Earlier this year Dennis Wingo wrote a very interesting piece for SpaceRef.com on Dennis Tito's announced plan to send two space passengers on a commercial flyby of Mars in 2018.

Mr. Tito has created the Inspiration Mars Foundation which he appears to be personally funding. Although the plan is still evolving, it is clearly inspirational and innovative. Most importantly, it is motivating and exciting, two things that have been missing the U.S. civil space program. While NASA is spending billions and getting little in return, Mr. Tito is proposing to spend a lot of money, but the returns are potentially huge.

Consider the impact on national space policy, increased national prestige, improved numbers of engineers and scientists graduating from universities and the expansion of other innovative space projects and ventures. In addition, surely Mr. Tito has some ideas about how to offset the cost of this mission with some clever marketing and merchandising schemes. More power to him.

The proposed launch date of 2018 was likely derived through a consideration of the expected spacecraft development time and the fact that the minimum energy Mars transfer window opens only on 26-month intervals. And, the 2018 opportunity appears to offer a relatively short time of flight (TOF).

Thus, it would appear that Mr. Tito has selected the first window in which he thinks the spacecraft can be ready. Since the proposed mission will use a minimum energy impulsive transfer trajectory between Earth and Mars, the mission duration is estimated to be 501 days.

A key element of the mission is a free-return trajectory. This term was first used for the lunar Apollo missions in which the trajectory of the spacecraft traveling away from Earth was influenced by moon's gravity, causing the spacecraft to return to Earth without propulsion.

For safety reasons the early lunar Apollo flights used a circumlunar free-return trajectory around the Moon in which the perilune was on the lunar far-side. The Apollo capsule moved through the perilune in a direction opposite to that of the Moon, giving the appearance of a path shaped like a figure 8. Thus, a cislunar free-return trajectory is one in which a spacecraft goes beyond the orbit of the Moon, returns to inside the Moon's orbit and continues on to Earth by Earth's gravity.

In the case of a free-return Mars mission, the concept is some more complicated by the fact that the spacecraft encounters three gravity fields, i.e., those of Earth, Mars and the Sun. More specifically, the spacecraft leaves Earth's influence and enters a heliocentric path around the Sun.

Thus, the initial trajectory must be designed to take the spacecraft on a ballistic flight away from Earth and through a heliocentric arc such that the spacecraft approaches Mars without the use of any propulsive maneuvering. When the spacecraft encounters Mars' gravity it must pass the planet in such a way that its departure path will allow a ballistic flight back to Earth.

Once the Earth departure launch vehicle injects the spacecraft into an Earth-escape trajectory, no propulsive maneuvering is allowed until the spacecraft returns to Earth's gravity field. In order to succeed, the departure must be precisely time to coordinate with planetary positions, a precise departure direction and velocity must be achieved and the passage of Mars must be geometrically exact.

In a 1998 paper, "Mars Free Return Trajectories," Patel, et al., reported on automated software used to analyze these gravity-assisted trajectories. The results indicated that low-energy free returns with TOFs around 3 years (or less) are plentiful and occur every synodic (26 Months) period. Of particular interest here are fast free returns that occur in the next several years. The shortest TOF is about 501 days.