oooooo! well there you go. I remember the initial findings from some years ago but never caught up with the more recent theories.
That’s what we’ve got science for.
They look similar to the Carolina Bays, equally not fully understood and also once thought to be impact craters. Sets of aligned oblong depressions are found in several areas in The Americas.
Hmm. Are there such asymmetrical craters on the moon or other airless bodies?
Edit: Saw post 9 above. Still curious.
From your link and the wiki articles it seems like the Argentinian craters are complicated by the possible wind-created features occurring over the top of a previous impact site (hence the finding of meteorite debris in the associated lakes). I do find such geological forensics rather fascinating.
You are mistaking gravitational acceleration (9.8 m/s[sup]2[/sup] on Earth) for escape velocity (11.2 km/s ditto). If I drop a rock from just above shoulder height (2 meters), it will hit the ground at 6.26 m/s. Even dropped from 100 km, the usual “edge of space”, (with no orbital velocity) it will hit at 443 m/s, still less than a typical rifle bullet.
The Apollo CM had the capability to fly a “skip reentry”, and that was the official term for it. There was definitely a risk of skipping off the earth’s atmosphere and back into space, as can be seen from this NASA paper “A Study of Ballistic Reentry Trajectories at a Velocity of 50000 Feet Per Second”
That is why the paper calls those “skip out trajectories”:
However Apollo never did perform an actual skip reentry. It was capable of that and skip guidance was programmed in the Apollo Guidance Computer, but they never used it. I think they felt it was too risky and reserved it for contingency-only use.
As asteroid or comet can apparently “skip off the earth’s atmosphere” and its trajectory can be significantly altered by that encounter.
A well known case which many people saw (and was photographed and filmed) was the 1972 Great Daylight Fireball. It is described in the below NASA site as “bouncing off Earth’s atmosphere, much like a skipping stone can bounce off of a calm lake”.
Still photo: https://apod.nasa.gov/apod/ap090302.html
This part:
brings to mind stranding surfaces in Antarctica.
The thing is, over the course of thousands of years, meteorites falling near some general area isn’t that uncommon. But the meteorites tend to weather away faster than native rocks because they usually contain elemental iron that rusts and expands when exposed to water, breaking them up. But if you have an area with very little rainfall (such as deserts) meteorites can remain mostly intact for thousands of years, and accumulate to such a degree that you basically just have to start walking while staring at the ground, and if you know what to look for (especially if there are few native rocks around) you have a fair chance of finding a meteorite sooner or later. (Many thousands of meteorites have been recovered in the past 20 years or so from the deserts of Northern Africa that way.)
The pampas where those elongated craterish features are isn’t dry enough to preserve the meteorites intact and obvious like a desert, but since “The soil consists chiefly of fine sand, clay, and silt washed down toward the Atlantic by the great rivers or blown in dust storms from the west.” (from Britannica.com) even weathered meteorites would be more findable than in an environment where there were plenty of other native rocks for the meteorites to hide amongst.
(tl;dr: finding unrelated meteorites in the “craters” isn’t as big a coincidence as one might think.)
BTW: there is a strewn field that has yielded some of the largest meteorites known in Argentina, but it is hundreds of miles away from the elongated “craters.”
Meant to include that there are 87 recorded meteorites from Argentina. Don’t know which ones are the ones found in the “craters.”
Sounds like a place where you can get your oil changed and have a Chic-fil-A while you’re waiting!
Vaguely related to any one of the recent asteroid impact threads (and this is the most recent), a newly discovered major impact.
I am convinced that NASA uses the term “skip” with regards to elliptical and hyperbolic orbits with perigees within the upper atmosphere, but it’s a misleading term at best. What’s happening is that since these orbits are less curved than the earth’s atmosphere they lose altitude till they reach perigee (generally above about 70km, because lower than that and they’ll be captured/burn up) and then begin to gain altitude again. This is not because they’ve been deflected upwards by the atmosphere. It is because the atmosphere is curving down away from their orbital path. The only forces the atmosphere can apply to a meteoroid or spacecraft are aerodynamic. So, drag and lift. In the case of a meteoroid, we’re talking pretty much exclusively drag. Some spacecraft, notably the shuttle, can generate non-trivial amounts of lift (that is, force perpendicular to the direction of travel, not necessarily upwards), but this lift is not what is involved in a “skip re-entry”.
If you poke a needle into the skin of an orange at a very oblique angle, and it emerges from the skin without ever reaching the meat of the orange, this is not because the orange peel deflected the needle outwards. The needle is straight, and the peel is curved. Now do the same thing with a curved needle where the curve of the needle has a larger radius than the orange, and you have a “skip off the orange peel” orbit.
I think you are right regarding meteors, but mentioning the NASA Apollo spacecraft confuses this because it had a significant lift/drag ratio and could fly a skip reentry. If this skip reentry was flown incorrectly it could burn up, exceed max g, or skip back into space. This is one reason the programmed “skip guidance” was never used on an actual mission. The shuttle also had this capability to a greater degree but (like Apollo) it was never used on an actual mission but reserved for contingencies. It could have stretched the hypersonic reentry glide much further, but like a rock on a pond, each skip off the atmosphere would have subjected the shuttle to high g force and also high heat loads.
By contrast a meteor or asteroid is presumably a purely ballistic object. If it had any lift vector it would be randomly oriented, and just as likely to deflect the atmospheric trajectory down vs up.
As you stated, the “rock off a pond” analogy is misleading. The rock skips because the rock’s lower surface exerts a downward force on the water which in turn exerts an upward force. That’s only possible because two different substances (water and air) are adjacent. If the water was covered with foam of gradually increasing density, the rock wouldn’t skip. The case of an asteroid entering the atmosphere is more like that.
The apparent asteroid “skipping” behavior is (as you said) likely due to the trajectory being tangent to the earth’s atmospheric shell that curves away from the linear trajectory. I doubt the asteroid trajectory is materially deflected in any vector perpendicular to its flight path (due to the atmosphere – Earth’s gravity would likely alter the trajectory but this isn’t “skipping”). However its orbit could be changed due to losing velocity from friction during the atmospheric encounter.
On the other hand, the Silbervogel Amerika Bomber would actually have, in some sense, “bounced” off of the atmosphere, dipping down into denser regions of air multiple times before coming back up due to aerodynamic forces, and eventually making it far enough doing so to fly a mission from Germany to (say) New York.
No, I am not mistaking anything for anything else.
From your very own link “The escape velocity from Earth is about 11.186 km/s (6.951 mi/s;”. I did round up a bit, from 6.951 to 7, but I also said “about”. ( I also used imperial units rather than SI, but that is not what you are attempting to correct me on.)
The “usual edge of space” is not where an asteroid is being dropped from, it is being dropped from effectively infinity. It is not as if the asteroid only notices Earth’s gravity when it is 100 miles away.
I am not sure why you would think that I have escape velocity, which I quoted and used correctly (7 miles per second), confused with the Earth’s gravitational acceleration, as I didn’t use any numbers anywhere similar to 9.8, nor did I talk about acceleration, but only final impact speed.
Yes, if you drop something from infinity (and anything that is not in orbit around earth is close enough to “infinity” that impact speed is going to be within a few percent), it will impact with 7 miles per second, (or 11ish kilometers per second) vertical velocity (minus air friction which could be a factor on smaller debris, but not going to affect a dinosaur killer noticeably).
Also, your link you used to calculate impact speeds uses 9.8m/s[sup]2[/sup] as a constant, regardless of how high you are dropped from. Useful enough when you are talking about distances less than a few miles or so, but it’s inaccuracy gets greater and greater as the acceleration due to gravity should be decreasing, but in your cited calculator, it acts as a constant. Not useful in the least for these sorts of situations.
So I did a GQ thread on this a few years back, and the best answer was:
To me this makes a convincing case that there is aerodynamic lift that ejects the craft into a shallower orbit for a short time… in other words, exactly what a skipping stone does.
The stone skips because the water touching the bottom is about 800 times more dense than the air touching the top. IOW the lack of symmetry between the two sides of the stone. That situation does not exist with an asteroid entering earth’s atmosphere – all sides are enveloped with the same pressure. The asteroid also has no coefficient of lift – it’s not like a stone spun horizontally to maintain a flat surface to the water surface.
The stone strikes the water as a collision. This depresses the water which creates a rebound back against the stone. This also does not happen with an asteroid entering earth’s atmosphere.
An asteroid is totally different from a hypersonic spacecraft which does have a lift/drag ratio. Even the Apollo capsule had a lift/drag ratio of about 0.7 to 1, and this was oriented toward one side of the base, so just by rolling the capsule it could fly left or right or a longer or shorter trajectory. It could even use this to “skip” back into space. So in the case of either the space shuttle or an Apollo capsule intentionally “skipping off the atmosphere”, they are not really skipping like a stone, but using aerodynamic lift at hypersonic speed to apply an upward lift vector and fly upward out of the atmosphere. That is the intentional “skip out” case, which both Apollo and shuttle were capable of but this was never used.
For the Apollo unintentional skip out case (as Gorsnak mentioned), this would be from having an excessively shallow lunar return trajectory and “skipping back into space”, but it’s not like a stone skipping. Rather it is a trajectory tangent to just above the earth’s surface. If excessively shallow the capsule guidance would attempt to compensate by rolling the left vector downward and pulling itself deeper into the atmosphere. However if the trajectory was too shallow (and the margin was only two degrees wide) it would fly through the atmosphere and out into space before the downward left vector could achieve aerocapture. In this situation it’s not “skipping” out into space but flying through and exiting the curving atmospheric shell before the combination of drag and limited downward lift vector can bend the trajectory sufficiently for aerocapture.
This paper discusses the physics of a stone skipping off water. It appears totally different from an asteroid entering the earth’s atmosphere: http://www.phys.ens.fr/~lbocquet/AJPricochets.pdf