Recent comments in /f/askscience

janoc t1_j5xxjvp wrote

Reply to comment by stickmanDave 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

You have completely missed my point. The orbital mechanic is a completely different issue and certainly can be calculated so that the two objects meet - we are doing rendezvous and dockings routinely.

The point is that even if you do all of this, carry all that extra fuel (and equipment!) required to decelerate and enter the orbit permitting to dock with the space station - would you want to take the risk?

It is not about "failing to decelerate" and hitting the station as some sort of space projectile. The problem is it would be a spacecraft that has likely not been tested to do this before - and will likely never do this after (deep space missions are usually one-offs). What if something goes wrong during the final approach and puts the station at risk?

We have seen what could happen when the Russian Mir got punctured by some ill-thought maneuvering. Only quick thinking and some crazy heroics by the crew has saved the station. And that was a spacecraft actually designed to dock with the station, equipped for that and one that has just undocked, so it was known to be in working order. Unlike something coming from deep space after who knows how many years - and in who knows what state.

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tforss t1_j5xwwnv wrote

Reply to comment by Asylumdown in What determines whether we can create a vaccine for an illness or not? by ShelfordPrefect

HIV is hard to make a vaccine for because without consistent treatment there is no way for the body to fully eradicate it, and it will only progressively get worse. Even with medication, it only suppresses the virus. The body is unable to replicate this protection on its own without the assistance of medication. If a vaccine existed that could stimulate the immune system to produce this defence naturally, it wouldn't have to rely on medication, at least not entirely.

So yes its hard to make a vaccine for something not fully understood and why the immune system cant develop any form of natural resistance. No matter how long someone has been diagnosed as positive, their immune system has no ability to ever learn to protect itself from this.

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SirCB85 t1_j5xv9yr wrote

Reply to comment by dWintermut3 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

Wait, my understanding of their job description was that they actually responsible for making sure WE don't contaminate let's say Mars before we are really really really sure that us contaminating it doesn't destroy any indigenous life we might like to study before we annihilate it?

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Vishnej t1_j5xtv5k 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

Reentry is easy; You just plop right into the atmosphere & ocean, barely any work needed. Getting to the ISS' orbit is hard; Coming in from interplanetary space, you're looking at >4000m/s adjustment that needs to be made to reach orbit. Making it using fuel requires several times your probe's weight in propellant, and making it using aerocapture / aerobraking requires basically the same heat shield as when you do reentry (slightly thicker), but with high-precision orbital windows that need to be hit very precisely to make the rendezvous, involving likely hundreds of m/s dV capability since those windows are transient solutions of a dynamic thermosphere.

Edit: While technical specs are always hard to find, one is left to believe that rather dramatic increases in capability per mass of heat shield material have occurred since the Shuttle program and then under SpaceX.

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dWintermut3 t1_j5xtmlg 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

now my knowledge of orbital mechanics comes entirely from video games, but couldn't you use a partial aerobrake?

one tactic I often use is to plan a trajectory that has a periapsis that is barely in the atmosphere, just brushing it such that there is only a tiny bit of drag from very thin atmosphere. this acts to lower my periapsis just a bit, the effect being exaggerated by the fact the relative orbital velocity is at maximum at periapsis and minimum at apoapsis. over a few successive orbits I can bleed a significant amount of velocity with minimal heat and stress. I never used this to attempt a rendezvous, though I want to try now, but I do use it to bleed enough speed that a drogue chute is viable to go the rest of the way

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electric_ionland t1_j5xte0y wrote

Reply to comment by TryingNot2BeToxic 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

Yes stopping on the surface of the moon is a pretty terrible idea. However you could envision missions where lunar ice is mined to create propellant and that fuel is sent to a convenient orbit that is more "on the way".

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waylandsmith t1_j5xtcqe wrote

Reply to comment by TryingNot2BeToxic 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

Naively, on paper, yes. Launching from the Moon vs Earth saves you about 9km/s in delta-v, more than making up for the 4km/s to slow down, and then another 2.4km/s to actually land on the moon. But the problem is a lot more complicated than that. Without aerobraking, the propellant needed to land would need to be sent with the spacecraft on its entire journey. Fuels that are stable for long, long journeys are typically significantly less efficient than those that can be refreshed/topped-up until the moment of launch, so more of that stable fuel is needed, requiring more launch fuel to get it into space. The delta-v budget would be turned on its head and the vast majority of it would be needed to be spent right at the end, instead of at the beginning. And this, of course, ignores the problem of how to get the spacecraft onto the moon and the payload back to Earth.

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