine thrusters and the reaction wheels provide the controlling mechanism for altitude. Hydrazine and NTO oxidizer required for the functioning of the spacecraft are loaded in the thrust system. Three fuel tanks and two oxidizers present in the thrust system carry 209 kilograms and 109 kilograms of Hydrazine and oxidizer respectively. The solar panels fixed in the windmill pattern form the source of power and each panel is 1.8 meter by 1.2 meter equipped to produce power to store in a nickel cadmium battery with rechargeable provision. The solar panels produce 400w at 2.2 au, the distance at which NEAR is possibly the farthest from the sun. The output of the solar panels is 1800 w at 1 au at optimal distance from the sun. The power generated is stored in 22 storage cells at 9 amps per hour output. Spacecraft navigation is facilitated by a sensor suite, inertial measurement unit and camera specialized in star tracking. The sensor suite has 5 altitude detectors capable of digital transmission, solar driven and positioned away from the instrument facing direction. IMU consists of resonator gyros and accelerometers designed and implemented for the first time in the spacecraft. Reaction wheels are used for control and management of three-axis and also as controller of the altitude. Thrusters neutralize the reaction wheels provided angled momentum and used for faster, thrust driven maneuvers. Command center and data management system is mirrored with 2 command and telemetry processors and state recording units and an alternate power switcher coupled with an interface for dual1553 standardized data buses for internal communication. State recorders are based on 16Mbit IBM Luna-C DRAMS with 1.1 Gbits and 0.67 Gbits storage capacity. NEAR mission marked the beginning of NASA’s Discovery program aimed at faster turn around involving smaller spacecrafts. The cost of the mission involving the development, test, launch and a month of monitoring is approximately USD 122 million well below the USD 150 million allocated for the mission. Controllers commanded the spacecraft into a four hour intensive pre touch down operations to land on the asteroid Eros. The target landing location was a saddle shaped area named Himeros, a part of the asteroid with size of Manhattan. NEAR Shoemaker collected high resolution images and information from asteroids through flyby and touch down on Eros through a one year mission targeted at asteroid 433 Eros. NASA made history with its NEAR shoemaker landing on the asteroid 433 Eros surface for the first time. It was the first landing of a space craft in a small area. It marked the first in approach, design, testing from the engineering side, information gathering from scientific point of view and a success story from cost effective mission launch and execution. Read More