INCOMING!, Detection and observation of Earth-approaching asteroids. Options

[Explorer1]

Reminds me of the Peru crater from a few years back, but no water filling the bottom of this one.
Closest approach of the other rock was above New Zealand, so there's probably not much relation....

[ngunn]

Doubts have been cast on whether this was a meteorite because no fireball was seen and no fragments have been found so far. This set me wondering if it might have been an icy object. A mini-comet arriving from directly overhead at 60 km/s would spend only two seconds in the atmosphere, so there would be little time to notice a flash and too little to turn around, run to a window or get out a camera. Also an icy projectile would not leave solid fragments in the crater.

But whatever is the real cause of this Nicaraguan crater I'd like to pose a few purely hypothetical questions, hoping for informed replies. Take a lump of pure ice and drop it into the inner solar system at 60 km/s. Undoubtedly sublimation would begin before it reached Earth. We want the object to be a few metres across when it enters our atmosphere, so here are my questions:
1. How big would it have to be to start with?
2. Would it be visible as a comet before it reached Earth?
3. Once in the atmosphere would it produce a visible fireball, or would cooling by evaporation/ablation keep the trail too cool to be luminous? Maybe only a vapour trail?
4. Could enough of it reach the ground (assuming a vertical trajectory) to form a crater?

Of these questions the most interesting to me is number 3: What would a lump of ice hitting the atmosphere at 60km/s look like?

[fredk]

The original BBC story reports that the site is near an air force base. We can imagine an accident or shenanigans...

[Gerald]

...Take a lump of pure ice and drop it into the inner solar system at 60 km/s...

Some guesstimates:

After entering the upper atmosphere, it would ionize the surrounding atmosphere like any meteor, not necessarily visible at day, hence the answer to 3. is a "maybe", if you knew it in advance. The thunder would occur after the impact, so looking for the meteorite after hearing/feeling the shock wave would be too late.
Between 60 km and zero height, it would be decelerated from about 60 km/s to almost zero, resulting in a mean deceleration of almost 60,000 m/s² or about 6,000 g. I doubt, that pure ice would survive the resulting force.
Hence the answer to 4. is likely "no".

The answer to 2. is probably "no", since the thickness of the coma would be too small to scatter enough sunlight.
The answer to 1. is ambiguous, since the implicite premise, that a small icy body could reach ground and cause a small crater, likely doesn't hold.
An iron-nickel metorite would probably need an initial diameter of a few meters to reach the ground with supersonic speed.
A body made of ice, and sufficiently large to cause damage, would probably cause a shock wave on the ground due to the kinetic energy, but not a small crater.
A body made of ice, and large enough to hit the ground (tens of meters at least), would probably be davastating, not limited to a small crater.

[nprev]

Sure looks like a bomb crater to me. Possible that it was a long-buried bomb or a BIG land-mine that suddenly decided it was time to explode, but I think the crater would look a bit different (steeper slopes).

[Gerald]

Agreed, a bomb/land mine looks more likely.
The crater is probably too small for an impact of a single meteorite. A sufficiently large fragmented body to reach ground with high velocity would have caused several craters.
And it's unlikely, that the trees immediately beside the crater would be intact after an impact.

[Mongo]

There is security camera footage, though, that show what looks like a bolide descending, with a very loud boom just after it has faded out but before it would have struck the ground. I would guess that the sound is from an explosive disintegration at altitude, probably from when the bolide was at its brightest, in which case the crater would be from a large fragment of the original object, dropping at terminal velocity (hence the lack of severe damage to the nearby trees).

[mto]

There is security camera footage, though, that show what looks like a bolide descending, ...

The caption on the photo reads "Stunned Spaniards rushed outside to snap photos of the fireball in the sky" and describes a separate event in Spain. The article is poorly composed mixing details between the events in Spain and Nicaragua.

[fredk]

Yep, the photos appear to be of the Spanish event:
http://www.thelocal.es/20140908/video-fire...oss-spanish-sky

[Mongo]

It looks like the article is mixing together two events, and the photos are of the Spanish event.

The security camera footage, though, appears to be of a different event, probably the Managua event.

[fredk]

No, that's the Spanish bolide as well:
http://www.youtube.com/watch?v=LYFti-dPEvU

[Mongo]

No, that's the Spanish bolide as well:
http://www.youtube.com/watch?v=LYFti-dPEvU

That Youtube link is obviously video of the same bolide as that in the photos. The video in the article I had linked to was of a different event. It looks nothing like the Youtube bolide you had linked to.

[fredk]

Ah, I see - my old browser wasn't rendering the video in the story you linked to.

There are several reasons to be suspicious of that video. It mentions no source, location, or other details. The person who posted it also posted loony videos about people on the moon and flying monsters. If you look closely, you can see that the video was manually brightened to coincide with the explosive sound (it does not look like an autoexposure adjustment, especially since the true scene brightness doesn't change at that time). The explosive sound is heard before the object would land, and probably before you'd hear a sonic boom. There were no reports of a fireball that I've heard.

[ngunn]

Some guesstimates:

Thanks for taking the trouble with my hypothetical questions, very interesting and much appreciated.

[Mongo]

Chelyabinsk meteor #2? Massive flash over Russia’s Urals stuns locals & scientists

An extraordinary bright orange flash has lit up the sky in Russia’s Sverdlovsk region in the Urals. While locals captured the massive ‘blast’ on numerous cameras, both scientists and emergency services still struggle to explain the unusual event.

Dark evening skies in the town of Rezh in Sverdlovsk region near Russia's Ekaterinburg turned bright orange for some ten seconds on November 14, with the event being caught on several cameras by the locals.

A driver filmed the massive flash with his dashcam, later posting the video on YouTube, with more people commenting they’ve seen it too. Teenagers in the town of Rezh also filmed the phenomenon with a mobile phone.

Theories of what might have caused the “blast” appeared both on social and traditional media, with a new meteorite or military exercise in the region being among the top guesses. Regional emergency services said no accidents in connection with the event had been recorded. No sound of explosion has been reported either.

According to E1.ru, the emergency officials suggested the military were behind the flash, as they might have had a scheduled explosive ordnance disposal procedure. The city administration has also said such ammunition disposal might have taken place, while the military themselves denied they were behind the mystery.

“No exercise and training were underway on that day, and no military units are based in the region, so we have nothing to do with it,” a military press service told E1.ru.

A fireball caused by an asteroid’s collision with the Earth's atmosphere is among other presumed reasons for the burning sky.

“Looks like a falling bolide, which invaded us. Because of the low cloud cover it ceased to exist above the clouds and lit up the whole sky,” a member of the meteorites committee of the Russian Academy of Sciences Viktor Grokhovsky told 66.ru.

Another astronoma, Vadim Krushinsky, doubted his colleague's theory, saying the color of the flash does not support the asteroid speculation. The shade of light depends on the body’s temperature, and flashes caused by bolides are usually whiter, he explained to Ekburg.tv. The observatory engineer suggested his own theory, saying a space rocket launch might have been the cause.

A path of launches from the Plesetsk cosmodrome lies above the area, Krushinsky said. But, according to Russian Federal Space Agency's website, the latest launch from the Plesetsk cosmodrome happened on October 29, with the next one planned for November 24.

People in the Urals witnessed a space ‘invasion’ event a year and a half ago, when the famous Chelyabinsk meteorite hit the region. A massive fireball explosion in February 2013 injured over a thousand people with shattered glass mostly, and damaged many residential and industrial buildings.

https://www.youtube.com/watch?v=XwznErZ1H-M

https://www.youtube.com/watch?v=GZsQKqwDU1k

I don't know that the observed yellow-orange color is that significant. The explosion is close to the horizon in all the video I've seen, and I would expect that just as with sunsets, the blue wavelengths of light would be more heavily scattered.

edit -- The Bad Astronomy blog points out that there is evidence that the explosion happened on the ground, and the light we see is a reflection off the base of the clouds -- not light being transmitted through the clouds. Still no word on what caused the light show, though.