Permanent Human Settlement of the Earth, Space and Ocean Frontiers

Thursday, March 09, 2006

Islands of Space

Ralph Buttigieg

Sydney, NSW


While the Moon and Mars are NASA's goals for human exploration there are other destinations which are easier to reach then Mars and would be of great interest to private explorers. These are the asteroids, the islands of Space.

Most of the asteroids are found in the Asteroid Belt, between the orbit of Mars and Jupiter, however in 1898 the asteroid Eros was discovered and astronomers realised its orbit brought it close to Earth. Since then several hundred other close approachers have been found and are known as the Near Earth Asteroids (NEA). The NEAs are classified into three groups, the Atens which have average orbital diameters closer than one AU and are usually inside the Earth's orbit, the Apollos which have average orbital diameters greater than that of the Earth and cross Earth's orbit and the Amors which have average orbital diameters in between the orbits of Earth and Mars and never cross Earth's orbit. New NEA are discovered every year and its estimated only about 10% have been found so far.

They are of interest to scientists because of the real posibility of a collision with Earth and as a source of raw materials. Something like 25% of the NEAs appear to be former comets and could contain substantial amounts of water. As Dennis has written, water is the most important resource in Space so their significance can not be underestimated.

Various unmanned spacecraft have flown past asteroids and two have attempted landings. In 2002 NEAR Shoemaker touched down on Eros's surface. Last year the Japanese ion drive spacecraft Muses-C managed to land on Hayabusa and hopefully will return a sample to Earth.

Because of their closeness they are good targets for human explorers not just robots. One potential target is 1991 VG. This tiny world can be reached in as little as 15 days. With a 30 stay the mission takes 60 days and the Delta Vee cost is 6.1 km/s. Stretch the mission out to 90 days and Delta Vee goes down to 4.9 km/s. Another good candidate is KY26, this worldlet, only 30 meters in diameter, appears to be loaded with water and might serve as provisions stop for explorers.

Landing on an asteroid will be have unique problems. Any dust stirred up could take hours to settle because of the light gravity. It would be more like docking a spacecraft then a planetary landing. Astronaut Thomas D Jones describes working on an asteroid as follows:

What would the encounter phase of an NEA mission be like? The cruise vehicle would orbit from a safe distance while sending field explorers down to the surface. In the very low gravity field-measured in thousandths of a g - these EVA's would be much more like my delicate tiptoeing around the ISS than the hard-charging marathons of the Apollo moonwalks. Tethering and delicate control will be all-important, since an astronaut and his tools could easily drift away from the surface. More reminiscent of Apollo will be the dust problem. Stirred up by sampling work or thruster firings, the fine particles will take minutes to settle. Planetary scientist Dan Durda of Southwest Research Institute suggests that the astronaut experience will be analogous to cave diving: near-weightless, with the slightest wrong move stirring up a vision-obscuring cloud that will dissipate only slowly. Instead of trying to stand and walk on such a surface, an astronaut might be equipped with a portable scaffold. The strong, light framework could anchor him within arms' length of the work at hand, yet prevent an embarrassing facedown drift into the primordial dust. Imagine a souped-up MMU jetpack, with adjustable struts to prop an astronaut in any desired orientation. Each EVA would end at a central work platform, where each spacesuit would get a thorough - and welcome - dust-off.

You can expect the explorers of the 21st Century will use the asteroids as stepping stones Mars and beyond.