Depiction of Mariner 2 in space
|Mission type||Planetary flyby|
|Operator||NASA / JPL|
|Harvard designation||1962 Alpha Rho 1|
|Mission duration||4 months, 7 days|
based on Ranger Block I
|Manufacturer||Jet Propulsion Laboratory|
|Launch mass||202.8 kilograms (447 lb)|
|Power||220 watts (at Venus encounter)|
|Start of mission|
|Launch date||August 27, 1962, 06:53:14UTC|
|Rocket||Atlas LV-3 Agena-B|
|Launch site||Cape Canaveral LC-12|
|End of mission|
|Last contact||January 3, 1963|
|Perihelion||105,464,560 kilometers (56,946,310 nautical miles)|
|Epoch||December 27, 1962|
|Flyby of Venus|
|Closest approach||December 14, 1962|
|Distance||34,773 kilometers (18,776 nautical miles)|
Mariner 2 (Mariner-Venus 1962), an American space probe to Venus, was the first robotic space probe to conduct a successful planetary encounter. The first successful spacecraft in the NASA Mariner program, it was a simplified version of the Block I spacecraft of the Ranger program and an exact copy of Mariner 1. The missions of Mariner 1 and 2 spacecraft are together sometimes known as the Mariner R missions. The probes had originally been intended to launch on the Atlas-Centaur, but that vehicle was not ready in time for the missions, forcing NASA to use the Atlas-Agena which had a much smaller payload capacity and necessitated greatly simplified probes with a minimum of instrumentation. The Mariner 2 spacecraft was launched from Cape Canaveral on August 27, 1962 and passed as close as 34,773 kilometers (21,607 mi) to Venus on December 14, 1962.
The Mariner probe consisted of a 100 cm (39.4 in) diameter hexagonal bus, to which solar panels, instrument booms, and antennas were attached. The scientific instruments on board the Mariner spacecraft were two radiometers (one each for the microwave and infrared portions of the spectrum), a micrometeorite sensor, a solar plasma sensor, a charged particle sensor, and a magnetometer. These instruments were designed to measure the temperature distribution on the surface of Venus, as well as making basic measurements of Venus' atmosphere.
The primary mission was to receive communications from the spacecraft in the vicinity of Venus and to perform radiometric temperature measurements of the planet. A second objective was to measure the Interplanetary Magnetic Field and charged particle environment.
En route to Venus, Mariner 2 measured the solar wind, a constant stream of charged particles flowing outwards from the Sun, confirming the measurements by Luna 1 in 1959. It also measured interplanetary dust, which turned out to be scarcer than predicted. In addition, Mariner 2 detected high-energy charged particles coming from the Sun, including several brief solar flares, as well as cosmic rays from outside the Solar System. As it flew by Venus on December 14, 1962, Mariner 2 scanned the planet with its pair of radiometers, revealing that Venus has cool clouds and an extremely hot surface.
Spacecraft and subsystems
The Mariner 2 spacecraft was designed and built by the Jet Propulsion Laboratory of the California Institute of Technology. It consisted of a hexagonal base, 1.04 meters across and 0.36 meters thick, which contained six magnesium chassis housing the electronics for the science experiments, communications, data encoding, computing, timing, and attitude control, and the power control, battery, and battery charger, as well as the attitude control gas bottles and the rocket engine. On top of the base was a tall pyramid-shaped mast on which the science experiments were mounted, which brought the total height of the spacecraft to 3.66 meters. Attached to either side of the base were rectangular solar panel wings with a total span of 5.05 meters and width of 0.76 meters. Attached by an arm to one side of the base and extending below the spacecraft was a large directional dish antenna.
The power system of Mariner 2 consisted of two solar cell wings, one 183 cm by 76 cm and the other 152 cm by 76 cm (with a 31 cm dacron extension (a solar sail) to balance the solar pressure on the panels), which powered the craft directly or recharged a 1000 Watt-hour sealed silver-zinc cell battery. This battery was used before the panels were deployed, when the panels were not illuminated by the Sun, and when loads were heavy. A power-switching and booster regulator device controlled the power flow. Communications consisted of a 3 Watt transmitter capable of continuous telemetry operation, the large high gain directional dish antenna, a cylindrical omnidirectional antenna at the top of the instrument mast, and two command antennas, one on the end of either solar panel, which received instructions for midcourse maneuvers and other functions.
Propulsion for midcourse maneuvers was supplied by a monopropellant (anhydrous hydrazine) 225 N retro-rocket. The hydrazine was ignited using nitrogen tetroxide and aluminum oxide pellets, and thrust direction was controlled by four jet vanes situated below the thrust chamber. Attitude control with a 1 degree pointing error was maintained by a system of nitrogen gas jets. The Sun and Earth were used as references for attitude stabilization. Overall timing and control was performed by a digital Central Computer and Sequencer. Thermal control was achieved through the use of passive reflecting and absorbing surfaces, thermal shields, and movable louvers.
Only 40 pounds (18 kg) of the spacecraft could be allocated to scientific experiments. The following scientific instruments were mounted on the instrument mast and base:
- A two-channel microwave radiometer of the crystal video type operating in the standard Dicke mode of chopping between the main antenna, pointed at the target, and a reference horn pointed at cold space. It was used to determine the absolute temperature of Venus' surface and details concerning its atmosphere through its microwave-radiation characteristics, including the daylight and dark hemispheres, and in the region of the terminator. Measurements were performed simultaneously in two frequency bands of 13.5 mm and 19 mm. The total weight of the radiometer was 22 pounds (10 kg). Its average power consumption was 4 watts and its peak power consumption 9 watts.
- A two-channel infrared radiometer to measure the effective temperatures of small areas of Venus. The radiation that was received could originate from the planetary surface, clouds in the atmosphere, the atmosphere itself or a combination of these. The radiation was received in two spectral ranges: 8 to 9 µm (0.00031 to 0.00035 inches) (focused on 8.4 μm) and 10 to 10.8 µm (0.00039 to 0.00043 inches) (focused on 10.4 μm). The latter corresponding to the carbon dioxide band. The total weight of the infrared radiometer, which was housed in a magnesium casting, was 1.3 kg, and it required 2.4 watts of power. It was designed to measure radiation temperatures between 200 and approximately 500 K.
- A three-axis fluxgate magnetometer to measure planetary and interplanetary magnetic fields. Three probes were incorporated in its sensors, so it could obtain three mutually orthogonal components of the field vector. Readings of these components were separated by 1.9 seconds. It had three analog outputs that had each two sensitivity scales: ± 64 γ and ± 320 γ (1 γ = 1 nanotesla). These scales were automatically switched by the instrument. The field that the magnetometer observed was the super-position of a nearly constant spacecraft field and the interplanetary field. Thus, it effectively measured only the changes in the interplanetary field.
- An ionization chamber with matched Geiger-Müller tubes (also known as a cosmic ray detector) to measure high-energy cosmic radiation.
- A particle detector (implemented through use of an Anton type 213 Geiger-Müller tube) to measure lower radiation (especially near Venus), also known as the Iowa detector, as it was provided by the University of Iowa. It was a miniature tube having a 1.2 mg/cm2 mica window about 0.3 cm in diameter and weighing about 60 g. It detected soft x-rays efficiently and ultraviolet inefficiently, and was previously used in Injun 1, Explorer 12 and Explorer 14. It was able to detect protons above 500 keV in energy and electrons above 35 keV. The length of the basic telemetry frame was 887.04 seconds. During each frame, the counting rate of the detector was sampled twice at intervals separated by 37 seconds. The first sampling was the number of counts during an interval of 9.60 seconds (known as the 'long gate'); the second was the number of counts during an interval of 0.827 seconds (known as the 'short gate'). The long gate accumulator overflowed on the 256th count and the short gate accumulator overflowed on the 65,536th count. The maximum counting rate of the tube was 50,000 per second.
- A solar plasma spectrometer to measure the spectrum of low-energy positively charged particles from the Sun, i.e. the solar wind.
The magnetometer was attached to the top of the mast below the omnidirectional antenna. Particle detectors were mounted halfway up the mast, along with the cosmic ray detector. The cosmic dust detector and solar plasma spectrometer were attached to the top edges of the spacecraft base. The microwave radiometer, the infrared radiometer and the radiometer reference horns were rigidly mounted to a 48 cm diameter parabolic radiometer antenna mounted near the bottom of the mast. All instruments were operated throughout the cruise and encounter modes except the radiometers, which were only used in the immediate vicinity of Venus.
In addition to these scientific instruments, Mariner 2 had a data conditioning system (DCS) and a scientific power switching (SPS) unit. The DCS was a solid-state electronic system designed to gather information from the scientific instruments on board the spacecraft. It had four basic functions: analog-to-digital conversion, digital-to-digital conversion, sampling and instrument-calibration timing, and planetary acquisition. The SPS unit was designed to perform the following three functions: control of the application of AC power to appropriate portions of the science subsystem, application of power to the radiometers and removal of power from the cruise experiments during radiometer calibration periods, and control of the speed and direction of the radiometer scans. The DCS sent signals to the SPS unit to perform the latter two functions.
The scientific objectives were:
- Radiometer experiment.
- Infrared experiment.
- Magnetometer experiment.
- Charged particles experiment.
- Plasma experiment.
- Micrometeorite experiment.
Besides the experiments with the scientific instruments, the objectives of both the Mariner 1 and 2 probes included also engineering objectives:
- Evaluation of the attitude control system.
- Evaluation of the environmental control system.
- Evaluation of the entire power system.
- Evaluation of the communication system.
Mariner 2 was launched from Cape Canaveral Air Force Station Launch Complex 12 at 06:53:14 UTC on August 27, 1962 by a two-stage Atlas-Agena rocket. The two-stage Atlas-Agena rocket carrying Mariner 1 had veered off-course during its launch on July 22, 1962 due to a defective signal from the Atlas and a bug in the program equations of the ground-based guiding computer, and subsequently the spacecraft was destroyed by the Range Safety Officer. Mariner 2 nearly met the same fate as its predecessor when one of the Atlas's verniers moved to maximum stop shortly before booster engine cutoff. This caused a rapid roll of the launch vehicle that quickly approached one revolution per second. With the structural integrity of the booster in jeopardy, the range safety officer prepared to issue the destruct command, but almost as soon as it started, the rolling motion stopped and the launch proceeded uneventfully. The incident was traced to a loose wire in the guidance computer which was pushed back into place by the centrifugal force of the roll.
Five minutes after lift-off, the Atlas and Agena-Mariner separated, followed by the first Agena burn and second Agena burn. The Agena-Mariner separation injected the Mariner 2 spacecraft into a geocentric escape hyperbola at 26 minutes 3 seconds after lift-off. The NASA NDIF tracking station at Johannesburg, South Africa, acquired the spacecraft about 31 minutes after launch. Solar panel extension was completed approximately 44 minutes after launch. The Sun lock acquired the Sun about 18 minutes later. The high-gain antenna was extended to its acquisition angle of 72°. The output of the solar panels was slightly above the predicted output. As all subsystems were performing normally, as the battery was fully charged, and the solar panels were providing adequate power, the decision was made on August 29 to turn on cruise science experiments. On September 3, the Earth acquisition sequence was initiated, and Earth lock was established 29 minutes later.
The accuracy of the Atlas-Agena was such that a mid-course correction was required to satisfy the mission requirements. The mid-course correction consisted of a roll-turn sequence, followed by a pitch-turn sequence and finally a motor-burn sequence. Preparation commands were sent to the spacecraft at 21:30 UTC on September 4. Initiation of the mid-course maneuver sequence was sent at 22:49:42 UTC and the roll-turn sequence started one hour later. The entire maneuver took approximately 34 minutes.
Due to the mid-course maneuver, the sensors lost their lock with the Sun and Earth. At 00:27:00 UTC the Sun re-acquisition begun and at 00:34 UTC the Sun was reacquired. Earth re-acquisition started at 02:07:29 UTC and Earth was reacquired at 02:34 UTC.
Loss of attitude control
On September 8 at 12:50 UTC, the spacecraft experienced a problem with attitude control. It automatically turned on the gyros, and the cruise science experiments were automatically turned off. The exact cause is unknown as attitude sensors went back to normal before telemetry measurements could be sampled, but it may have been an Earth-sensor malfunction or a collision with a small unidentified object which temporarily caused the spacecraft to lose Sun lock. A similar experience happened on September 29 at 14:34 UTC. Again, all sensors went back to normal before it could be determined which axis had lost lock. By this date, the Earth sensor brightness indication had essentially gone to zero. This time, however, telemetry data indicated that the Earth-brightness measurement had increased to the nominal value for that point in the trajectory.
Solar panel output
On October 31, the output from one solar panel (with solar sail attached) deteriorated abruptly. It was diagnosed as a partial short circuit in the panel. As a precaution, the cruise science instruments were turned off. A week later, the panel resumed normal function, and cruise science instruments were turned back on. The panel permanently failed on November 15, but Mariner 2 was close enough to the Sun that one panel could supply adequate power; thus, the cruise science experiments were left active.
Encounter with Venus
Mariner 2 was the first spacecraft to successfully encounter another planet, passing as close as 34,773 kilometers (21,607 mi) to Venus on December 14, 1962.
After encounter, cruise mode resumed. Spacecraft perihelion occurred on December 27 at a distance of 105,464,560 km. The last transmission from Mariner 2 was received on January 3, 1963 at 07:00 UTC, making the total time from launch to termination of the Mariner 2 mission 129 days. Mariner 2 remains in heliocentric orbit.
The data produced during the flight consisted of two categories, namely tracking data and telemetry data.
The microwave radiometer made three scans of Venus in 35 minutes on December 14, 1962 starting at 18:59 UTC. The first scan was made on the dark side, the second was near the terminator, and the third was located on the light side. The scans with the 19 mm band revealed peak temperatures of 490 ± 11 K on the dark side, 595 ± 12 K near the terminator, and 511 ± 14 K on the light side. It was concluded that there is no significant difference in temperature across Venus. However, the results suggest a limb darkening, an effect which presents cooler temperatures near the edge of the planetary disk and higher temperatures near the center. This also supported the theory that the Venusian surface was extremely hot or the atmosphere optically thick.
The infrared radiometer showed that the 8.4 μm and 10.4 μm radiation temperatures were in agreement with radiation temperatures obtained from Earth-based measurements. There was no systematic difference between the temperatures measured on the light side and dark side of the planet, which was also in agreement with Earth-based measurements. The limb darkening effect that the microwave radiometer detected was also present in the measurements by both channels of the infrared radiometer. The effect was only slightly present in the 10.4 μm channel but was more pronounced in the 8.4 μm channel. The 8.4 μm channel also showed a slight phase effect. The phase effect indicated that if a greenhouse effect existed, heat was transported in an efficient manner from the light side to the dark side of the planet. The 8.4 μm and 10.4 μm showed equal radiation temperatures, indicating that the limb darkening effect would appear to come from a cloud structure rather than the atmosphere. Thus, if the measured temperatures were actually cloud temperatures instead of surface temperatures, then these clouds would have to be quite thick.
The magnetometer detected a persistent interplanetary magnetic field varying between 2 γ and 10 γ, which agrees with prior Pioneer 5 observations from 1960. This also means that interplanetary space is rarely empty or field-free. The magnetometer could detect changes of about 4 γ on any of the axes, but no trends above 10 γ were detected near Venus, nor were fluctuations seen like those that appear at Earth's magnetospheric termination. This means that Mariner 2 found no detectable magnetic field near Venus, although that didn't necessarily mean that Venus had none. However, if Venus had a magnetic field, then it would have to be at least smaller than 1/10 the magnetic field of the Earth. In 1980, the Pioneer Venus Orbiter indeed showed that Venus has a small weak magnetic field.
The Anton type 213 Geiger-Müller tube performed as expected. The average rate was 0.6 counts per second. Increases in its counting rate were larger and more frequent than for the two larger tubes, since it was more sensitive to particles of lower energy. It detected 7 small solar bursts of radiation during September and October and 2 during November and December. The absence of a detectable magnetosphere was also confirmed by the tube; it detected no radiation belt at Venus similar to that of Earth. The count rate would have increased by 104, but no change was measured.
It was also shown that in interplanetary space, the solar wind streams continuously and the cosmic dust density is much lower than the near-Earth region. Improved estimates of Venus' mass and the value of the Astronomical Unit were made. Also, research, which was later confirmed by other explorations, suggested that Venus rotates very slowly and in a direction opposite that of the Earth.
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- Mariner 2 Mission Profile by NASA's Solar System Exploration
- Full-scale engineering prototype of Mariner 2 in the Smithsonian Air and Space Museum, Washington, D.C.
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