Recent comments in /f/askscience

Beetin t1_j5zutlb wrote

Reply to comment by rabidferret in Why do sample return missions such as OSIRIS-REx use their own reentry vehicles instead of just going to the space station for pickup and return with ISS equipment? by PromptCritical725

I mean, when you have a nice long trip like these missions, we get REALLY accurate pretty quick, and there are smaller more reliable thrusters we can use to make small course corrections once we get the data on the initial thrust errors.

We've got really good computers compared to even 10 years ago.

For example, the dart mission accurately hit a 530 foot object orbiting another 2500 foot object which was 11 million kilometers from earth (1/10th of the distance from earth to mars). All were travelling at several km/s. While that isn't the type of rendezvous the ISS is looking for :) it shows the extreme accuracy we are able to achieve aligning with objects and doing orbital mechanics.

There are no technological limitations on docking with the ISS, but huge practical disadvantages as talked about above. We aren't going to spend the money designing a return ship that can slow down into a stable orbit near the ISS and then correct into a docking procedure when we can just slam into the atmosphere with a heat shield and get the data back faster, easier, and WAY cheaper.

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colt61986 t1_j5zc9af wrote

Reply to comment by radioactive_dude in How hot is the steam coming out of nuclear power plants? by ivy-claw

Thank you so much for the clarification. It’s been a question of mine for 20+ years. The most important part of your answer, to me, is that it can be done. I’m guessing the reason that it isn’t has more to do with nuclear energy fear than engineering. I’d imagine Arizona, California, and the Colorado river would greatly appreciate a couple of nuclear powered desalination/power generation plants on the Texas gulf coast piping in as much fresh water as they can make west to the desert, where people apparently could never have foreseen a water shortage…..in the desert.

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chancellortobyiii t1_j5zbkjd wrote

Reply to Why do sample return missions such as OSIRIS-REx use their own reentry vehicles instead of just going to the space station for pickup and return with ISS equipment? by PromptCritical725

I think the real reason is that these sample missions want their success to not be tied to any other infrastructure that would increase the number of possible failures the mission will have.

For example for the Osiris-REx mission, it launched last 2016 and will comeback this 2023. If it had to rendezvous with the ISS for its sample return, you would have to ensure that nothing would happen to the ISS that would jeopardize the sample return. Even if you would say that there are a lot of contingencies for the ISS, 8 years is long enough for a lot of things to happen.

Imagine if something did happen to the ISS then the OSIRIS-REx mission would all be for nothing. The increase in potential failures outweigh the savings.

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LazyLizzy t1_j5za8bk wrote

Reply to comment by electric_ionland in Why do sample return missions such as OSIRIS-REx use their own reentry vehicles instead of just going to the space station for pickup and return with ISS equipment? by PromptCritical725

Your answer has made me wonder. In the future if we have surface to space aircraft would they theoretically be able to enter Earth's atmosphere at a slow enough rate (with some form of propulsion assistance) to enter with little to no friction?

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Old_comfy_shoes t1_j5za47o wrote

Reply to comment by MTGamer in How hot is the steam coming out of nuclear power plants? by ivy-claw

If you have some water and it touches something very hot, the water can't exceed 100C, because as soon as it does, it becomes steam. But the steam can be crazy hot. So with enough temperature difference, if you plunge something crazy hot into water, the vapour won't instantly escape, and the heat transfer could be very rapid creating larger pockets of really hot steam, only the outside of which would be cooled by the water. So, the steam can be much hotter. But the water must stay at 100C in order to be water.

Steam, should always be hotter than 100C, which is why it converted to steam. It's essentially water that is hotter than 100C at 1 ATM, by definition. But usually it would only be a little bit hotter in conventional use. 300C seems like a lot though. Idk anything about reactors, but I think they plunge massive insanely hot rods into water?

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Busterwasmycat t1_j5z9ok9 wrote

Reply to How hot is the steam coming out of nuclear power plants? by ivy-claw

It isn't steam, it is condensed water vapor in air (fog or a cloud). The temperature depends on a few things but generally somewhere around 35 degrees C based on info I just looked up. The water being pushed out of the tower is not water from the steam-generation system used to power the electric turbines. That steam is not usually released except in emergency (the steam is recovered as liquid and reheated to make new steam, in a big circle of use).

Instead, what happens is that the used steam from driving a turbine (now only hot water) is pumped to a radiator of sorts (some sort of closed unit that has air pass over it) located at the base of the tower. Air passing over this radiator heats up (takes heat form the hot water in the radiator, cooling that water more). the cool water is sent back to be made back into steam.

To help cool this air taking the heat from the "radiator", a spray of cold water is typically used. This spray is released near the top of the tower and descends down over the upflowing hot air (cold water is used as a counter flow to the rising hot air to help cool it faster). The water is usually taken from a nearby river or other similar water body. it is "fresh" water, different water from the water used by the electricity-generating system.

Because of this water spray, the hot air gets saturated (maximum humidity), and because maximum humidity decreases as temperature of the air drops, the water vapor condenses into droplets, making a fog or a cloud. This is what we see leaving the plant tower. The water in the discharge fog is not radioactive and it is not all that hot.

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