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09/12/2002
Twin Travelers
Today, September 11, is a beautiful September day, much like
the day a year ago when our world was shaken to its core. With
the coverage on last Sunday''s "60 Minutes" of our own town''s
losses and of the lives of victims'' families since 9/11, this
anniversary hits even closer to home. There is talk of making
September 11 a holiday. To my mind, holidays tend to be
"celebrated" and September 11, like December 7, deserves to be
remembered but certainly not celebrated.
I thought I should try to find some sort of anniversary that one
could truly celebrate. I think I''ve found one that fits the bill.
Last week, September 5th marked the 25th anniversary of the
launch of the spacecraft Voyager 1 from Cape Canaveral. Just a
couple weeks earlier, on August 20, 1977, Voyager 2 had been
launched. A quarter of a century later, these twin travelers are
still sailing through space and sending back data. I''m indebted to
Brian Trumbore for calling my attention to an article on the
Voyagers by John Wilford of the New York Times dated August
14, which Brian found in the International Herald Tribune. The
article spurred me to go to the Jet Propulsion Laboratory Web
site and also to my copy of Carl Sagan''s book "Cosmos" for
more detailed information on these intrepid travelers.
Having worked on batteries, I wondered about the source of
power for such long-lived spacecraft. Voyager 1 is now some 8
billion miles from Cape Canaveral and there isn''t enough
sunlight out there to allow solar panels of any reasonable size to
be used. The electrical power is actually supplied by so-called
RTGs (Radioisotope Thermoelectric Generators), which rely on
the decay of a radioactive isotope of plutonium. The plutonium,
in the form of plutonium oxide bricks, emits alpha particles. An
alpha particle is simply a helium atom minus all its electrons. In
this decay process, heat is generated. The heat is used to heat up
the junctions of a bunch of thermocouples, which convert the
heat to electricity.
You may be familiar with the use of thermocouples to measure
temperature. Take two wires of different metals or alloys and
twist or weld the wires together at one end to form a junction.
Place the junction in your furnace or wherever it is you want to
measure the temperature. Keep the other ends at room
temperature or some other constant temperature and hook up to a
voltmeter of some kind. You''ll find a voltage that is larger, the
higher the temperature of your furnace. Measure the voltage and
you can look up the temperature in tables for your particular
combination of metals and/or alloy wires. The voltage of a
single thermocouple is not very high but by hooking up a bunch
of them you can get the 30 volts necessary to power a Voyager.
With the plutonium supplying the heat, the Voyagers started off
life with 470 watts of power. In your home, 470 watts will let
you light four 100-watt light bulbs plus one 70-watter.
After 25 years, the power output on the Voyagers is down to only
about 300 watts. Why? As the plutonium decays, there are
fewer and fewer particles given off and less heat is generated.
The thermocouples may also degrade somewhat. What does this
mean for the Voyagers? As time passes, various functions will
have to be turned off to conserve power. For example, this is the
year that the ultraviolet measurements have to be shut off in
Voyager 1 (no UV in Voyager 2 for four years, since 1998). To
make UV measurements the apparatus has to be heated. The
heater has been turned off - hence no more UV. Over the years,
the power will drop to the point where only a single instrument
can be powered at a time and, sometime after 2020, the Voyagers
will become totally silent as they head out to the stars.
We''ve all seen the results of the Voyagers'' tours of Saturn,
Jupiter, Uranus and Neptune and various moons of these planets.
Among the highlights have been gorgeous pictures of the storms
on Jupiter and its famed Red Spot; discovery of volcanoes in
action on Io, one of Jupiter''s moons; the pale blue color of
Neptune and its crackling electrical storms; icy volcanoes on
Triton, a moon of Neptune; etc. etc. The list goes on and on.
Although some might rank landing on the moon as NASA''s
biggest achievement, a strong case can be made that Voyager is
the most successful of all of NASA''s projects. Originally, the
project called for only Jupiter and Saturn to be visited. However,
the Voyagers'' durability and the reprogramming of the onboard
computers by the ingenious ground crew allowed the missions to
be expanded to include the more distant Neptune and Uranus.
The one planet that both Voyagers missed is the outermost
planet, Pluto. However, since they were launched, there has
been the feeling among some astronomers that Pluto no longer
deserves to be called a planet. Now that one of them is twice as
far away as Pluto is from earth, you might think the Voyagers''
job is done. But, hey, those RTGs are still putting out heat and
NASA has decided there''s one more job. The mission has been
renamed the Voyager Interstellar Mission, with the objective of
answering the question "Where does the solar system end?" Or,
alternatively, "Where does interstellar space begin?" We don''t
know. The Voyagers might have just enough power left to
deliver some answers.
Let''s look at the solar wind. The solar wind is the outward flow
of all the particles and magnetic field from the sun. It''s like one
continuous bubble expanding outward and it''s some wind. Its
speed is a million miles an hour! However, as it gets farther and
farther away from the sun, the speed of the wind drops until it
hits what''s known as the heliopause. At the heliopause, the sun''s
dominance over its solar system starts to weaken and the speed
slows to a piddly 250,000 miles an hour. This is in the region
characterized as termination shock. The slowdown of the solar
wind indicates that it''s beginning to feel the effects of another
wind, the interstellar wind. I''m assuming this wind is generated
by all those other zillions of stars in our Milky Way galaxy.
Speculation is that Voyager 1 might reach this region of
termination shock within the next couple of years.
After hitting the termination shock Voyager will enter the
"heliosheath". Here the sun still rules the roost with its solar
wind particles and magnetic field. It may take Voyager some
time, even a decade or two in the heliosheath, before reaching
true interstellar space, where the sun no longer has any influence.
Nobody really knows how big the heliosheath is and where true
interstellar space begins. Hopefully, one or both Voyagers will
provide the answers before they run out of steam.
I haven''t mentioned another limiting factor, aside from the power
generator dying down. That''s hydrazine, the fuel to maneuver
the attitude of the Voyagers. It seems that a couple hundred
pounds of hydrazine were in the voyagers at launch, based on the
fact that about 175 pounds have been used to date (calculated
from weights at launch and today). Fortunately, the NASA
engineers had hoped for the Voyagers to still be functioning after
Jupiter and Saturn and planned wisely for the possible vastly
extended tour to the limits of our solar system.
The Voyagers are now headed off in different directions, one
above and one below the plane in which most of the planets
orbit. Voyager 2, after another 260,000 years, will be a mere 25
trillion miles away from Sirius, the brightest star in our sky.
Much sooner, after only 40,000 years, Voyager 1 will pass only
about 9 trillion miles from that familiar star, AC+79 3888, in the
constellation Camelopardalis. Just in case there are other
intelligent beings out there who might intercept one of our twin
travelers, each contains a Voyager Golden Phonograph Record
with messages and music from Earth. Carl Sagan and his
collaborators assembled the contents of that record and I''m sure
that Sagan would have made those contents quite interesting for
those beings to mull over.
As the Voyagers continue their remarkable journey of discovery
and exploration, we can only wish them, "Bon voyage. You''ve
served us well!" I find it sobering, and inspiring on this 9/11
anniversary, that these two products of man''s ingenuity and
curiosity may still be sailing out there among the stars long after
mankind has disappeared from this Earth.
Allen F. Bortrum
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