NASA's plan is to use two separate missions to bring home the drill core samples that the Perseverance rover will take. The samples will in plan be picked up by a landed mission and launched to Mars orbit. There the samples will dock with another spacecraft that brings them to Earth.

Why not instead dock two launches in Earth orbit, so that enough mass can be landed on Mars to both pick up the samples AND bring them home? For example, first launch a fully fueled upper stage that can bring the thing to Mars, then launch the pick up, ascent and return vehicle to dock the two. If there's a failure in that process, a new attempt can be made without losing the samples that Perseverance will spend several years to collect.

Docking in LEO has become routine with hundreds of successes and I think only two failures in history. But an autonomous docking in Mars orbit has never been tried. Why try to do it the hard way?

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    $\begingroup$ So you are landing the fuel for return on mars then launching it back to mars orbit, for what? $\endgroup$ – user3528438 Aug 14 at 4:04
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    $\begingroup$ When conquering the South Pole here on Earth, the explorers were using a similar approach: instead of bringing all the supplies along, so they'd arrive at the South Pole with half of them remaining and use them up on the way back, they first started an expedition that left 'bases', small depots of supplies along the way roughly halfway to the pole, then return - and on the next o they'd use half the supplies from the bases, then past the last base they'd start leaving new ones half as big, so by the time they reached the pole they were almost out of supplies. $\endgroup$ – SF. Aug 14 at 5:25
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    $\begingroup$ On the way back they just depleted the bases, while traveling light, with only enough supplies to reach the next base. $\endgroup$ – SF. Aug 14 at 5:26
  • $\begingroup$ @user3528438 Because it would be simpler and safer to not risk losing the precious Mars mud. $3 billion is invested in collecting the samples on Mars' surface, so perhaps another quarter of a billion or so investment in extra launch vehicles would be good risk management. $\endgroup$ – LocalFluff Aug 14 at 5:27
  • $\begingroup$ Another problem is keeping the rocket usable in Martian conditions. Realistically it should be able to wait for martian soil delivery for months if not more, because of landing point uncertainty. Unfortunately not so much details are avalable about Mars Sample Return atchitecture study. But from artist's concepts it looks like the ascent rocket should be thermally conditioned. $\endgroup$ – Heopps Aug 14 at 5:46

Earth Orbit Rendezvous is a method for applying brute force. Mars Orbit Rendezvous actually improves efficiency, potentially by a lot.

A Mars sample return (or, for that matter, a straight-up crewed mission to Mars) needs to do the following in order:

  1. Launch from Earth (*)

  2. Get on transfer orbit to Mars (*)

  3. Land on Mars with an ascent vehicle ready

  4. Take off from Mars and get into, at an absolute minimum, Mars orbit (*)

  5. Get on transfer orbit to Earth (*)

  6. Either land on Earth or be recovered in Earth orbit.

The starred items absolutely require a significant amount of delta-V -- any vehicle that does them, with present or near future technology, will be mostly fuel and will have a mass several times as large as their payload, at a minimum. This is the tyranny of the Tsiolkovsky Rocket Equation.

Items 0 and 3 absolutely need a high-thrust rocket, which regrettably today means chemical fuel, which means abysmal specific impulse and enormous boosters in relation to the payloads.

Earth Orbit Rendezvous provides a method to assemble a very, very large spacecraft in Earth orbit when you don't have a booster large enough to launch it in one piece. This implies at least two expensive heavy-lift booster launches, and we have some pretty heavy lift boosters today, and will have some even heavier ones (Starship, SLS) in the future.

A slight complication is that when sending unmanned spacecraft to other planets, we often use the booster to provide the initial impulse into their transfer orbit (the payload is a lot less than the booster's max payload to LEO) rather than having them fly from LEO to their destination under their own power. However, this is mostly just an operations detail.

But lets look at that list of maneuvers. The rocket for getting on a transfer to Earth (#4) is something fairly substantial (at an absolute minimum, several times the mass of the payload delivered to Earth), and it's not needed on tasks 2 and 3.

If we launch into Mars orbit, and then jettison the Mars ascent vehicle and dock with an Earth return vehicle, then both the Earth return vehicle and the Mars ascent vehicle may be roughly equal weight: several times that of the payload going to Earth.

If we make a staged rocket to launch into Mars orbit carrying the Earth return vehicle, and then light off the Earth return vehicle to go back to Earth, the Mars ascent vehicle needs to be several-squared times that of the payload going to earth. That's a LOT more -- especially if "several" is more like 10 than, say, 3.

You can bet that making this rocket larger and heavier will also complicate landing a great deal, and will cost for getting it all to Mars in the first place.

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    $\begingroup$ Nice answer, and well written. The Mars Orbit Rendezvous approach echoes the Lunar Orbit Rendezvous approach the Apollo Program adopted after recognizing the mass savings. $\endgroup$ – Tom Spilker Aug 14 at 5:21
  • $\begingroup$ I'd like to see a slightly better calculation of how much more delta-V, or fuel mass, would be required to land the full Earth return vehicle on Mars, rather than keeping the Earth return vehicle in Mars orbit. Is it really closer to 10 than 3 times "squared" more? Soviet's space program could return samples directly from the Moon in the 1970s. The Moon's surface gravity is 0.16 that of Earth, on Mars it is 0.37. Wouldn't two Falcon 9 rockets be enough to do that? $\endgroup$ – LocalFluff Aug 14 at 5:21
  • $\begingroup$ @LocalFluff I need to do some calculations. This ends up having a lot of assumptions, mostly about the dry mass of various objects. The number-squared figure is fuel mass or total vehicle mass, not delta-V. It's worth noting that boosting into orbit and then on a transfer wastes some delta-V, potentially. $\endgroup$ – ikrase Aug 14 at 5:33
  • $\begingroup$ @LocalFluff what exactly are you doing with those two Falcon rockets? $\endgroup$ – ikrase Aug 14 at 5:34
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    $\begingroup$ @LocalFluff about that single launch: space.stackexchange.com/q/36677/6944 It wasn't even guided. Impressive. $\endgroup$ – Organic Marble Aug 14 at 12:47

In this question I suggested, that, on the face of it, the SS520-5 sounding rocket could get about 5kg from the surface of Mars onto a trajectory to hit Earth. That rocket (a three stage solid fuel rocket) masses 2.6 tons at liftoff. Let's make the rather large assumption that I haven't missed any problems (someone has already suggested the cold as an issue). Then you would also need to land some kind of lightweight version of the launch rail and associated systems that are normally used with an SS520. On Earth that's a truckload of stuff, but let's be very optimistic and say we can get it down to 1.4 tons, making 4 tons that we need to land on the Mars surface.

Working backwards, we need to wrap that in a heatshield and landing system. For Curiousity, that system massed about 3 times the mass of the mass of the lander, so let's, again optimistically -- landing large things on Mars is hard -- go for the same ratio, so we need 16 tons launched into Mars Transfer orbit. This is actually just within the capacity of a Falcon Heavy, a rocket that was not available when these missions were designed, but there's not much to spare.

Working forwards, our 5kg payload has to manage any trajectory corrections on its way to Earth, communications and power for the cruise, a reentry at escape velocity, and some kind of landing, as well as payload, all in a mass budget about 1/3 of what the MarCo cubesats had.

So the mission profile you designed isn't actually against the laws of physics, and doesn't even need an Earth Orbit Rendezvous, although that might buy a few more kg, but it would need new and untried technology to land a multi-ton spacecraft on Mars and then to build a 5kg payload capable of getting anything useful back to Earth intact. Mars orbit rendezvous looks a lot less risky to me.

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    $\begingroup$ It's definitely a significantly smaller spacecraft if you don't have to land your mars-earth transfer fuel with your ascent vehicle, though I can't help thinking that it'd be useful to land something significantly larger on the martian surface as a practice run for a manned Mars Ascent vehicle. $\endgroup$ – Ruadhan2300 Aug 14 at 13:01

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