sealPurdue News

August 2000

Mars launch would offer safety option for astronauts

WEST LAFAYETTE, Ind. – The best opportunity in the near future to launch the first human mission to Mars will come in 2014 because an alignment of planetary bodies that year provides an ideal escape route back to Earth, in case of an Apollo 13 type of accident.

So concludes a research paper written by a Purdue University engineer and one of his students.

Owing to the timely orbital alignments of Earth, Mars and Venus, slingshot maneuvers requiring only minor course adjustments would be possible, bringing the spacecraft home safely. But the emergency flight path would be possible only if the spacecraft were launched within a few days of Jan. 14, 2014. No similar escape option exists for at least a decade before or after that time frame, meaning astronauts might be forced to attempt a landing on the red planet even if their spacecraft became crippled in an accident on the way to Mars, says James Longuski, a professor of aeronautics and astronautics at Purdue.

"This trajectory is remarkably fortuitous as it does not exist for many years prior to or after the 2014 date," Longuski says in the paper, which will be presented Aug. 15 during the Astrodynamics Specialist Conference and Exhibit,in Denver. The conference is co-sponsored by the American Institute of Aeronautics and Astronautics and the American Astronomical Society.

Coincidentally, NASA had identified 2014 as a possible launch date for the first human mission to Mars in a 1997 study. That study, Human Exploration of Mars can be accessed on line.

Longuski and graduate student Masataka Okutsu discovered that the safest route to take would be one that permitted a quick return trip, via Venus, in case of an accident that forced the Mars landing to be aborted. The Martian gravity would bend the spacecraft's trajectory, hurtling it toward Venus, where another gravity assist would guide the ship to Earth. Because of the gravity-assisted trajectories, the spacecraft could make the return trip with only minor attitude adjustments from small thrusters, even if its main engine were disabled, Longuski says.

Apollo 13, launched in 1970, was to be the third mission to land on the Moon. An explosion in one of the oxygen tanks crippled the spacecraft during flight, raising fears that the crew might be lost. But it was brought home safely, in part by using the moon's gravity to slingshot the craft and crew back toward Earth.

Longuski and Okutsu discovered the Mars option using a software program, originally developed by engineers at NASA's Jet Propulsion Laboratory but then improved by the Purdue engineer and his students, who made it hundreds of times more powerful.

"My contribution, along with my students, was to automate the software and make it the potent tool that it is," Longuski says. "Without that automation, it has one-thousandth the power."

The software program, which is called STOUR (pronounced Ess Tour), automatically identifies the myriad possible routes that a spacecraft might travel, considering its launch date, rocket power and ultimate destination, and then displays the flight paths on a graph. It enables engineers to calculate complex spacecraft trajectories within hours or days, instead of the months or even years it would take with conventional methods.

"If you are going to Mars, Venus and back to the Earth, you can look at a launch window 20 years wide and compute every possible gravity-assist trajectory that exists," Longuski says.

The conventional method requires engineers to compute each possible trajectory one at a time.

"It's like people doing accounting by hand and then suddenly having computers to do it," he says.

A faster technique for mapping the trajectories is needed because space missions often require a series of several gravity assists from planetary bodies, precisely strung together in just the right way so that the spacecraft arrives properly at its final destination. Engineers highly skilled in celestial mechanics may take months or years to plan the complex "tours," only to see their methodical calculations discarded because of launch delays that require entirely new tours to be calculated, possibly with little time to spare, Longuski says.

Source: James Longuski, (765) 494-5139,

Writer: Emil Venere, (765) 494-4709,

Purdue News Service: (765) 494-2096;

NOTE TO JOURNALISTS:  A copy of the Purdue research paper referred to in this release is available from Emil Venere, (765) 494-4709,



Mars Free Returns Via Gravity Assist From Venus

Masataka Okutsu and James M. Longuski
Purdue University

The safety of the crew is the top priority for human exploration of Mars. If an unexpected emergency occurs, a free-return trajectory brings the spacecraft back to the Earth without a deterministic maneuver. We use an automated design tool to search for Mars free-return trajectories, which satisfy NASA's Design Reference Mission (DRM) constraints. While no cases exist for Mars alone, a Mars-Venus free return does meet the DRM requirements for a launch in 2014. This trajectory is remarkably fortuitous as it does not exist for many years prior to or after the 2014 date.

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