In ancient times heavenly alignments foretold doom. Nowadays they set the
schedule for space exploration.

July 19, 2001: “Beware the Ides of March,” the crone intoned to the Roman
dictator in 44 B.C. But Caesar, secure in his divinity and power, ignored
her and shortly thereafter was sent from this Earth by some of his closest
“friends.” The position of heavenly objects played a role in the
assassination because, by most accounts, it was an astrologer who foretold
his demise. Emboldened by her prediction, Caesar’s assassins turned it into
a self-fulfilling prophecy.

“There was a similar case about 140 years after Caesar met his end,” says
Florian Himmler, a researcher in ancient history at the University of
Regensburg in Bavaria, Germany. “On September 18, 96 A.D., the Roman emperor
Titus Flavius Domitianus was also sent packing by assassins — some were his
closest friends and courtiers. His assassins chose the date and hour of his
departure based upon the position of the planets … including Mars, which
was positioned to make his ‘divine protection’ weakest.”

Centuries ago monitoring the stars and planets was a popular way to plan
daily events. Some say it still is! But the scientific method has shown that
astrology holds little, if any, predictive power. As a result the belief in
astrology is now far less universal than it was in Titus’ day.

Nevertheless there are certain endeavors that are absolutely dependent upon
the positions of the planets. In fact, some of our civilization’s most
advanced organizations, like NASA and its sister space agencies around the
world, sometimes do nothing without first consulting the stars!

In this case, however, it’s not for luck. NASA’s mission planners carefully
check the heavens to assure that their targets — usually planets, comets or
asteroids — are in the right place to make journeys there as short and
inexpensive as possible.

Such checks are rarely done in science fiction. When Star Trek’s Captain
Kirk wants to go someplace he never waits for a propitious alignment — he
just points the Enterprise in the right direction and cries “Warp Speed,
Sulu!” Or in Star Wars, when Han Solo wants to travel to the Alderaan
District, he simply pushes a few buttons and off he goes.

Unlike the mighty vessels of Kirk and Solo, however, our present-day space
ships harbor limited power. Even the awesome Saturn V rocket, which carried
45,000 kg to lunar orbit during the Apollo program, didn’t completely escape
the pull of Earth’s gravity. (Remember, the Moon is trapped by our planet’s
gravitational field and that’s as far as the Saturn V went.) Nowadays the
space shuttle can haul about 25,000 kg into low Earth orbit. Without extra
propulsion built in, however, those payloads are still tightly bound to
Earth’s gravitational field.

Of course, some real-life spacecraft can reach escape velocity and travel to
other worlds. Delta 2 rockets — often used to send missions to Mars — can
loft about 700 kilograms free of Earth’s gravity. But we can’t send those
700 kg anywhere we want, for two reasons. First, such payloads remain bound
to the Sun’s gravitational field. Even after escaping Earth, they are still
trapped within the solar system! Second, once the rocket engine exhausts its
fuel, which happens quickly for chemical rockets, the payload can do little
but coast in the direction it was slung.

Interplanetary coasting can take a long time. The recently-launched 2001
Mars Odyssey, for instance, will reach the Red Planet fully six months after
it left Earth. During that interval Mars will have moved one-quarter of the
way around its orbit. Clearly, it’s vital that we understand not only where
the target is at launch, but also where it will be when the spacecraft
arrives. Present-day astronomers and mission planners find themselves
calculating planetary motions and alignments much as their ancient ancestors

NASA has been considering a human mission to Mars for years. Larry Kos, a
mission planner at NASA’s Marshall Space Flight Center, notes that timing is
everything. “The best time to launch a mission to Mars,” he says, “is
usually a few months before Earth and Mars are closest together — a time
astronomers call opposition. When Mars missions take off, they head toward
an apparently empty point in space. The planet isn’t there yet, but it will
be when the spacecraft arrives.” Of course, if humans go to Mars they will
need to come back, too. “For a return trip we would wait 26 months for a
similar Earth-Mars alignment and once again launch a few months before
opposition. That geometry would minimize the return propulsion needed.”

While Earth and Mars approach each other every 26 months, their minimum
separation varies over a 15 year cycle due to the ellipticity of each
planet’s orbit. Indeed, it can vary by almost a factor of two. Choosing the
right year to launch will have a significant impact on the propulsion power
required to fling a payload from Earth to Mars, and back again.

The next best times to go to Mars will come in 2003, 2018, and 2020 — years
when Earth and Mars will be unusually close together. Humans might finally
visit the Red Planet in 2018 or 2020, but alas, they won’t travel there
aboard vessels like the USS Enterprise or the Millennium Falcon. Our first
Martian explorers will probably blast off on chemical rockets after
intensive calculations of capability, aim points, and timing. In that
regard, human exploration of Mars will begin as have so many other
adventures in history … only when the planets are properly aligned.