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City Tech Physics Professor Says Hitching Ride on an Asteroid May be Safest Way to Travel to Mars

“Hitching a ride on an asteroid may be the safest way for humans to travel to Mars,” according to NASA consultant and City Tech Adjunct Associate Professor of Physics Gregory L. Matloff in a paper co-authored with student Monika Wilga and published in a special March-April 2011 issue of Acta Astronautics. Matloff calls this means of interplanetary travel “NEO-hitchhiking.”

Two dangers, in particular, confront astronauts on what could be a three-year round-trip voyage to the Red Planet by conventional, rocket-propelled means. Such a trip would take nine months or so each way, and the ship’s crew would have to remain on Mars for well over an Earth year until the two planets were again in the proper relationship for the return journey to their home world to commence.

The dangers to life and limb stem from two significant barriers to long-duration travel far from Earth. The least significant of these – although the one that has received the most publicity – is bone degradation stemming from prolonged weightlessness. Solving this problem involves spacecraft design, whereby spinning an interplanetary craft to provide at least partial artificial gravity would almost certainly lessen these damaging health effects.

A more significant impediment to rocket-propelled interplanetary travel was experienced by Project Apollo astronauts between 1968 and 1972. The first and only humans to travel beyond the influence of Earth's magnetic field, to date, they reported visible flashes when their eyes were closed in a darkened spacecraft. These flashes were produced by the impact on the ship of high-energy high-Z cosmic rays.

“At the present time,” say Matloff and Wilga, “little is known about the long-term health effects of exposure to these particles. On a trip to Mars requiring nine months or more, it’s possible that galactic radiation might impair mental acuity or result in life-threatening cancers.

“Two methods have been suggested to shield interplanetary voyagers or space-habitat dwellers from cosmic radiation,” they add. “The most technically sophisticated utilizes magnetic fields on the ship to simulate the shielding effects of Earth’s magnetic field, but this ‘active’ approach is not yet in an advanced stage of development.”

A second and far simpler “passive” approach is mass shielding. “Here,” say the authors, “astronauts would be protected by layers of water, sand, aluminum or other material thick enough to sufficiently attenuate the high-Z galactic cosmic-ray flux. But a lot of mass is required!”

According to one NASA study the paper cites, about 5,500 kg/m2 of shielding would be required to simulate Earth-surface radiation levels, although humans today dwelling on mountain tops and in the orbiting space station, Matloff said more recently, show no noted increase in bad radiation effects.

The approximate thickness of an aluminum shield would have to be about 2 m, Matloff and Wilga state in their paper, a thickness which would greatly increase the mass of an interplanetary spacecraft. Consider, for example, a craft consisting of two joined cylindrical modules with a diameter of 4 m and a length of 16 m. The area of this assembly, not counting the end caps, is about 200 m2. The shield mass is a whopping one million kilograms!

“The projected costs of Mars expeditions with ‘dry’ (unfueled) masses of 100,000 kg, give or take,” the authors note, “is approximately $100 billion. So if we are to launch our Mars-bound astronauts with passive cosmic ray shielding, trillion-dollar costs per mission might be more realistic.”

Matloff and Wilga suggest that before sticker shock sets in, “we might consider an alternative to launching a fully shielded interplanetary mission. This approach makes use of space resources located not too far from the Earth – those small celestial bodies dubbed NEOs or near-Earth objects.

“Most of those celestial icebergs we call comets,” they say, “reside in two locations very distant from the sun: the Kuiper Belt and the Oort Cloud. Most of the rocky and stony minor planets or asteroids reside between the orbits of Mars and Jupiter. In recent decades, however, increasing numbers of extinct comet and asteroid-like objects have been observed in orbits that bring them close to the Earth.”

Although the inner solar system has by now been pretty well cleared of objects like the 10 km object that impacted the Earth 65 million years ago and hastened the demise of the dinosaurs, they note, many smaller NEOs exist. At intervals of centuries, one large enough to destroy a city impacts the Earth. It has been estimated that there may be as many as 100,000 such objects in the 0.1-1.0 km size range. Many of the small NEOs have not yet been discovered. American NEO observers operate under a Congressional mandate to detect 90 percent of the NEOs larger than 140 m by 2020, concurrent with the planned introduction of new human interplanetary capabilities.

For use as stepping stones to Mars, Matloff and Wilga add, it is necessary to locate NEOs near the ecliptic since Mars’ inclination is about 2°. The perihelion of a candidate NEO should be about 1 AU, or the mean distance of Earth from the sun; its aphelion should be about 1.5 AU, or Mars’ mean distance from the sun. A survey of NEOs known prior to 1994 indicates that a number of Apollo-class NEOs (those with orbits that cross Earth's orbit) satisfy this requirement. For readers unfamiliar with the terms “perihelion” and “aphelion,” the closest point to the Sun in an object’s orbit is called perihelion and the furthest point in its orbit is called aphelion. Moreover, the object moves fastest at perihelion and slowest at aphelion.

The authors then suggest that the sizable number of NEOs that fit this requirement raises the possibility of a mode of travel to Mars that would substantially reduce a crew’s exposure to galactic cosmic radiation. In this approach, a NEO is located that orbits close to the ecliptic plane with a perihelion near Earth and an aphelion near Mars.

After Earth escape, a velocity increment is applied to the spacecraft which allows it to rendezvous with the NEO within 2 to 3 months. During the balance of the interplanetary flight and after imbedding itself within the NEO, the object’s material is used as a cosmic-ray shield. Approaching Mars, the spacecraft emerges from the protective cover of the NEO, alters course for the Red Planet and decelerates upon arrival by aerocapture, a process that uses the drag created by the atmosphere of a celestial body to decelerate. During the ship’s return to Earth, a similar strategy is followed.

In August 2008, there were approximately 5,500 NEOs in the data base. Since this is thought to represent only a small percentage of NEOs large enough to shield an interplanetary spacecraft from galactic cosmic rays, many more candidate NEOs are likely to be located as detection sensitivity increases.

“There are many issues and opportunities to be considered regarding this approach to interplanetary travel,” say Matloff and Wilga. “First, is the obvious trade of cosmic-ray shielding for flight time. Interplanetary trips using NEO-hitchhiking will be longer in duration and considerably longer in distance than those trajectories that humans are likely to fly. For the approach considered here to be applied, the spacecraft would have to be buried under 2π of NEO material to provide adequate shielding. Unless massive mining equipment was carried aboard the spacecraft, asteroid equivalents to iron meteorites would probably be inappropriate for this application. A volatile-rich NEO or one with a thick regolith layer [unconsolidated solid material covering the bedrock of a planet or other celestial object] would probably serve better. Moreover, in such a long-duration flight, microgravity will almost certainly be an issue. It might be necessary to place the spacecraft in a small crater near the pole of a slowly rotating NEO and spin [the craft] slowly to create partial gravity after covering it with regolith, rubble or ice.”

Many asteroids are suspected to be volatile-rich, according to the authors, or composed of chemical elements, that is, with low boiling points. If water-ice-rich NEOs were used in the application described here, they conclude, water could be gathered by the crew and stored aboard the spacecraft. Using solar-electrolysis equipment, hydrogen and oxygen could be separated for use as rocket fuel. This material could be applied during Mars approach after separation from the NEO or perhaps used during the return voyage to Earth.


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