Advanced propulsion systems are not required for the direct implementation of this strategy. However, if they were to be available the structural mass of the spacecraft and the amount of time required for a given mission would be decreased.

Nuclear Propulsion

The main idea behind the utilization of nuclear power can best be explained in terms of specific impulse. With the traditional chemical hydrogen-oxygen reaction the upper-limit of the specific impulse is about 480s. This performance can be improved by raising the temperature in the combustion chamber and by lowering the molecular weight of the exhaust gases. In a nuclear fission reactor the specific impulse can be doubled to about 800 to 1000 s. Liquid hydrogen is used as the working fluid, which reduces the molecular mass of the rocket exhaust to about one-eight of that of a hydrogen-oxygen engine.

Solar-Sails

Solar sails use radiation pressure exerted by solar photons to provide thrust for the spacecraft. Even if that thrust is small, the sail never runs out of fuel. Over a long period of time, the spacecraft can be accelerated up to very high speeds. For these reasons, such a system is ideal for the shuttling of interplanetary cargo or rendezvous missions to asteroids.

In order to harness the radiation pressure, large lightweight reflectors are attached to the spacecraft. The resulting force is normal to the back of each reflector, so that by tilting the sail you can change the direction of the spacecraft. Based on the design used for the sail (ie. disc sail, heligyro sail, square sail), the maneuverability of the spacecraft can be increased. However, the use of more supporting structures reduces the overall performance of the system.

For example, a NASA study on a 12 bladed heligyro with a characteristic acceleration of 0.01m/s2 estimated that this configuration would be able to reach Jupiter in 900 days with a 1500kg payload (3).

Artificial Gravity

Considering the fact that future space travelers will likely be staying aboard their spacecraft for several months (if not years), we should take a closer look at a very interesting concept: artificial gravity. A fair number of studies has been done to date which show that humans may suffer some serious effects when exposed to microgravity for extended periods of time. These effects include cardiovascular deconditioning, orthostatic intolerance, muscular atrophy, and bone demineralization. In order to minimize these effects, an artificial gravity environment may be required.

According to bed rest studies, no more than four hours of exposure to 1 g with exercise is required each day to maintain a positive calcium balance. In contrast, control subjects without this daily 1g exposure, experienced a significant negative calcium balance and bone loss (Schneider, 1987).

Only two systems capable of generating artificial gravity are currently known: spinning systems and accelerating systems.

• Spinning Systems

Artificial gravity provided by rotating environments is not the same as that felt on Earth. The introduction of a rotating environment poses special problems for sensory systems involved in spatial orientation and consequently also for sensory-motor performance because gravitational forces are not constant for an object or human being moving within it (Howard and Templeton, 1966).

Nevertheless, it has been shown in previous studies that even untrained humans are able to adapt to environments with rotation periods of less than 3-4 rpm.

This figure shows that in order to keep rotation levels low and gravity levels sufficiently high, large structures are required. In general, either tethers or trusses may be used for such configurations. Even though tethers have been subject of recent studies, their dynamic behavior in space is still rather poorly understood compared to that of rigid truss structures. For the International Mars Mission developed during the ISU SSP '99 design project, a truss structure with a radius of 85 m and a rotation rate of 2 rpm generating an artificial gravity of 0.38 g (ISU, 1991).

A small centrifuge placed inside the spacecraft may prove to be a valuable alternative to big spinning structures. In this case, rotation rates as high as 24 rpm and radii of 1.5 to 2 meters may be considered. However, studies have shown that subjects placed inside these centrifuges were able to sleep and exercise regularly (Diamandis, 1988).

• Continuous Linearly Accelerating Systems

To make use of this technique, the spacecraft would have to be accelerated during the first half of the mission, then turned around and slowed down for the second half of the mission. In this manner, a constant force of artificial gravity can be provided.

Imagine a spacecraft accelerating at 1 g. After only 35 days you would reach relativistic speeds and the resulting mass increase would slowly decrease the apparent level of gravity. A trip to Mars aboard such a spacecraft would last only three days, considering both acceleration for the first half and deceleration for the second half. Unfortunately, existing space transportation systems do not have the technology to maintain this order of acceleration.

Table2-2: Comparison of Different Artificial Gravity

	Systems Spinning systems with tethers	Spinning systems with truss structure	Accelerating systems
Pros	Significant g-levels with acceptable rotation rates Mass penalty of only 10% to 20% Stability ok, once spun up	Stability control system less complex than for tethers Operation for spinning up and down easier	Constant gravity on the whole system  Very high speeds can be attained
Cons	Stability questionable during spin up/down  Difficult dynamics as mass changes Major catastrophe if tether breaks Expensive on-orbit testing required	About 6 times the mass of a comparable tether May have vibration problems	Technology not yet available

In conclusion, we recommend that spinning systems with truss structures are the preferred option in create artificial gravity environments in the near future. They come with a relatively high mass penalty but are easier to control from an attitude dynamics standpoint.

Asteroids Under Control

As mentioned earlier, techniques for deviating asteroids appear in different branches of the strategy steps, namely in terms of Earth protection (section 2.4.3), for the extraction of resources (section 2.5.2), for the future scientific research (section 2.6.3), and for the outlook step (section 2.6.4).

Several studies have analysed possible techniques to deflect the trajectory of near-Earth objects (NEOs). Traditional destruction and deflection techniques include kinetic weapons, nuclear weapons, solar sails, and mass drivers. On the one hand, nuclear weapons have a very large specific energy (energy per unit mass), but they could raise several legal and public concerns in their development. It means that even if no concerns could stand against an imminent crash, they can appear during previous test needed to handle this technique.

On the other hand, the other techniques mentioned above are not as powerful. Some other possible techniques do not appear to have received any attention. In any case, the "ideal" technique to be used in order to deflect the trajectory of any NEO depends on several factors such as its mass, its orbit, its composition, its structural integrity (monolith Vs rumble pile), and the warning time before impact.

Moreover, whereas the deflection of an earth menacing asteroid does not need an accurate method, some missions may need a precise control of the asteroid's trajectory. That is the reason why we also have to look closer to other techniques which do not involve nuclear weapons.

The main idea of the techniques mentioned above is to fix a rocket on an asteroid and then try to get propellant for it. In order to reduce the cost of the method, techniques such as steam rockets or mass drivers use the resources available on board, namely water and rocks, to feed the propulsion engine. This can be a problem if you want to use those resources for other purposes.

Here we try to describe a new method that has the advantage of not wasting the resources of the asteroid.

Electromagnetic tethers: could benefit from the "Faraday" effect and use magnetic fields to alter the trajectory of asteroids. On the Figure -1 is explained the principle of the technique. Two spacecraft in rendez-vous are necessary to attach the conductive tether to the asteroid.

Assuming that the electromagnetic field B is perpendicular to the plane of this sheet, going trough it, being given that the conductor is displaced at the velocity v, it undergoes an electro-motive force "EMF" by the simplified generator law "EMF=Blv" where l is the length of the conductive tether (see figure).

The circuit is closed by ionized gas (plasma) in the space environment. That means that this method can work only in a plasma dense enough to be able to close the loop (planetary magnetosphere for example). Then a current "I" takes place in the conductor and this latter undergoes a force "F" by the simplified motor law F=IBl acting as a drag.

In a first approximation, we will neglect the resistance of the plasma closing the circuit. Expressing I as a function of EMF, l, S the section of the conductive tether and p its resistivity, the force is given by the equation F=B^2vlS/p. Taking an average velocity of 10 km/s, a length of 100 km, a section of 10 cm diameter and a resistivity of 10E-4 Wm , the equation yields to F=1E11B^2.

The magnetic field in the interplanetary medium is 5E-9 T which is not enough for our technique to be effective. However the planetary magnetic fields can be much stronger than that, from 3E-5 T for the Earth to 4E-4 T for Jupiter . We could then use that technique in planetary magnetospheres to put asteroids into orbit or direct them into close moons.

The rough result presented above is unfortunately the best we can obtain due to the fact that we did not consider the resistance of the plasma guide as well as the non-perpendicularity of the magnetic field and the velocity vector. The force, even for the electromagnetic field is tiny. The only way to improve the performance is to count on superconductor tether and mostly on long term deviation for which even a weak force can have a significant effect.

In conclusion, we can state that we don't have any efficient methods with good specific impulse for controlling accurately the trajectory of asteroid. A serious endeavour has to be done in this sense and to be considered as quick as possible by the present strategy.

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