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- If Robert Frost had been the poet laureate of space flight,
he might have written, "Something is there that doesn't like a Mars
probe." And a comic cartoonist once drew an ugly, hungry space beast
lurking near Mars to devour Earth's space vehicles (the painting hung on
the wall of a mission manager at NASA's Jet Propulsion Laboratory for years).
You get the picture.
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- This past decade has not been kind to Earth's Mars probes.
There was NASA's expensive Mars Observer blowing up in 1993 as it warmed
its rocket engines up to slow into orbit. And we've just seen both of NASA's
1999 missions fail.
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- Russia lost another ambitious probe in 1995 when its
upper stage failed, dumping radioactive fragments onto the Andes Mountains.
And a Japanese mission, their first to Mars, went off course right out
of the gate in December 1998.
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- But the most bizarre loss of a Mars probe is unarguably
the case of Phobos-2 (or Fobos-2, in the Russian spelling). It "disappeared"
in March 1989, under very unusual circumstances that still mystify and
excite many people.
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- Recent developments in the Russian space program have
opened new insights into that failure. But first, here's some history.
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- Trying to lift the curse
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- The Soviet Union launched two probes towards Mars in
mid-1988, trying to break a decades-old jinx. Its initial series of small
probes (1960-1965) had been a total disaster, and a series of heavier probes
(1969-1973) didn't do much better. But this third generation was much more
promising.
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- The spacecraft "bus" -- the main body -- was
of an entirely new design. It had new engines, new computers, new communications
gear. And this new mission carried subsatellites to be dropped onto the
inner Martian moon, Phobos.
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- But the old jinx still prevailed. The first probe was
lost due to an erroneous command on the outbound leg. The second vehicle
was crippled by electronics failures and by the time it reached Mars on
January 30, 1989, it was operating on its last and lowest-powered radio.
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- Nonetheless, it slipped into orbit around Mars and slowly
matched its path with Phobos. As it closed in, it also made observations
of Mars.
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- A dozen times, it turned its cameras away from Mars and
towards Phobos. This required the whole spacecraft to turn, since a movable
"scan platform" hadn't been installed. The maneuver also turned
the dish antenna away from Earth, cutting communications for several hours
each time.
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- On March 27, 1989, the probe began another Phobos photo
maneuver, and as expected radio signals ceased. But after the planned maneuver,
when listeners on Earth expected to reacquire the signal, nothing was heard.
More careful listening picked up brief bursts of radio signals, as if the
dish antenna were swinging wildly through space and only occasionally beaming
back towards Earth. Then -- only silence.
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- Strange shadows explained
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- But not for long. Soon a strange and wonderful story
grew and spread, about mysterious structures observed on the surface of
Mars. The probe's last view, so the tale continued, showed a miles-long
oval object closing in. The object's elliptical shadow could be seen on
the surface of Mars thousands of miles below.
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- And indeed, cigar-shaped shadows were plainly visible
in many of the 37 photographs that the doomed probe sent back to Earth
during the 60 days it survived circling Mars.
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- Such images are not unusual in the archives of American
Mars orbiters, from Mariner-9 to the Viking Orbiter twins, to the Mars
Global Surveyor, still at work there to this day. In those cases, what
was seen were shadows of the moonlet Phobos, stretched by being projected
at a low angle to the Martian surface.
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- But Fobos-2 was on a different orbit, and for the last
few weeks it was fairly close to the moon Phobos. Thus, any view of that
object's shadow on Mars would have to be fairly circular. Think about the
geometry -- use props if you want to. It's an illuminating exercise in
3-D perspectives. If you're next to a roundish object casting a shadow,
the shadow will look more-or-less roundish to you no matter how steeply
tilted the surface it's projected on is lying.
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- So where did the cigar-shaped shadow come from?
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- Several years ago, mission scientist Aleksandr Selivanov
explained the cigar-shaped shadow in the Fobos-2 images this way.
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- The imaging system is a "scanning radiometer,"
not a camera, Selivanov pointed out. A rotating mirror moves perpendicular
to the line of the probe's motion over Mars. As a result, "a picture
is generated by the motion of the spacecraft in its orbit." The probe
did NOT gather an entire image in one snap, but accumulated it over a period
of time, line by line.
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- Fobos-2 was staring straight back along the Sun-to-Mars
line, to get the best infrared readings. In contrast, visible light imagers
prefer to look for shadows cast by surface features, so they are aligned
at large angles to the Sun's rays. This made the visible-light images from
Fobos-2 look washed out.
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- Now, Fobos-2 was quite near the moon Phobos in the last
days of its flight, both circling Mars along the same path. So the roundish
shadow of Phobos was on Mars's surface, within the field of view of the
scanner, when the scanner was looking "down sun" at Mars.
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- Selivanov explained that if the probe had been rock steady,
the Phobos shadow would have left a dark streak right through the entire
center of each image, as the image was assembled line-by-line over the
course of each orbit. Because of a slight rocking of the probe, however,
the scanning beam "sliced" the Phobos shadow at different points,
from back to front, over the course of each imaging session.
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- The resulting elongated shadow is thus an artifact of
the imaging technology, and of the probe's motion through space and around
its own axis. Selivanov argued that since these shadows are all precisely
aligned along the probe's flight path over Mars, they are unquestionably
not shadows of other objects near Mars. They show the shadow of Phobos.
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- One supposed photograph, the "last one before the
attack," shows Phobos and a bright vertical line below it. Since the
line runs right along the telemetry scan lines, space experts are confident
it is some sort of transmission flaw, not a real object in space. Besides,
the date on the image is March 25, two days before the probe's loss.
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- Hurry and other human errors
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- The breakdown of Fobos-2 was disappointing to experts
associated with the program, but not surprising. They had seen human error
doom its sister craft, Fobos-1, before it even got to Mars, and they had
seen signs that Fobos-2 wasn't in much better shape.
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- Dr. Larry Soderblom was one of the American scientists
who had instruments aboard the probe. "There is a feeling in the American
space science community the Russians were in too much of a hurry,"
he later told a reporter. "The two satellites lost were launched without
much thought to a system of checks and balances that might have prevented
such problems."
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- And even the design was questionable. "Soviet scientists
at the Space Research Institute in Moscow complained that the new, sophisticated
spacecraft actually was designed for purposes other than those for which
it was being used on the missions to Phobos," wrote a British space
expert. "Engineers adapted it for the mission in order to flight-test
it for future missions to which it was considered better suited."
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- These problems were recognized even as the missions were
launched. I remember telling another reporter: "I'd be surprised if
both make it -- and I wouldn't be surprised if neither do."
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- Everything returns
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- But space is full of surprises, and 11 years later, Fobos-2
has suddenly been reborn. A rocket stage based on its design was launched
into orbit on February 9 and performed perfectly. A second and more ambitious
test on March 20 also went perfectly.
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- The stage is called "Fregat," Russian for "Frigate"
-- and in fact, on many Fobos-2 photographs from 1989, one can read the
designation "Phobos-Fregat." In 1995, project manager Vladimir
Ashushkin described to me his hopes for a commercial deal to carry paying
customers into space by adapting and improving the interplanetary module.
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- That deal has now been signed with a number of European
customers and Fregat spacecraft will soon be heading off into space again,
into high Earth orbits, out to the moon, and even back to Mars -- a Fregat
is slated to carry the European Space Agency's Mars Express probe in 2003.
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- With a better design, and better luck, the curse of Fobos-2
may be dispelled. But Mars may have its own ideas about that!
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