The Phobos (Фобос, Fobos) program was an unmanned space mission consisting of two probes launched by the Soviet Union to study Mars and its moons Phobos and Deimos. Phobos 2 became a Mars orbiter and returned 38 images with a resolution of up to 40 meters. Both probes suffered from critical failures.
Phobos 1 and 2 were of a new spacecraft design, succeeding the type used in the Venera planetary missions of 1975-1985, last used during the Vega 1 and Vega 2 missions to comet Halley.
Phobos 1 was launched on July 7, 1988 and Phobos 2 on July 12, 1988, each aboard a Proton-K rocket. They each had a mass of 2600 kg (6220 kg with orbital insertion hardware attached).
The program featured co-operation from 14 other nations including Sweden, Switzerland, Austria, France, West Germany, and the United States (who contributed the use of its Deep Space Network for tracking the twin spacecraft).
The objectives of the Phobos missions were to:
- conduct studies of the interplanetary environment;
- perform observations of the Sun;
- characterize the plasma environment in the Martian vicinity;
- conduct surface and atmospheric studies of Mars; and,
- study the surface composition of the Martian satellite Phobos.
The main section of the spacecraft consisted of a pressurized toroidal electronics
section, surrounding a modular cylindrical experiment section. Below these were mounted four spherical tanks (the Fregat vehicle) containing hydrazine
for attitude control
and, after the main propulsion module was to be jettisoned, orbit
adjustment. A total of 28 thrusters
(twenty-four 50 N
thrusters and four 10 N thrusters) were mounted on the spherical tanks, with additional thrusters mounted on the spacecraft body and solar panels
. Attitude was maintained through the use of a three-axis control system, with pointing maintained with Sun and star
Phobos 1 operated nominally until an expected communications session on 2 September 1988 failed to occur. The failure of controllers to regain contact with the spacecraft was traced to an error in the software uploaded on 29 August/30 August, which had deactivated the attitude thrusters. By losing its lock on the Sun, the spacecraft could no longer properly orient its solar arrays, thus depleting its batteries.
A natural question is "Why would a spacecraft have instructions that turn off the attitude control, normally a fatal operation?" In this case, these instructions were part of a routine used when testing the spacecraft on the ground. Normally this routine would be removed before launch. However, the software was coded in PROMs, and so removing the test code required removing and replacing the entire computer. Because of time pressure from the impending launch, engineers decided to leave the command sequence in, though it never should be used. However, a single character error in constructing an upload sequence resulted in the command executing, with subsequent loss of the spacecraft.
Phobos 2 operated nominally throughout its cruise and Mars orbital insertion phases on January 29, 1989, gathering data on the Sun, interplanetary medium, Mars, and Phobos. Shortly before the final phase of the mission, during which the spacecraft was to approach within 50 m of Phobos' surface and release two landers, one a mobile "hopper", the other a stationary platform, contact with Phobos 2 was lost. The mission ended when the spacecraft signal failed to be successfully reacquired on March 27, 1989. The cause of the failure was determined to be a malfunction of the on-board computer.
The Phobos design was used again for the long delayed Mars 96 mission which ended in failure when the launch vehicle's fourth stage misfired.
Systems and sensors
Phobos probes carried several instruments: solar x-ray
telescopes, a neutron spectrometer
and the Grunt radar experiment designed to study the surface relief of Phobos.
The lander had an x-ray/alpha spectrometer
to provide information on the chemical element composition of the surface of Phobos, a seismometer
to determine the internal structure of Phobos, and the "Razrez
" penetrator with temperature sensors and an accelerometer
for testing the physical and mechanical properties of the surface.
The Phobos 2 infrared spectrometer (ISM) obtained 30 000 spectra in the near infrared (from 0.75 to 3.2 µm) in the equatorial areas of Mars, with a spatial resolution ranging from 7 to 25 km, and 400 spectra of Phobos at 700 m resolution. These observations made it possible to retrieve the first mineralogical maps of the planet and its satellite, and to study the atmosphere of Mars. ISM was developed at IAS and DESPA (Paris Observatory) with support from CNES.
List of instruments:
- "VSK" TV imaging system
- PROP-F "hopping" lander. Only carried by Phobos 2.
- ARS-FP automatic X-ray fluorescence spectrometer
- ferroprobe magnetometer
- Kappameter magnetic permeability / susceptibility sensor
- temperature sensors
- BISIN conductometer / tiltmeter
- mechanical sensors (penetrometer, UIU accelerometer, sensors on hopping mechanism)
- "DAS" (long-lived autonomous station) lander
- TV camera
- ALPHA-X Alpha-Proton-X-Ray Spectrometer
- LIBRATION sun sensor (also known as STENOPEE)
- RAZREZ anchor penetrometer
- Celestial mechanics experiment
- "ISM" thermal infrared spectrometer/radiometer - 1-2 km resolution
- near-infrared imaging spectrometer
- thermal imaging camera; magnetometers
- gamma-ray spectrometers
- X-ray telescope
- radiation detectors
- radar and laser altimeters
- Lima-D laser experiment - designed to vaporise material from the Phobos surface for chemical analysis by a mass spectrometer
- "Grunt" imaging radar - Only carried by Phobos 1
- R. Z. Sagdeev & A. V. Zakharov (1989). "Brief history of the Phobos mission". Nature 341 581–585.
- Articles in Nature 341 (1989) pages 581 - 619