DART result and deflection of dangerous NEAs

Feb 2, 2023
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Regarding the «DART mission having been declared a resounding success».
Briefings, emotional interviews, impressive photos, PR-type publications - all this can be important for discussions and conclusions in politics, but not in science. Before the appearance of publications in peer-reviewed journals with detailed data and analysis of the collision consequences for Dimorphos`s orbit, there is nothing to discuss seriously and essentially.
The following must also be taken into account. One million kilograms of ejected fragments is only less than one percent of the total mass of the target. Since the ejection was "fan-shaped", its effect was most likely extremely insignificant, as well as the impact itself. This follows from the existence of an unavoidable completely inelastic reaction for impulse transmission to Dimorphos, which is a loose pile of rubble, stones and dust, but not a monolithic rock.
Finally, during photometric observations, nature can played a «cruel joke» with observers who used the mutual eclipses of Didymos and Dimorphos to determine of its orbit changes. The matter is that an asymmetric (one-side) and optically dense ejection of large asteroid fragments could partially remain in the orbit around the Dimorphos for a long time. With a small value of its first cosmic velocity around of 4 cm/s, this is quite probable and is confirmed by well known video animation - after 50th second (compressed in 500 times):
View: https://www.youtube.com/watch?v=bfqVqOl9S9w

In turn, this could lead to optical-geometric distortions in the measurements of the difference between the moments of mutual eclipses due to the displacement of the photometric center with respect to the center of gravity. Using only different types of eclipses, occurring after 6 hours, lead to distortions of around 30 minutes, which could be interpreted as a real change in orbital period. Since large fragments were most likely ejected into different orbits around Dimorphos, the asymmetry of the ejection may disappear after some time. In this case, measurements of the orbital period owing to observations of any eclipses will show that it remained unchanged after the impact.
 
Looking at the linked video, the puff of debris resutling from the impact does not look like it "remainded in orbit" around either object. And other imagry seems to indicate that what was dislodged from the surface was then pushed by solar radiation in to a long tail going away from the target, not orbiting it as a ball of haze.

So, this argument about the optical center being displaced from the center of mass sufficiently to mislead observers of the eclipse period into thinking that the change in orbit velocity was larger than actual seems unlikely. But, we will eventually see if the eclipse period changes back to the undisturbed frequency, And, hopefully, there will be a follow-up observation mission to look at Didymos close-up (if the Russian invasion of Ukraine has not effectively eliminated the launch).
 
Feb 2, 2023
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These are additional details needed to understand.

Please see this video RIGHT after 50th second ignoring the relatively far-flung and expanding "cloud". It shows that Dimorphos`s image has approximately doubled in size, which leads to an estimate of the photometric center displacement by approximately 150 m. The animation compressed 500 times (according to its authors), so it seems that this process happened abruptly. It also can be seen from it that such a VISIBLE increased size remained unchanged, which indicates the "hanging" of the ejection around Dimorphos on the various orbits. In addition, the fact that it did not fly far indicates the relatively large size of the ejected fragments.

Naturally is that «orbital» largest fragments ejected into various orbits around Dimorphos will not co-spinning along with it. Therefore, this ejection can remain on one side of asteroid for a long time. This means that at the first eclipse (passage of the Dimorphos`s shadow on the Didymos`s image) they will be located AHEAD of Dimorphos, and at the second eclipse (passage of the Dimorphos`s image through the Didymos`s shadow - after 6 hours) they will be located BEHIND him. This will cause the observed "photometric centers" to shift towards each other by around 30 minutes, based on the orbital velocity of Dimorphos and its «doubled» size, distorted by the ejection. On the contrary, when observing the same types of eclipses (occurring approximately 12 hours between them), the relative shift of the "photometric spots" will not be fixed, that is, the orbital period will not change despite the distortion of the Dimorphos`s actual dimensions.

Actually, for me as an astrophysicist, the described reasoning is so far only a justified hypothesis. So, let's wait for publications in the refereed journals.
 
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There seems to be a lot of speculation in your argument about how the heavy material behaves, some of which does not seem reasonable.

For one thing, you argue that the rubble pile is not monolithic, then seem to argue as if it is, except for the ejecta. And you argue that the heavy ejecta does not rotate with the main body, which would then mean that it is not in orbit around Didymos. I would expect it to be in some sort of tidal relationship with Dimorphos if it is still attracted enough to Didymos to remain close to its surface. So, I would not expect the behavior you are hypothesizing.

Regarding the doubling of Didymos's apparent size: Is that really unexpected for a "rubble pile" hit with a kinetic "bomb"?

For me, the real measure of "success" in changing the orbital trajectory of the center of mass while not creating a "shot gun blast" of fragments that still have trajectories that encompass the original trajectory. That is what we would need to protect Earth if faced with a similarly composed but much larger rubble pile asteroid on a collision course with us.

And, then there is the potential for something like Pysche, which might be a cast iron bowling ball heading for our strike zone.

So, yes, there is still a lot to learn about what happened at Didymos on impact. So hopefully the Hera probe will eventually get completed, launched and arrive there and tell us what we need to know. (see https://www.space.com/dart-asteroid-crash-requires-follow-up-mission )

But, it seems unlikely from the elementary physics of the impact and the observation of a lot of ejecta moving away at high velocity that Didymos's orbit could have remained unchanged. Too bad that Hera was delayed and was not there to catch the event in detail. But, it might not have fully survived the passage of the ejecta, either.

All good reasons to keep spending money on investigating asteroids.
 
What if we confirmed that a killer rock will collide with earth in 6 months, would any here think that we could stop it? How about 1 yr. notice? Two yrs. notice? Do you believe our science, engineering and weapons could save us? Even at two yrs. notice? And don't forget the needed travel time.....for any solution.

This should be a world wide priority. This is not a theoretical threat.

If one showed up tomorrow, two yrs. out, all we can do is panic. Our only choice now is series of nuc explosions. And that might mitigate, but could possibly pollute the earth.

Let's find a big, fast one and nuc it. And see what happens. Let's find out now.
 
Once we get something like SpaceX's Starship operational, we would have a serious chance for fending off an oncoming asteroid or comet, because we would have nearly instant launch capability by the time we configured a payload. Even using their Falcon Heavy, we might be able to cobble together a mission in a few months, but it would be more limited in transit time. But once we can put a Starship payload into orbit, we can really cut down transit time, especially if we can refuel in orbit on a routine basis.

Surely thinking out ahead of time and maybe even fabricating parts to stand-by for such a mission would be a good idea. And, we may very well need to test the effects of blasting some asteroid with a nuclear explosive before we actually need to do it.

There is so much vaiability in asteroids and comets that we really are not yet ready to design a good deflection mission. We need more research missions, some of which are underway, already. But, at least we are making progress.
 
Absolutely. We need a battery of deflectors on station. My scenario was an ideal one. The one that hits us, will give us very short warning. We don't have the necessary eyes to spot the one that will get us. We have blind spots. And Murphy knows it.
 
Feb 2, 2023
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There seems to be a lot of speculation in your argument about how the heavy material behaves, some of which does not seem reasonable.

For one thing, you argue that the rubble pile is not monolithic, then seem to argue as if it is, except for the ejecta. And you argue that the heavy ejecta does not rotate with the main body, which would then mean that it is not in orbit around Didymos. I would expect it to be in some sort of tidal relationship with Dimorphos if it is still attracted enough to Didymos to remain close to its surface. So, I would not expect the behavior you are hypothesizing.

Regarding the doubling of Didymos's apparent size: Is that really unexpected for a "rubble pile" hit with a kinetic "bomb"?

For me, the real measure of "success" in changing the orbital trajectory of the center of mass while not creating a "shot gun blast" of fragments that still have trajectories that encompass the original trajectory. That is what we would need to protect Earth if faced with a similarly composed but much larger rubble pile asteroid on a collision course with us.

And, then there is the potential for something like Pysche, which might be a cast iron bowling ball heading for our strike zone.

So, yes, there is still a lot to learn about what happened at Didymos on impact. So hopefully the Hera probe will eventually get completed, launched and arrive there and tell us what we need to know. (see https://www.space.com/dart-asteroid-crash-requires-follow-up-mission )

But, it seems unlikely from the elementary physics of the impact and the observation of a lot of ejecta moving away at high velocity that Didymos's orbit could have remained unchanged. Too bad that Hera was delayed and was not there to catch the event in detail. But, it might not have fully survived the passage of the ejecta, either.

All good reasons to keep spending money on investigating asteroids.

Dear colleague, unfortunately your statements “you argue that the rubble pile is not monolithic, then seem to argue as if it is, except for the ejecta. And you argue that the heavy ejecta does not rotate with the main body" are wrong. First, looseness (rubble pile) for NEAs is currently well known. Being familiar with scientific publications on this topic, I could not say otherwise. Secondly, there is nothing unusual in the ejection of large rubble into orbits around Dimorphos after the impact, taking into account the low first space velocity compared to the estimated rate of Dimorphos`s «doubling» (see video and my estimation in the first text). And thirdly, the term "spinning" means the rotation of the asteroid around its own axis, but not relating the orbital rotation. As a result, Dimorphos will rotate along with its new "micro-satellites", the behavior of which is practically independent of the parent body`s properties except for its mass. The probability of maintaining its one-sided location also does not depend on these properties (Dimorphos`s spinning, orbital period, etc.). Therefore, my hypothesis remains justified as before, and, by the way, the need for the HERA mission is doubtful. Emotionally, I understand your belief in the "success" of the DART mission, but nature cannot be deceived ...
 
Feb 2, 2023
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It seems that avoiding collisions with the help of an extremely risky and contrary to international law nuclear explosion, alas, remains popular ... However, more optimal options are needed, in addition to sci-fi (for example, laser with the PRACTICAL impossibility of cooling powerful lasers in space) or inefficient (for example, use of a gravitractor when it is impossible to attract a pile of gravel AS A WHOLE). So we need to use the deflecting approach, which would be able to transfer much more kinetic energy to rubble pile asteroid regardless of its naturally cushioning, and thus to nudge it away enough. Such a proposal, in particular, was made in my work, material about which with a short video and the corresponding link are in the Wikipedia:
 
Not understanding why you think that a gravitractor would be any less effective on a rubble pile than on a more solid body of the same mass. Yes, there would be a tidal force, but a gravitractor of sufficient mass should still be able to move the entire asteroid.

To me, the idea of a gravitractor is less appealing because it requires a large mass to be launched and a long period for it to interact with the asteroid. That just doesn't seem feasible to me for any asteriod that is massive enough to require deflection unless we find it and predict its impact on Earth is in the very distant future.

Regarding the need for the Hera mission, it seems that it is nothing like "doubtful" if your speculation has any merit. We would need to verify the resulting condition of Didymos as well as its actual orbit.

In that regard, are we seeing anything different in the eclipse period as time after the impact increases? I expect that people must be continuing to monitor it.
 
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I agree with your assessment of the gravitractor method efficiency . For the sake of brevity, I have given only one of its shortcomings, which has manifested itself recently, thanks to an understanding of the NEAs internal structure. To illustrate it, one can do (or imagine) the following simple experiment, which is a relatively adequate analogy. If a small magnet is brought sufficiently close to a large pile of small metal parts, then it will be able to move only the parts closest to it, but not the entire pile. Therefore, for effective influence, masses are needed that are much larger than actually possible.

In addition to this and the shortcomings you indicated, the gravitational approach has no less significant ones. With my full respect for its authors (former astronauts) and the rating of the journal Nature, where it was published. For example, according to calculations carried out within the already “forgotten” European NEOshield-1 project, even for an asteroid size of about 200 m, it is necessary to place a tractor at distances of only tens of meters from the asteroid surface. At present, there is an experimental fact according to which the dimensions of individual stones on the surface are close to this distance. Given this fact, as well as the typical periods of NEAs axial rotation (several hours), this leads to the conclusion that there is a high probability of damage to the gravitractor.

The question also arises about the effectiveness of its ion engines, preventing the "merger" with the asteroid. The latter becomes obvious if you depict this scheme in the correct mutual scale. In such a scheme, the asteroid will shield most of the hemisphere and therefore the possible directions of ion jets (that is, passing by it) will be far from optimal.

Of course you are also right in saying that "people must be continuing to monitor it". Unfortunately, today this asteroid pair has already moved away from the Earth at distances at which detailed ground-based observations are hardly possible – see:
https://theskylive.com/how-far-is-d...stance of Asteroid 65803,GT) and arrive to us.

As for need the HERA mission. Of course, any assumptions without access to observational data are vulnerable, even for specialists in astrophysics. However, if as a result of their complete analysis it turns out that testing of the impact method showed it to be unsuitable for deflecting dangerous asteroids (i.e., those with dimensions greater than 140 m), then this mission may be useful only to clarify the internal structure of NEAs as such, but no more.

Finally, doesn't it look too strange so far (more than four months!) the complete absence of serious publications regarding the detail analysis of DART results? Indeed, this is truly unprecedented given the importance of the global problem and the considerable budgetary funds spent on this mission.
 
A couple of quibbles:

1. Magnetic attraction is not a good model for gravitational attraction. First, it is a dipole effect and thus diminishes with the cube of the distance instead of the square of the distance. But, it also has inductive effects in ferromagnetic materials such as the "large pile of small metal parts" in your thought experiment, so that tends to modify the field by inducing magnetic effects in the "responding" material that are not mimicked by gravitational effects.

2. With respect to "serious papers" analyzing the DART impact effect, it seems that the initially reported results were pretty specific, so I am not understanding where you are looking for more "serious" writers to get journal articles, unless there is new, contradictory information. And, if the asteroid pair has really moved to a distance where more data would be too inaccurate to be of value, then doesn't that answer your question (as well as mine) regarding additional analyses, at least for about a year?

I see 3 potentially feasible ways to prevent an asteroid from hitting Earth:

1. Something like the DART impact scenario, which requires that the asteroid be cohesive enough to react as one mass, even if a somewhat fluffy mass that changes shape.

2. A slow, steady shove from an active reaction motor, such as an ion drive that is landed on the surface and fired radially outward, perhaps at only those times when the rotation of the asteroid points the thrust in the useful direction(s).

3. An explosive that pulverized the asteroid so that no large pieces remain to fully penetrate the atmosphere and the pieces disperse well enough before reaching Earth that only a small fraction enter the atmosphere and burn up as fireballs (which might still cause some damage).

The physical nature of the target will be the determinant of what can work and what would fail. And, the potential targets seem to vary from loose piles of rocks an dust to dirty snow balls to balls of metal. Clearly, a rocket engine that could move Psyche could be and would need to be able to produce a lot more thrust force than something that we could expect to work on Didymos without probably working its way through the rubble pile.

So, I expect we are going to have to do a lot more work on investigating asteroid and comet physical characteristics, both to understand what would actually work on each type and how to quickly identify the type of any threat that is discovered, maybe without verry much warning.
 
Feb 2, 2023
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Dear colleague, if you do not know which methods of disclosure regarding scientific or technological results are evidence-based and credible (see my first post), do not agree with the experimentally established internal structure of NEAs (which excludes the use of any "pulse" approaches and simplicity of their pulverize - see: Jourdan, Fred, Rubble pile asteroids are forever, Proceedings of the National Academy of Sciences (2023) DOI: 10.1073/pnas.2214353120), and also you suppose that it is possible to install and to keep working of anу «active reaction motor» on a rapidly spinning rubble pile (after all, it will immediately fly away, even without taking into account the need to its turn-off every few hours ...), does it make any sense to continue the discussion?
 
Apparently it is not worth continuing with this particular discussion. I am not convinced of your position, and you seem to not want to consider mine. So, I will continue to think that there are things that we should continue to explore, and will continue to advocate for them where I think they are relevant. Just because some techniques would not work for some targets does not mean they would fail, or even be suboptimal on other types of targets. As I already posted, there are very large differences in the physical characteristics of the types of objects that potentially threaten Earth. So, I am going to continue to consider all sorts of potential engineering approaches to dealing with them, looking at the feasibility of different approaches to different types of objects. I am not ready to buy-into a single plan, at this point.
 

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