Starship Asterisk*

SpaceCadet wrote: ↑Thu Feb 06, 2020 6:16 am When i see romantic pics like this it amazes me there isn't a booming moondustry yet getting ppl up there to see it for themselves and experience the open and vast void of space. Who wouldn't want to go up and walk the moon?

SpaceCadet wrote: ↑Thu Feb 06, 2020 6:16 am When i see romantic pics like this it amazes me there isn't a booming moondustry yet getting ppl up there to see it for themselves and experience the open and vast void of space. Who wouldn't want to go up and walk the moon?

orin stepanek wrote: ↑Thu Feb 06, 2020 2:06 pm

SpaceCadet wrote: ↑Thu Feb 06, 2020 6:16 am When i see romantic pics like this it amazes me there isn't a booming moondustry yet getting ppl up there to see it for themselves and experience the open and vast void of space. Who wouldn't want to go up and walk the moon?

Thanks but no thanks; I'll stay right here! Maybe if I was a lot younger!SouthernMoonscapeClaviusTycho1024.jpg

Southern terrain pretty rugged though, what with thr heavy cratering !

TheZuke! wrote: ↑Thu Feb 06, 2020 2:22 pm

SpaceCadet wrote: ↑Thu Feb 06, 2020 6:16 am When i see romantic pics like this it amazes me there isn't a booming moondustry yet getting ppl up there to see it for themselves and experience the open and vast void of space. Who wouldn't want to go up and walk the moon?

"I'm just a man, an average man
Doing everything the best I can
But if I could, I'd give the world to you

I'd like to someday be the owner of
The first house on the moon
There would be no neighbors
And no population boom
You might say that all I do
Is dream my life away
I guess it's true
‘Cause I'm stone in love with you"

"I'm stone in love with you" -The Stylistics 1972
https://www.youtube.com/watch?v=bj0aAYDXebo

SpaceCadet wrote: ↑Thu Feb 06, 2020 2:32 pm

orin stepanek wrote: ↑Thu Feb 06, 2020 2:06 pm

SpaceCadet wrote: ↑Thu Feb 06, 2020 6:16 am When i see romantic pics like this it amazes me there isn't a booming moondustry yet getting ppl up there to see it for themselves and experience the open and vast void of space. Who wouldn't want to go up and walk the moon?

Thanks but no thanks; I'll stay right here! Maybe if I was a lot younger!SouthernMoonscapeClaviusTycho1024.jpg

Southern terrain pretty rugged though, what with the heavy cratering !

Not even for just a few days? Maybe a week? I can't imagine a person being the same after such an experience!

SpaceCadet wrote: ↑Thu Feb 06, 2020 6:16 am
When i see romantic pics like this it amazes me there isn't a booming moondustry yet getting ppl up there to see it for themselves and experience the open and vast void of space. Who wouldn't want to go up and walk the moon?

• to pull off my boots and feel that warm moon sand between my toes
  and then to lift off my helmet and smell the roses

SpaceCadet wrote: ↑Thu Feb 06, 2020 6:16 am When i see romantic pics like this it amazes me there isn't a booming moondustry yet getting ppl up there to see it for themselves and experience the open and vast void of space. Who wouldn't want to go up and walk the moon?

MarkBour wrote: ↑Thu Feb 06, 2020 8:39 pm
I also labelled Wilhelm, even though this is
space, so no one can hear Wilhelm scream.

https://en.wikipedia.org/wiki/William_IV,_Landgrave_of_Hesse-Kassel wrote:

Click to play embedded YouTube video.

<<Wilhelm IV of Hesse-Kassel (24 June 1532 – 25 August 1592), also called Wilhelm the Wise, was the first Landgrave of the Landgraviate of Hesse-Kassel. Wilhelm was born at Kassel, the eldest son of Landgrave Philip the Magnanimous and Christine of Saxony. He was the founder of the oldest line, which survives to this day.

Wilhelm was a pioneer in astronomical research, and perhaps owes his most lasting fame to his discoveries in this branch of study. Most of the mechanical contrivances which made instruments of Tycho Brahe so superior to those of his contemporaries were adopted in Kassel about 1584. From then on the observations made in Hesse-Kassel seem to have been about as accurate as those of Tycho.

The principal product of the astronomical observations was the Hessian star catalogue, a catalogue of about a thousand stars. The locations were determined by the methods usually employed in the 16th century, connecting a fundamental star by means of Venus with the sun, and thus finding its longitude and latitude, while other stars could at any time be referred to the fundamental star. It should be noticed that clocks, on which Tycho depended very little, were used at Kassel for finding the difference of right ascension between Venus and the sun before sunset. Tycho preferred observing the angular distance between the sun and Venus when the latter was visible in the daytime. The Hessian star catalogue was published in Historia coelestis (Augsburg, 1666) by Albert Curtz, and a number of other observations are to be found in Coeli et siderum in eo errantium observationes Hassiacae (Leiden, 1618), edited by Willebrord Snell.>>

dlw wrote: ↑Thu Feb 06, 2020 7:04 pm Perhaps a naive question but how did the centers of the oldest craters become flat? Tycho seems bowl shaped with a central peak but most of the larger ones don't appear to be like that.

dlw wrote: ↑Thu Feb 06, 2020 7:04 pm
Perhaps a naive question but how did the centers of the oldest craters become flat?

Tycho seems bowl shaped with a central peak but most of the larger ones don't appear to be like that.

• "small complex craters" called central peak craters (like Tycho).

https://en.wikipedia.org/wiki/Lunar_craters#Lunar_crater_categorization wrote:
<<In 1978, Chuck Wood and Leif Andersson of the Lunar & Planetary Lab devised a system of categorization of lunar impact craters. They used a sampling of craters that were relatively unmodified by subsequent impacts, then grouped the results into five broad categories. These successfully accounted for about 99% of all lunar impact craters.

The LPC Crater Types were as follows:

ALC — small, cup-shaped craters with a diameter of about 10 km or less, and no central floor. The archetype for this category is Albategnius C.

BIO — similar to an ALC, but with small, flat floors. Typical diameter is about 15 km. The lunar crater archetype is Biot.

SOS — the interior floor is wide and flat, with no central peak. The inner walls are not terraced. The diameter is normally in the range of 15–25 km. The archetype is Sosigenes.

TRI — these complex craters are large enough so that their inner walls have slumped to the floor. They can range in size from 15–50 km in diameter. The archetype crater is Triesnecker.

TYC — these are larger than 50 km, with terraced inner walls and relatively flat floors. They frequently have large central peak formations. Tycho is the archetype for this class.

Beyond a couple of hundred kilometers diameter, the central peak of the TYC class disappear and they are classed as basins.

Large craters, similar in size to maria, but without (or with small amount of) dark lava filling, are sometimes called thalassoids.>>

https://en.wikipedia.org/wiki/Impact_crater#Modification_and_collapse wrote:
<<Contact, compression, decompression, and the passage of the shock wave all occur within a few tenths of a second for a large impact. The subsequent excavation of the crater occurs more slowly, and during this stage the flow of material is largely subsonic. During excavation, the crater grows as the accelerated target material moves away from the point of impact. The target's motion is initially downwards and outwards, but it becomes outwards and upwards. The flow initially produces an approximately hemispherical cavity that continues to grow, eventually producing a paraboloid (bowl-shaped) crater in which the centre has been pushed down, a significant volume of material has been ejected, and a topographically elevated crater rim has been pushed up. When this cavity has reached its maximum size, it is called the transient cavity.

The depth of the transient cavity is typically a quarter to a third of its diameter. Ejecta thrown out of the crater do not include material excavated from the full depth of the transient cavity; typically the depth of maximum excavation is only about a third of the total depth. As a result, about one third of the volume of the transient crater is formed by the ejection of material, and the remaining two thirds is formed by the displacement of material downwards, outwards and upwards, to form the elevated rim. For impacts into highly porous materials, a significant crater volume may also be formed by the permanent compaction of the pore space. Such compaction craters may be important on many asteroids, comets and small moons.

In large impacts, as well as material displaced and ejected to form the crater, significant volumes of target material may be melted and vaporized together with the original impactor. Some of this impact melt rock may be ejected, but most of it remains within the transient crater, initially forming a layer of impact melt coating the interior of the transient cavity. In contrast, the hot dense vaporized material expands rapidly out of the growing cavity, carrying some solid and molten material within it as it does so. As this hot vapor cloud expands, it rises and cools much like the archetypal mushroom cloud generated by large nuclear explosions. In large impacts, the expanding vapor cloud may rise to many times the scale height of the atmosphere, effectively expanding into free space.

Most material ejected from the crater is deposited within a few crater radii, but a small fraction may travel large distances at high velocity, and in large impacts it may exceed escape velocity and leave the impacted planet or moon entirely. The majority of the fastest material is ejected from close to the center of impact, and the slowest material is ejected close to the rim at low velocities to form an overturned coherent flap of ejecta immediately outside the rim. As ejecta escapes from the growing crater, it forms an expanding curtain in the shape of an inverted cone. The trajectory of individual particles within the curtain is thought to be largely ballistic.

Small volumes of un-melted and relatively un-shocked material may be spalled at very high relative velocities from the surface of the target and from the rear of the impactor. Spalling provides a potential mechanism whereby material may be ejected into inter-planetary space largely undamaged, and whereby small volumes of the impactor may be preserved undamaged even in large impacts. Small volumes of high-speed material may also be generated early in the impact by jetting. This occurs when two surfaces converge rapidly and obliquely at a small angle, and high-temperature highly shocked material is expelled from the convergence zone with velocities that may be several times larger than the impact velocity.

In most circumstances, the transient cavity is not stable and collapses under gravity. In small craters, less than about 4 km diameter on Earth, there is some limited collapse of the crater rim coupled with debris sliding down the crater walls and drainage of impact melts into the deeper cavity. The resultant structure is called a simple crater, and it remains bowl-shaped and superficially similar to the transient crater. In simple craters, the original excavation cavity is overlain by a lens of collapse breccia, ejecta and melt rock, and a portion of the central crater floor may sometimes be flat.

Herschel Crater on Saturn's moon Mimas

---

Above a certain threshold size, which varies with planetary gravity, the collapse and modification of the transient cavity is much more extensive, and the resulting structure is called a complex crater. The collapse of the transient cavity is driven by gravity, and involves both the uplift of the central region and the inward collapse of the rim. The central uplift is not the result of elastic rebound, which is a process in which a material with elastic strength attempts to return to its original geometry; rather the collapse is a process in which a material with little or no strength attempts to return to a state of gravitational equilibrium.

Complex craters have uplifted centers, and they have typically broad flat shallow crater floors, and terraced walls. At the largest sizes, one or more exterior or interior rings may appear, and the structure may be labeled an impact basin rather than an impact crater. Complex-crater morphology on rocky planets appears to follow a regular sequence with increasing size: small complex craters with a central topographic peak are called central peak craters, for example Tycho; intermediate-sized craters, in which the central peak is replaced by a ring of peaks, are called peak-ring craters, for example Schrödinger; and the largest craters contain multiple concentric topographic rings, and are called multi-ringed basins, for example Orientale. On icy (as opposed to rocky) bodies, other morphological forms appear that may have central pits rather than central peaks, and at the largest sizes may contain many concentric rings. Valhalla on Callisto is an example of this type.

RocketRon wrote: ↑Mon Feb 10, 2020 6:24 am Might be a bit late, so may get no replies.

Someone on talkback radio (subject astronomy) posed this question,
to which I have added a bit.

In the view shown here, almost all the craters shown are near perfectly circular.
Now why would that be ? Given that plenty should be less than perpendicular hits. ?

And also that the Moon is tidally locked to the Earth, so this 'side' of the Moon is shielded somewhat by the Earth,
so direct in perpendicular hits are quite unlikely - more so in the past when the Moon was (much) closer to the Earth.