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| 2 Strategy - A Concept | 2.4 Step 1 - Leaving Low earth Orbit |
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| 2 Strategy - A Concept | 2.4 Step 1 - Leaving Low earth Orbit |
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The figure below shows the strategic roadmap for NEOs. As will become apparent, NEOs are very interesting targets in terms of space resources, as well as space science. Considering the fact that they appear to be more numerous than we originally thought, their exploration becomes more and more attractive as well as necessary.  Strategic Roadmap for NEOs The technical requirements for expeditions to NEOs are intermediate between those for lunar and those for Martian missions. A particularly attractive aspect is that human space flight beyond the Moon will probably require incremental steps and an expedition to a NEO will be considerably shorter duration, lower risk, and lower cost than one to Mars (see "target selection" later in this section). Therefore, NEOs could serve as a test bed for systems necessary for the human Mars missions. Appreciation of the fact that some Near-Earth Objects (NEOs) can collide with Earth has led to increased support from NASA for systematic surveys of potentially hazardous objects (Committee on Planetary Exploration, 1988). They are classified into three categories, the Apollo asteroids, which have their orbit crossing the Earth's orbit, the Atens asteroids, which have the semi-axis of their orbit inside the Earth's orbit and the Amor asteroids which have the perihelia of their orbit between the Earth's orbit and 1.3 AU.  Figure 2-4: Location of the Average Orbit for Each Category The vast majority of Near Earth asteroids are thought to have originated in two ways. Firstly, asteroids from the Main Belt (between Mars and Jupiter) which dropped down to lower orbits due to gravitational perturbations, or occasionally due to a collision with another asteroid in the Main Belt. Other NEOs include comets whose orbits were lowered when they passed too close to a planet. It is generally estimated at this time that more than about 25% of the near-Earth asteroids are captured comets and the rest came from the asteroid belt. Astrophysicists' calculations have led to a consensus that over the next 100 million years, most near-Earth asteroids will have been thrown back out by close gravitational encounters with the inner planets or will collide with the inner planets. Conversely, a new supply will be constantly generated from the Main Belt and incoming comets will replace these losses. The estimated population of NEOs includes approximately 1400 Earth crossers (Atens, Apollos, Earth-crossing Amors) of at least 1-km in size and an additional ~1500 non-crossing Amors (Figure 2-4). As of April 1998, about 140 known Earth crossers and some 120 non-crossing Amors were of this size. Thus, the discovery of NEOs is estimated to be about 9% complete for objects larger than 1km in diameter. Although the task is well begun, the vast majority of NEOs remain to be found (Committee on Planetary Exploration, 1988). The starting point of our strategy is based on the detection of asteroids, which contain water. Asteroids are classified as a function of their spectroscopic signature. The C-Class asteroids "carbonaceous chondrites" are the most water rich with an average of about 10% water in a clay mineral (~12% for the class C1 and 5.7% for the class C2) (Committee on Planetary Exploration, 1988). A robotic mission will then be necessary in order to confirm spectroscopic data and initiate further exploration. The surface of an asteroid has not yet been touched by a man made probe. However, there have been analyses of asteroids by flyby probes (e.g. NEAR probe which flew to Mathilde and is currently en route to the asteroid Eros), and future asteroid flybys and landers (e.g. Clementine 2), including sample return missions, are underway, planned or proposed. The database today is far from complete, and the abundance of NEOs with usable water is still not known. If little water is found, it will speed up water research and extraction on Mars and its moons and restrain NEOs exploration to science and mining. Nevertheless, the strategy assumes that usable water is available on some of these bodies (see "target selection" in the following). Water extraction and exploitation is described in section 2.5. In case of lack of water on the Moon (e.g. for lunar bases), water from NEOs could be supplied by cargo missions, since the Dv requirement is considerably lower than in the case of terrestrial supply.  (During Close Flyby of Earth in 1992) Source: J.H. King, "The NSSDC Photo Gallery: Asteroids", http://nssdc.gsfc.nasa.gov/image/planetary/asteroid/ idadactyl_color.jpg (current September 5 1997). Scientific research on these bodies will be focused on geology. This is key to understanding aspects of the solar system's formation. Robotic and human studies are critical for determining whether complex asteroids have inherited accretional structure or have acquired heterogeneity by internal geological processing or by random collisions that resulted in rubble pile objects. These studies are dedicated to providing some crucial information about the formation of planetesimals at an early time in the Solar System's evolution, and are fully developed in section 2.6.3 (future science: "space billiards"). In the long-term perspective, asteroids can actually be useful for human settlement. They have a natural shielding against radiation and require minimal Dv to launch spacecraft from them. They could even be used as large-scale space habitats (section 2.6.4). Earth Protection Another important point that still needs to be developed is Earth protection. The danger of collisions between near-Earth objects and the Earth is real . Although such catastrophic impacts remain few and far between, the impact of smaller but equally threatening objects occurs a lot more frequently (Steel, 1995). The threat posed by such planetary collisions ought to be taken seriously. Indeed, the probability of death due to an asteroid impact is comparable to the probability of death in floods, tornadoes, or even jetliner crashes (Steel, 1995).. Although plane crashes and tornadoes occur a lot more frequently than major asteroid impacts, when a large asteroid hits the Earth, it is as if all humankind were on a single plane about to crash. When averaged over millions of years, the current yearly number of humans expected to die as a result of asteroid impacts ranges between 5,000 and 10,000 (Steel, 1995). However, the overall annual American effort to detect NEOs is approximately $12 million. In fact, "the total number of people engaged in (NEO detection) world-wide is less than the staff of an average McDonald's restaurant" (Steel, 1995). For example, the Spacewatch team currently discovers 20 to 30 NEOs per year . Other efforts to detect and track NEOs are also underway in other countries such as Australia, Germany, and Russia (to name only a few). As of now, astronomers believe that none of the NEOs discovered so far will strike the Earth within the next 200 years. Nevertheless, if a large asteroid were to strike the Earth anytime soon, there is a 91% probability that we would not even discover it before the catastrophic impact (Canavan, et al, 1993). Over the last few years the world scientific community has reached a consensus: asteroids and comets do pose a significant threat to humankind and greater efforts should be made to detect Earth-approaching objects. Furthermore, techniques to deflect the trajectory of these celestial bodies should be studied in greater detail (see section 2.6.2). As this problem concerns all of humankind, detection and deflection efforts should be international in nature. Finally, most specialists agree that testing any deflection system at this time is dangerous. Indeed, such a technology could potentially be used to redirect an Earth-approaching asteroid toward an "enemy" nation (Canavan, et al, 1993). We disagree with this view because the world political climate has become "relatively" peaceful since the end of the Cold War. The strategy recommends the development and testing of NEO deflection and destruction technologies far from Earth (in the main asteroid belt or at least at a safe distance) to avoid any political complications. Should a NEO ever threaten to collide with the Earth after such tests, humankind would be ready to tackle the challenge. To summarise, the threat posed to human civilization by Earth-approaching objects is real and 91% of the most threatening objects remain undiscovered. The strategy recommends increased funding for NEO detection and tracking programs. Furthermore, the strategy calls for further studies and missions to test trajectory deflection techniques on asteroids sufficiently far away from Earth (to avoid any danger of impact with the Earth in case of failure). Due to the global nature of the problem, such efforts should be internationally co-ordinated by the NEO Working Group of the proposed International Human Exploration Consultative Group. To finish, after good experience of the deviation techniques, these can be applied to bring resources to Earth orbit or Lagrange points by deviating asteroids from their current orbit. Target Selection This section will consider which targets will allow us to answer the needs of our strategy, based on the most updated data available.  Figure 2-6: Near Earth Objects In recent years, the discovery of hundreds of formerly unknown NEOs has significantly increased the number of objects that are interesting in terms of mining. At the same time, the Dv requirements for reaching representatives of this class of objects have been reduced considerably. The best candidates are NEOs with a slightly larger semi-major axis than the Earth, moderate eccentricity, and high H2O content; most probably an Apollo or Amor C1 carbonaceous chondrite with a small spin rate. The spin rate is typically of the order of several hours, but can be as small as 10.7 minutes in the case of 1998 KY26 . Short Period Comets (SPCs) would probably have a higher H2O content, but it looks like they are more difficult to reach (delta V - wise) - unless some NEO SPCs with semi-major axes smaller than 1.5AU and low eccentricity are found. If we are more interested in minerals / metals than water, then the situation changes only slightly. Instead of a C1 chondrite, the best choice is probably an "iron meteorite" - with the same orbit requirements. As a first step of downselecting, the list was screened using the following two requirements:
The following graph shows the Dv requirement for a roundtrip from Earth's escape velocity to the object and back as a function of semi-major axis and numerical eccentricity (with aerocapture, no Lunar Swing-by):  As can be seen from this graph, the lowest delta v numbers are lying in a "valley" with the lowest point at (a=1, eps=0) - which is of course Earth orbit (no trip at all). Accordingly, the best targets lie in this "valley". The second requirement, a small mean time between launch windows, prohibits targets with orbits very similar to Earth's orbit (in this case, the time between launch windows is of the order of several years). Therefore, from the lists of hundreds of NEOs, only Amors and Apollos with an eccentricity between 0.1 and 0.3 and a semi-major axis between 1.1 and 1.3 AU were considered The area of special interest is marked in Figure 2-4 by two concentric ellipses. Objects other than these were regarded as too difficult to reach (delta v wise) or having too long a mean time between launch windows. The result of the screening is shown below: Apollos:
Amors:
The abbreviations used in the listing have the following meanings:
Since a high inclination (> 10 degrees) seems undesirable (because it increases the delta v requirement as a plane change would be necessary), we can further downselect to the following NEOs: Apollos:
Amors:
From this data, the delta v requirement to reach these objects from Earth's orbit and also the mean time between launch windows (MTBLW) can be calculated. If we limit the delta v requirement to 3 km/s and the mean time between launch windows to 3.4 years, four targets remain:
This graph shows that the most interesting targets were all found within the last 6 years. Many more discoveries of (maybe even more) interesting objects can be anticipated in the near future. Among the four, information on the composition was only available for 1998 KY26 (which is a water-rich C1 carbonaceous chondrite). If 1993 KA turns out also to be a C1 chondrite, than it would probably be a better choice, since 1998 KY26 is well known for its high spin rate (10.7 min.), which would make landing more complicated. In any case, for now we can assume, that with 3km/s it should be possible to reach a body which contains 10 to 20% water. These calculations assume a Hohmann transfer. In reality, the required Dv can be significantly reduced by one or more lunar swing-bys prior to trans-NEO injection. With one swing-by alone this number can already be decreased by up to 1.5 km/s (G. Landis, 1999). A lunar swing-by could also be used to perform a plane change. This would make targets with highly inclined orbits accessible, too. NEXT > [Home] [ISU] |