Colllisions lcp 11: Part II spacewatch Fi

Asteroid and comet collisions became a popular topic in the 1990s. Public awareness of the potential for a collision with a rogue asteroid or a comet was raised because of several recent occurrences that were all highly publicised. Many things have contributed to a more collective and political awareness of the dangers of asteroid collisions: from the Alvarez’ team promotion of the “dinosaur extinction by impact”, the reports of a “near miss” by 1889 FC asteroid, the Jovian impact of SL-9.that could be watched on TV in real time, to the premature announcement that the Asteroid 1997XF11 was on a collision course with Earth in 2028. Clearly, the movies Armageddon and Deep Impact have also contributed to capturing the imagination and the concern of the general public. As early as 1990 the U.S. House of Representatives, in its NASA Multiyear Authorization Act stated in part:

The primary goal of this group is

.... the study of statistics of comets and asteroids in order to investigate the collisional evolution of the solar system’ the discovery of target asteroids for space missions, such as Clementine, whose aim is the exploration and the protection the Earth from asteroid impact

The Spacewatch program was the result of action taken by Congress in the US that asked for national workshops in 1990. The astronomer and asteroid expert, Tom Gehrels, of the University of Arizona, started systematically looking for asteroids using a special telescope, using the newly developed CCD’s (charge-coupled devices) which are superior to photographic emulsions.

In 1992 a NASA study the Spaceguard Report, partly an outgrowth of the Spacewatch program, showed that all potential Earth impactors down to a 1 kilometer in size could be discovered and tracked in a program costing only $300 million, spread over 25 years. As we have seen, 1 km in diameter is the size of an impactor that would cause mass destruction, killing about 25-50% of mankind. Astronomers argue that this is a small investment for buying security against a potentially global destructive agent. Two committees were struck: The Detection Committee and the Interception Committee. The first covered the astronomical aspects of the problem:

How do you search these objects out and then determine their orbits with sufficient precision such that any potential impact in the foreseeable future might be identified?

The second committee’s task was to find an answer to the question:

Or more realistically:

Using nuclear energy seems the obvious answer, but let us consider other sources of energy. One kilogram of TNT could be represented as the maximum chemical energy we could deliver per unit mass, or 4.1 x 10⁶ Joules. It is easy to show (see problem below) that a 1 kilogram mass travelling with a speed of 2.88 km/s has the same kinetic energy as the chemical energy available from one kilogram of TNT. Therefore, if a ton projectile of TNT were to hit an asteroid, 90 % of the total energy would come from the kinetic energy of the mass and only 10% from the ch chemical explosion! It looks like chemical weapons would be unsuitable for asteroid deflection. Thus, the options we have are:

1. 
   Large kinetic interceptor, with a mass of at least 10, 000 tons, and
   

Granted, we need only a modest change in speed, maybe only about 10 cm/s, but to accomplish that requires a large amount of energy. For example, to change the speed of a 1 kilometer asteroid by 10 cm/s would require about 4 x 10 ¹⁵ of energy, or about 1 megaton of TNT. This might convince us that a comparatively small weapon would do the job. Unfortunately, a great deal of the energy would be inelastically absorbed by the asteroid, and only a small amount left over to change its speed. This problem was investigated by Thomas Ahrens and Alan Harris of the California Institute of Technology.

They considered al sizes of bolides of diameters 0.1, 1, and 10 km. The smallest is the minimal size to cause significant damage, and the largest is about the size of the biggest Earth-crossing asteroids and periodic comets. Comets smaller than 0.1 km (100m) could be diverted by kinetic means, but the larges ones would require alternative methods. The following were suggest

1. Land a device or mechanism on the asteroid or comet that would throw off material (a mass driver) continuously and at a very high speed,

2. Choose from various methods of deploying nuclear charges:

a. Burying nuclear explosives in the center of the body which would blow it apart.

b. Deposit charges on the surface so that the explosion would produce a recoil force that in turn would change the velocity of the asteroid or comet.

c. Use an explosion about 100 m above the bolide’ s surface to cause a skin layer to be ejected that would then produce a recoil force.

Let us consider these in turn. The first approach, blowing up an asteroid, sounds very dramatic but it really is not the preferred way to divert it. Many large fragments would be produced with unpredictable trajectories. Digging deep into an asteroid may present unforeseen technological problems. We must also remember that the centre of mass of the individual parts of the asteroid after explosion will always travel along the original orbit.

The second idea is more attractive, provided the speed with which the mass is ejected is greater than the the escape velocity of that asteroid. We have already calculated the escape velocityfrom various sizes of planets, planetoids and asteroid, so we know that for asteroids of 1 - 30 km across , these are of the order of 1-20 m/s. The problem is, however, that we still do not know enough about the composition of the asteroid. The explosion may just fragment the asteroid rather than give it a desired velocity change.

The third approach is the one the researchers recommended. Exploding a nuclear weapon at roughly 0.40 times the diameter of the asteroid, about 30% of its surface would be “bathed” in the neutron radiation of the explosion. They think that the sudden heating of the surface to a high temperature would produce the ejection of material above the escape velocity (of that particular asteroid). They found that the required explosive energy is from about 100 kilotons of TNT for a 1 km asteroid to about 10 megatons for a 10 km body. Using our present nuclear capabilities we should be able to meet these requirements.

There are, of course, also suggestions that are quite exotic. These include:

1. 
   Attaching a giant “solar sail” to provide a small but continuing impulse that would push the asteroid out of a collision course.
   
2. 
   Alternatively, using a very large sail as a solar collector to focus Sunlight onto the asteroid surface in an effort to evaporate material that would then produce a jet force as the gas expanded away from the asteroid.
   

Icarus is a very interesting asteroid. It was discovered accidentally in 1949 by the famous American astronomer Walter Baade, using the new 48 inch Schmidt telescope on Palomar Mountain. The asteroid was about 1 km across, is an Earth-crossing asteroid and has a high eccentricity. In 1949 it was the closest any observed asteroid came to the Earth: about 6 million kilometers.

A surprising result of orbit deflection calculations is that it takes a very small velocity change to alter the orbit of an asteroid significantly, if we have sufficient time before the predicted collision. The following example will illustrate this:

Or more realistically:

How do you shove a 1 kilometer asteroid with a mass of more than one billion tons, and travelling at 30 km/s, out of the way of the Earth?

The CCD ( charge-coupled devices) scanning observations are conducted 20 nights each “lunation” with the Stewart Observatory 0.9 m Spacewatch Telescope on Kitt Peak Using these data, the organization:

Studies the orbital element distribution of Trojan and Main-Belt asteroids
First of all, it must be made clear that predicting an orbit of a comet or asteroid beyond 100 years is tricky and beyond 200 years almost impossible. The best that can said is something like: “Asteroid X will pass somewhere within five times the radius of the Earth, or that asteroid X has about a 4% chance of hitting the Earth This information is very important because if you make the decision to intercept a 1 km asteroid or comet that is predicted to pass within 5 Earth radii, then you must be able to deflect that object by much more than that amount.

Where would you have to intercept the object to have the greatest effect? Most people seem to think that pushing an asteroid sideways would be the best thing to do. Surprisingly, the most effective thing to do is to give it a push along the trajectory in which it is already moving. But the push (impulse) can be applied either to the trailing end or the leading end, speeding it up or slowing it down. This will be illustrated in the problems below.

To deliver the appropriate push, what sort of explosive should we use? Using nuclear energy seems the

obvious answer, but let us consider other sources of energy. One kilogram of TNT could be represented as the maximum chemical energy we could deliver per unit mass, or 4.1 x 10⁶ Joules. It is easy to show (see problem below) that a 1 kilogram mass travelling with a speed of 2.88 km/s has the same kinetic energy as the chemical energy available from one kilogram of TNT. Therefore, if a ton projectile of TNT were to hit an asteroid, 90 % of the total energy would come from the kinetic energy of the mass and only 10% from the chemical explosion! It looks like chemical weapons would be unsuitable for asteroid deflection.

Thus, the options we have are:

1. Large kinetic interceptor, with a mass of at least 10, 000 tons, and

2. An interceptor with a nuclear warhead.

Granted, we need only a modest change in speed, maybe only about 10 cm/s, but to accomplish that requires a large amount of energy. For example, to change the speed of a 1 kilometer asteroid by 10 cm/s would require about 4 x 10 ¹⁵ of energy, or about 1 megaton of TNT. This might convince us that a comparatively small weapon would do the job. Unfortunately, a great deal of the energy would be inelastically absorbed by the

asteroid, and only a small amount left over to change its speed. This problem was investigated by Thomas Ahrens and Alan Harris of the California Institute of Technology.

They considered al sizes of bolides of diameters 0.1, 1, and 10 km. The smallest is the minimal size to cause significant damage, and the largest is about the size of the biggest Earth-crossing asteroids and periodic comets. Comets smaller than 0.1 km (100m) could be diverted by kinetic means, but the larges ones would require alternative methods. The following were suggested:

1. Land a device or mechanism on the asteroid or comet that would throw off material (a mass driver) continuously and at a very high speed,

2. Choose from various methods of deploying nuclear charges:

a. Burying nuclear explosives in the center of the body which would blow it apart.

b. Deposit charges on the surface so that the explosion would produce a recoil force that in turn would change the velocity of the asteroid or comet.

c. Use an explosion about 100 m above the bolide’ s surface to cause a skin layer to be ejected that would then produce a recoil force

Let us consider these in turn. The first approach, blowing up an asteroid, sounds very dramatic but it really is not the preferred way to divert it. Many large fragments would be produced with unpredictable trajectories. Digging deep into an asteroid may present unforeseen technological problems. We must also remember that the centre of mass of the individual parts of the asteroid after explosion will always travel along the original orbit.

The second idea is more attractive, provided the speed with which the mass is ejected is greater than the

the escape velocity of that asteroid. We have already calculated the escape velocity from various sizes of planets, planetoids and asteroid, so we know that for asteroids of 1 - 30 km across , these are of the order of 1-20 m/s. The problem is, however, that we still do not know enough about the composition of the asteroid. The explosion may just fragment the asteroid rather than give it a desired velocity change.

The third approach is the one the researchers recommended. Exploding a nuclear weapon at roughly 0.40 times the diameter of the asteroid, about 30% of its surface would be “bathed” in the neutron radiation of the explosion. They think that the sudden heating of the surface to a high temperature would produce the ejection of material above the escape velocity (of that particular asteroid). They found that the required explosive energy is from about 100 kilotons of TNT for a 1 km asteroid to about 10 megatons for a 10 km body. Using our present nuclear capabilities we should be able to meet these requirements.

There are, of course, also suggestions that are quite exotic. These include:

a. Attaching a giant “solar sail” to provide a small but continuing impulse that would push the asteroid out of a collision course.

b. Alternatively, using a very large sail as a solar collector to focus Sunlight onto the asteroid surface in an effort to evaporate material that would then produce a jet force as the gas expanded away from the asteroid.

The first suggestion must be immediately rejected because most asteroids spin and tumble so that it would be impossible to attach a sail. The second one, however, seems possible. They showed that a sail of 0.5 km in diameter could deflect an asteroid up to 2 km in size, assuming continuous operation for a year. The reflective material would only have a mass of about one ton.

Already in 1967 there were groups of scientists concerned about a possible asteroid collision with the Earth. At MIT a group of engineering students was given the problem of diverting the asteroid Icarus, which was imagined to be on a collision course with Earth. The conclusion of the students was that the asteroid could be diverted by using a Saturn V heavy-lift capability (then available for the Apollo program) and six 100 megaton hydrogen bombs. The scenario was based on the assumption that there was only about a year left before impact, and that meant high velocity changes were required.

Icarus was discovered accidentally in 1949 by the famous American astronomer Walter Baade, using the new 48 inch Schmidt telescope on Palomar Mountain. The asteroid was about 1 km across, is an Earth-crossing

asteroid and has a high eccentricity. In 1949 it was the closest any observed asteroid came to the Earth: about 6 million kilometers.


Fig. 4 Deflecting an asteroid.

IL **** An excellent source of videos on asteroid defence

http://planetarydefense.blogspot.com/2007_12_01_archive.html
Deflection calculations: Examples

IL *** Discussion of deflecting asteroids

http://pdf.aiaa.org/preview/CDReadyMPDC04_865/PV2004_1433.pdf
A surprising result of orbit deflection calculations is that it takes a very small velocity change to alter the orbit of an asteroid significantly, if we have sufficient time before the predicted collision. The following example will illustrate this:

Imagine a 1 km asteroid, asteroid X 2001 (density about 3.0 cm ³) in a collision course with the Earth. The asteroid is found to have a perihelion of near 1 AU and aphelion at 4 AU. It is expected that the asteroid will pass within 5 Earth radii unless the asteroid is diverted.

a. Sketch the elliptical orbit of the asteroid.

b. Show that the semi-major axis is 2.5 AU.

c. What is the eccentricity of the asteroid?

d. Using the vis-viva equation show that the speed of the asteroid (relative to the Sun) at perihelion will be 37.6 km/s a

e. Now calculate, using Kepler’s second law, the speed at aphelion.

1 Give an argument that the change of velocity at perihelion will have a greater effect in changing the elliptical orbit than changing it (by the same amount) at aphelio

2. Imagine that asteroid X is nudged by a large, 1000 ton projectile which hits the asteroid in front, with a contact speed of 30 km/s. The projectile is imbedded in the asteroid.

3. What will be the change in velocity of the asteroid?

4. Show that the change in the semi-major axis will be slight, from 2.500000 to 2.5000531 AU., a distance of about 8000 km. But this is a little more than the radius of the Earth, so how can it improve our collision probability?

Actually, the asteroid will miss the Earth by much more than 8000 km. Remember, the change of the semi-major axis will increase the orbital period by a significant amount.

5. Sow that the orbital period increase of the asteroid is about 66 minutes.

6. It is now easy to calculate the distance the Earth travels during this time. Calculate this distance.

7. Approximately how many Moon radii away will the asteroid pass us?

8. Debate the following conclusion: “Intercepting dangerous objects is best done near perihelion, either by increasing or decreasing orbital speed by at least about 10 cm/s”.

Although Hollywood blockbusters have popularised the idea of using nuclear weapons to blow up asteroids, the study will investigate more realistic alternatives such as space mirrors. These would be angled to focus sunlight onto the incoming object. The intense heat would boil away a section of the asteroid, creating a natural rocket that pushes the asteroid in the opposite direction. The study will also look into high-speed collisions to literally knock an asteroid out of the way using no explosives, just a ‘battering ram’ spacecraft.

The internal arrangement of Near Earth Objects (NEOs) can critically affect the deviation strategy. Some asteroids, known as rubble piles, are not solid slabs of rock but loose assemblages. Slamming an object into a rubble pile would not be very effective in altering its course, because the rubble would absorb the energy of impact rather like a crumple zone on a car absorbs a crash. Instead, scenarios which melt part of the surface, such as space mirrors, producing jets of gas that gradually ease the object into a new orbit, are favoured.

Yet this is about more than just diverting asteroids, no matter how critical that need may one day become. The biggest part of the study concerns how to intercept such targets. In conventional space exploration, everything is precisely worked out beforehand and targets are chosen that have well-known orbits. That’s how NASA recently bulls-eyed comet Tempel 1 with its Deep Impact mission.

However, a dangerous object is likely to be newly discovered and that means its orbit will be poorly known. “We’d probably have to launch a deflection mission without a clear idea of where we’re aiming,” says McInnes. So, the study will seek to find the best strategies for launching space missions into approximate intercept orbits that can be adjusted later.

Such seat-of-the-pants flying could result in more versatile spacecraft across the board. These would be better able to respond to a variety of unexpected situations. As well as fuel considerations, the team will investigate ‘general purpose’ orbits and flexible navigation strategies that keep a spacecraft’s options open for longer, before committing it to a final destination.

Natasha Richardson | Quelle: alphagalileo
Weitere Informationen: www.epsrc.ac.uk