Why People Should Read

[SimonDoom]

According to NASA's tracker, Artemis is now over 250,000 miles from Earth (all time record, I think), and still getting farther away, and around 8700 miles away from the moon, and drawing closer.

Does anyone know the answer to this: On lunar missions, does the Earth typically rendezvous with the moon when it is on the opposite side of the earth from the sun? That appears to be the case now. If so, is there a reason for this? Gravity? Safety?

 

[TheLobster]

On lunar missions, does the Earth typically rendezvous with the moon when it is on the opposite side of the earth from the sun?

You mean the spacecraft rendezvous with the Moon, right? And "it" is the Moon there, I reckon. Just making sure we understand what you're asking.

 

[SimonDoom]

You mean the spacecraft rendezvous with the Moon, right? And "it" is the Moon there, I reckon. Just making sure we understand what you're asking.

Yes. Of course. My typo. Is it standard for the spacecraft to reach its closest point to the moon when the moon is on the opposite side of the earth from the sun, as opposed to some other location in the moon's orbit? We're about 4 days past full moon right now, so not quite the opposite side.

 

[SimonDoom]

It's 1:45 PDT and Artemis is approaching its farthest point from Earth and closest position to the moon. Its current velocity is only about 1000 mph, which is slower than I would have guessed and much slower than the velocity it achieved after leaving Earth. I understand the physics of why it must slow down as it curves around the moon -- but does anyone know HOW it does it? Does it use reverse thrusters or rely principally on the moon's gravity?

 

[anthrodisiac]

Yes. Of course. My typo. Is it standard for the spacecraft to reach its closest point to the moon when the moon is on the opposite side of the earth from the sun, as opposed to some other location in the moon's orbit? We're about 4 days past full moon right now, so not quite the opposite side.

I was gonna say, gosh, I sure hope the Earth doesn't rendezvous with the Moon

 

[anthrodisiac]

It's 1:45 PDT and Artemis is approaching its farthest point from Earth and closest position to the moon. Its current velocity is only about 1000 mph, which is slower than I would have guessed and much slower than the velocity it achieved after leaving Earth. I understand the physics of why it must slow down as it curves around the moon -- but does anyone know HOW it does it? Does it use reverse thrusters or rely principally on the moon's gravity?

It's almost exclusively gravity. It's slowed down by the pull of the Earth even as it starts to get farther away and no longer is putting more energy into the system with engines. Because of that, Earth's gravity keeps trying to tug it back, which slows it down. By the time they get to the moon's influence, they'll start to speed back up as they fall toward the moon, due to the moon's gravity, but the trajectory will slingshot them past it instead. The only thing you would need thrusters for is pinpoint maneuvers at this point.

 

[anthrodisiac]

It's crazy to think the moon is so bright they can barely look at it directly.

 

[Bramblethorn]

Speaking of Apollo astronauts, before his death, Jim Lovell recorded a wakeup message for the Artemis crew:

"Hello Artemis II! This is Apollo astronaut Jim Lovell. Welcome to my old neighborhood. When Frank Borman and Bill Andrews and I orbited the moon on Apollo 8, we got humanity's first up close look at the moon and got a view of the home planet that inspired and united people around the world. I'm proud to pass that torch on to you as you swing around the moon and lay the groundwork for missions to Mars, for the benefit of all. It's a historic day, and I know how busy you'll be, but don't forget to enjoy the view. So, Reid and Victor and Christina and Jeremy, and all the great teams are supporting you, good luck and Godspeed from all of us here on the good Earth."

https://www.cbsnews.com/chicago/news/jim-lovell-wakeup-message-artemis-ii/

 

[SimonDoom]

It's almost exclusively gravity. It's slowed down by the pull of the Earth even as it starts to get farther away and no longer is putting more energy into the system with engines. Because of that, Earth's gravity keeps trying to tug it back, which slows it down. By the time they get to the moon's influence, they'll start to speed back up as they fall toward the moon, due to the moon's gravity, but the trajectory will slingshot them past it instead. The only thing you would need thrusters for is pinpoint maneuvers at this point.

It's so obvious now that you mention it, but I wasn't considering the impact of Earth's gravity in slowing it down as it went to the moon. Here's the interesting thing: right now it's still slowing down even though it's only 6153 miles from the moon and my understanding is it won't get closer than 4,000. So I assume it will quickly accelerate as it goes around the back side of the moon. Then the Earth's gravity will catch it as it approaches.

 

[Voboy]

You can hear in their voices how excited they are to be up there.

 

[anthrodisiac]

It's so cool to think that when they lose contact from Earth, they will be the most distant and isolated people in history.

I can't wait to see some of those videos and pictures from when they get the solar eclipse, and really just everything!

 

[SimonDoom]

You can hear in their voices how excited they are to be up there.

It must be awesome.

I think it would be hard to do this. I try to imagine what it's like being in a relatively small, confined spacecraft for 10 days, knowing that only a thin layer of metal separates you from the vastness, coldness, darkness, and certain death of space. It's hard to imagine.

 

[anthrodisiac]

It must be awesome.

I think it would be hard to do this. I try to imagine what it's like being in a relatively small, confined spacecraft for 10 days, knowing that only a thin layer of metal separates you from the vastness, coldness, darkness, and certain death of space. It's hard to imagine.

And if anyone offered it to me I'd take it in a heartbeat!

 

[anthrodisiac]

The terminator is freakin' gorgeous!

 

[anthrodisiac]

"Hey, babe." Probably the farthest "Hey, babe" in all of human history.

 

[ElectricBlue]

It's 1:45 PDT and Artemis is approaching its farthest point from Earth and closest position to the moon. Its current velocity is only about 1000 mph, which is slower than I would have guessed and much slower than the velocity it achieved after leaving Earth. I understand the physics of why it must slow down as it curves around the moon -- but does anyone know HOW it does it? Does it use reverse thrusters or rely principally on the moon's gravity?

All gravitational forces, as I understand it. Newtonian physics doing something. We need a space-boffin to explain.

The spacecraft will need to get some acceleration from somewhere, or it's going to take a long time to get home.

 

[Voboy]

It must be awesome.

I think it would be hard to do this. I try to imagine what it's like being in a relatively small, confined spacecraft for 10 days, knowing that only a thin layer of metal separates you from the vastness, coldness, darkness, and certain death of space. It's hard to imagine.

I can imagine it with little trouble, and things I've done in the past lead me to believe I'd handle it well enough if I had enough brains to become an astronaut.

I know it's difficult for some people to imagine, but those probably aren't the people NASA would choose for the manned program. Their "casting agents" look for people with a propensity for being able to cope with this kind of environment, and there are probably quite a few out there.

 

[TheLobster]

Like others said, it’s all just gravity. Check the previous page of this thread for a Wikipedia link that describes the free-return trajectory they’re following.

By the way, this is also why it’s a longer trip. Apollo 8 made it in less than a week, and they did a few orbits around the moon, too. Artemis II will take 10 days with only a short flyby. But they need to carry less fuel, so that probably translates to more equipment and one extra crew member.

 

[Voboy]

Wild, the clarity of their observations from 5k miles out.

With no atmosphere to get in the way and a low sun, the terrain appears to be very very clear.

 

[TheLobster]

It must be awesome.

I think it would be hard to do this. I try to imagine what it's like being in a relatively small, confined spacecraft for 10 days, knowing that only a thin layer of metal separates you from the vastness, coldness, darkness, and certain death of space. It's hard to imagine.

It would probably be around 10 days, maybe more, if I added up all the transatlantic flights I’ve been on, and this spacecraft has waaaaay more space per person than what you get in economy.

So yeah, zero problems imagining it

 

[BeechLeaf]

But how much free red wine do they get to cope with it? That's how I deal with flight.

(And has anyone done the 'Roger, Victor, we reid you' shtick yet?)

 

[TheWritingGroup]

The spacecraft leaves Earth at maximum speed, slows down due to Earth's gravity as it travels away, then starts to speed up again as soon as the force of lunar gravity exceeds that of Earth's, hits periluna (closest approach to the moon), starts to travel back toward Earth (since periluna for this trajectory is on the far side of the moon), slows down again as long as the moon's gravity overpowers Earth's, then starts to speed up once more as it falls toward Earth, peaking just before atmospheric contact.

 

[TheLobster]

The spacecraft leaves Earth at maximum speed, slows down due to Earth's gravity as it travels away, then starts to speed up again as soon as the force of lunar gravity exceeds that of Earth's, hits periluna (closest approach to the moon), starts to travel back toward Earth (since periluna for this trajectory is on the far side of the moon), slows down again as long as the moon's gravity overpowers Earth's, then starts to speed up once more as it falls toward Earth, peaking just before atmospheric contact.

Pretty much. And while it does all that, it follows this kind of figure-8 trajectory with Earth and the Moon in its foci:

 

[anthrodisiac]

That double crescent is a thing of beauty... Wow...

 

[anthrodisiac]

Ooph, that LoS gave me chills, man!