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The Phobos mysteries:


The Jan/Feb 1993 issue of the Planetary Society's "The Planetary Report" contains a brief note written by A.S. Selivanov and U.M. Gektin of the Institute of Space Device Engineering, Moscow, on the mysterious end of Phobos 2.

Main section on the Phobos mysteries.


"Last year I noticed more than one mention in the media of a 'mystery object' that appeared in the last images returned by the Russian spacecraft Phobos 2. It was suggested that aliens were responsible for the object and perhaps even for the demise of the mission. What actually happened?" -Len Seymour, Elko, Nevada.

The "mystery object" was actually the shadow of one of Mars' moons. The spacecraft Phobos 2 reached Mars orbit and began to approach the Martian satellite Phobos in February 1989. In the middle of March, Phobos and the spacecraft were several hundred meters apart, and they moved synchronously in the same orbit. At that time there were several surveys of the Martian surface by the Termoscan equipment on board the spacecraft.

Termoscan is a two-channel scanning radiometer that can receive images in the visible and near-infrared region of the spectrum and at the same time in the thermal, or far-infrared, region. A Termoscan image is produced by a scanning mirror moving perpendicular to the spacecraft trajectory with a frequency of one scan line per second. Thus a picture is generated by the motion of the spacecraft in its orbit.

The survey of the Martian surface was made with a constant Sun-to-spacecraft orientation. The centerline of the image is in the anti-Sun direction to an accuracy of one to two scan lines. Since the spacecraft was near Phobos and the Sun-Phobos directions was approximately the same as the Sun-spacecraft direction, the Phobos shadow on the Mars surface can be seen in the Termoscan field of view (Figure 1).

The shadow.

The length of this shadow was about 21 kilometers (13 miles). Termoscan's field of view on the Martian surface was 650 kilometers (400 miles) wide, and the resolution was 1.8 kilometers (about 1 mile). The moonlet's shadow came into Termoscan's field of view when the spacecraft was 200 kilometers (about 120 miles) away from Phobos. At this point the termoscan instrument was pointed at Mars' surface in the same direction as the Sun's rays. On March 26, 1989, the shadow appeared as shown in Figure 2.

The factors that influenced the form and dimensions of the shadow included Phobos' orientation (Phobos has an irregular shape); distortion from Mars' surface curvature, especially near the planet's limb (the edge of its disk as seen from the Phobos spacecraft); and the dispersion of radiation and other atmospheric processes.

Another factor -- and probably a more important one -- was deviation of the axis caused by the spacecraft's instability. The spacecraft's axis moved about 40 minutes of arc during the experiments.

If the spacecraft's orientation and the distance from it to Phobos had been perfectly constant, Phobos' shadow would have been an even line. But because of the deviation of the spacecraft's axis, Termoscan's lines moved ahead of the shadow or dropped behind it as the shadow moved on the planet's surface. The scanning line overtook the shadow, going through its center, then passed it (see Figure 2).

This process caused 250-to-300-kilometer (155-185-mile) motion on the Mars surface in the direction of motion, and the shadow was elliptically stretched in the resulting picture.

Figure 2 shows images from two different spectral regions. Figure 2a, taken in the infrared part of the spectrum, shows Phobos' shadow. In Figure 2b, a visible image, the shadow image drops behind. This image indicates that the shadow lowered the local surface temperature 4 to 6 degrees Celsius.

The temperature variation depends on how fast Mars' thin surface layer (several millimeters) cools. After analyzing the results from the two spectral channels, we were able to calculate the thermal inertia (the measure of the object's resistance to changes in temperature) of the surface layer and draw conclusions about its physical characteristics.

The calculations showed that almost all of the observed surface is covered with a layer of dust. The thermal inertia is two to three times lower than it would be without the dust. Further analysis will tell us more about Mars' surface properties.

The death of Phobos 2 resulted from a failure in the control system of its on board computer. this caused reorientation of the solar cells such that they did not receive sunlight power. The storage battery was drained of its charge, and all the spacecraft's subsystems lost power. Phobos 1 went out of control earlier when its control lock with Earth was lost after an operator's error.

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This page was last updated on February 27, 2001.