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?

Submitted by PromptCritical725 t3_10l6lls in askscience

All the sample return missions I've seen have their own reentry systems. Seems like a large weight penalty and extra complication to do so when the craft can rendezvous with the ISS, be taken on board, and returned to earth on a scheduled supply or crew return mission.

Seems that not having to use it's own reentry vehicle would reduce mission costs, or allow for more or bigger scientific equipment to be carried on the probe.

Am I mistaken here or would this not be a nice effective use of having a continuously manned space vehicle in orbit?

1,469

Comments

You must log in or register to comment.

electric_ionland t1_j5v0opz wrote

The main issue is that you would need to match the ISS orbit.

A probe coming in from deep space will have a velocity of more than 11.5km/s. ISS orbits Earth at only around 7.5km/s. This means you need enough fuel to slow down by more than 4km/s (nearly 9000 mph!). This propellant would be way heavier than a heatshield. The few deep space missions that have brought things back have not bothered to slow down to orbital speed. They just slam into the atmosphere and let it do all the braking for free.

Heatshield are actually really convenient (if technically difficult to build) ways to slow down. Think of how big a rocket needs to lift something to orbit. If you did not have atmosphere to slow you down on you way back you would need nearly as big of a rocket to land.

1,328

Drzhivago138 t1_j5v7fy6 wrote

>They just slam into the atmosphere and let it do all the braking for free.

It helps that unmanned probes don't have to worry about pesky things like crew survivability during reentry.

824

crazunggoy47 t1_j5wu80j wrote

In a pinch, they can also lithobrake!

307

DoubleDot7 t1_j5wuusq wrote

Does that mean letting the ground do the breaking for free?

218

danielrheath t1_j5wxivw wrote

Yes, as in "Craft underwent rapid unplanned disassembly after an unintentional lithobraking maneuver"

324

Controlled01 t1_j5xofup wrote

Is that how they described that Martian lander that plowed into the dirt all those years ago

73

gandraw t1_j5xqxmn wrote

That's why it's important to remember the difference between aerobraking and areobraking when you tell the NASA engineers to build something.

124

Sergio_Morozov t1_j5y2d4y wrote

Did you mean aerobraking and aerobreaking? If so, are YOU a NASA enigeer perchance?

46

gandraw t1_j5y2mk6 wrote

Ares is the greek name of Mars, so "areobraking" is the equivalent of "geobraking" and technically means "braking using the Mars surface"

122

anomalous_cowherd t1_j60adde wrote

Nice, I'll use that when I want to outnerd somebody. (Said in an affectionate way, I like outnerding people!)

3

tdmonkeypoop t1_j63v923 wrote

Ok.. Socrates!! Dudes over here playing 4D chess like he's dust in the wind or something

2

LionST1 t1_j5xbury wrote

Using elastic deformation of structural materials and the landing zone to rapidly dissipate kinetic energy, very clever.

111

Equoniz t1_j5xh962 wrote

I’m pretty sure lithobraking often entails deformation significantly past the elastic limits of the materials involved.

79

LetterBoxSnatch t1_j5z15vc wrote

I suppose it depends on just how elastic your definition of “elastic” is…the materials may still be capable of reattaining their prior size and shape, given enough energy and engineering dollars

5

oz6702 t1_j5xl67o wrote

This is the kind of engineering we're trying to perfect over at /r/KerbalSpaceProgram

60

Iz-kan-reddit t1_j5xlwoe wrote

> Does that mean letting the ground do the breaking for free?

I'd correct you with braking, but breaking is also correct in it's own way.

49

NetworkLlama t1_j5xeh9a wrote

That actually happened with the Genesis mission to collect samples from the solar wind. It hit the atmosphere at 11 km/s, but after slowing down, the parachutes never deployed. It impacted the ground at 86 m/s, contaminating most but not all of the collectors, and some of the science was salvaged.

43

overlydelicioustea t1_j5y8rvv wrote

mars sample return mission will not do this in a pinch, its the planned reentry mode to just have the container be sturdy enough and , well, just let it hit the deck.

11

BaldBear_13 t1_j5ww4tg wrote

If Sci Fi movies taught me anything, it's that when a probe returns from uncharted reaches of deep space, lack of survivability is a plus, and is called sterilization.

87

dWintermut3 t1_j5xt71k wrote

NASA has a specialist for that actually, with the most badass job title in history of "planetary protection officer".

30

SirCB85 t1_j5xv9yr wrote

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?

19

dWintermut3 t1_j5xw7in wrote

yes, that's a very important job of the PPO. but my understanding is as the staff's highest-ranked microbiologist control of potential alien lifeform risks to earth is also in their purview.

19

b33r_engineer t1_j5yifcj wrote

That’s what they want you to think, yes…

(It’s also true, but they do both)

1

BaldBear_13 t1_j5yi0tj wrote

are they issued a flamethrower? A cool suit? A memory wipe device?

5

PromptCritical725 OP t1_j5v47hy wrote

Really good point about the delta-V. I hadn't thought of that.

80

fishling t1_j5v6yas wrote

Note that the problem is larger than they made it sound, as those are vectors as well. The sample return mission is almost certainly not going to be coming on a path aligned with the ISS orbit that only needs to slow 4 km/s to meet it.

Also, I would think that it would increase the risk to ISS, some of the return capacity is already booked, and that doing the load/transfer of samples and ensuring everything is balanced and secured appropriately for reentry is hard.

149

CountingMyDick t1_j5x2tjs wrote

To be slightly annoyingly pedantic, the actual return trajectory is most likely nowhere near the ISS, but if they were planning to dock with the ISS, they would presumably cheaply adjust their incoming trajectory to be as close to the ISS orbit as possible while still far away. If they were rather good at it, presumably they could get pretty close to only that 4km/s of total DeltaV to match orbits.

Of course that's still a hell of a lot of DeltaV versus aerobraking.

38

cjameshuff t1_j5yfrbi wrote

> they would presumably cheaply adjust their incoming trajectory to be as close to the ISS orbit as possible

That's a pretty major presumption. It can be hard enough just intercepting Earth, requiring that interception to also occur with the probe trajectory aligned with the orbital plane of the ISS would greatly restrict the set of targets we could retrieve samples from.

2

UnamedStreamNumber9 t1_j5zb3na wrote

They can be precise enough to know where it’s coming down. That said, Osiris ain’t stopping at the earth. It’s just dropping off a package and heading back out to intercept Apopthis

2

KingZarkon t1_j5z8y96 wrote

Isn't it just Newtonian math? I mean, we know the weight of the craft, how much thrust it has etc, so why is it so hard to calculate?

0

rabidferret t1_j5zfqvx wrote

Measuring a spacecraft's position and trajectory has a margin of uncertainty. Engines are not devices that can produce a known exact amount of thrust for an exact amount of time.

4

Beetin t1_j5zutlb wrote

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.

2

cjameshuff t1_j602f2y wrote

Yeah, the issue isn't accuracy. It wouldn't be that difficult to hit the ISS. The solution space for a rendezvous with near-zero relative velocity is rather more restrictive.

For Earth, there's vast areas suitable as landing locations, where it doesn't really matter what direction we approach them from. We just need atmospheric entry to happen at a reasonable angle and velocity.

1

cjameshuff t1_j600x1g wrote

It's not a matter of it being hard to calculate, it's a matter of the solar system not being physically arranged to conveniently allow it.

A minimum-energy transit will come at Earth roughly aligned with its orbital motion. To match planes with the ISS, the return must happen at one of the two times a year where the ISS orbit is also aligned with that motion, which means the trip must have started on the opposite side of the sun from that point, half a transfer orbit earlier. But we don't control where other solar system objects are or what their motions are. Windows to/from Mars occur every 26 months. If by chance things are properly aligned one year, it will be 60 degrees off the next time, and most Earth-Mars windows will be unusable: it will be 3 launch windows, 6.5 years, before they align again. And they in reality don't line up in such nice whole numbers, so in reality you're going to have a substantial plane correction to make on arrival, even with such limited windows.

1

fishling t1_j5xmq9a wrote

You're still assuming that the path is coming up from behind the ISS, in the same direction ISS is moving. If it's moving in the opposite direction, it would have to come to a stop and then accelerate to catch up to the ISS. Or, if it coming in at a right angle, it would have to shed all that extra perpendicular velocity and add all the parallel velocity. Only in the most perfectly aligned case could it be 4 km/s.

−14

jinxbob t1_j5xtox4 wrote

Choosing whether to enter prograde or retrograde is relatively easy if you're far enough away from earth

21

Mispunt t1_j5xnwyd wrote

There isn't really an opposite direction or a perpendicular direction though. If you circularize on the 'right' side you go left around the planet, on the 'left' side right. Edit: once you match the orbital plane of the ISS you can catch up or slow down for a rendezvous by changing orbit height.

12

Nemisis_the_2nd t1_j5y7igp wrote

That's really not a big problem. Coming back fro somewhere like Mars, you'd need to alter the tragectory by a fraction of a degree to flip the reentry trajectory 180^0.

From there, you could bleed off speed like in early space missions with rounds of aerobraking.

The original commenter makes a good point about fuel weight, but its also got less to go wrong if you just slam the vehicle into the atmosphere one time.

6

Skipp_To_My_Lou t1_j5vra6n wrote

Even if the sample tube attaches to the exterior of the space station, at some point they will have to bring it inside to transfer it to the reentry vehicle & every set of hands touching it is another opportunity to contanimate the sample.

35

oz6702 t1_j5xlhp5 wrote

> The sample return mission is almost certainly not going to be coming on a path aligned with the ISS orbit that only needs to slow 4 km/s to meet it.

To be fair, this is a trivially easy problem to solve. A change of a few cm/s when you're a million clicks away can result in huge differences in your destination, so setting things up such that your incoming deep space probe lines up with the ISS' direction of orbit and plane of orbit and whatnot would be quite easy, and pretty cheap as far as dV is concerned. Still, slowing down to match orbit with the ISS is something that's gonna cost you a ton of fuel either way - unless you aerobrake, in which case you might as well just do that instead of bring the fuel along to begin with.

2

Nemisis_the_2nd t1_j5y7nt7 wrote

> Still, slowing down to match orbit with the ISS is something that's gonna cost you a ton of fuel either way

Aerobraking, as you suggest, is the answer. It's how we used to do it for a long time before more effective technology and better understanding of orbital trajectories came along.

5

sinspawn1024 t1_j5wep8o wrote

also, don't forget that if something fails to work on slow-down, you have an 11.5 km/s projectile heading straight for the ISS... Full of astronauts...

50

notjordansime t1_j5wtq6o wrote

If it didn't slow down at exactly the correct rate, it'd more likely miss than anything

57

FolkSong t1_j5x2jme wrote

Yeah, the risk scenario would probably be more like everything goes perfectly until the last second and then it explodes.

11

sinspawn1024 t1_j5wyzal wrote

Even if the probability of collision was very low, do you think Congress will fund a NASA mission where there was a small chance the craft might smash the International Space Station, all its active experiments, and the astronauts of multiple countries into the Pacific Ocean for all the world to see?

−3

FriendlyDespot t1_j5x8bme wrote

The risk would be substantially lower than any number of other risks that are accepted daily for the ISS mission. With the maneuvers required to match an orbit, any failure would put the intercepting vehicle somewhere other than where the ISS is.

Consider that the scenario you're describing is a risk that's faced every single time a crew or supply mission is launched to the station.

22

paaaaatrick t1_j5xztog wrote

You are overestimating what “very low” looks like for something like this

14

R3lay0 t1_j5z7t18 wrote

The risk of it crashing into the ISS is just as high when just going directly into the atmosphere

3

WeDrinkSquirrels t1_j5zc0p8 wrote

You mean like every resupply mission they send up?

2

sinspawn1024 t1_j6kpdgy wrote

Resupply missions are moving up earth's gravity well. If an engine malfunctions, the craft will lose velocity and altitude due to Earth's gravity. Return missions are falling into Earth's gravity well, so engine malfunction results in continued acceleration. Also, retropropulsion is fundamentally unstable (the force balance is the same as balancing a ruler vertically on your finger), which means that if a system loses attitude control, the craft will much more likely enter a tumbling condition, which if not arrested, will dramatically widen the cone of possible collision.

1

lethal_rads t1_j5x2p6x wrote

It wouldn’t go straight at the iss. It’d have to go into a different orbit first, and then do the standard approach for capture/docking.

11

LordOverThis t1_j5wz0uy wrote

It’d be pretty unlikely to collide with the ISS, since it’s already like trying to shoot a bullet with a smaller bullet on a completely different trajectory. Just guesstimating that it’s probably a margin of error of like a milliarcsecond between “intercepted successfully” and “it flew by so far away it couldn’t be seen”, which would put the difference between “intercepted successfully” and “everyone aboard was killed” at even smaller.

8

dWintermut3 t1_j5xtmlg wrote

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

4

Kantrh t1_j5xwy1v wrote

If you're partially aerobraking you might as well do it fully rather than trying to catch up to the ISS

16

Alblaka t1_j5y6ikr wrote

I'd suggest that performing a precise aerobrake in reality might be slightly more difficult than in an abstracted simulation, i.e. due to the simulation always being 100% accurate, whilst any modelling of our actual atmosphere might be less precise.

It's easy to do a full aerobrake, and it's relatively easy to avoid aerobraking. But treading that fine line between might be a bit unfeasible in an unsimulated environment.

6

gdshaffe t1_j5ydp8g wrote

I see someone other than me has spent the last drop of their fuel getting that Kerbin periapsis down to 50km or so and just used the "aerobrake 200 times" maneuver to get back home.

1

ninthtale t1_j5wwnrm wrote

>If you did not have atmosphere to slow you down on you way back you would need nearly as big of a rocket to land

So missions from a lunar post would still need quite a lot of braking fuel?

2

fyrstormer t1_j5xdz36 wrote

The Moon has a tiny amount of gravity compared to the Earth, so lander modules falling towards the Moon don't speed up nearly as much and don't need nearly as much fuel to slow them down before they land. The Apollo Lunar Module was a single-stage-to-land/single-stage-to-orbit aluminum box with a little rocket motor strapped to the underside; the same setup on the surface of the Earth wouldn't even be able to lift its own weight.

16

FolkSong t1_j5x2ytl wrote

A similar amount as they use for takeoff, but it's still very little compared to getting out of Earth's gravity well.

9

cjameshuff t1_j5ygqwh wrote

A landing on an airless Earth, launched from the moon? It would be a bit more efficient than the moon landing, because the spacecraft would be at its heaviest (with a full stack of stages fully loaded with propellant) in low lunar gravity and would be doing its final braking in Earth's heavy gravity after burning most of its propellant and discarding most of its stages, but gravity losses are fairly small in comparison to the overall acceleration/deceleration requirements. You'd need something of similar size, just with fewer first stage engines to get it off the moon.

1

LeatherCode2624 t1_j5yx36j wrote

Why did sci-fi lie to me about the ease of which zipping around in space would be when I got older?

2

TryingNot2BeToxic t1_j5xew3p wrote

Oh this brings up an interesting problem when it comes to an intermediary moon base/launch platform. Would the propellant needed in order to reduce speed to land back on the moon offset the propellant saved from launching in lower gravity/no atmosphere?

1

waylandsmith t1_j5xtcqe wrote

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.

5

TryingNot2BeToxic t1_j5zwnns wrote

Aight, new question. Would the space elevator concept be viable on the moon? Like we'd have a moon base, an elevator, and then a station in orbit connected.

1

waylandsmith t1_j60w9pt wrote

A lunar space elevator is possible, but would be quite different than one that could be built on Earth. But the moon's lower gravity makes a lot of the benefit of a space elevator moot. A lack of atmosphere on the moon also makes some sort of railgun launch possible and economical, at least for cargo.

2

electric_ionland t1_j5xte0y wrote

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".

2

Alerith t1_j5yopiy wrote

I assume there are a bunch of calculations accounting for the Earth moving around the sun and through space in general? I can't imagine the math that goes into hitting Earth from deep space.

1

UnamedStreamNumber9 t1_j5ytgex wrote

To add to that, the osris-Rex probe itself isn’t going to end its mission on return to earth. It’s just dropping off the reentry vehicle and heading back off into space where it will be targeted to flyby the asteroid apophis (the one that will have a very close earth encounter in the late 2020’s). This wouldn’t be possible if it slowed down to earth orbital speeds

1

electric_ionland t1_j5yukz7 wrote

Sure but AFAIK this was never a primary mission goal. It's just a nice bonus because they had the margins.

1

LazyLizzy t1_j5za8bk wrote

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?

1

odddutchman t1_j5w958r wrote

Strictly speaking, they don't get the atmospheric braking for free...still needs a heatsheild and guidance system; but they don't have to drag extra mass of fuel, rocket engines, etc with them during the ascent part of the flight...which adds up REALLY fast. Look up "the tyranny of the rocket equation".

−2

17934658793495046509 t1_j5wpij1 wrote

Also you would need a heat shield to slow down in the atmosphere. It’s heavy but weighs less than the fuel you would need to do the same thing.

5

Hokulewa t1_j5x1h2j wrote

You could really improve your payload mass margin by using aerobraking to match velocity with the Earth, but you would need a shielding device of some kind to protect you from the heat. That would weigh a lot less than the propellant you'd need to use, otherwise.

5

janoc t1_j5v7tqc wrote

In addition to what /u/electric_ionland said, there is the whole safety thing. If anything goes haywire with the complicated rendezvous and/or docking, you have just put an irreplaceable space asset and 7 people in danger - that in addition to losing whatever samples the spacecraft was returning.

Anything flying to ISS has to be specifically certified for it and the whole approach and docking process needs to be extensively tested before even the first test flight is allowed. Obviously not an option for a one-off deep space mission.

That doesn't mean that this couldn't be done sometime in the future but it is just too impractical and risky today.

181

stickmanDave t1_j5xc4kn wrote

If it was coming in on a trajectory to decelerate and end up at the space station, then failing to decelerate would result in it passing nowhere near the space station.

Remember, the space station isn't a stationary target. It's tooling a long at 7km/second or so. Everything has to go exactly according to plan for the two to end up at the same place at the same time.

5

janoc t1_j5xxjvp wrote

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.

20

gridsandorchids t1_j5z56m5 wrote

He's talking about something going wrong in the final approach to dock, which also shows how complicated the whole concept is. Once you get to a matching orbit, and close enough on a stellar scale, it's still highly delicate and specialized to actually safely recover it.

MIR had a Progress craft smash into it and almost kill the station and inhabitants and it was just docked and then pushed out a bit and brought back in.

1

Dunbaratu t1_j5vukxk wrote

It's because using air drag to slow you down saves enormously on payload fuel mass.

If you are coming down near Earth from a very high position, you will sweep by the Earth going way too fast to stay in low earth orbit. You'll just fling past Earth and rise up high again. So if you want to dock with something in low earth orbit, like the ISS, you have to decelerate a lot. That excess speed has to be removed to slow you down into a relatively circular orbit at low altitude.

And if you are going to do that decelerating in space, not touching the atmosphere, you have to provide all the deceleration yourself. With your own fuel.

But if you pass really low to Earth, so you are scraping the atmosphere, then the atmospheric drag can provide all the deceleration you need without you having to spend fuel doing it yourself. Granted, that means you need protection from the heating effects of the pressure shock, but if you can handle that heat you gain the advantage of needing no propulsion to decelerate with - that means not needing a powerful engine and not needing fuel for that powerful engine.

Merely having a very small bit of fuel for a little steering engine that can merely slightly deflect your path is sufficient. All you have to do is slightly bend your path long before you get to Earth, to ensure you hit the atmosphere just right. That's more of a computing challenge than a delta-V challenge.

And you don't even need to put that computing power on the probe itself. You can have the probe have just enough smarts to obey any maneuver command you send it, and then use more powerful computers on Earth to calculate the needed maneuver to beam to the probe. (This is basically how they did this sort of thing in the 70's when the math required room-sized computers.)

I didn't cover it yet, but if you try to do your own self-propelled deceleration to turn a high-speed Earth flyby into a low Earth orbit, it's not sufficient to have a little bitty weak steering engine. You need to ensure that engine has a significant level of thrust because it doesn't just need to spend a lot of delta-V, it has to spend it fast. If it's one of these super-efficient but also super slow engine designs, such as a weak little ion engine, the probe won't slow down fast enough and will slingshot past the Earth back up to a high altitude before it has caused enough delta V to do much.

tl;dr - Needing a heat shielding arrangement for the bit where the probe hits atmosphere hard does add a little bit of mass, but not nearly as much mass as it would take to have a high-thrust engine and the fuel to run it so you can provide that deceleration yourself.

And since we're talking about the mass at the very end of the mission, in the final payload that comes home, that's mass you have to carry at the tip of the rocket through all the stages along the way. The very last stage of the mission is the most important place to save on mass. 1 kg more mass in the final stage can mean more tonnes of mass added across the other stages that come before it.

47

quetric t1_j5w6n75 wrote

In addition to what others have said about delta-v required to match the orbital speed of the ISS, there's also the challenge of matching its inclination. Most likely a sample return mission will occur roughly in the solar system invariable plane, since that's where most of the planetary mass is. The ISS orbits at a relatively high inclination and I suspect it would take some creative gravity assists or a lot of delta-v to get in alignment with it.

21

a10t2 t1_j5wfte2 wrote

Bear in mind though that any sample return would be coming in on a hyperbolic trajectory. Given the time scales involved the ∆v required to intercept the ISS would be negligible when compared to that needed to rendezvous.

1

censored_username t1_j5ythbs wrote

Eh, you would design your approach so you intersect the ISS orbit at your perigee. Then decelerate just enough to get a highly eccentric orbit (or a weakly stable one), match inclination at apogee and then decelerate further at perigee.

Still, that would require a restartable motor with >4km/s delta V, which implies at least like 80% of your vehicle being storable propellants. A heat shield is simply lighter.

1

Avery_Thorn t1_j5vpw4u wrote

Sometimes the answers also come for political or project management reasons, as much as physics and scientific reasons.

The ISS is slightly risky. It is hard to know how much longer the ISS will be there for. It is currently funded and authorized through 2030, with deorbit scheduled for 2031.

There are other reasons that could potentially shorten the lifespan of the ISS further.

A sample return mission might take 10-15 years (or more) to plan, get approval and funding for, to build the hardware for the mission, to launch that hardware, and the time to get to and from the sample location. (In addition, often these are tacked on to other missions; which might include a science package that has more duration.)

If a mission would use the ISS as a critical component for it's return journey, that means that the mission assumes extra risk based on the likelihood of the space station being available for it's role in the mission at the end of the mission.

And while we can certainly assume that the Space Station will be funded for longer... the PM and approval teams cannot assume this. This means that if the project is expected to take 10 years, the project cannot use the ISS, because on paper the ISS will be gone by then.

So the question becomes: does using the ISS as a return link outweigh the risk and scheduling complications? Given u/electric_ionland's very nice response, my guess is "no".

Note that I don't work for NASA, any governmental agency, or any governmental contractors.

15

IKnowWhoYouAreGuy t1_j5x9e5v wrote

With sample return, you're talking about a craft Pringles can trying to carry enough fuel to return to Earth, let alone trying to locate and synchronize with the ISS, which is the size of a house moving at thousands of miles per second (depending on your perspective). Its much easier to hit the earth-sized object than attempt to throw a baseball at a moving car from millions of miles away...

3

Vishnej t1_j5xtv5k wrote

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.

3

fyrstormer t1_j5xdodu wrote

The return vehicle would have to carry a HUGE amount of fuel to slow down enough to dock with a space station in orbit. The return vehicle has been falling towards Earth for millions of miles and it's moving incredibly fast by the time it gets here. The most economical way to bring it to a stop is to let it shove a few thousand miles of Earth's atmosphere out of the way, rather than firing expensive retro-rockets.

2

Westerdutch t1_j5xzuyk wrote

tl;dr because falling down even if you have to carry something bulky to survive the fall is still a lot easier than trying to exactly meet up with the fastest moving manned structure in existence. Not only does the latter take a LOT of speed but the precision required is also absolutely non-trivial (and if it goes wrong youll still fall down so you would still need the bulky fall protection).

2

Dusty923 t1_j601hpm wrote

Space is big, so a probe needs to go pretty fast to get anywhere in years instead of decades. Plus, a returning probe picks up a lot of kinetic energy falling back into Earth's gravity well. Carrying a heat shield and carefully smacking into the Earth's atmosphere is way more doable than carrying all the fuel it wound need to decelerate enough to park in low earth orbit. That fuel would need to be added to the payload at launch and be carried along for every maneuver, which means you'd need more fuel to move that fuel, then more fuel to move that fuel (the so-called tyranny of the rocket equation) Not to mention having to come in aligned with the ISS's orbit, or even worse having to do a plane change maneuver on top of decelerating, all of which would need fuel.

2

AboveTheCarmanLine t1_j5yak2w wrote

Relative velocity at earth is extremely high. The minimum velocity from LEO to escape is ~3350 m/s, which you would need to burn off on the trip back. That‘s a lot of fuel you‘d need to burn off that delta v, so immediate reentry from escape trajectory is the way to go

1

chancellortobyiii t1_j5zbkjd wrote

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.

1

hogey74 t1_j5wp51a wrote

Because human space flight looks powerful but is actually super weak. The chemical reactions we use are dangerously powerful of course but compared to the physics of gravity, required reaction mass etc, they're barely enough to get us off the planet. Those Saturn moon rockets were the most crazy impressive things we'd ever made but NASA couldn't get us to and from the moon until they did things like remove the seats from the lunar module. That's how close it was.

And the amount of money being spent similarly is a lot yet is also barely enough. The US briefly hit 4% of GDP on the moon program, a massive effort for something other than a world war. It only happened because the space program was essentially a military operation that was an extension of WW2. Space mostly still is today.

In short, we can just barely do space stuff. Every gram and every dollar is tight.

You are not mistaken in the idea. It's doable and preferable for biosecurity etc.

−1