APOD: To Fly Free in Space (2020 Feb 09)

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APOD: To Fly Free in Space (2020 Feb 09)

Post by APOD Robot » Sun Feb 09, 2020 5:06 am

Image To Fly Free in Space

Explanation: What would it be like to fly free in space? At about 100 meters from the cargo bay of the space shuttle Challenger, Bruce McCandless II was living the dream -- floating farther out than anyone had ever been before. Guided by a Manned Maneuvering Unit (MMU), astronaut McCandless, pictured, was floating free in space. McCandless and fellow NASA astronaut Robert Stewart were the first to experience such an "untethered space walk" during Space Shuttle mission 41-B in 1984. The MMU worked by shooting jets of nitrogen and was used to help deploy and retrieve satellites. With a mass over 140 kilograms, an MMU is heavy on Earth, but, like everything, is weightless when drifting in orbit. The MMU was later replaced with the SAFER backpack propulsion unit.

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Re: APOD: To Fly Free in Space (2020 Feb 09)

Post by Ann » Sun Feb 09, 2020 6:39 am

As a color commentator, I wonder about the (changing?) color of the Earth's atmosphere from space. The picture that is today's APOD was taken in 1984, and the Earth looks very blue. But in December 2014, when ISS filmed the Earth from space, the sky had taken on a very different color.

The blue skies of Earth as seen from space in 1984. Image credit: NASA, STS-41B
The green skies of Earth as seen from space in 2014. Photo: ESA/NASA.


















An even more interesting ISS image is actually the one at left. The Earth's sky is seen to be a shade of murky green, overlaid with a brilliant thin line of vivid green. Above the green atmosphere of the Earth, many bluish stars pierce the blackness like pale sapphires.

What has changed? Why does the Earth's (night) sky so often look green these days? Is the color just a consequence of new ways of photographing the atmosphere?

Ann
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EricX

Re: APOD: To Fly Free in Space (2020 Feb 09)

Post by EricX » Sun Feb 09, 2020 12:01 pm

Hi Ann!
The green tint to the atmosphere is called “airglow” and it’s always there as far as I know. It’s caused by various interactions with the solar wind and cosmic rays (I’m probably butchering this explanation :oops: ). I’ll just copy and paste...

“ The night-time emission of green light, with a wavelength of 557.7 nanometres, was detected in the Earth’s upper atmosphere more than a century ago. Most researchers believed that the light comes from oxygen atoms in the ionosphere. The idea was that during the day, oxygen molecules are ionised by ultraviolet light from the Sun to form O2+ ions. When these ions recombine with electrons at night, they create unstable molecules which split into energetic atoms. These can emit energy as a green glow.



Read more: https://www.newscientist.com/article/mg ... z6DSODADre

Cheers!

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Re: APOD: To Fly Free in Space (2020 Feb 09)

Post by neufer » Sun Feb 09, 2020 1:23 pm

EricX wrote:
Sun Feb 09, 2020 12:01 pm

The green tint to the atmosphere is called “airglow” and it’s always there as far as I know.

“ The night-time emission of green light, with a wavelength of 557.7 nanometres, was detected in the Earth’s upper atmosphere more than a century ago. Most researchers believed that the light comes from oxygen atoms in the ionosphere. The idea was that during the day, oxygen molecules are ionised by ultraviolet light from the Sun to form O2+ ions. When these ions recombine with electrons at night, they create unstable molecules which split into energetic atoms. These can emit energy as a green glow.

Read more: https://www.newscientist.com/article/mg ... z6DSODADre
https://en.wikipedia.org/wiki/Airglow wrote: <<The airglow phenomenon was first identified in 1868 by Swedish physicist Anders Ångström. Airglow is caused by various processes in the upper atmosphere of Earth, such as the recombination of atoms which were photoionized by the Sun during the day, luminescence caused by cosmic rays striking the upper atmosphere, and chemiluminescence caused mainly by oxygen and nitrogen reacting with hydroxyl free radicals at heights of a few hundred kilometres. It is not noticeable during the daytime due to the glare and scattering of sunlight.

Even at the best ground-based observatories, airglow limits the photosensitivity of optical telescopes. Partly for this reason, space telescopes like Hubble can observe much fainter objects than current ground-based telescopes at visible wavelengths.

Airglow at night may be bright enough for a ground observer to notice and appears generally bluish. Although airglow emission is fairly uniform across the atmosphere, it appears brightest at about 10° above the observer's horizon, since the lower one looks, the greater the depth of atmosphere one is looking through. Very low down, however, atmospheric extinction reduces the apparent brightness of the airglow.

One airglow mechanism is when an atom of nitrogen combines with an atom of oxygen to form a molecule of nitric oxide (NO). In the process, a photon is emitted. This photon may have any of several different wavelengths characteristic of nitric oxide molecules. The free atoms are available for this process, because molecules of nitrogen (N2) and oxygen (O2) are dissociated by solar energy in the upper reaches of the atmosphere and may encounter each other to form NO. Other species that can create air glow in the atmosphere are hydroxyl (OH), atomic oxygen (O), sodium (Na), and lithium (Li).>>
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Re: APOD: To Fly Free in Space (2020 Feb 09)

Post by orin stepanek » Sun Feb 09, 2020 1:56 pm

freeflyer_nasa_960.jpg
It feels so alone out here! :ohno:
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Re: APOD: To Fly Free in Space (2020 Feb 09)

Post by Chris Peterson » Sun Feb 09, 2020 3:19 pm

Can't believe I beat Art to this recollection...
_
2001-poole-floating-in-space.jpg
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Re: APOD: To Fly Free in Space (2020 Feb 09)

Post by Ann » Sun Feb 09, 2020 3:57 pm

Chris Peterson wrote:
Sun Feb 09, 2020 3:19 pm
Can't believe I beat Art to this recollection...
_
2001-poole-floating-in-space.jpg
Let's see, was that 19 years ago?

Or 52 years ago?

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Re: APOD: To Fly Free in Space (2020 Feb 09)

Post by Chris Peterson » Sun Feb 09, 2020 3:59 pm

Ann wrote:
Sun Feb 09, 2020 3:57 pm
Chris Peterson wrote:
Sun Feb 09, 2020 3:19 pm
Can't believe I beat Art to this recollection...
_
2001-poole-floating-in-space.jpg
Let's see, was that 19 years ago?

Or 52 years ago?

Ann
Either way, it ended better for McCandless than it did for Poole! At least the pod bay doors stayed open.
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Make a wish, Dear, make a wish.

Post by neufer » Sun Feb 09, 2020 4:11 pm

Click to play embedded YouTube video.
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Re: APOD: To Fly Free in Space (2020 Feb 09)

Post by sillyworm 2 » Sun Feb 09, 2020 6:14 pm

The outhouse is around here I just know it is...

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Re: APOD: To Fly Free in Space (2020 Feb 09)

Post by neufer » Sun Feb 09, 2020 6:57 pm


Ann wrote:
Sun Feb 09, 2020 6:39 am
sillyworm 2 wrote:
Sun Feb 09, 2020 6:14 pm

The outhouse is around here I just know it is...
Why does the Earth's (night) sky so often look green these days?
  • Tom Hanks relieves hi... Dale Gardner retrieves Westar 6 :arrow:
https://en.wikipedia.org/wiki/Manned_Maneuvering_Unit wrote:
<<The Manned Maneuvering Unit (MMU) is an astronaut propulsion unit that was used by NASA on three Space Shuttle missions in 1984. Two aluminium tanks with Kevlar wrappings contained 5.9 kilograms of nitrogen propellant each, enough propellant for a six-hour EVA depending on the amount of maneuvering done. Typical MMU velocity capability was about 25 m/s. There were 24 nozzle thrusters placed at different locations on the MMU. To operate the propulsion system, the astronaut used their fingertips to manipulate hand controllers at the ends of the MMU's two arms. The right controller produced rotational acceleration for roll, pitch, and yaw. The left controller produced translational acceleration for moving forward-back, up-down, and left-right. After a safety review following the Space Shuttle Challenger disaster, the MMU was judged too risky for further use and it was found many activities planned for the MMU could be done effectively with manipulator arms or traditional tethered EVAs.>>
https://en.wikipedia.org/wiki/Milli_mass_unit wrote:
<<The milli mass unit or (mmu) is used as a unit of mass by some scientific authors. It is a short form of the tongue-breaking but formally more correct "milli unified atomic mass unit" (mu) and equivalent to ​a thousandth of the unified atomic mass unit (u). A more modern name is the millidalton (mDa). Since 1961 the unified atomic mass unit "u" has been defined as ​1⁄12 the mass of 12C. Before that the atomic mass unit "amu" was defined as ​1⁄16 the mass of 16O (physics) and as ​1⁄16 the mass of O (chemistry). Thus the publication date in literature ought to be heeded when reading about the milli mass unit as its name does not reveal whether it refers to the old amu or the newer u. The mass excess is usually indicated in mu or mmu.

In mass spectrometry the mass accuracy of a mass analyzer is often indicated in mu, even though a more correct unit would be mTh (millithomson) since mass spectrometers measure the mass-to-charge ratio, not the mass.>>
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Re: APOD: To Fly Free in Space (2020 Feb 09)

Post by MarkBour » Mon Feb 10, 2020 12:31 am

When I learned a few basic facts about orbital mechanics, I found them surprising / counter-intuitive.

For example, if you are following behind a spaceship and want to catch up to it, a single forward blast would increase your speed, but this would cause you to go into a higher orbit and eventually fall further behind the spacecraft.

As I think about an astronaut attempting to maneuver freely with his/her MMU, I wonder if this is an issue. Perhaps they needed to undergo special planning and training about these surprising facts, in order to behave correctly in the event of an emergency resulting in a widening distance. But, without trying to get deeper into the physics of it, I'm hoping that the effects such as I mentioned are not really a problem over short distances, or with real-time ongoing control by the astronauts. For example, perhaps they just angle their thrust downward a bit, perhaps even without having to really think about it. Optimistically, in the local frame of reference where they're flying around and about a spaceship, intuitive jetting around works fine.

I guess I'm wondering the following: Can an astronaut just "aim back at the ISS" and he/she'll get there without a problem, or is it complicated?
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Re: APOD: To Fly Free in Space (2020 Feb 09)

Post by Ann » Mon Feb 10, 2020 4:56 am

MarkBour wrote:
Mon Feb 10, 2020 12:31 am
When I learned a few basic facts about orbital mechanics, I found them surprising / counter-intuitive.

For example, if you are following behind a spaceship and want to catch up to it, a single forward blast would increase your speed, but this would cause you to go into a higher orbit and eventually fall further behind the spacecraft.

As I think about an astronaut attempting to maneuver freely with his/her MMU, I wonder if this is an issue. Perhaps they needed to undergo special planning and training about these surprising facts, in order to behave correctly in the event of an emergency resulting in a widening distance. But, without trying to get deeper into the physics of it, I'm hoping that the effects such as I mentioned are not really a problem over short distances, or with real-time ongoing control by the astronauts. For example, perhaps they just angle their thrust downward a bit, perhaps even without having to really think about it. Optimistically, in the local frame of reference where they're flying around and about a spaceship, intuitive jetting around works fine.

I guess I'm wondering the following: Can an astronaut just "aim back at the ISS" and he/she'll get there without a problem, or is it complicated?
I have no idea, Mark, but recently Swedish newspapers wrote a lot about Swedish-American astronaut Jessica Meir, who performed a spacewalk in order to do some repair (or something) on the ISS. One Swedish paper asked, in a quiz, how many meters or kilometers Jessica Meir had moved during her spacewalk. The correct answer was, She didn't move very far, but it required an effort corresponding to running many kilometers.

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Re: APOD: To Fly Free in Space (2020 Feb 09)

Post by Ann » Mon Feb 10, 2020 5:00 am

neufer wrote:
Sun Feb 09, 2020 6:57 pm

Ann wrote:
Sun Feb 09, 2020 6:39 am
sillyworm 2 wrote:
Sun Feb 09, 2020 6:14 pm

The outhouse is around here I just know it is...
Why does the Earth's (night) sky so often look green these days?
  • Tom Hanks relieves hi... Dale Gardner retrieves Westar 6 :arrow:
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Re: APOD: To Fly Free in Space (2020 Feb 09)

Post by Chris Peterson » Mon Feb 10, 2020 5:34 am

MarkBour wrote:
Mon Feb 10, 2020 12:31 am
When I learned a few basic facts about orbital mechanics, I found them surprising / counter-intuitive.

For example, if you are following behind a spaceship and want to catch up to it, a single forward blast would increase your speed, but this would cause you to go into a higher orbit and eventually fall further behind the spacecraft.

As I think about an astronaut attempting to maneuver freely with his/her MMU, I wonder if this is an issue. Perhaps they needed to undergo special planning and training about these surprising facts, in order to behave correctly in the event of an emergency resulting in a widening distance. But, without trying to get deeper into the physics of it, I'm hoping that the effects such as I mentioned are not really a problem over short distances, or with real-time ongoing control by the astronauts. For example, perhaps they just angle their thrust downward a bit, perhaps even without having to really think about it. Optimistically, in the local frame of reference where they're flying around and about a spaceship, intuitive jetting around works fine.

I guess I'm wondering the following: Can an astronaut just "aim back at the ISS" and he/she'll get there without a problem, or is it complicated?
You don't need to jet about in some non-intuitive direction. Sure, as you move away from the ISS you are in a different orbit- slowly drifting behind if you're farther from the Earth, slowly moving forward if you're closer. But these orbital rates are extremely small given maximum distances on the order of 100 meters. At a 100m Earth-radial distance from the ISS, an astronaut will experience a drift rate compared with the ISS of 62 mm/s. The MMU is capable of a lateral acceleration of 91 mm/s2. So while there is a small counterintuitive motion vector, I doubt it's enough for the astronauts to even be aware of. They pretty much aim where they want to go, because they're pretty much operating in the same frame of reference as the ISS (i.e. both are moving in a orbit at about 7900 m/s). That's quite different from spacecraft which are matching velocities or which are separating with the intent of having very different velocities.
Chris

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Re: APOD: To Fly Free in Space (2020 Feb 09)

Post by neufer » Mon Feb 10, 2020 2:50 pm

MarkBour wrote:
Mon Feb 10, 2020 12:31 am

When I learned a few basic facts about orbital mechanics, I found them surprising / counter-intuitive.

For example, if you are following behind a spaceship and want to catch up to it, a single forward blast would increase your speed, but this would cause you to go into a higher orbit and eventually fall further behind the spacecraft.

As I think about an astronaut attempting to maneuver freely with his/her MMU, I wonder if this is an issue. Perhaps they needed to undergo special planning and training about these surprising facts, in order to behave correctly in the event of an emergency resulting in a widening distance. But, without trying to get deeper into the physics of it, I'm hoping that the effects such as I mentioned are not really a problem over short distances, or with real-time ongoing control by the astronauts. For example, perhaps they just angle their thrust downward a bit, perhaps even without having to really think about it.
Yes... angle one's catch-up velocity (δv) downward a bit (Θ)
to best catch up with an object you are trailing by distance D.
  • where tan(Θ) = 2πD/(δv x orbital period).
(If your destination is behind you then aim slightly above the object.)

If Θ is small enough it is probably best to simply play it by ear as Chris suggests
(keeping in mind that you don't want to smash into your final destination too fast).
Last edited by neufer on Mon Feb 10, 2020 5:04 pm, edited 1 time in total.
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Re: APOD: To Fly Free in Space (2020 Feb 09)

Post by Ann » Mon Feb 10, 2020 3:11 pm

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Re: APOD: To Fly Free in Space (2020 Feb 09)

Post by MarkBour » Mon Feb 10, 2020 6:59 pm

Ann wrote:
Sun Feb 09, 2020 6:39 am
As a color commentator, I wonder about the (changing?) color of the Earth's atmosphere from space. The picture that is today's APOD was taken in 1984, and the Earth looks very blue. But in December 2014, when ISS filmed the Earth from space, the sky had taken on a very different color.

... https://apod.nasa.gov/apod/image/2002/f ... sa_960.jpg
... https://compote.slate.com/images/498a34 ... 10349b.jpg
... https://www.nasa.gov/sites/default/file ... 110462.jpg
What has changed? Why does the Earth's (night) sky so often look green these days? Is the color just a consequence of new ways of photographing the atmosphere?

Ann
Capture.JPG
You may be comforted by this image, from the ISS about 14 hours ago ...
http://www.ustream.tv/recorded/125722115
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Re: APOD: To Fly Free in Space (2020 Feb 09)

Post by Ann » Mon Feb 10, 2020 7:11 pm

MarkBour wrote:
Mon Feb 10, 2020 6:59 pm
Ann wrote:
Sun Feb 09, 2020 6:39 am
As a color commentator, I wonder about the (changing?) color of the Earth's atmosphere from space. The picture that is today's APOD was taken in 1984, and the Earth looks very blue. But in December 2014, when ISS filmed the Earth from space, the sky had taken on a very different color.

... https://apod.nasa.gov/apod/image/2002/f ... sa_960.jpg
... https://compote.slate.com/images/498a34 ... 10349b.jpg
... https://www.nasa.gov/sites/default/file ... 110462.jpg
What has changed? Why does the Earth's (night) sky so often look green these days? Is the color just a consequence of new ways of photographing the atmosphere?

Ann
Capture.JPG
You may be comforted by this image, from the ISS about 14 hours ago ...
http://www.ustream.tv/recorded/125722115
Thanks, Mark! :D

Ann
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