Bright Explosion on the Moon | Hacker News Reader

Bright Explosion on the Moon

science.nasa.gov

277 pointsColinWright61 comments

I saw this once. It was quite a few years back, through my 8" dob in my front yard. I saw a flash and couldn't imagine what it could have been except an impact.

After some research online that night I began to doubt myself, most people concluded it was too rare and would not be visible.

The next day I saw someone asking in a newsgroup about the mysterious flash he had seen the night before while observing the moon through his telescope. Again, the general consensus was that he couldn't have seen an impact, it was too rare and would not be visible.

I checked with him and we both saw the flash at the same time.

I also saw a satellite transit the moon once with that same telescope, it was so cool.

And to think I don't even pull it out anymore. Sad....

You know what you must do now. Pull that sucker out and start looking at the skies again, with your kids too if you have them.

well, if he doesn't pull out - he will be grateful in a few months to have kids to share his discovery.

Just wanted to say that I helped build this site. It's 2009/2010-era Django site built on feincms with a PostgreSQL backend.

Originally it was a Plone site but after a year we realized it was too hard to meet the deadlines of NASA VIPs. So we moved it to Django. Also, myself and the other developers have NEVER been happy with the flash on the front page. :P

To some this might come off as simple bragging, but I can relate to the sense of accomplishment (and appreciate the little insight into the tech stack).

Well, to be honest, acdha did most of the work for this project. At the time I was stuck being yanked back and forth between several projects per day.

This doesn't strike me as bragging at all. I always appreciate the chance to connect people with projects.

I have learned from some of your posts about developing with Django, so I took a second look at the site after realizing you had helped build it.

Why not replace the flash banner with an animated .gif?

Because the designer who insisted on the flash banner wanted to use flash. :P

I would be remiss in not thanking the Varnish cache developers, too: it's using CloudFront now but Varnish came in quite handy when we suddenly jumped from mostly images to some large videos like http://science.nasa.gov/science-news/science-at-nasa/2010/21... shortly after launch.

The scariest bit of code was probably the scraper + html5lib normalizer I used to migrate something like 15 years of legacy static HTML content from two separate into the CMS using a large pile of selectors and cleanup heuristics.

http://chris.improbable.org/2009/10/16/deploying-django-site... describes how the site was packaged using RPM so we could fit within the more traditional IT model used by the production sysadmins.

One open-source tool which came out of this was https://github.com/shentonfreude/webcompare which we used to bulk-compare the old and new sites to confirm that all of our legacy URLs either redirected or were intentionally 404/410ed.

Thanks for speaking up! FWIW, you did most of the work for the migration and rebuild. I apologize for not naming you when most of this was your work. :-)

No apology necessary - I find it feels weird to name people unless they're already part of the conversation.

I found the footnote particularly interesting:

"The Moon has no oxygen atmosphere, so how can something explode? Lunar meteors don't require oxygen or combustion to make themselves visible. They hit the ground with so much kinetic energy that even a pebble can make a crater several feet wide. The flash of light comes not from combustion but rather from the thermal glow of molten rock and hot vapors at the impact site."

(As you know Bob,) Explosions are kind of like rocket engines. They (generally) don't require external oxygen to happen. Quite a lot of explosions are solids (e.g. TNT) turning into a bunch of N2.

On the other hand, if something creates a bunch of gas in space, there's really no medium for a shock wave, other than the gas created. So it's not going to be as damaging. Though, there's no reason a sub-lunar explosion would be much different that what we get on the earth.

Combustion doesn't make things explode; it just heats things up so they vaporize. But there are other ways to heat things up, too, and the end result is the same.

I thought it was a weird footnote. I mean, do people really think that the rock would burn even if there was oxygen?

I suspect many meteors can and will combust. Lot's of things love oxygen bonds once they get enough energy and there is not a lot oxygen or heat in space.

PS: Think Class D Fire http://en.wikipedia.org/wiki/Fire_classes

Back in 1982, I saw a very bright flash on the Moon - naked eye! It was early Sunday morning (5.22 am) on July 18th.

To this day, I'm not sure what caused it, and I've always eagerly followed up on any reports of 'transient Lunar phenomena', or, as in this instance, 'bright explosions'.

I once spoke to Patrick Moore about it, with a full description (I remember him asking if it was summer time - yes, it was). He was similarly intrigued, and promised to look into it, but I never managed to follow up...

And that ladies and gentlemen is why having an atmosphere is a very good thing.

I wonder if the meteorite that exploded in the atmosphere over Russia did more or less damage than if it had struck the earth intact.

Clearly Atmosphere makes a good shield for burning up the small meteorites - but I wonder if it exacerbates the damage from medium sized ones?

Had that impact hit the ground it would have released over 100 kilotons of energy. It would have been far, far more destructive and deadly.

But without an atmosphere the shockwave from that 100 kiloton explosion should be pretty minimal (through the ground only), and any debris flung into the atmosphere would fall right back down on parabolic trajectories (or go into orbit), not linger in the sky for months/years.

The danger posed by such an explosion, off the top of my head, would be getting hit by the object itself, getting hit by falling debris ejected by the explosion, structural damage from the shockwave that traveled through the ground... I'm not sure what else.

Of course without an atmosphere everyone would be dead anyway, so I suppose this is kind of a silly thing to think about.

Of course without an atmosphere everyone would be dead anyway, so I suppose this is kind of a silly thing to think about.

Just never walk out of your front-door without wearing your space suit.

Hmm. Living in pressure vessels makes structural damage to buildings a great deal more dangerous.

Sort of, there's still a blast wave from the vaporized rock, and there's still the ejecta blanket (which could easily ruin your day if any of it hit you).

How could it release more energy if it hits the ground? The total kinetic energy released must be the same whether it is released in the atmosphere or on impact with the ground.

The toal energy released is the same. The total energy released on the impact is quite different.

100 kilotons of energy released along a several hundred kilometer long tube in the upper atmosphere, at altitudes of tens of kilometers poses much less of a threat to people who live on the surface of the Earth than if all that energy is released at once on the ground.

Define "damage", I guess. It's spread over a larger area, but there's a qualitative difference between widespread broken windows and a local 440 kiloton (per wikipedia) explosion on the ground that will level the surrounding terrain.

People tend to focus on the size of projectiles from space, but their great speed is often overlooked - from a simulator at: http://impact.ese.ic.ac.uk/ImpactEffects/

>Impact Velocity - This is the velocity of the projectile before it enters the atmosphere. The minimum impact velocity on Earth is 11 km/s. Typical impact velocities are 17 km/s for asteroids and 51 km/s for comets. The maximum Earth impact velocity for objects orbiting the sun is 72 km/s

For comparison, the velocity of the ISS is about 7.7 km/s, about equal to the speed of detonating cord.

The minimum impact velocity on Earth is 11km/s??? I beg to differ -- bodies can impact the Earth at arbitrarily low velocities. It depends on where they originate. There are clearly some assumptions being made there that aren't spelled out.

Starting from rest at a distance infinity, any object (regardless of it's own mass) necessarily undergoes a minimum increase in kinetic energy due to the gravitational potential of the mass of the target object (in this case the Earth, which has a known mass and so this is calculable). So I think the assumption is that the object isn't propelling itself away from the earth, and the object hasn't experienced any major competing gravitational forces inside the radius of the Moon's orbit.

Also, 11km/s isn't terribly quick on the celestial scale, I'd imagine.

What you say is true but most objects are not starting from rest at infinity. Things thrown up from the Earth, for example.

It also neglects that there is an atmosphere...

>Impact Velocity - This is the velocity of the projectile before it enters the atmosphere. The minimum impact velocity on Earth is 11 km/s.

I think it's implied that the impacting objects come from outside the Earth-Moon System. Otherwise there would obviously be no minimum impact velocity.

No kidding. Shoot an 8-foot 2x4 at the Earth at 100km/sec and guess how fast the carbon-fluff will be going when it hits. The pieces of anything that breaks up as it descends will quickly slow to their terminal velocity at that height.

NASA says the February Russian fireball was going about 11.6 miles/sec when it blew up (14 miles up) but there's a lot of denser atmo in that 14 miles.

http://neo.jpl.nasa.gov/news/fireball_130301.html

That, and breathing is nice.

Don't forget cosmic rays!

In the video on the website:

  Date: US date format
  Weight: metric
  Size: metric
  Speed: imperial
  Explosion: metric

Consistency is key.

Well NASA has history with making $300 million mistakes with units of mesure:

http://en.wikipedia.org/wiki/Mars_Climate_Orbiter#Cause_of_f...

  The primary cause of this discrepancy was engineering error. 
  Specifically, the flight system software on the Mars Climate 
  Orbiter was written to take thrust instructions using the 
  metric unit newtons (N), while the software on the ground that 
  generated those instructions used the Imperial measure 
  pound-force (lbf). This error has since been known as the 
  "metric mixup" and has been carefully avoided in all missions 
  since by NASA.

Are we even sure of the type of ton used in the explosion measurement?

I think the guidance about how deep you have to bury a moon base to be safe just got revised.

Cosmic rays? Nope - it's the meteors you really have to watch out for.

Can someone explain to me why this would be bright? I understand that this should send up a fair amount of ejecta but wouldn't most of the light be just from a reflection of the sun off of the particles?

If so, this isn't much of an explosion - it's just the most significant impact we've seen so far. I guess explosion is more interesting to the every-man.

Read rkaplan's comment above. In short, it's hitting with so much energy that the force of the impact alone is enough to leave the rock glowing-hot.

And even vaporize some of the material, thus an explosion.

The massive kinetic energy must be dissipated on impact. Some is consumed in displacing the material on the lunar surface. Some is converted into thermal energy, resulting in hot enough material to radiate energy in the visible spectrum.

The ejecta aren't tossed about due to recoil they are lifted out of the ground by an expanding sphere of vaporized rock. And that vaporized rock glows because it is superheated to thousands of degrees, that's what creates the bright flash.

Do you have any reference for this? It seems like you're saying the vaporized rock is imparting force to the impacted surface, which is in turn ejected. I had not understood that the vaporized rock played a central role.

I thought the impactor imparted force directly to the surface. Basically there is a shock wave in the underlying rock which pushes ejecta out of a region underneath the impact site.

Best easy source: http://www.impact-structures.com/understanding-the-impact-cr...

Let me backtrack a bit and say, more accurately that "the ejecta aren't just tossed about due to recoil", both effects play a role. My point was that the flash is caused by superheated/vaporized rock, which you will pretty much always get at such speeds due to the amount of energy released.

Consider the absolutely insane energies involved - the report says "The 40 kg meteoroid measuring 0.3 to 0.4 meters wide hit the Moon traveling 56,000 mph."

That's a thing roughly half the size of a fridge. Traveling at a speed of about 25 kilometers, per second, coming to a full stop.

And this response, which is quite typical for laypeople like myself, shows why the site put that footnote on that article.

Apparently, the impact would be so hard that the rock heats up quite significantly.

"Lunar meteor showers have turned out to be more common than anyone expected"

what does that mean? Is there more stuff flying through space than we thought? Does this have some consequence in the odds of hitting something in space?

It's remarkable to me that someone peering up at the moon right when this struck could have noticed the flash.

It wasn't noticed live, the moon was recorded with the help of a telescope and the explosion was noticed later by an analyst. I also doubt anyone could see that explosion without a telescope as the moon itself is quite bright.

In the video, the narrator says that someone looking at the moon would have seen it, even without a telescope.

The Moon - it's like have BatWings of steel to protect us from evil.

It seems Decepticons have landed on the moon!

Soon, there will be Brights everywhere!

oh gawd, transformers has started...

oh gawd, reddit is leaking...

Aaaand obligatory,

> If your account is less than a year old, please don't submit comments saying that HN is turning into Reddit. (It's a common semi-noob illusion.)

http://ycombinator.com/newsguidelines.html

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