Atmospheric Blowoff Cooling and Mining

Although Table 7.1 makes it seem as though Venus has many similarities with Earth, it actually differs greatly from our home world in not having a natural satellite. Our Moon was produced within the first few million years of the Earth's formation and is most probably the result of a glancing blow struck by a wayward Mars-sized proto-planet (Figure 7.6). As a consequence of this impact, material ejected from the mantle mingled with the debris from the disrupted proto-planet and formed a ring of boulders and

Figure 7.6. A Mars-sized proto-planet collides with the young Earth to produce our Moon. Similar such impacts appear to have taken place on Mercury and Venus, which both disrupted the majority of the former planet's outer mantle and tipped the spin axis of the latter over by nearly 180°. Artwork by Lynette R. Cook, for the Gemini Observatory, Hawaii.

Figure 7.6. A Mars-sized proto-planet collides with the young Earth to produce our Moon. Similar such impacts appear to have taken place on Mercury and Venus, which both disrupted the majority of the former planet's outer mantle and tipped the spin axis of the latter over by nearly 180°. Artwork by Lynette R. Cook, for the Gemini Observatory, Hawaii.

dust around the Earth, from which the Moon was able to coalesce and grow. Indeed, it is perhaps appropriate that our terrestrial muse of romance and love was formed through the fleeting union of two colliding bodies.

No permanent moon resides in orbit around Venus, but the planet has nonetheless undergone some of the same intensive battering that produced our Moon. That Venus must have suffered a massive collision or close encounter with a large planetesimal shortly after it formed is betrayed by its high obliquity (177.36°; see Table 7.1) and resultant retrograde spin. For, indeed, to tip the planet's spin axis over from the expected ~0° obliquity to its observed value requires a close encounter with an object comparable in mass to Mars (that is, ~one-tenth the mass of the Earth; see Table 6.1) is required.

If, as is apparently the case, massive collisions were an important agent in shaping the properties of the young terrestrial planets, then, as many researchers have suggested over the years, why not engineer additional collisions to shape their future properties? It is now known, for example, that there is an abundant supply of multiple 100-km-sized bodies in the outer Solar System, the Kuiper Belt objects (KBOs), that could be utilized as celestial cannonballs.

Made predominantly of water ice and silicates, the orbits of some of these KBOs could be altered to produce either direct or grazing collisions with Venus. Direct collisions would result in the ejection of atmospheric gases, while grazing, that is, off-center collisions, could be arranged so as to increase the planet's spin rate, or to generate a planet-encircling ring of material or numerous small Venusian moons.

Direct impacts onto the Venusian surface could partially satisfy conditions 2 and 3 of the terraforming requirements listed earlier, although the process is not likely to be overly efficient. At best, a large impact could eject the atmospheric material located above the so-called tangent plane (see Figure 7.7). In the early 1990s, Sagan and his former student, the late James Pollack, estimated that perhaps a few ten-thousands of the mass of the Venusian

Figure 7.7. Atmospheric-mass-reducing scenarios. Large impacts can at best eject the atmospheric material situated above the tangent plane (TP-TP). The ramscoop would make multiple passes through the atmosphere before taking its cargo to another location in the Solar System, possibly the Earth's Moon (as described in Chapter 8).

Figure 7.7. Atmospheric-mass-reducing scenarios. Large impacts can at best eject the atmospheric material situated above the tangent plane (TP-TP). The ramscoop would make multiple passes through the atmosphere before taking its cargo to another location in the Solar System, possibly the Earth's Moon (as described in Chapter 8).

atmosphere might be ejected during a large body impact. This result indicates that multiple thousands of impacts would be required to reduce the planet's atmospheric mass to something similar to that of the Earth's. There is a ready supply of KBOs within the outer Solar System that might be diverted to achieve this end, but one is left somewhat uncomfortable at the possibility of so many impacts being engineered on a body that has an orbit inside of the Earth's. Certainly a few impacts might well be arranged, but perhaps atmosphere mining via ramscoops (Figure 7.7) is a more esthetically pleasing and practical solution5 to the problem of reducing the planet's atmospheric mass.

Rather than engineer thousands of direct KBO impacts onto Venus to reduce its atmospheric mass, it might be more practical to engineer a few grazing impacts to generate a circumplanetary debris disk. Such a disk might then be maintained through the addition of material mined from the asteroid belt to produce a partial sunscreen over the equatorial regions of Venus. This would have the effect of cooling the atmosphere through a reduction in the solar insolation.

As such, the formation of a circumplanetary debris disk won't appreciably reduce the mass of the Venusian atmosphere, but its cooling effect might well be important in the long-term maintenance of a terraformed atmosphere (as described below). In fact, an asteroid-debris ring located about the Earth has been described by Jerome Pearson and co-workers (Star Technology and Research, Inc., Mount Pleasant, South Carolina), who suggest that such a structure might be used to offset global warming. This approach also reduces the potential asteroid-impact risk on the Earth (as well as on Venus if the idea is adopted there, too), since the best material to utilize in the ring construction is that which comes naturally close anyway.

An alternative debris cloud method for cooling Venus was proposed in the early 1980s by Christian Marchal (Office National d'Etudes et Recherches Aerospatiales, France), who advocated the destruction of one or more asteroids at the so-called Venusian Li point. The first Lagrange6 point (L1) is located on the Venus-Sun line at the point where the gravitational attractions of the two bodies, on a zero-mass test particle, are equal and opposite in the rest frame rotating with the planet. Accordingly, the L1 point is located about 1.03 x 106 km, or 169 planetary radii, away from the center of Venus.

The reason for engineering a debris cloud at L1 is that it is a relative good spot with respect to stability. Material placed at Lj tends to stay close to L1. The material will eventually drift away from the L1 position, but the residency time there should last for at least a few years. This latter point unfortunately means that the debris cloud will need to be repeatedly replenished, but, on the brighter side, there is a large reserve of asteroids within the Solar System. Just as with the collisional induction of Venusian atmosphere loss, the debris cloud at L1 cooling idea also lacks esthetic appeal, but sometimes brute force is the only cost-effective way of getting the job done.

The most recent incarnation of the debris cloud-shielding concept, developed again as a means for offsetting global warming, is that proposed by astronomer Roger Angel7 (University of Arizona, Tucson). Rather than produce an asteroid debris cloud, however, Angel proposes to launch a swarm of some 16 trillion pico-sats (that is, small, semi-autonomous satellites that are just a few tens of centimeters across), each weighing in at perhaps 1 gram, to the Earth's L1 point (see Figure 7.8). Each of the Sun-fliers, as the pico-sats have been called, will have fins that can be controlled to maintain an L1 location for periods of perhaps up to 50 years, and the combined swarm of Sun-fliers will cover an approximately rectangular area with sides 6,200 km by 7,200 km.

The cost of constructing a pico-sat swarm to stave off the effects of global warming are clearly going to be large, and Angel estimates that it would carry a price tag of perhaps a few trillion dollars spread over a time interval of several decades. (The cost is equivalent to about 10 years' worth of the current annual US Defense Department budget.) Compared to the accumulated costs that will likely result from the damages wrought by unchecked global warming, however, the price tag for the swarm is actually a very competitive one. One would hope that by the time the terra-forming of Venus begins, perhaps several hundreds of years from now, the cost of manufacturing the individual fliers and the expenditure of launching them will have fallen dramatically from those of the current day. Time, of course, will tell how this particular technological approach to solving global warming, and possibly the cooling of Venus, will play itself out.

Figure 7.8. Artist's impression of Sun-flier pico-satellites. Each flier is equipped with small fins to allow for orbit and orientation control, and in this illustration consist of a transparent substrate that spreads any incident light into a diffuse ring. A swarm of some 16 trillion such pico-sats could be placed at the Earth's L1 point and collectively act as a giant solar shade (or more correctly a giant solar light diffuser), alleviating the effects of global warming. A similar such swarm of pico-sats placed at the Venusian L1 could be used to cool its atmosphere. Image courtesy of the university of Arizona and Steward Observatory.

Figure 7.8. Artist's impression of Sun-flier pico-satellites. Each flier is equipped with small fins to allow for orbit and orientation control, and in this illustration consist of a transparent substrate that spreads any incident light into a diffuse ring. A swarm of some 16 trillion such pico-sats could be placed at the Earth's L1 point and collectively act as a giant solar shade (or more correctly a giant solar light diffuser), alleviating the effects of global warming. A similar such swarm of pico-sats placed at the Venusian L1 could be used to cool its atmosphere. Image courtesy of the university of Arizona and Steward Observatory.

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