posted 08-15-2013 01:33 PM
Did the Apollo moon missions coincide with the moon at perigee?
SpaceAholic Member
Posts: 4494 From: Sierra Vista, Arizona Registered: Nov 1999
posted 08-15-2013 01:42 PM
The launch window was not established contingent upon earth/moon distance.
Robert Pearlman Editor
Posts: 43576 From: Houston, TX Registered: Nov 1999
posted 08-15-2013 02:22 PM
Here is a 1967 NASA film explaining the Apollo launch window constraints:
YankeeClipper Member
Posts: 622 From: Dublin, Ireland Registered: Mar 2011
posted 08-16-2013 11:35 AM
If you seek out the various Apollo TransLunar/TransEarth Trajectory Plotting Charts (ATT) you will see great diagrams for each mission showing lunar orbital positions during possible launch dates, mission trajectories and distances.
Dietrich Member
Posts: 68 From: Registered: Jul 2009
posted 08-16-2013 03:29 PM
On the charts, the lunar orbit is clockwise. What is the reason for this?
The moon's rotation is anti-clockwise (to face always the same side to Earth). Would not the braking impulse and the re-start to lunar orbit by the lunar module be less if these two maneuvers are performed not counter the movement of the lunar surface but in the same direction of the moon's rotation?
Practically all rockets are launched towards East on Earth (except retrograde and polar orbits and launches from Israel) to utilize the speed of Earth's rotation.
moorouge Member
Posts: 2458 From: U.K. Registered: Jul 2009
posted 08-17-2013 12:40 AM
Just to confuse the issue folks. The conception we all have of the moon (or satellites for that matter) in nice, tidy paths going round our planet is an over simplification to make it understandable.
In fact, the moon weaves in and out of the path Earth follows as it seems to go round the sun and does not "go round" us at all.
Jim Behling Member
Posts: 1488 From: Cape Canaveral, FL Registered: Mar 2010
posted 08-17-2013 07:40 AM
The moon does orbit Earth and does "go round" it, just as Earth orbits the sun. However, when looked at from the reference of the sun, the moon weaves in and out of the path Earth follows as it goes round the sun. Looking at the solar system from outside it, and the motion of the solar system makes the moon's path is even more complex.
The point of reference in inertial space is very important when describing motion of objects in relation to each other.
moorouge Member
Posts: 2458 From: U.K. Registered: Jul 2009
posted 08-18-2013 02:05 AM
To us, sitting here on Earth, the moon appears to go round us. In fact, the truth is that the moon, like us, is in its own orbit round the Sun. That this orbital path is influenced by Earth's gravity causing it to weave in and out of Earth's orbital path creates the illusion that it goes round us.
Whilst on the vagaries of orbital mechanics, and getting back to somewhere near the original query, the furthest humans have been from Earth are the crews of Apollo 8 or Apollo 13 depending on which source one looks at. However, because the distance between the Earth and Moon is constantly changing, even during the course of a flight, this is a figure it's almost impossible to compute.
For the same reason of changing orbits, the closest humans have been to the Sun are not the Apollo astronauts. This distinction belongs to the inhabitants of our planet who lived some 70,000 years ago.
Robert Pearlman Editor
Posts: 43576 From: Houston, TX Registered: Nov 1999
posted 08-18-2013 02:57 AM
To cite Phil Plait from Bad Astronomy:
Every now and again I get an email from someone who tries to tell me that the moon orbits the Sun more than it orbits the Earth.
On the face of it, their claim appears to hold water. For example, they'll say that if you compare the orbital velocity of the Moon around the Earth (about 1 km/sec) to the orbital velocity of the Moon around the Sun (the same as the Earth's velocity around the Sun, about 30 km/sec), you'll see that the Moon's orbit is always convex to the Sun; in other words, it doesn't make a loopy pattern around the Sun, as you might expect...
Moreover, if you calculate the force of the Sun's gravity on the Moon, you find it's more than twice the force of Earth's gravity on the Moon!
Whoa. So does the Moon orbit the Earth, or the Sun?
Turns out, it orbits the Earth, despite these claims. The above claims are true, but are not important in this argument. Instead, you have to look at something called the Hill sphere. Basically, it's the volume of space around an object where the gravity of that object dominates over the gravity of a more massive but distant object around which the first object orbits.
OK, in English — and more pertinent to this issue — it's the volume of space around the Earth where the Earth's gravity is more important than the Sun's. If something is orbiting the Earth inside Earth's Hill's sphere, it'll be a satellite of the Earth and not the Sun.
The derivation of the math isn't terribly important here (and it's on the Wikipedia page if you're curious), but when you plug in the numbers, you find the Earth's Hill sphere has a radius of about 1.5 million kilometers. The Moon's orbital radius of 400,000 km keeps it well within the Earth's Hill sphere, so there you have it. The Moon orbits the Earth more than it orbits the Sun. In reality it does both, and saying it orbits one and not the other is silly anyway....
Kite Member
Posts: 855 From: Northampton UK Registered: Nov 2009
posted 08-18-2013 08:22 AM
The way I have always understood is that Earth and the moon revolve around their common centre of gravity, which is well underneath Earth's surface, which explains the bobbles in the moon's and Earth's orbits.
canyon42 Member
Posts: 238 From: Ohio Registered: Mar 2006
posted 08-18-2013 09:53 AM
It's really not that complicated. Does the moon orbit Earth? Yes. Does it orbit the sun? Yes. Does it orbit the Milky Way? Yes. Does it orbit some ridiculously larger cluster of galaxies/objects than that? Undoubtedly, yes.
You have to determine what frame of reference you are specifying. The simplest one — and the one that matches our own everyday experience — is that the moon does indeed orbit Earth (or as was previously noted, they both orbit the pair's center of gravity). The fact that it also "orbits" other objects in other frames of reference does not nullify this.
The claim that its "actual" path through space is much more complex is actually sort of meaningless in the sense that there is no ultimate backdrop against which it can be measured. It all depends on the viewpoint one selects, and that is always going to be somewhat arbitrary.
Blackarrow Member
Posts: 3160 From: Belfast, United Kingdom Registered: Feb 2002
posted 08-18-2013 12:59 PM
Does it help to look to the examples of Apollo SIV-B stages that have slipped from heliocentric orbit to Earth orbit and back again? I'm thinking in particular of the Apollo 12 SIV-B which was spotted a few years back.
Gonzo Member
Posts: 596 From: Lansing, MI, USA Registered: Mar 2012
posted 08-19-2013 06:29 PM
quote:Originally posted by Robert Pearlman: Here is a 1967 NASA film explaining the Apollo launch window constraints...
Thanks for the film. While it reminds me of all the bad science films I had to watch in high school (many years ago), it also very elegantly explained something that has puzzled me for those years as well.
I never really understood why the launch windows were where they were and why they were so short. I mean, from one point of view, the moon isn't going anywhere. It's been there for billions of years and will continue to do so for the foreseeable future! So thanks for the education.
Whether the moon revolves around Earth or the sun is relative. Both statements are correct if taken from the proper relative view. Because in it's simplest form, the moon goes around Earth, and the Earth goes around the Sun.
However, when you start understanding orbital mechanics, the fact is, Earth and the moon orbit around a central point due to their gravities and that point orbits around the sun as a system.
You can argue definitions, but the fact is, Earth and the moon are a system. What one does effects the other.
Look at it this way, how would Earth's orbit around the sun change if the moon wasn't there? How would that path change? What about the moon's orbit if Earth wasn't there? (And by extension, the path of the sun itself if the Earth-Moon system didn't exist?)
While these perturbations may be small, you can't dismiss them either. If you can show that nothing would change (which you can't), only then could you claim that one orbits the other.
SpaceAholic Member
Posts: 4494 From: Sierra Vista, Arizona Registered: Nov 1999
posted 08-19-2013 09:21 PM
quote:Originally posted by Gonzo: ...Earth and the Moon orbit around a central point due to their gravities and that point orbits around the Sun as a system.
Since the barycenter continuously resides within Earth's radius the moon still revolves around Earth regardless of frame of reference.
Robert Pearlman Editor
Posts: 43576 From: Houston, TX Registered: Nov 1999
posted 08-20-2013 01:31 PM
What we're all discussing here is a what is generically referred to as the n-body problem, which "is the problem of predicting the motion of a group of celestial objects that interact with each other gravitationally."
In particular, we're referring to the three-body problem, in this case the Earth, moon and Sun. Even more specifically, it is the "circular restricted three-body problem," where two of the bodies are in circular orbits (i.e. the Earth and the moon).
And while it is true the three-body problem is not entirely solved, for the narrative purposes of explaining the Earth-moon relationship to another person, the Hill sphere — the the region around an astronomical body in which it dominates the attraction of satellites — establishes that the moon circles the Earth.
Rocketman! Member
Posts: 122 From: Redmond, Washington, USA Registered: Dec 2007
posted 08-21-2013 12:23 AM
quote:Originally posted by Dietrich: ...the lunar orbit is clockwise. What is the reason for this?
I believe your insight is correct regarding the additional braking force, and subsequent additional launch acceleration, that the lunar module would need to attain as a result of its east-to-west orbital direction. But, the amount of the disadvantage is very small compared to launching east-to-west on Earth, from what I have been able to find out.
The relevant facts that follow are gathered from a quick survey of reliable internet sources...
Spacecraft launched from Cape Canaveral, Florida, Earth, attain a "speed boost" by launching easterly which contributes 915 mph to their orbital speed. Thus, to attain low Earth orbit, the rocket need only add 16,585 mph to get to 17,500 mph. In this case, the Earth's rotation contributes about five percent of the total required orbital speed.
The speed of a point on the moon's equator moves at about 10 miles per hour due to its rotation. (Circumference divided by 27 day period of rotation divided by 24 hours per day.) Since Apollo lunar modules landed and launched in a lunar westerly direction, it follows that they had to overcome about 10 miles an hour due to the moon's rotation in the direction opposite to travel.
To orbit the moon at 60 miles above the surface, a speed of 3330 mph is needed. That figure is derived from the Apollo 8 orbital period of 129 minutes at 60 miles. So, the 10 mph surface motion is only a 0.3% disadvantage relative to the required lunar orbital speed.
As I have come to understand it, and as shown in the video shared above, the primary reason they approached the moon during landing in the direction they did is that they wanted a low sun angle to help highlight the lunar terrain, and they also wanted the sun at the pilot's back for visibility.
I believe there may also be a secondary advantage to orbiting in the direction the command modules did that uses the moon's orbital motion to help decelerate when entering lunar orbit (the spacecraft moving opposite the direction of the moon's orbit). The effect would be somewhat opposite to the "slingshot" trajectories used by the Voyager and Pioneer probes to accelerate.
This last notion is mainly speculation on my part, and I again invite those who may have more orbital mechanics knowledge to weigh in if I am in error.
Peter downunder Member
Posts: 57 From: Lancefield, Victoria, Australia Registered: Apr 2012
posted 08-26-2013 05:58 AM
quote:Originally posted by Tykeanaut: Did the Apollo moon missions coincide with the moon at perigee?
This question has been one that has intrigued me for years.
Without going back to check, I understand Apollo 8 had a launch window that was a couple of weeks (a month?) ahead of a potential Russian launch. My question is: without a concern for lighting conditions at the landing site, what is taken into consideration for a lunar orbit launch?
After all, in 1968 it was paramount to be the first into orbit. Would finding the correct lighting conditions on arrival even be considered if losing the race was a possibility?
With my limited understanding, I would have thought once in orbit, whether launched from Russia or New Zealand, on the first of the month, or the 29th, the TLI burn could be adjusted to find the moon three days later. Why did it matter what time of the month we took off?
Jim Behling Member
Posts: 1488 From: Cape Canaveral, FL Registered: Mar 2010
posted 08-26-2013 06:49 AM
There was a lighting concern. They wanted the Apollo 8 crew to be able to see the potential landing sites while lit by sunlight.
Headshot Member
Posts: 891 From: Vancouver, WA, USA Registered: Feb 2012
posted 08-26-2013 07:24 AM
The Soviet lunar launch window actually opened on Dec. 7, 1968, several weeks ahead of Apollo 8's launch window. Technical problems with the unmanned Zond 6 test mission in November 1968 prevented the Soviets from launching their manned mission around the moon ahead of Apollo 8.
Peter downunder Member
Posts: 57 From: Lancefield, Victoria, Australia Registered: Apr 2012
posted 08-27-2013 01:57 AM
Okay, this is the crux of my question: Why is there a launch window for Moscow (or thereabouts) and Kennedy Space Center's opens days/weeks later?
Keep it simple! I failed high school physics.
Andy Anderson Member
Posts: 87 From: Perth, Australia Registered: Dec 2009
posted 09-03-2013 09:57 PM
The following is quoted from NASA MSC Internal Note No. 70-FM-78 - Apollo 13 Lunar Trajectory Notes.
LAUNCH WINDOWS.
There are a number of considerations which determine the unique time period called the launch window from which the lunar mission is flown. These considerations are as follows.
Daylight launch from Kennedy Space center
Launch azimuth (direction) from Kennedy Space Center restricted to 72 (degrees) to 108 (degrees) from north
Translunar injection to occur over the Pacific Ocean (as opposed to the Atlantic Ocean)
Low sun elevation at the lunar landing site
Goldstone, California, radar coverage of the lunar landing phase
Daylight earth landing in the prime recovery area.
By the time Apollo 17 launched, the location of the lunar landing site combined with item 4 and confidence in the launch procedure was probably the reason that item 1 was not complied with.
As for...
quote:Originally posted by Dietrich: ...the lunar orbit is clockwise. What is the reason for this?
My uneducated guess is that, to have a free return trajectory in case of a translunar abort (as occurred on Apollo 13) you have to approach the moon from the leading edge of the orbit (clockwise around the moon) in order to get a moon gravity assist to slow your approach speed and "curve" the trajectory back towards earth.
Finally as to what goes round what, from the same document, NASA says,
The moon rotates 360 (degrees) about the earth in a near circular orbit every 27.32 days. The distance from the earth to the moon varies between 222,000 miles at the nearest point and 253,000 miles at the furthest point, distances equivalent to 28 to 32 earth diameters.
The earth revolves about the sun every 365.25 day; leap year every fourth year compensates for the small difference. The earth's orbital plane about the sun, called the ecliptic plane, is inclined 23.5 (degrees) to the earth's equator, the latter defined by the earth's rotation about the north pole.
...and with a little help from Captain REFSMMAT, that seemed to work out OK for them.
Explorer1 Member
Posts: 180 From: Los Angeles, CA, USA Registered: Apr 2019
posted 04-12-2020 02:21 AM
Were the launch dates of Apollo lunar missions always timed according to the moon's closest point to Earth?
Editor's note: Threads merged.
nasamad Member
Posts: 2141 From: Essex, UK Registered: Jul 2001
posted 04-12-2020 02:38 AM
No, they were timed for having an optimal sun position at each respective landing site.
Explorer1 Member
Posts: 180 From: Los Angeles, CA, USA Registered: Apr 2019
posted 04-12-2020 05:42 AM
But were they timed only with the optimal sun position in mind or were the missions timed with a combination of sun position and the proximity of the moon in mind as well?
nasamad Member
Posts: 2141 From: Essex, UK Registered: Jul 2001
posted 04-12-2020 06:49 AM
I’m pretty sure it was based just on sun angle.
They needed a low angle for a number of things such as thermal control (hence the differing patterns of materials on the lunar module), defining the landing site and obstacles during descent, and being able to complete the mission before lunar noon. The highest sun angle during an Apollo mission was 13.3 degrees on Apollo 17
Explorer1 Member
Posts: 180 From: Los Angeles, CA, USA Registered: Apr 2019
posted 04-12-2020 08:46 AM
But did the low angle create long shadows during the landing descent?
oly Member
Posts: 971 From: Perth, Western Australia Registered: Apr 2015
posted 04-12-2020 10:31 AM
Yes, that is why the launch dates were selected. The low sun angle allowed the astronauts to see their own shadow as they approached the surface, giving them a reference of scale against the lunar surface (they had never been there before and had no reference how big the craters and boulders were).
You intercept your shadow at touchdown. Plus, the low sun angle make identifying craters and hills easier.
Watch the landing films, the shadow of the lunar module is an important cue for landing.
posted 08-15-2013 01:33 PM
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