Tuesday, August 24, 2010

Early vs Late Human Missions To Deep Space


Anyone who has enjoyed my recreational attempts at designing a human mission to a near-Earth asteroid should check out the newly released mission to an asteroid by a team at Lockheed Martin*. The report ends with these important words:

The Plymouth Rock study shows that the first visits to asteroids can be easier and earlier than we have previously thought. The United States does not need to wait for more advanced technologies or develop expensive dedicated deep space vehicles. We can explore the asteroids within a decade, using spacecraft already being developed and tested.

This is a reasonable statement which I agree with. As far back as Apollo the question of "are we ready?" has been asked, and despite the success of Apollo it is still being asked. I have tried to make the argument that a Dragon capsule would be sufficient for a bare-bones mission to an asteroid, assuming some modifications to life support systems, dual use of propellant and supplies as radiation shielding and a whole lot of vigorous hand-waving. This Lockheed Martin study makes the same argument, obviously using their own hardware, and with a level of detail and rigorous breadth of study that puts my amateur efforts to shame. If we don't go we'll never be ready.

That said, they make some statements that go a little further than I have previously, or probably would. For example, consider this statement, which at first blush seems to be making a purely technical argument, but is certainly saying something a little more rhetorical.

As a consequence of their orbital similarity to Earth, there are only a few opportunities per decade to visit known asteroids. The number of opportunities will likely increase as more asteroids are discovered, but for now the limited number of opportunities has profound implications for asteroid mission planning.

It drives the timing of a human asteroid program since it may not make sense to plan a program which provides an initial operational capability during a period such as 2021-2024 when there are no known mission opportunities.

The small number of opportunities means that it may not make sense to design a spacecraft system dedicated to asteroid missions. Rather, asteroid missions would be performed occasionally by spacecraft that are also designed to perform other missions, such as going to the Moon or other deep space destinations.

While I can't find fault with the logic, I think I see a deeper message here. The second paragraph, while representative of a simple fact that I too was surprised to discover in my Prospector's Skymap efforts, is unnecessarily vague about what, precisely, it is recommending. I don't think it is saying NASA should not bother to field an asteroid mission until 2025, as is the President's current plan. This paragraph meant to hint that NASA should hurry up, perhaps even aim for the (2008 HU4) approach in 2016, the year the Senate's proscribed heavy lift vehicle is supposed to come online, lest the schedule slip and slip into the 4 year "asteroid gap".

The third paragraph above, however, is an outright stab at supporters of technology development, and is the "take home" message - going to the near-Earth asteroids is not a technology development program.

Bob Zubrin of the Mars Society recently made this argument in a little more alarmist way - it would be a terrible thing if VASIMR and propellant depots became "gateway technologies". The fear is that missions to deep space targets, Mars obviously being the intention, will be delayed until these technologies come online. "We can't go to Mars because we don't have VASIMR yet" they'll say. Unfortunately people are saying that, and the Plymouth Rock study comprehensively shows why it is not the case, at least for 6 month asteroid missions.

However, buried in the technical detail of this study is the answer why NASA is not yet ready to go all the way to Mars with existing technology and vehicles currently under development. The delta-v requirements are bigger for Mars missions than near-Earth asteroid missions, this is true, but the technology development required for a bigger Earth departure stage is minimal - with or without heavy lift. To see the real need for technology development we have to turn to page 13 of the report, and the section titled "Mission Duration".

Unlike Apollo or the Space Shuttle, the Orion spacecraft includes design features which support long missions, such as solar arrays rather than fuel cells for power, and regenerative amine beds rather than single-use lithium hydroxide canisters to remove CO2. Orion is designed to support four astronauts for 18 days going to and from the Moon, with a 180 day unoccupied period in lunar orbit while the astronauts are at the lunar outpost, plus 30 days of contingency loiter capability for a mission extension. This built-in long duration capability is a critical enabler for an asteroid mission.

Orion hardware is already designed for the same mission duration needed for an asteroid mission, addressing issues such as reliability, leak rates, hardware radiation tolerance, and micrometeroid protection. Micrometeroid and orbital debris (MMOD) protection has turned out to be one of the most challenging requirements to meet for long duration missions, since longer missions have higher cumulative probability of impacts.

The duration of a Mars mission is more like 32 months - 8 months there, 500 days on the surface (or, for a Phobos/Deimos mission, in orbit!), and 8 months back, and the crew size is almost certainly going to be bigger than an asteroid mission. The next section on "Life Support" addresses this.

Despite Orion's long duration capability, a five to seven month mission requires more food, water, oxygen, and nitrogen than Orion is presently designed for. Reducing the crew size from four to two astronauts and pairing up two spacecraft quadruples the number of days the astronauts can be supported, to approximately 80 days. A further factor of 2 to 2.5 increase in consumables is required for a 6 month class mission.

And goes on to state that although the Orion's open-loop life support system is adequate and the right choice for 6 month duration missions, it acknowledges that it is on the knife's edge and a closed-loop life support system is inevitably going to be necessary for the much longer duration missions.

Finally, no discussion of deep-space human missions is complete without addressing the radiation exposure problem. Section 10 on page 32 takes the problem head on and contains a great summary of the analysis Lockheed Martin took in the placement of components in the Orion spacecraft to provide good coverage for radiation shielding without adding additional mass. It also advocates an on-need strategy for deployment of supplies as additional shielding in the event of a solar storm. Flying asteroid missions is suggested as an important precursor to a Mars mission, and on this criteria alone that's a good argument.

In short, I think there's a false choice here. Although I agree with the basic statement that NASA doesn't need to design a dedicated vehicle now, and doesn't need fundamentally new technologies to reach the more favorable near-Earth asteroids in early missions, a sustained technology program to improve capability will widen the choice of near-Earth asteroids that are favorable and beat the path to Mars.

* If it's not already up there it should be soon. Thanks to Josh Hopkins for an advance copy of the report.

21 comments:

  1. Trent!

    Thanks for the news. It seems that there are people at LM and Boeing that are more than just bean counters. The dream lives on in many hearts, and it may be that our time has come.

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  2. I would dearly love to see something fly soon too. It seems possible that a mission could be put together from some combination of Orion or Dragon capsules and Bigelow Inflatable modules. However, it would take a lot of propulsion to launch even a tiny mission from LEO. Perhaps multiple Centaur-derived upper stages could be used? At least one of them would have to be equipped for long-duration, on-orbit storage to provide the boost back to Earth.

    However, this would be a fairly risky mission with tight margins. I'm skeptical of NASA and Congress being that bold without some urgent national need.

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  3. Heinrich Monroe12:28 PM

    I too think this study is novel and far reaching.

    But the lack of targets is a major problem. Yes, there are likely to be more targets identified in a decade or two, but the situation right now is somewhat desperate. You do understand that 2008 SU4 is thought to be less than 10 meters across, no? So this rock isn't much bigger than the dual Orion that is sent to visit it. That diameter doesn't correspond to a whole lot of "exploration". In fact, according to the paper, the available targets for the next couple of decades are similarly tiny.

    My understanding is that NASA has informally adopted a qualified asteroid target as one that that is at least as large as ISS. Why? I really don't know, but at least on such an object there is room for a flagpole and a few footprints. There are no such asteroids within the range of this dual Orion concept for many decades.

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  4. 10m is a minimum.

    Some common asteroid densities (from meteor data) are:

    C2-type: 3.3 g/cm^3
    C1-type: 2.0-2.8 g/cm^3
    S-type: 3.5-3.8 g/cm^3
    M-type: 7.0-7.8 g/cm^3

    So the total mass of a 10m asteroid ranges between 20,000 kg and 78,000 kg.

    The primary reasons to go to an asteroid are: science, mining, planetary protection.

    In terms of scientific samples, this mass is more than sufficient to gain knowledge about the formation of solar system.

    In terms of mining, this mass can be used to prove in-situ resource utilization, propellant production, characterization of net present value, etc.

    In terms of planetary protection, it's important that initial missions not be to planet killers are disturbing them may be what actually causes them to hit Earth in the future. Demonstrating the ability to move a 10m rock is just as valuable as demonstrating the ability to move a 1km rock.

    Ultimately, the purpose of flying to an asteroid is to demonstrate the capability of doing so. Should NASA ever get into the enviable position of having a deep-space capability without an asteroid target to aim for, it will only be slightly less climatic to go out to a similar deep-space orbit than it will be to actually rendezvous with a deep-space target.

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  5. a manned NEO mission is extremely risky and so expensive, that, with the same money, we can send many robotic missions toward ALL the most interesting NEOs coming close the Earth and, each robotic mission, can be hundreds times longer (with a deep exploration of a NEO) than a (few days long) manned mission

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  6. Gaetano, hey, thanks for the comment.

    Recently, NASA held the Exploration of Near Earth Objects Objectives Workshop (http://www.nasa.gov/exploration/new_space_enterprise/home/neoworkshop.html) where the project leads for all the robotic asteroid missions that have flown discussed their successes and failures.

    They were all adamant that a human mission to an asteroid could teach us a lot more about these objects than any robotic mission for the foreseeable future. Robotic exploration is just really that hard.

    Regardless, there is no serious humans vs robotics argument. The purpose of going to an asteroid is simply that it is a target beyond the Moon but not so far as Mars. Human exploration of the solar system isn't about science. It isn't about spinoffs or international cooperation either. Sending humans into space is about preparing us for our destiny.

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  7. I agree that spaceflights are mainly an human adventure rather than only a scientific or commercial thing but a manned mission to NEOs is also useless since there is no way to extract and bring to Earth any NEO mineral (at competitive prices) within, at least, the next 50 years, or more, when low cost commercial space vehicles will fly everyday to/from space... it's better to devote all efforts and use all funds to more rational targets like the Moon and Mars

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  8. Umm.. obviously NASA isn't planning to fly humans to asteroids to economically return material to Earth. Any commercial mining efforts would be done robotically, but just as the Apollo Experiences documents have been invaluable to SpaceX in developing their commercial capsule, the future Asteroid Experiences documents will be invaluable to future commercial asteroid prospectors.

    Just because you can't close the business case today does not mean it is "impossible". The funny pages is full of people making famous quotes like that.

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  9. true... one of the early IBM managers has said, 60+ years ago, that computers was "useless" and no more than two-three computer would been enough for the entire world... :)
    .
    however, personally, I'm not particularly "excited" by a manned mission to an asteroid and I don't see enough scientific reasons to take the risks and the costs of these missions
    .
    of course, this is only my opinion, since, the last word about them will come from the US politics, that should "pay the bill"

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  10. "taking the risks" is the entire point. As for not being excited, this is why I've previously called you a robot. What's wrong with you? I can understand someone saying that it's not worth the money, that's a mostly rational argument.. but how can you possibly not get excited about going beyond the Earth-Moon system? Don't you have a soul?

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  11. maybe, when I will watch the mission in TV it could "excite" me... but, now, I see more risks and costs, than "excitement", especially since all NEOs are only big rocks... however, I did not want to see these missions scrapped to to nothing, but only to use the money to accomplish other (and not less "exciting") missions... please be aware that NASA will have a so little amount of money for the next budgets, that, probably, ALL these missions (NEO, Moon, Mars, etc.) could be impossible to do

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  12. Wow.. there is a spark in there. Dare to dream.

    Staying in LEO is cheaper, why not advocate for that? I think the reason is excitement, it's the risk. Going beyond LEO is exciting, so where do we go?

    Many say "the Moon", meaning the surface, what they don't understand, and what I think the Plymouth Rock study tries to say, is that NASA does not yet have that capability. The capability NASA almost has is described in the study, it's the bleeding edge of the capabilities NASA can expect to have in the next decade.

    Yes, that's disappointing to people who think NASA should be able to do now what Apollo did in the 60s, but that was a different NASA in a different environment. Today, NASA has to make do with and sustain limited capabilities. Apollo only flew a handful of missions. The shuttle program has flown 132! The Orion will need to be flown at least as many times to justify its cost.

    If we're terribly lucky, NASA will get the funding to develop a lander before Orion has flown 50 missions, and return to the Moon. Clearly, not all these missions can be to asteroids, but thankfully there's geostationary orbit, telescope servicing missions, and lagrange points.

    Political fighting is bad because it makes people myopic. While arguing over Moon vs Mars vs Asteroids they ignore the details of station, depots, infrastructure - sustainability.

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  13. A reader points out to me that a 10m asteroid is 1 billion cm^3, so we're talking 2000 tons to 7800 tons. I was only off by 100x. :)

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  14. nice article QG

    i know we know how to get to the moon. but the US has forgotten how technologically speaking. we don't have the equipment/hardware/infrastructure.

    i don't understand why the legs would get knocked off a program that would be capable of doing any of the proposed missions. after the program is already started.

    i was saddened when Apollo was canceled. but i was excited when the shuttle took off that first time and every time since. i could get excited to go to an asteroid, really, i could. it just doesn't sound as exciting as a moon mission and neither did the 'space plane'.

    it just doesn't make sense to me to start a whole new program when we were already developing hardware. all that money and time wasted. if we start from square one what's the point? gawd i hate politics.

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  15. Stuart,

    If the US decided to fund a manned mission to a near earth asteroid, it really and truly wouldn't be starting from scratch. The Constellation Program hardware that NASA was developing (and more-or-less still is) would work just fine for this mission.

    NASA itself was touting this type mission not long ago as something to do before a lunar lander is available. Strictly speaking we do not even need Ares 1 or Ares 5. Some sort of earth departure stage is needed for propulsion, but that could possibly be cobbled together out of Centaur upper stages. Proven propellant depot technology would be useful here, but may not be necessary.

    In some important ways a mission to an asteroid would be *easier* than landing on the moon.

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  16. Note also that a lot less hardware was being developed for NASA's Constellation Program than what you probably think. The Orion capsule, Ares 1 launch vehicle, and CSSS spacesuit are the *only* parts that were well underway. Both Orion and Ares 1 have been proving much harder to develop than planned.

    Many important parts including the Ares 5 super heavy launch vehicle, the Altair lunar lander, the earth departure stage (think Saturn IVb upper stage), and host of other items had all been "deferred" years further down the road. A few studies have been done here and there, but no contracts were signed.

    From its first announcement the Constellation Program has had a "success" oriented schedule. That has simply not worked out and it is currently being trashed/reconstituted because it was taking a LOT more time and money to do MUCH less than what was promised-- and things were getting worse.

    In many ways the Constellation Program was an unfortunate triumph of government bureaucracy (look up Pournelle's Iron Law of Bureaucracy). Just about every group at NASA and many, many congressional districts had a piece of the pie. New ideas that might be easier to accomplish and employ less people have been strictly forbidden.

    This might be okay if NASA's plans were key to winning a war or settling easily colonized planets, but they are not. Funding NASA has become a strange combination of tradition and entertainment that is looking less and less sustainable.

    The Obama Administration's proposed plan for NASA looked fairly good to me (the only thing that I liked about this administration!), but they were a pretty big change of course that seem to have caught important parts of Congress off guard. Worse still, this administration has done a very poor job of selling it to either Congress or the American people.

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  17. Lets see, this mission would require two heavy lift launches timed extremely closely together and would go out to a small asteroid to plant flags and put a few footprints there.

    Five to 8 billion dollars minimum costs.

    Now that's a bargain.

    On the other hand, if you went to the Moon and drove around, you could obtain samples of thousands of asteroids and it is only three days away.

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  18. Let's start from LEO. Cost is driven by delta v, mass and time. Mars is easy - huge in all three factors. Further, I don't think that the human race is expanding into space until driven or lead economically, by the likes of Musk or Rutan; certainly not NASA. The free market really is the Golden Goose, let's hope it survives. Anyway, think of a product produced on Mars and sold for a profit on Earth.

    That leaves Luna and the NEOs. VASIMIR and fuel depots are needed for both. The Moon trades delta-v for time, but has water, connectivity and requires no breakthroughs for human survivability. With railgun-like technology, H3 could (literally) be launched to Earth for pennies. Learning how to survive in open space without a great deal of mass will be difficult. So will how to extract, refine and return material to Earth from a very distant and weightless environment. I think that the Moon just makes more sense.

    I'd go tomorrow.

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  19. But that's not the choice. The choice is: do we do something in the 10 years it takes to build a lunar lander or don't we? And if we chose to do nothing then will the capsule and the booster even be around when the lander is finished?

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  20. The LM paper shows that we could go to an asteroid before we have a lunar lander, but it also assumes a super heavy booster or some sort of cryogenic propellant depot. I would like to see a followup study of what you could do with various propellant depots and NO super heavy boosters.

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  21. the 10 years it takes to build a lunar lander

    That's the saddest thing I've ever heard you say. I know in today's world you're right, but...

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