Wikipedia:Reference desk/Archives/Science/2018 March 29

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March 29

Trip to Moon

when the man needs so strong missiles for escaping earth gravity ; which missile helped Armstrong to escape from moon gravity field? --Akbarmohammadzade (talk) 06:16, 29 March 2018 (UTC)

• We have a bit of information at Moon landing, in particular the section "scientific background". The Moon is smaller but also much less massive than the Earth, thus its escape velocity is smaller (by about 5 times, at 2.38km/s instead of 11.19km/s for Earth) and much less thrust is needed (as a first approximation, 25 times less, since kinetic energy grows as the square of the speed). Tigraan^{Click here to contact me} 07:47, 29 March 2018 (UTC)

There was no "missle"; instead the Lunar Excursion Module only needed a remarkably small "ascent engine" to escape the moon's gravity. —2606:A000:4C0C:E200:51D9:C715:1A1D:7336 (talk) 07:55, 29 March 2018 (UTC)

Strictly the LEM did not "escape" lunar gravity: when it docked with the command module it was orbiting in lunar gravity. It was a burn from the service module that took Apollo away from lunar gravity.--Phil Holmes (talk) 09:24, 29 March 2018 (UTC)

possible? — Preceding unsigned comment added by Akbarmohammadzade (talk • contribs) 10:03, 29 March 2018 (UTC)

Yes. ←Baseball Bugs ^{What's up, Doc?} carrots→ 10:21, 29 March 2018 (UTC)

• Some rough figures for rocket thrust:

• Saturn V first stage - 8,000,000 lbf thrust
• Apollo Service Module - 20,000 lbf thrust
• Apollo Descent Stage - 1,000–10,000 lbf thrust (throttleable, for control during the landing)
• Apollo Ascent Stage - 3,000 lbf thrust

The Ascent Stage managed with so little thrust for several reasons. It only had to lift the absolute minimal mass - just the two astronauts, life support, moon rocks and enough fuel to get into a low Moon orbit. They even left the spacesuit backpacks behind on the Moon to save weight. Also the lunar orbit was very low, so they weren't going far. Then the Moon's low mass helps enormously. For the flight to and from the Moon, and entering and leaving lunar orbit, they relied on the much more powerful Service Module engine (which didn't have to land on the Moon). Andy Dingley (talk) 11:00, 29 March 2018 (UTC)

Yes, not only possible. Besides the factors mentioned above, there is also the effect (or lack thereof) of atmospheric drag on rocket launches.Doroletho (talk) 11:49, 29 March 2018 (UTC)

And 8 million lbf means it has the ability to levitate 8 million pounds (1.5 million pounds excluding itself) till it runs out of fuel unless it tips over or something breaks. Since the rocket burns up to 20 tons of fuel per second you'd have to find some way to gently add weight at the same rate it's released. Sagittarian Milky Way (talk) 19:15, 29 March 2018 (UTC)

More directly to the point: the reason the Saturn V was so massive is it had to lift all the fuel for the entire mission—along with the craft of course, but most of the mass was fuel. If you add fuel, you now need fuel to lift that fuel, and up and up your fuel requirements go. This is referred to as the "tyranny of the rocket equation". At some point it actually becomes impossible to launch a rocket because you can't carry enough fuel. This is what makes rocket launches so expensive, and why there's interest in things like reusable rockets and non-rocket spacelaunch. As noted above, for Apollo, by contrast, when lifting off from the Moon, the lunar ascent stage only needed enough fuel to lift off from the Moon (which has much lower gravity than Earth) and rendezvous with the command module. A perhaps more intuitive way of putting it, stolen from someone else but I don't remember who: if you were going on a long automobile trip, would it be cheaper to tow all the fuel for your entire trip with you, or refuel at stations along the way? The problem is there are no refueling stations in space. --47.146.60.177 (talk) 20:55, 29 March 2018 (UTC)

Interested readers may find joy in comparing the rocket equation to the jet aircraft range equation ... this is exemplary of the many times during the advanced study of aeronautics and astronautics when the Astro- student must turn to his peer, the humble Aero- student, and confess to her that he has taken the easy way out by deciding to travel exclusively in space. Nimur (talk) 15:36, 30 March 2018 (UTC)

To answer our OP's question explicitly, the Lunar Module departed the moon's surface under the power of the Lunar Module Ascent motor; and as described above, the returning astronauts then transferred to the Command Module and departed moon's gravitational potential energy well under the power of the Service Module's primary mover, the Apollo_Command/Service_Module#Service_Propulsion_System.

This grand scheme was called Lunar orbit rendezvous; it is the necessary and obvious extension of the staged rocket beyond the simple launch vehicle and into the interplanetary domain. This key concept - the in-orbit rendezvous of multiple independent manned propulsion modules - was the Ph.D. thesis of one Edwin Aldrin: Line-of-sight guidance techniques for manned orbital rendezvous (PDF), a technical overview that bridges the very unusual technological and psychosocial divide between airplane-pilot and the space-man. (The kind of astronaut who wants to "see where he's going" is the same kind of astronaut who demands an artificial horizon to be built into the space-ship, and there is a great deal of technical irony in both considerations that belies the absolute insanity of putting mammals in space).

If I may wax poetic and speak metaphorically, the missile that is necessary for a successful moon-mission is not even involved in the space-flight: rather, it is the very existence of the nuclear intercontinental ballistic missile - the weapon that makes possible the literal act of planetary destruction - that makes a manned flight to the Moon a fundamentally rational economic decision for Earth life-forms to undertake. The greatest and most powerful missile ever constructed on Earth was a statement-piece - an artist's response to a very difficult time on Earth - a great moving monument built from titanium and kerosene, dedicated to the principle that our species does not need to make devastating weaponry out of every monumental accumulation of high-energy-density material. For six billion years, various life-forms on this tiny wet rock have converted energy and transformed matter through chemical mechanisms; but prior to the breaching dawn of the Unix era, Life on Earth had never successfully concentrated sufficient fossilized fuel-energy required for orbital escape velocity: this was only possible by virtue of billions of years of evolution toward an intelligent life form that could evolve enough Intelligence to end itself.

Nimur (talk) 16:06, 30 March 2018 (UTC)

Earth is only around 6,000 4.5 billion years old. --2600:1:B0AB:F55F:C4AE:90C:A494:5A81 (talk) 20:53, 30 March 2018 (UTC)

I don't really want to quibble over the details, and of course I generally accept the peer-reviewed scientific consensus regarding the timeline of formation for both the Earth and the rest of the solar system, to within some moderately-high-likelihood confidence-interval - but it's pretty arbitrary to suggest that the exact value of "4.5 billion years" is meaningful to define the "age" of Earth. This is true for a variety of reasons, not the least of which is the troublesome definition of the word year when we're describing the material that accretes into a proto-planetary object. After all, the molecules inside Earth, for the most part, have been around for a lot longer than the time during which they've been bound gravitationally to each other; and the atoms inside those molecules, for the most part, have been around since ... well, most educated guesses suggest that they come from one or two suns ago; and as for the subatomic particles in those atoms... at least one scientist jested that we can't meaningfully define an age for an indistinguishable fundamental particle. I prefer to think of the formational history of Earth as a continuous series of causally-related events; and if scientific authors want to call out a few key moments or entire eras that are hallmarks of qualitative formational characteristics, I fully respect their estimates - as long as they describe those estimates using valid SI units. Nimur (talk) 21:46, 30 March 2018 (UTC)

Nobody suggested an exact value. The precise value is 4.54 ± 0.05 billion years, so "4.5 billion years" is a perfectly acceptable rounding. And there is nothing troublesome about the definition of "year". Astronomy uses the Julian year, defined as exactly 365.25 days of exactly 86,400 seconds, totaling exactly 31,557,600 seconds in the Julian astronomical year. Seconds, of course, are exactly 9,192,631,770 cycles of a Cesium atomic clock. Even if you decide to use the Gregorian mean year (averages roughly 365.2425 days) it won't change that "4.5 billion years" to 4.4 or 4.6. And nobody suggested that the age of the earth is the age of the atoms that comprise the earth. That would be silly. --Guy Macon (talk) 06:18, 31 March 2018 (UTC)

How many orbits around the sun do you think have been completed by the constituent matter that now comprises the bulk of Earth's mass? I would propose that this difficult consideration presents a more interesting conundrum than quibbling over whether geologists and astronomers can keep their units coordinated. Nimur (talk) 16:00, 31 March 2018 (UTC)

That's not the definition of "year" being used here, and is thus irrelevant to the question of how old the earth is. (It's 4.5 billion years old). --Guy Macon (talk) 16:04, 31 March 2018 (UTC)

If you're very interested, here's Age of the Earth from the USGS, which also gives 4.5 billion years - in agreeement with Guy Macon and our article. So, I will formally apologize that I betrayed this fact by virtue of my casual tone. However, I can't emphasize strongly enough that there is a danger of false precision and a great deal of subjectivity in the definition of these scientific ideas. For example, USGS couches the age of Earth metric with the following caveats: "So far scientists have not found a way to determine the exact age of the Earth directly from Earth rocks because Earth's oldest rocks have been recycled and destroyed by the process of plate tectonics...." and regarding the very precise 4.54 billion years figure: "To be precise, this age represents the last time that lead isotopes were homogeneous througout the inner Solar System and the time that lead and uranium was incorporated into the solid bodies of the Solar System." If you want to define this specific metric as the age of the planet, you're in fine company; but it's certainly not the only way to define planetary age; and it isn't even a particularly universal method, because some planets - most planets - don't contain enough rock to measure a radiological signal. So, shall we really choose to define a methodology to ascertain "planetary age" that cannot possibly be applied to most planets?

I am in good company when I decline to emphasize the "age" of Earth. Consider this discussion of Earth's moon: although many ages are proffered, the esteemed author instead describes an "interpretive evolution ... in seven major stages." Worlds are quite complex - even when they are too small to be planets - and to abbreviate the immensity of entire geological eras into a single numeral would be doing a great disservice to the reader and to scientific truth.

Or to put it in more concrete terms: let us suppose that the Earth is precisely 4.54 billion years old. What exactly do you think occurred during the interval between 4.54001 billion years ago until 4.54000 billion years ago? If the event occurred during that time, then this short period of only ten thousand years must surely have been quite dramatic! But that is illogical, and that is not how planets form. There was no "instant" of creation; there was in fact a lengthy period of gradual transition; and like all gradual occurrences, the exact boundaries are not clear. We can select a token "age" to demarcate an era, but this "age" isn't a timestamp for any specific event.

By now we're way off topic from the original question. I regret that my lengthy narrative has diverged so far from answering the question; and I apologize that my literary flourishes may have resulted in a less-than-airtight summary of some very complex scientific topics. My key point remains, irrespective of the numerical details: humans are unique on planet Earth, in that we comprise the only process that has existed in our entire planet's geological history that was capable of synthesizing and concentrating the quantity of energy necessary to reach escape-velocity; and that this energy-density has manifested in two forms: as rocket propulsion suitable for lunar voyage, and as weaponized nuclear fission. No other geological process on Earth has accomplished this phenomenon since planetary accretion. If you wish to take exception to any part of my commentary, I recommend that you ignore the numerics, and assault my unconventional equation of human life-forms with other geological processes; but this is a world-view that is shared by many great scientists.

Here's a great book: Intelligent Life in the Universe. A lot of the science is decades old; so if you want to quibble over irrelevancies, you'll find dozens of minute errors in the work; no one short of Carl Sagan himself writes, in the chapter on planetary age, that we have almost no empircal data on planets, save one data point; and that great philosophical minds "of the caliber of Kant and LaPlace have wrestled with this problem..." and that qualitatively, it seems the consensus on basic facts of our solar system appear to change every hundred years or so ...but if anything, it only proves the higher point: your knowledge of correct scientific truth is possible only because you are an intelligent life form, and that manifestation of intelligent life will quantitatively exist for a very very short period of time compared to the remainder of the universe outside of your existence. The universe is large and has lasted a long time; and you, like I, are very small mammals who shall only live for a few orbits, and shall only visit our sentience upon one or two worlds, plus or minus. You will only see a few orbits of this planet around its star, and from these, you will attempt to inductively reason about billions of orbits that you have not seen; and will not see; you will estimate a little bit; and compute the greatest mathematics that can be performed by the brain of a life-form that evolved on this Earth; and you can cite the art conducted by other smart mammals who have tried the same estimations, a few orbits prior; and you will bank a lot in your innate belief in consistency, determinism, and causality of this universe. Maybe a little less hubris is due, with respect to the demand for extraordinary precision on topics of such immensity?

Nimur (talk) 06:16, 1 April 2018 (UTC)

Even the claim "humans are unique on planet Earth, in that we comprise the only process that has existed in our entire planet's geological history that was capable of synthesizing and concentrating the quantity of energy necessary to reach escape-velocity" can run aground on the rocky shore of definitions. One could say that simple machine intelligences have reached escape velocity -- in fact only the MIs have reached the escape velocity of the sun and not just the earth. The counterargument is that the MI would not exist if not for the humans, but of course humans would not exist if not for various earlier hominids. --Guy Macon (talk) 11:49, 2 April 2018 (UTC)

Very true. The Voyager space probes did not build themselves; but it is equally true to say that the Apollo astronauts did not themselves design the basic chemical mechanisms that enable cellular respiration; and so we have the conundrum summarized in the original Cosmos series: "if you wish to make an apple pie from scratch, you must first invent the universe...". Nimur (talk) 15:06, 2 April 2018 (UTC)