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A Propitious Alignment of Planets

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A Propitious Alignment
of Planets

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

NASA
Marshall Space Flight Center

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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.

Right: Gaius Julius Caesar was one of the most powerful men of his time. He was assassinated at a time based on the location of the planets and stars.

"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."

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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.

Above: Heavy-lifting chemical rockets like the ones that propel the space shuttle have trouble escaping Earth's clingy gravity. Pictured: STS-101.

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 did!

Left: Space ships can coast to Mars by following a Hohmann Transfer Orbit. [learn more from JPL's "Basics of Space Flight"]

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.

Above: The closest distance between Mars and Earth varies by almost a factor of two over an 18 year period. Note that 1 AU (Astronomical Unit) is the average distance between the Sun and the Earth (about 150 million kilometers).

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.


A Propitious Alignment of Planets
July 19, 2001
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The lessons and activities below are based on the Science@NASA story
"A Propitious Alignment of Planets."

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Web Links

The Basics of Space Flight - How do you get to another planet? Can gravity assist you? Find out the answers at this excellent web resource from the Jet Propulsion Laboratory.

Beware the Ides of March - a brief historical overview of Caesar's assassination on March 15, 44 B.C., from about.com (external site).

A Close Encounter with the Planet Mars -- Earth and Mars are closer together than at any time since 1988. Stargazers won't want to miss the Red Planet blazing bright in the midnight sky.

The Great Mars Rush -- Once in about every fifteen years a startling visitant makes his appearance upon our midnight skies --a great red star that ... mounting higher with the deepening night, blazes forth against the dark background of space with a splendor that outshines Sirius and rivals the giant Jupiter himself. -- from Mars by Percival Lowell (1895)


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