Conceptual drawing of
Mars Reconnaissance Orbiter over Mars
NASA's Mars Reconnaissance Orbiter (MRO) is a multipurpose spacecraft, launched August 12, 2005 to advance knowledge of Mars through detailed observation, to examine potential landing sites for future surface missions, and to provide a high-data-rate communications relay for those missions. It is intended to orbit for four or more years, and to become Mars' fourth active artificial satellite (joining Mars Express, Mars Odyssey, and Mars Global Surveyor), and its sixth active probe (the satellites plus the two Mars Exploration Rovers), in an historic scientific focus on Mars.
Overview
MRO will conduct its science mission for an expected two-year period after aerobraking and technical checks are completed in November 2006. After that, extended science and communications relay missions are expected.
The Mars Reconnaissance Orbiter will lay the groundwork for NASA's planned surface missions: a lander called Phoenix selected in a competition for a 2007 launch opportunity, and the Mars Science Laboratory, a highly capable rover being developed for a 2009 launch opportunity. The MRO's high-resolution instruments will help planners evaluate possible landing sites for these missions both in terms of science potential for further discoveries and in terms of landing risks. The MRO's communications capabilities will provide a critical transmission relay for the surface missions; MRO will even be able to provide critical navigation data to these probes during their landing. Also it may provide evidence which could help to uncover the reasons behind the failure of past Mars missions such as NASA's Mars Polar Lander, and the British Beagle lander.[1]
Launch and cruise
Mars Reconnaissance Orbiter launched on August 12, 2005. Between August 10 and August 30, two-hour launch windows were available almost every day. It was launched from Space Launch Complex 41 at Cape Canaveral Air Force Station, aboard an Atlas V-401 rocket equipped with a Centaur upper stage. Fifty-six minutes after launch the Centaur completed its burns placing MRO in interplanetary transfer orbit towards Mars.
MRO cruised through interplanetary space for 7½ months before reaching Mars. During the cruise testing and calibrations of most of the scientific instruments and experiments were conducted. Four trajectory correction maneuvers were planned for any need to correct the trajectory for proper orbital insertion upon reaching Mars. Only three trajectory correction maneuvers were undertaken, a fourth correction was deemed unnecessary; also a fifth emergency trajectory correction option was skipped.
Launch and cruise timeline
- On April 30, 2005, the spacecraft was delivered to the launch site.
- On August 9, 2005, the earliest launch opportunity on August 10 was postponed due to reliability concerns over the Atlas V's gyroscopes.
- On August 10, 2005, concerns over the gyroscopes were resolved. Launch was scheduled for 7:50 am EST August 11.
- On August 11, 2005, concerns over weather cause a rescheduling of the launch to 9:00 am EST August 11. Conflicting sensor readings during fueling of the Centaur stage's liquid hydrogen fuel tank could not be corrected in time, causing the launch to be scrubbed, rescheduling launch to 7:43 am EST August 12.
- At 7:43 am EST August 12, 2005, MRO was launched. There were no significant anomalies reported during launch and deployment into interplanetary transfer orbit.
- On August 15, 2005, the MARCI (see below) was tested and calibrated.
- Mars Reconnaissance Orbiter was 100 million kilometers away from Mars at August 25, 2005 15:19:32 UTC.
- On August 27, 2005, the first trajectory correction maneuver was executed. The burn lasted 15 seconds and used the same main thrusters that are needed for the orbital insertion maneuver and they worked as expected. A velocity change of 7.8 meters per second was achieved.
- On September 8, 2005, MRO completed calibration and testing of the HiRISE and CTX by taking pictures of the Moon from 10 million km away.
- On November 18, 2005, MRO underwent its second course correction as scheduled: firing its 6 medium thrusters for 20 seconds and changing its velocity by 75 centimeters per second.
- Mars Reconnaissance Orbiter was 10 million kilometers away from Mars at January 29, 2006 06:59:24 UTC.
- On February 3, 2006, Mars Reconnaissance Orbiter began Approaching Phase.
Orbital insertion and aerobraking
Artwork of
MRO aerobraking
The start of the orbital insertion occurred as MRO approached Mars for the first time on March 10, 2006, passing above the Martian southern hemisphere at an altitude of about 370–400 km (190 mi). All six of the orbiter’s main engines burned for 27 minutes reducing the speed of the probe (relative to Mars at closest approach) from ~2900 m/s (~6500 mph) to ~1900 m/s (~4250 mph).
The helium pressurization tank was colder than expected which caused about 21 kpa (3 psi) less pressure in the fuel tank, which caused the engine thrust to be reduced by 2%. MRO automatically compensated for the reduced thrust by extending the burn time by 33 seconds. The final speed of MRO relative to Mars was only 0.17 m/s (0.4 mph) faster than expected.[2],[3],[4]
Orbital insertion has placed the orbiter in a highly elliptical polar orbit. The periapsis, the closest point in the orbit to Mars, is 3709 km from the center of Mars (about 329 km from the surface). The apoapsis, or farthest point in the orbit from Mars, is 47,972 km from the center of Mars. The orbital period is approximately 35.5 hours.
Aerobraking will be conducted soon after orbital insertion to bring the orbiter to a lower, quicker orbit. Aerobraking cuts the fuel needed to reach the desired orbit roughly in half, and consists of three steps:
- MRO will drop the periapsis of its orbit to aerobraking altitude using its thrusters. Aerobraking altitude will be determined at that time depending on the thickness of the Martian atmosphere (Martian atmospheric density changes over the seasons on Mars). This step will take about five orbits or one Earth week.
- MRO will remain in aerobraking altitude for 5½ Earth months, or less than 500 orbits. Correct aerobraking altitude will have to be maintained with occasional corrections in periapsis altitude using its thrusters. Through aerobraking the apoapsis of the orbit will be reduced to 450 km (280 mi).
- To end aerobraking, the MRO will use its thrusters to move its periapsis out of the edge of the Martian atmosphere.
After aerobraking another week or two will be spent to make additional adjustments in the orbit with thrusters. These corrections will likely occur before solar conjunction when Mars will appear to pass behind the Sun from Earth perspective, between October 7 and November 8, 2006. After this, science operations will begin. Final or science operations orbit will be at approximately 255 km (160 statute miles) to 320 km (200 mi) above the Martian surface. After reaching science operational orbit the SHARAD will be deployed.
Orbital insertion timeline
- On March 10, 2006, Mars Reconnaissance Orbiter successfully completed orbital insertion.
- In the next few weeks MRO's controllers will begin the "walk in" phase of aerobraking, where the periapsis is lowered into Mars' atmosphere. [5]
Science operations and extended mission
From November 2006, science operations will be conducted for a nominal period of two Earth years. After this extended mission operations will include communication and navigation for lander and rover probes.
Instrumentation
Science instrumentation of
MRO
The broad goals of the Mars Reconnaissance Orbiter are to search for evidence of water, and characterize the atmosphere and geology of Mars.
Six science instruments are included on the mission along with two "science-facility instruments", which use data from engineering subsystems to collect science data. Three technology experiments are also included to demonstrate new technologies for future missions.[6]
- Cameras
- HiRISE (High Resolution Imaging Science Experiment)
- CTX (Context Camera)
- MARCI (Mars Color Imager)
- Spectrometer
- CRISM (Compact Reconnaissance Imaging Spectrometer for Mars)
- Radiometer
- MCS (Mars Climate Sounder)
- Radar
- Science-Facility
- Technology experiments
Science instrumentation
HiRISE
The High Resolution Imaging Science Experiment (HiRISE) camera consists of a 0.5 meter reflecting telescope, the largest of any deep space mission, and has a resolution of 1 microradian, or 1 meter at a height of 300 km. (For comparison purposes, satellite images of Earth are generally available to a resolution of 0.1 meter, and satellite images on Google Maps are available to 1 meter.[7]) It can image in three color bands, 400-600 nm (blue-green or B-G), 550-850 nm (red) and 800-1000 nm (near infrared or NIR).[8]
Red color images are at 20 264 pixels wide (6 km in a 300 km orbit), and Green-Blue and NIR are at 4048 pixels wide (1.2 km). HiRISE's onboard computer reads out these lines in time with the orbiter's ground speed, meaning the images are potentially unlimited in height. Practically this is limited by the onboard computer's 28 Gb memory capacity. The nominal maximum resolution of red images is 20 000 × 40 000 pixels, or 800 megapixels and 4000 × 40 000 pixels (160 megapixels) for the narrower images of the B-G and NIR bands. A single uncompressed image will use 16.4 Gb. However, these images will be transmitted compressed, at a total size of 5 Gigabytes. These images will be released to the general public on the HiRISE website via a new format called JPEG 2000. [9],[10],[11]
The general public will be allowed to request sites to take pictures of Mars with the HiRISE camera. For this reason, and due to the unprecidented access of pictures to the general public, shortly after they have been received and processed, the camera has taken the philosophy "The People's Camera". [12]
To facilitate the mapping of potential landing sites, HiRISE can produce stereo pairs of images from which the topography can be measured to an accuracy of 0.25 meter.
|
|
A worker prepares HiRISE before it is shipped for attachment to the spacecraft
|
Comparison of resolution of MRO HiRISE camera with predecessor, the MOC aboard MGS
|
CTX
The Context Imager (CTX) will provide grayscale images (500 nm to 800 nm) up to 40 km wide with a pixel resolution of 8 meters. The CTX is designed to operate in conjunction with the other two imaging devices to provide (as the name describes) context maps for the areas being observed.
The optics consist of a 350 mm focal length Maksutov Cassegrain telescope with a 5064 pixel wide line array CCD, similar to the HiRISE instrument. It has enough memory on board to record a 160 km long "track" before the image is loaded into the main computer with its 20GB storage.[13]
MARCI
The Mars Color Imager (MARCI) will image Mars in five visible and two ultraviolet color bands. MARCI will produce a global map to help characterize daily, seasonal and year-to-year variations in Mars' climate, as well as providing daily weather reports for Mars.
CRISM
The Compact Reconnaissance Imaging Spectrometers for Mars[14] (CRISM) instrument is an infrared/visible light spectrometer, to produce detailed maps of the mineralogy of the surface of Mars. It has a resolution of 18 meters at a 300 km orbit. It will operate from 400 nm to 4050 nm, measuring the spectrum in 560 channels, each 6.55 nm wide.
MCS
The Mars Climate Sounder (MCS) is a nine-channel spectrometer with one visible/near infrared channel (0.3 to 3.0 micrometers) and eight far infrared (12 to 50 micrometers). These channels are selected to measure temperature, pressure, water vapor and dust levels.
It will observe the atmosphere on the horizon of Mars (as viewed from MRO), breaking it up into vertical slices, and taking measurements within these slices at heights separated by about 5 km (3 miles).
These measurements will be assembled into daily global weather maps, showing the basic variables of Martian weather: temperature, pressure, humidity and dust density.
SHARAD
An artist's concept of MRO using SHARAD to "look" under the surface of Mars
The orbiter's Shallow Subsurface Radar (SHARAD) experiment is designed to probe the internal structure of Mars' polar ice caps, as well as to gather information planet-wide about underground layers of ice, rock and, perhaps, liquid water that might be accessible from the surface.
SHARAD operates between 15 and 25 MHz HF radio waves. It will have a vertical resolution as low as 7 m and penetration depth up to 1 km deep. It will have a horizontal resolution as low as 0.3 by 3 km. SHARAD is designed to operate in conjunction with Mars Express' MARSIS radar which has lower resolution but much greater penetrating depth. Both instruments were made by the Italian Space Agency.[15]
Science facility
Gravity Field Investigation Package
Variations in the gravitational field of Mars can be deduced from variations in MRO's velocity. The velocity of the MRO will be detected using the doppler shift of the Orbiter's radio signal as received on Earth.
Atmospheric Structure Investigation Accelerometers
Sensitive accelerometers aboard the Orbiter will be used to deduce the in situ atmospheric density. This experiment will only be conducted during aerobraking, when MRO is lower, in denser parts of the atmosphere.
Technology experiments
Electra
The Electra, a UHF antenna, is designed to communicate with spacecraft as they approach, land and operate on Mars, and will also aid in landing navigation and positioning.[16]
Optical Navigation Camera
The Optical Navigation Camera will image Phobos and Deimos against background stars, to precisely determine MRO's orbit. This is not mission critical, and has been included to test the system for future orbiting and landing spacecraft as a means of making more accurate orbital insertions and landings.[17]
The Optical Navigation Camera was tested succesfully during February and March 2006. [18]
Engineering data
Size comparison of
MRO with predecessors
Structure
Workers at Lockheed Martin Space Systems in Denver, assembled the spacecraft structure and attached instruments. The instruments were built for it at the University of Arizona, Tucson; at Johns Hopkins University Applied Physics Laboratory, Laurel, Md.; at the Italian Space Agency, Rome; at Malin Space Science Systems, San Diego, Calif.; and at JPL.
The structure is made of mostly carbon composites, as well as aluminum honeycombed plates. The titanium fuel tank takes up most of the volume and mass of the structure and provides a large percentage of structural integrity.
- Total mass is less than 2,180 kg (4,806 lb)
- Dry mass (without fuel) is less than 1,031 kg (2,273 lb)
Power systems
Mars Reconnaissance Orbiter's solar panel
Mars Reconnaissance Orbiter gets all of its electrical power from two solar panels. Each panel can move independently around two axes of movement (up-down, or left-right rotation). Each solar panel measures 5.35 × 2.53 m and on the front side, 9.5 m² (102 ft²) of the surface of each panel is covered with 3,744 individual photovoltaic cells. The very high efficiency triple junction solar cells are able to convert more than 26% of the Sun's energy directly into electricity, and are connected together so that the power they produce is at 32 V, which is the voltage that most devices on the spacecraft need in order to operate properly. At Mars, the two panels together produce 2,000 watts of power (6,000 watts in Earth orbit).
Mars Reconnaissance Orbiter uses two nickel metal hydride rechargeable batteries. The batteries are used as a power source when the solar panels are not facing the Sun (such as during launch, orbital insertion and aerobraking) or when Mars blocks out the Sun during a period in each orbit. Each battery has an energy storage capacity of 50 ampere-hours (180 kC). The spacecraft can't use this total capacity, because as the batteries discharge their voltage drops. If the voltage drops below about 20 volts the computer will stop functioning. So, only about 40% of the battery capacity is planned to be used.
Electronic systems
Mars Reconnaissance Orbiter’s main computer is a 133 MHz, 10.4 million transistor, 32-bit, RAD750 processor. This processor is a radiation hardened version of a PowerPC 750 or G3 processor, with a specially built motherboard. The RAD750 is a successor to the RAD6000. This processor may seem underpowered in comparison to a modern PC or Mac processor but it is extremely reliable and resilient, and can function in solar flare ravaged deep space.
Data is stored in a 160 Gbit (20 GB) flash memory module consisting of over 700 memory chips, each with 256 Mbit capacities. This memory capacity is not actually that large, considering how much data is going to be acquired; for example, a single image from HiRISE camera can be as big as 28 Gbit.[19]
The operating system software is VxWorks and has extensive fault protection protocols and monitoring.
Navigation systems
Navigation systems and sensor will provide data on position, course and attitude throughout the mission.
- Sixteen Sun sensors (eight are backups) are placed around the spacecraft to measure the direction of the Sun relative to the spacecraft's orientation.
- Two star trackers are used to provide full knowledge of the spacecraft orientation and attitude. Star trackers are simple digital cameras used to map the position of catalogued stars autonomously.
- Two inertial measurement units are onboard (the second for backup purposes) provide data for any spacecraft movement. Each inertial measurement unit is a combination of three accelerometers and three ring-laser gyroscopes.
Telecommunications system
The Telecom Subsystem uses a large antenna to transmit at the normal Deep Space communications frequency (X-band, 8 GHz), as well as demonstrating the use of the Ka-band, at 32 GHz, for high data rates. Maximum transmission speed from Mars is projected to be as high as 6 Mbit/s, a rate ten times higher than previous Mars orbiters. Two amplifiers for the X-band radio frequency transmit at 100 watts, the second is a backup. One amplifier Ka-band radio frequency transmits at 35 W. Two transponders are carried in total.
Two smaller low-gain antennas are present for lower-rate communication during emergencies and special events, such as launch and Mars Orbit Insertion. These antennas do not have focusing dishes and can transmit and receive from any direction.
MRO High Gain Antenna installation
|
|
Propulsion system
A 1175 L (310 US gallon) fuel tank filled with 1187 kg (2617 lb) of hydrazine monopropellant. Fuel pressure is regulated by adding pressurized helium gas from an external tank of helium. Seventy percent of the fuel will be used for orbital insertion alone.
A total of 20 rocket engine thrusters.
- Six large thrusters, mainly meant for orbital insertion. Each producing 170 N (38 lbf) of thrust; total 1,020 N (230 lbf) of thrust. (These thrusters were originally designed for the Mars Surveyor 2001 Lander.)
- Six medium thrusters, for performing trajectory correction maneuvers and attitude control during orbit insertion, each producing 22 N (5 lbf) of thrust.
- Eight small thrusters, for attitude control during normal and all operations, each producing 0.9 N (0.2 lbf)
Four momentum wheels are also used for precise attitude control, such as during high-resolution imaging, where the slightest unwanted motion could cause blurring of the image. Each wheel is used for one axis of motion. The fourth wheel is for backup, in case one of the other three wheels fails. Each wheel weighs 10 kg (22 lb), and can be spun as fast as 100 Hz (6000 rpm).
References
- This article contains material and/or images that originally came from a NASA website. According to their site usage guidelines, "NASA material is not protected by copyright unless noted". For more information, please review NASA's use guidelines.
- ↑ Mars Reconnaissance Orbiter Overview. Mars Reconnaissance Orbiter Website. URL accessed on February 11, 2005.
- ↑ NASA TV coverage of JPL MRO Control Room. (2006). "NASA TV [TV subscriber channel/webcast]." USA:JPL/NASA.
- ↑ NASA TV coverage of MRO post-insertion press conference. (2006). "NASA TV [TV subscriber channel/webcast]." USA:JPL/NASA.
- ↑ "Spaceflight Now" MRO Mission Status Center. URL accessed on March 12, 2006.
- ↑ MRO Timeline: Aerobraking. Mars Reconnaissance Orbiter Website. URL accessed on March 11, 2006.
- ↑ Spacecraft Parts: Instruments. Mars Reconnaissance Orbiter Website. URL accessed on February, 2005.
- ↑ "Google Earth FAQ" Google Earth Website.
- ↑ MRO HiRISE Camera Specifications. HiRISE website. URL accessed on 2 January 2006.
- ↑ HiRISE: Instrument Development. NASA Ames Research Center website. URL accessed on 7 February 2006. (PDF)
- ↑ Fact Sheet: HiRISE. National Air and Space Museum. URL accessed on 18 February 2006. (PDF)
- ↑ Luner and Planetary Sciences, University of Arizona (2005). "HiRISE Facts Sheet"
- ↑ HiRISE. Lunar and Planetary Sciences, University of Arizona. URL accessed on 19 March 2006.
- ↑ MRO Context Imager (CTX) Instrument Description. Malin Space Science Systems website. URL accessed on 17 August 2005.
- ↑ CRISM Instrument Overview. CRISM Instrument Website. URL accessed on April 2, 2005.
- ↑ Spacecraft Parts: SHARAD. Peer Review Papar website (PDF Format). URL accessed on August, 2005.
- ↑ Spacecraft Parts: Electra. Mars Reconnaissance Orbiter Website. URL accessed on February, 2005.
- ↑ Spacecraft Parts: Optical Navigation Camera. Mars Reconnaissance Orbiter Website. URL accessed on February, 2005.
- ↑ Optical Navigation Demonstration Near Mars Multimedia Feature. NASA Mars Reconnaissance Orbiter Website. URL accessed on March, 2006.
- ↑ Spacecraft Parts: Command and Data-Handling Systems. Mars Reconnaissance Orbiter JPL Website. URL accessed on 17 August 2005.
See also
External links
