Asteroid 99942 Apophis impacting the Earth in 2029

[D H]

Clearly much more needs to be done in the three spheres of identifying objects, determining their orbits with great accuracy and developing strategies for any that would constitute a real threat, but I cannot find the excellent work done to date to be inept.

I agree. Scientists are now seeing smallish lumps of coal in space (most asteroids have an albedo close to that of coal). Nobody would have thought was possible even twenty years ago, and they're doing so on a rather limited budget. This is the opposite of ineptitude.

 

[Chronos]

There is no disagreement our ability to detect and predict the trajectory of asteroids has greatly improved over the decades. We also still have a long way to go. The OIG released an audit of the NASA NEO program last September, which was incidentally is the first over the last 5 years. The report is here: http://oig.nasa.gov/audits/reports/FY14/IG-14-030.pdf. It does not paint a rosy picture. A couple quotes to consider:

1. "In addition, the NASA Authorization Act of 2005 required the Agency to implement a “program to detect, track, catalogue, and characterize the physical characteristics of near-Earth objects equal to or greater than 140 meters in diameter” and established a goal of cataloging 90 percent of these objects by 2020. However, even with a ten-fold increase in the NEO Program budget in the past 5 years – from $4 million in fiscal year (FY) 2009 to $40 million in FY 2014 – NASA estimates that it has identified only about 10 percent of all asteroids 140 meters and larger. Moreover, given its current pace and resources, the Agency has stated that it will not meet the goal of identifying 90 percent of such objects by 2020."

40 million is, IMO, less than a serious commitment to the NEO program.

2. "Mitigation. Mitigation is the means of defending Earth and its inhabitants from the effects of NEO impacts. Mitigation may take the form of civil defense efforts such as emergency evacuations or efforts to deflect the trajectory of an object such as “slow-push” or “slow-pull,” kinetic impact, or nuclear explosions in space. 21 The NEO Program devotes about $1 million annually or 7 percent of its overall funding to the study of mitigation strategies.

Are those rockets needed to intercept a threatening NEO with a nuke funded by the DOD? According to http://www.airpower.maxwell.af.mil/airchronicles/apj/apj97/sum97/nici.html,"Currently planetary defense is not itemized in the DOD budget. As with any organization, priorities set the budget. The apprehension from those not in DOD may be that any planetary defense could be just another excuse for an arms race since the cold war is over. The reality from the congressional perspective is that the money for any efforts specifically itemized for planetary defense should come out of DOD’s current budget."

 

[Ophiolite]

There is no disagreement our ability to detect and predict the trajectory of asteroids has greatly improved over the decades. We also still have a long way to go. <snip>

40 million is, IMO, less than a serious commitment to the NEO program.

So, just to confirm, your reference to ineptitude relates to political decisions over budget and not to the technical achievements that have been made to date?

 

[Chronos]

NASA's NEO program achievements are impressive given budgetary constraints, but, insufficient to provide any sense of security. USA appears virtually alone in offering even a token response to the NEO threat. It's probably a risk that will not draw attention without a catastrophe. It's safe to assume a great deal of criticism will emerge thereafter - assuming anyone is left to complain.

 

[Glenstr]

Interesting discussion - particularly the assertion that an explosion on the surface of an Earth bound object would be ineffective in the vacuum of space. I'm not a physicist, so I'm not familiar with exactly how the energy from an explosion is dissipated in such a vacuum, but if a short burst from a rocket engine can alter the trajectory of a large spacecraft hurtling through space I can't see why a nuke detonating close to or on the surface of a large solid object would not alter it's trajectory at all.

I'm very familiar with the terrestrial nature of explosives, having worked with them a lot on road construction in the logging industry. An explosion will dissipate *most* of it's energy towards the path of least resistance, but a significant amount of that energy is still directed towards the path of most resistance. I'll use the analogy of a large rock sticking above the surface of a road grade as an example, as I did this in real life many, many times.

Ideally to get rid of this one would drill a 2" hole a couple of feet into the rock, put in a relatively small charge at the bottom of the hole and pack it with mud. In this scenario a half stick of 2X12 explosive would likely shatter the rock enough it could be removed from the road surface. Now let's take the scenario where no rock drill is available but you still need to remove the top of the rock. In this case you could lay about 10 sticks of 2X12 explosive on top of the rock, detonate it and the force from the detonation would still shatter the rock, even though over 50% of the explosion is directed towards open air. Now say you pack about 6" of heavy mud on top of the explosive sticks and you can reduce the 10 sticks to about 5 and still get the same result. The mud provides nowhere near the resistance the rock does, but still diverts enough of the energy downwards that less explosive is needed.

Now I'm not sure how this would all apply in the vacuum of space - but the speed of the explosion in terrestrial blasting is also a big factor. IIRC a typical 75% explosive has a detonation speed of roughly 5 miles per second. This is good when wanting to shatter a solid object like granite etc. However when one would want to just "move" or loosen a large object from it's place rather than shattering it to smithereens so it could be moved by heavy equipment, then a slower speed explosive is much more effective. Large tree stumps fell into this category, simply placing a 75% high explosive under it's roots would shatter the stump, often leaving the roots intact and still firmly entrenched in terra firma. However, put a sizable charge of 35% or 40% lower speed explosive in the same place would have the effect of "lifting" the entire object while keeping it intact would move it rather than shatter it, thus enabling large equipment to now push it out of the way.

I'm thinking a nuke detonated on the surface of a 1km asteroid is going to have some effect, even more if it could be jettisoned in with enough impact to get it embedded in the surface. Even if it only moved it a fraction of a millimeter off it's trajectory, if done far enough out that would be a significant change.

 

[Chronos]

The shock wave generated by a nuclear detonation is responsible for most of its destructive force. On earth, superheated air provides the medium to convey blast energy. In the vacuum of space, you need another medium. The mass of the nuke would provide relatively few, but, hugely energetic particles. Peppering an asteroid in this manner is not an efficient way to move it. You need to hijack some of the asteroid mass to provide a huge number of lower energy particles to convey the force: much like a lower velocity explosive on Earth is a more efficient way to dislodge a stump.

 

[Glenstr]

Conventional explosives also utilize shockwaves, just not on the same scale nor do they have the superheated air a nuke does. Are you saying that a 1 megaton nuke detonated on the surface on an asteroid is just going to be a useless explosion because there's no air to carry the shockwave? I'm still thinking that yes most of the force from a large blast in space is going to dissipate into the empty void, but there is still going to be some residual force against the hard object it is detonated on.

 

[Chronos]

No, not at all useless. A significant fraction of the detonation energy will be transferred to the vaporized asteroid material. The composition of the asteroid is a variable in deciding how much energy it can withstand without shattering. The more intact it remains, the bigger the shove; which is why identifying composition is an important objective of the NASA NEO program.

 

[Ophiolite]

The more intact it remains, the bigger the shove; which is why identifying composition is an important objective of the NASA NEO program.

I think the point has been made already, but it is very important and merits repeating. Many asteroids are rubble piles. That is, they are loose agglomerations of bits and pieces of various sizes. They are held together by gravity, not by intimately interlinked crystal structures. The gravity of a small asteroid is sufficient to hold it together, but only just. Apply some other significant force, such as a nuclear explosion, and the chances are you will break the asteroid apart. Now, instead of a single devastating impact you have many devastating impacts. You might get lucky and find the net effects were less, but they might easily be worse.

 

[Chronos]

A rubble pile is more susceptible to deflection if you don't waste energy fragmenting it, although more difficult. If you can reduce fragments to under 100M, it's a plus. Large fragments may still detonate in the atmosphere, but, not reach the ground, and airbursts are less destructive. Atmospheric shock waves disperse energy much more evenly than solid matter.

 

[Ophiolite]

A rubble pile is more susceptible to deflection if you don't waste energy fragmenting it, although more difficult. If you can reduce fragments to under 100M, it's a plus. Large fragments may still detonate in the atmosphere, but, not reach the ground, and airbursts are less destructive. Atmospheric shock waves disperse energy much more evenly than solid matter.

The point is that a rubble pile is a chaotic mix of sizes whose cohesive properties are also random. I strongly suspect it would be practically impossible to deflect without fragmenting. If one has determined (which could be possible) that once fragmented the components would be small enough not to suffer airburst then you could go for detonation. We do not currently have sufficient data on size distribution from either an observational or theoretical basis to know how likely that would be.

 

[Garth]

A rubble pile will be fragile, however, it caught earlier enough, it may be possible to deflect it with a continuous tiny thrust, such as from a gravitational tractor beam.

Garth

 

[Ophiolite]

Gravity tractors, light sails, mass drivers, Yarkovsky effect, etc,: all are potential solutions and, in my view, preferable to nuclear explosions in the case of rubble piles. The downside is that they all require a long time to work, which loops us back to Chronos' concerns about our incomplete catalogue of potential threats.

 

[Glenstr]

Wouldn't a detonation powerful enough to either fracture it or a detonation inside an asteroid that is already a conglomerate of rubble would alter the trajectory of said pieces of rubble enough that they would probably miss us anyway?

 

[Ophiolite]

The simple answer is no. Some would miss, the majority would not.

 

[Glenstr]

The simple answer is no. Some would miss, the majority would not.

I guess that would depend on a lot of factors, the size & number of fragments in the rubble pile, or the number & size of fragments created from blowing up a solid object being a couple.

If I drop a 10' rock of a 1000' cliff it's going to hit one 10' diameter spot. If I set off a large charge in that same rock before it's 1/2 way down then few, if any fragments are going to hit that same 10' area it would have hit if it was intact - and consider that the exploded fragments trajectories are dealing with Earth's gravity.

If I drop a big bag of gravel off with a charge going off in its center the effect will be the same, only trajectory changes in the already loose fragments are going to be much greater than the first example. In fact, few would even hit within a 500' circle around the original point of impact.

Now do this to a pile of rubble a few million km's from us in the vacuum of space, where the altered trajectories won't be affected by Earth's gravity..

 

[Ophiolite]

You need to compare the velocity of the bolide, which is relatively very large, to the relatively small velocity change imparted by the explosion. In your example the explosion is orders of magnitude greater than what we could practically deliver to an incoming asteroid. I'm sure there is published research out there on this - time permitting I'll track something down and post a link.

 

[D H]

The shock wave generated by a nuclear detonation is responsible for most of its destructive force. On earth, superheated air provides the medium to convey blast energy. In the vacuum of space, you need another medium. The mass of the nuke would provide relatively few, but, hugely energetic particles. Peppering an asteroid in this manner is not an efficient way to move it. You need to hijack some of the asteroid mass to provide a huge number of lower energy particles to convey the force: much like a lower velocity explosive on Earth is a more efficient way to dislodge a stump.

This is completely wrong. There have been a number recent posts that carry the same misunderstanding.

There is no shock wave in space. I previously explained how nuclear detonation changes the trajectory of an asteroid or comet, so quoting myself,

The way nukes work to divert an impact by an asteroid or comet is simple. Nuclear explosions generate a lot of high frequency radiation (X-rays and gamma rays) and neutrons, preferably a lot of neutrons. A thin layer of the asteroid/comet absorbs this bath of incoming neutrons and high frequency EM radiation. The radiated material then evaporates, making for the equivalent of a rocket. Neutrons are best because they penetrate deeper than does the high frequency EM radiation. Ideally, a nuclear explosion will result in a few percent of the material of the asteroids/comets material vaporizing.

To elaborate on the above, because neutrons penetrate deeper than does EM radiation, it's the neutrons generated by a nuclear explosion that are most effective at diverting the object. This means that a fusion bomb would be more effective than a fission bomb because ounce for ounce, fusion produce a lot more neutrons than does fission.

 

[Chronos]

Marshall Eubanks, a physicist educated at MIT and former member of the Technical Staff at the Jet Propulsion Laboratory, had this to say at http://www.researchgate.net/post/What_would_be_the_effect_of_a_nuclear_explosion_in_space :

"[a nuclear] bomb might ablate a 10 meter layer from the entire bomb-facing side of the asteroid and send this material off at 10 km / sec, causing a 10 meter / second impulse to the rest of the body."

The National Research Council report "Near-Earth Object Surveys and Hazard Mitigation Strategies(2010) notes at
http://www.nap.edu/openbook.php?record_id=12842&page=78

"... [Dr. David S.P. Dearborn, Lawrence Livermore Labs] simulated the effect of a nuclear standoff detonation on homogeneous 1-kilometer-diameter NEOs with densities between 1.91 and 1.31 g/cm3. In these numerical models of a standoff burst about 150 meters above the NEO’s surface, about 40 seconds after the burst the NEO’s speed change ranged from 2.2 to 2.4 cm/s. Approximately 97.5 percent of each NEO remained intact (the NEO was held together by gravity only—it had no tensile strength), while about 2.5 percent of its mass was ejected at greater than escape speed by the rebound to the shock wave that passed through the body in reaction to the ejection of heated material."

Are these references sufficient to justify the inference that momentum is conveyed by material ablated from an asteroid in a nuclear detonation?

 

[mfb]

You need to compare the velocity of the bolide, which is relatively very large, to the relatively small velocity change imparted by the explosion. In your example the explosion is orders of magnitude greater than what we could practically deliver to an incoming asteroid. I'm sure there is published research out there on this - time permitting I'll track something down and post a link.

You also have to take into account the distance to earth, which is very large in realistic scenarios, with the size of earth.

If the fragements move away at a typical velocity of 1m/s (the escape velocity of an asteroid that would be a global threat), after one year their path changed by ~30000km* - enough to miss Earth with typical velocities. A few would happen to move in the direction where they still hit earth, but most would not.
Early warning is key. If you have ten years of warning time, a velocity change if 10cm/s (for the whole object or for fragments) is sufficient.

*not including the nongravitational effects that can alter the orbit differently for smaller objects

 

[GiantSheeps]

Couldn't we use a method like the one in this video http://www.nasa.gov/content/what-is-nasa-s-asteroid-redirect-mission/ ? Basically send a probe out there, wrap it in a bag, and bring it somewhere else? (I know Apophis is no longer a threat, but for other, future asteroids)

 

[Vanadium 50]

Basically send a probe out there, wrap it in a bag, and bring it somewhere else

Why not try it on Earth first. That should be easier. Use, say, the Rock of Gibraltar. Wrap it in a bag and move it to, say, the South Pole.

 

[GiantSheeps]

Why not try it on Earth first. That should be easier. Use, say, the Rock of Gibraltar. Wrap it in a bag and move it to, say, the South Pole.

Okay I was making a joke, but if you watch the video that NASA put out you'll see what I mean, I put a link to it in my post