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Posted by: abalone Jan 31 2006, 09:13 AM

What is the impact speed 1.5km/s? Probably more like a plane crash the a meteorite impact
http://www.spacedaily.com/reports/SMART_1_To_Crash_Into_Lunar_Surface_In_August.html

Posted by: paxdan Jan 31 2006, 12:23 PM

Smart-1 impact: 1.5 km/s x 367 kg

Lunar Prospector impact: 1.7 km/s x 160 kg

lets hope for a better show this time

Posted by: AndyG Jan 31 2006, 02:43 PM

Hmmm...413 MJ...almost exactly 100kg of TNT equivalent. That said, it's only 80% more energy than Lunar Prospector. But then the reports suggest it's going to be visible (i.e. on the near side, and not all-but hidden in a polar crater?)

Andy G

Posted by: tty Jan 31 2006, 08:47 PM

Actually I think the speed of a lunar satellite close to the surface is about 1.7 kms-1, but even taking 1.5 kms-1 I would like to point out that this is about equal to Mach 5, which is a rather higher speed than an APDS shell has when leaving a tank gun. Hardly your typical plane crash which normally occurs at about a tenth of the speed and a hundredth of the kinetic energy per pound.

tty

Posted by: Toma B Jan 31 2006, 09:35 PM

I just hate that kind of reporting...
-There is no mention about what kind of telescope would be appropriate to watch that event...KECK or my "Orion" Newton 4,5" reflector short-tube telescope...
-Will SMART-1 hit illuminated or not illuminated side of visible side of the Moon?
-From what part of Earth will this be visible?
...is there any real report about this event?

Posted by: djellison Jan 31 2006, 10:19 PM

I'd hope they do it on the dark side so that we can all get out our binos and watch for it.

Doug

Posted by: ljk4-1 Jan 31 2006, 10:34 PM

This reminded me of Robert Goddard's plan in 1920:

"to [send] to the dark part of the new moon a sufficiently amount of the most brilliant flash powder which, in being ignited on impact, would be plainly visible in a powerful telescope. This would be the only way of proving that the rocket had really left the attraction of the earth as the apparatus would never come back."

Quoted from here:

http://en.wikipedia.org/wiki/Robert_Goddard_%28scientist%29

Did Goddard say how much flash powder would be needed to make the flash visible from Earth? Would his plan have been possible?

Note: The 80th anniversary of Goddard launching the first liquid-fueled rocket is coming up on March 16:

http://en.wikipedia.org/wiki/Image:Goddard_and_Rocket.jpg

Posted by: abalone Jan 31 2006, 10:41 PM

From the report, it gives the impression that they will be looking for a dust cloud. I dont think that 1.7km/s is fast enough to vaporise anything and I assume it is almost out of any chemical propellants that might result in an explosion.

For these reasoms the best impact conditions are probably just over the dark side of the terminator to make any small explosion visible and illuminate any rising dust cloud against the dark background

Posted by: Phil Stooke Jan 31 2006, 11:01 PM

As things stand now the impact will be on the far side. If there is sufficient propellant left the orbit will be adjusted to make impact occur about 2 weeks earlier or later, so it will be on the near side. Since an exact position is not yet chosen (and depends on the fuel available) we can't say if it will be illuminated or not.

The only impact certainly known to have been seen from Earth was Hiten. Ranger and Apollo SIVB impacts were not seen, though efforts were made to see the Ranger impacts. But Luna 2, Luna 5 and Luna 7 impacts were reportedly seen or photographed. But most US astronomers discounted those reports. This would be a unique chance to assess the visibility of an impact using modern equipment.

The Hiten impact was seen in IR from Australia, just on the dark side of the terminator near Furnerius crater.

Posted by: RNeuhaus Feb 1 2006, 04:11 AM

Then, I am afraid that the HSB space telescope is the only which can see the impact flash?

Rodolfo

Posted by: AndyG Feb 1 2006, 12:26 PM

Let's see - back of an enveloping:

The Sun shines at mag -26.7. I'd personally want a good, bright flash that I could see without optical aid on the dark side of a thin crescent Moon. I'm aiming for mag +1.0.

Solar output is around 1380 watts/m2, and therefore mag +1.0 is equivalent to 1.15*10^-8 watts/m2.

My eye is about 3.85*10^8m away from the Moon, meaning the flash (if hemispherically propagated) must release about 10.7GW at source. Thinking of flash-powder explosions I've seen in old films, and with reference to http://photography.about.com/library/weekly/aa031901d.htm, once persistance-of-vision effects are factored in, I'd suggest the explosion would need to take a tenth of a second or so. Therefore I'd need 1.07 GJ of powdered magnesium and potassium perchlorate: given that it's basically an explosive, that's around about a quarter-tonne...

Andy (not obviously related to Robert) Goddard

Posted by: Bob Shaw Feb 1 2006, 02:18 PM

I think I'd rather not have been the range safety officer, or had to fuel the rocket!

I wonder if they'd have launched with an expected impact on July the fourth?

Bob Shaw

Posted by: ljk4-1 Feb 1 2006, 10:06 PM

I think Goddard expected to the flash to be seen with less powder. I recall reading one of his contemporaries criticising this aspect of his concept, not so much the idea of a rocket reaching the Moon.

Posted by: AndyG Feb 2 2006, 11:19 AM

Goddard might be right: I estimated a mag 1.0 flash would be visible with around 250kg of powder. But the joy of magnitudes, of course, is that they're logarithmic: a just-visible, edge-of-the-naked-eye-envelope magnitude 6.0 flash would require a hundred times less powder - i.e. 2.5kg. Conversely, that should produce a flash that would be easily visible in binoculars.

Intriguingly, the descent engine of the LEM produced a not dissimilar amount of power to the ignition of this smaller quantity of powder. Given that much of that energy was released in the form of heat, I wonder if - once the CEV is flying - a smallish aperture, IR-equipped, amateur telescope could be used to track future manned vehicles at the Moon?

Andy

Posted by: Bob Shaw Feb 2 2006, 11:25 AM

Andy:

Seems reasonable, especially if the burn takes place when the vehicle is against a cold, dark background, such as space or the pre-dawn surface. Maybe even optical...

Bob Shaw

Posted by: dvandorn Feb 2 2006, 01:12 PM

But, Bob -- not even the CMPs were able to see the LMs firing their engines from more than about 100 miles distance. Of course, they didn't use night-vision scopes, either...

-the other Doug

Posted by: Bob Shaw Feb 2 2006, 02:13 PM

oDoug:

I wonder whether they were looking at the engine from above, the side, or below? And against what sort of background?

I suppose that there's actually some virtue in this enquiry, as manned vehicles could act as a yardstick against which meteorite impacts on the moon could be measured!

Bob Shaw

Posted by: dvandorn Feb 3 2006, 01:55 AM

I've read several different accounts of CMPs watching LM engine operations, and of course we have the TV record of the three J mission LM lift-offs. From these, I've determined that you pretty much have to be looking straight down the throat of the engine bell to see much in the way of actual self-illuminated exhaust with the nitrazine-UDMH engines they used in Apollo. Otherwise, the exhaust expanded far too rapidly for there to be much illumination. Or even much in the way of visible exhaust at all.

The best view of such an engine exhaust is the TV record of the Apollo 17 lunar liftoff. The engine exhaust is very briefly visible just as the stages separate, due I'm sure to the reflection of exhaust from the descent stage forrming interference patterns in the plume. As the ascent stage lifts clear, you see absolutely no indication of exhaust or even illumation from the engine. At pitchover, however, the ascent engine brightens, and as the stage flies almost directly away from the TV camera, you see a dramatic brightening as we look straight down the engine bell. The brightness of the exhaust at that point overcomes the brightness of the sunlight reflected off the stage. It's fairly dramatic.

I imagine the exhaust was somewhat more noticeable when burns occurred in darkness -- but even so, Mike Collins wrote of being able to see Eagle's DOI burn (in darkness), but that he was looking through his telescope pretty much straight down the descent engine's throat, which we know is the optimal angle. So the question of dark-condition visibility is still not that well defined.

Of course, we *do* have the record of Shuttle attitude control and OMS engine burns, which produce quite bright and noticeable exhaust patterns, when viewed from the Shuttle windows. And those are also nitrazine-UDMH engines. Maybe it's just a matter of the lighting...

-the other Doug