Forehead Smacking Moments: NEO Delta-V Misunderstanding

This is kind of embarrasing to admit, but I had a long-time misconception about the delta-V requirements for reaching NEOs.  A long time ago, I read some figure for delta-V requirements for earth-crossing asteroids.  The figure was ridiculously low, something like 60m/s.  At the time I read it, I didn’t really have a lot of experience with orbital dynamics, so I just filed the number away.  I had assumed from what I read that that was the delta-V required to reach some near-earth asteroids.  Unfortunately, while I wish I was the only one dumb enough to have made that mistake, there’s a good chance I wasn’t.

Anyhow, I probably would’ve figured it out a little quicker if I had been more interested in NEOs.  I’ve always been a planetary chauvanist, and a Moon Firster at that.  I always just waived away the much easier access to NEOs (which turns out not to have been as much easier as I thought) with the argument that while the transportation delta-V requirements were less, the trip times were a lot longer, and the difficulty of operating that far from home would likely drive the costs up a lot higher than just shear delta-V numbers alone would indicate.

So this misconception sat uninvestigated (and fortunately mostly harmless) for several years until earlier this week I was running some numbers regarding the so-called “Flexible Path” approach that was discussed by the Augustine Committee.  To my surprise when I actually looked up the numbers, the closest and easiest to reach NEOs all required delta-Vs from Low Earth Orbit of greater than 3.8km/s (which is approximately delta-V needed to reach Earth-Moon L-1 or one of the Earth-Sun L-points).  In fact some required over 10km/s of delta-V just for rendezvous!  After thinking it through, it actually made plenty of sense.  NEOs aren’t orbiting earth, they’re orbiting the Sun.  So it makes sense that you would need to do an earth escape maneuver first (3.2km/s right there) plus some more to change your orbit to intersect there, and a final burn to match their orbits and rendezvous.

So where the heck did the 60m/s number come from?  It turns out that the 60m/s number is the delta-V needed to depart the closest of earth-crossing NEOs in a trajectory that intersects with earth’s atmosphere.  If you actually wanted to bring the returning vessel into LEO, unless you have a really good aerobrake you’re talking about at least 3.2km/s just to decelerate from an escape trajectory, and honestly it’s probably the same amount of delta-V to return from an NEO into LEO as it is to depart LEO and rendezvous with the NEO–as it typically is in orbital mechanics.

What this means to me is that the round-trip delta-V’s needed for NEOs, especially for missions that don’t just go directly to reentry, are actually a lot more demanding than I had ever suspected.  Without extensive aerobraking, for a round-trip you’re looking at at least 7.6km/s of delta-V, ie nearly SSTO levels of delta-V.  Even with aerobraking and in-situ propellant production at the NEO, you’re still talking at ~4km/s of delta-V on the outbound leg–which means that with a LOX/LH2 system, only about 1/3 of your LEO mass will even reach the asteroid.  This also makes a propellant depot/transportation node at one of the Earth-Moon L-points look a lot more interesting for missions to NEOs.  The delta-V from L-2 to most of those locations is around 1-2km/s, which means that most of your mass that leaves L-2 will arrive at the destination (about 65-80% for a LOX/LH2 stage, depending on your target).

In summary, I still think that NEOs have their place, and I still think that they do have some transportation advantages compared to going down into the lunar gravity well.   But now that I’ve cleared up my misconception, it looks like actual near-term commercial exploitation of NEOs is not likely going to be any easier than commercial exploitation of the Moon.

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Jonathan Goff

Jonathan Goff

President/CEO at Altius Space Machines
Jonathan Goff is a space technologist, inventor, and serial space entrepreneur who created the Selenian Boondocks blog. Jon was a co-founder of Masten Space Systems, and is the founder and CEO of Altius Space Machines, a space robotics startup in Broomfield, CO. His family includes his wife, Tiffany, and five boys: Jarom (deceased), Jonathan, James, Peter, and Andrew. Jon has a BS in Manufacturing Engineering (1999) and an MS in Mechanical Engineering (2007) from Brigham Young University, and served an LDS proselytizing mission in Olongapo, Philippines from 2000-2002.
This entry was posted in Lunar Commerce, Lunar Exploration and Development, NEOs. Bookmark the permalink.

26 Responses to Forehead Smacking Moments: NEO Delta-V Misunderstanding

  1. Bill Parkyn says:

    Jonathan
    Would you please explore the question of appropriate orbits and acceleration profiles to move a small NEO, such as Apophis, into solar L1 or L2? I would imagine that this is far safer to Earth than trying to get it to Lunar L4 or L5. Either a gravity tractor (beefed up with material from the asteroid) or a solar-sail & tether combo could provide the years of micro-gee to produce the delta V. What I want to know is the minimum acceleration capability needed during final approach and capture into a halo orbit. First small rocks such as Apophis would be tried to prove the technique, but the long lead time for delivery will motivate bigger rocks, too big to prudently let inside L1/L2 distances. Could a big nickel-iron rock supply all of Earth’s metals at lower costs than from down here? Assume on-site solar refining and EM catapulting of metal cargo in heat-shielded gliders that land at airports with 100 cargo-tons each. The world’s billion ton annual steel production would require 10 million deliveries, one every three seconds, major space traffic indeed. You’d need over a thousand airports to spread that out to one arrival per hour. The alternative is to have catchers in GEO that put them on some very heavy-duty space elevators. Will either ever be cheap enough to close down all our iron mines? Maybe in 200 years.

    PS: Regarding Apophis, how can anybody not want to do anything to it during such a close approach? At least install a beacon. If it turns out to be destined to thread the 2036 keyhole, just put 20 tons in its way, suitably spread out to prevent breakup, made of carbon fiber so it won’t vaporize. Payload to that altitude is the same as to LEO, so an ArianeV would be a cheap alternative to the collision. Amazingly, the power of kinetic energy is so great that a solar-sail 80 kps retrograde 800 kg impactor would be as effective as those 20 tons.

  2. Pete says:

    I recall that 60m/s figure – always wondered what it pertained to exactly.

    I still keep banging my head against the thrust to weight ratios required to get up and down to the moon. An ultra lean LH2/LOX rocket using lunar water is currently looking to me like the easiest short term infrastructure to develop.

    NEOs have the potential for some type of solar powered high ISP drive such that delta v requirements can become less significant. But this requires the development of a NEO sourced/based propellant depot and a compatible engine with an ISP of ~1000 and a reasonable P/W.

    I am still not sure which of these two systems would win out, even in the short term. Large scale lunar oxygen and hydrogen extraction and refining is probably not trivial. Bagging some NEO and turning some of it directly into a plasma for a high ISP rocket engine may not necessarily be harder.

  3. A 90 to 120 day round trip to a NEO would be practically as dangerous as a trip to Mars– as far as radiation exposure is concerned. So you’re going to have to add several hundred tonnes of mass shielding in order to protect the astronauts. And that’s a lot more massive than any 25 tonne Orion vehicle.

    The continuous capture and transport of tiny NEOs that are 100 tonnes or less to L1, L2, L4, or L5 by utilizing unmanned reusable light sail transport vehicles would be far more useful as a source of hydrocarbons, oxygen, and mass shielding.

  4. Martijn Meijering says:

    As so often Lagrange points and depots are the key. This holds for NEOs just as it does for interplanetary missions. Another advantage of NEOs over the moon is that you don’t need a full lander and you don’t need surface systems.

    I don’t understand your point about atmospheric reentry. This is a bad idea until you have aerobraking. Just return a reusable transfer craft to L1/L2, and have to capsule to return you to Earth. You need one anyway, since you want to return to Earth somehow eventually.

  5. Pardon the OT, guys, but have you seen this?

    Nanoaluminumand ice (water) propellant:
    http://www.space.com/businesstechnology/091021-tw-alice-rocket.html

    Don’t know about you, but I think this is WAAAAYYY cool!

  6. Pete says:

    Space industrialization is going to be one really big development program. So where in space will development costs be the lowest? I suspect LEO (low radiation, fast cheap access of goods and people from Earth, fast communication, direct markets, etc), and so I suspect that is where extra terrestrial resources need initially be delivered to.

    Either way, I suspect a highly effective extra terrestrial propellant transport system between the Moon/NEOs and LEO is required – whether it carries people and gear out or raw materials back. But I do suspect it will be cheaper and less constraining to ship raw material back to LEO than to ship people out – in the short term.

  7. Dwight Lasswell says:

    The large delta-V to a NEO is hardly indicative of a propulsive advantage to the Moon. If you’re going to the Moon, you need to get your Altair to 3.8 km/s! If you’re going to a NEO, you don’t need no Altair.

    The “hundreds of tons of shielding” required to visit a NEO is a complete and utter myth. That’s the shielding you’d need if you were going to hold irradiation down to terrestrial (sea level) fluxes. That’s totally unrealistic, and no astronaut has ever flown under such restrictive radiation caps. Some shielding will be required going to a NEO, and technology advances will present us with opportunities for doing it. Those technologies will be important for whatever we do outside of LEO.

  8. Pete says:

    There also appears to be significant progress being made in anti radiation drugs at the moment. This might greatly reduce the need for shielding.

  9. “The “hundreds of tons of shielding” required to visit a NEO is a complete and utter myth.”

    Yeah. You can read about this myth in Scientific American:

    http://www.scientificamerican.com/article.cfm?id=shielding-space-travelers

  10. MG says:

    I suspect one of the interesting aspects of NEO examination is that one can likely use the same spacecraft (high Isp, solar cell, etc.) to rendezvous with / flyby multiple objects. The transit time may make it a multi-year mission, but so what? One pays to get out of a big gravity well only once, and can characterize a number of objects.

    Now, if we constrain our discussion to manned vehicles, there seems to be relatively little rationale for an “out and back” flight to a NEO. The time / money might be better spent on Martian moon base setup. Especially if one used them as floors on a Martian space elevator system. Much less mass demanding than a terrestrial one.

    Sorry, got a bit afield of the topic at hand.

    PS: Welcome back to your blog after such a long time away. We dusted the corners a bit before you returned.

    And drank the Near Beer in your refrigerator. Sorry about that…

  11. Charles Grimm says:

    Land on an asteroid, and you will find lots of shielding material. Stay on the asteroid, and you will be shielded. If your stay is short, take some shielding with you for the return trip. If it’s long, your exposure is correspondingly shorter. “Sandbags” will work as shielding on a low thrust return trip.

  12. Continuously capturing small asteroids (10 tonnes to 100 tonnes) and transporting them back to one of the Lagrange points via light sail or solar or nuclear powered plasma rocket would make a lot more sense, than some potentially very dangerous human unshielded human adventure to a NEO. These small asteroid resources would allow us to properly shield future interplanetary vehicles for voyages to Mars and beyond while also supply lunar colonies with the hydrocarbon resources that they need. Check out a pdf copy of the Asterant concept:

    http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20040084672_2004088388.pdf

  13. Pete says:

    So say one wants 1000 ton of material in LEO a year to start playing with. Is it easier to get it from the Moon with a lunar LH2/LOX transport infrastructure, from NEOs with a solar powered high ISP rocket system or get it from Earth with a much lower cost launch vehicle?

    At $100/kg a 1000 ton/year launched from Earth to LEO would cost $100 million ($1 billion at $1000/kg). This actually seems difficult to beat with short term lunar or NEO infrastructure, and low cost launch from Earth is probably required anyway.

    Yet another option is a high ISP ram scoop in LEO that collects oxygen from the Earth’s atmosphere, this could largely supply propellant needs (high ISP engines and/or mix with a very little LH2 from Earth). A high ISP drive that is capable of transporting good to and from the Moon or NEOs is probably also capable of doing this skimming.

    Thinking about it from this perspective, it is probably cheaper to first get a 100 people or so living in LEO, intensively developing and proving all the systems required for life in space before going further afield for raw materials. There are also many hundreds of tons of raw materials available near LEO to get a started on (skimmed volatiles, retired satellites and stations…) Lunar and NEO resources should perhaps come later.

  14. Dwight Lasswell says:

    I’m delighted that you’re reading Bob Parker’s insightful essay in Scientific American, but you’re not reading it right. He says that 500 mT of water is required to shield an astronaut, in a liveable capsule, to radiation levels that would be be comparable to a human being on the surface of the Earth. That’s a lot of mass! But that terrestrial radiation level is about 5 mSv/yr. Say, a few hundred mSv in a lifetime. The career max dosage for astronauts is something a bit higher. About 5000 mSv actually. At least ten times higher. So while shielding will be needed for interplanetary travel, it’s nowhere near 500 mT to conform to astronaut radiation regulations. This certainly is a challenge for long duration travel outside of LEO, but it’s not going to prevent it. In fact, it’s well understood that judicious hab architecture, in which supplies for the trip are arranged as shielding, can go a long ways towards minimizing that exposure.

    The “hundreds of tons of shielding” required to visit a NEO is a complete and utter myth. Certainly if it’s considered “extra” material and you allow astronauts the kind of exposure that they have been allowed to have. There is rich technical literature on these strategies. Get past 2006 Sci Am and go find them.

  15. Mike Long says:

    Again.. another very fascinating discourse on the ‘how’ of doing interplanetary transport.. with very little mention of the ‘why’. This feeds back to the discussion of the ‘why’ of NASA… what the hell is the point of putting 100kg on Mars if you can’t use it to ‘do’ anything.. well.. Lagrangian points or no.. we will eventually figure out the ‘why’ and then the ‘how’ will no longer be very interesting to talk about, i.e… how much Laika cares about GPS.

  16. Jonathan Goff Jonathan Goff says:

    A couple of comments before bed (it’s been a busy week and I’m not going to get much rest until next Thursday).

    Dwight in comment #7:
    The large delta-V to a NEO is hardly indicative of a propulsive advantage to the Moon. If you’re going to the Moon, you need to get your Altair to 3.8 km/s! If you’re going to a NEO, you don’t need no Altair.

    I wasn’t trying to imply that NEOs didn’t have an advantage delta-V wise, just that the advantage is nowhere near as great as I had previously misunderstood to be the case. There are 1000 NEOs on that list that have delta-V’s to visit that are less demanding than the lunar surface. But there’s only 2 of them that are even noticeably easier to reach than the earth-moon L-points or lunar orbit. Sure you don’t need a lander to reach them, but the delta-V requirements are still in the range that you’re talking about non-trivial logistics streams to even begin to tap their resources…

    While I think the flexible path approach is valid in saying that NEOs are easier to reach than the lunar surface, I honestly still think the lunar surface is the first offworld surface that will be developed commercially.

    Mike Long in #15:
    Again.. another very fascinating discourse on the ‘how’ of doing interplanetary transport.. with very little mention of the ‘why’.

    This post was mostly me just trying to take the time for once to write down one of my ideas while it was still fresh in my head. I’ve been so buried in propellant depot conference papers, white papers, conference panels, and in designing the propulsion system for our Level 2 vehicle that I’ve really been neglecting my blog. I figured that the observation would at least be interesting.

    That said, your point is valid. The reason there aren’t so many posts about the “why” of space travel is that there aren’t many easy answers. As you point out, it will be a lot more obvious in hindsight what the “why” is once we get there. That’s not saying there aren’t reasons, they’re just a lot harder to articulate without spending more time than I have at the moment.

    ~Jon

  17. Mike Long says:

    Ahh… that is exactly the response I would hope to get from someone who is dedicated to the ‘how’, instead of the ultimately far more mundane ‘why’… God Speed.. and I really do mean that! I think there are a lot of people out there who think that NASA means National Aught to Spend our Asses into debt administration… you know.. $700 hammers and all that. When in reality we are spending an almost insignificant fraction of our GDP on space exploration. Did you or you readers know that the NSF spends close to 7 billion dollars on things like making pig kidneys work in people, verses the 3 billion that NASA spends on basic exploration?.. now the National Ignition Facility is one of my favorite pet projects, but seriously, NASA has much more street cred.. 🙂

  18. Dwight Lasswell says:

    “Did you or you readers know that the NSF spends close to 7 billion dollars on things like making pig kidneys work in people, verses the 3 billion that NASA spends on basic exploration?”

    What “things like” were you thinking of? Actually, for people with serious kidney disfunction, using pig kidneys has real world value. Unlike the “basic exploration” you say gives NASA “street cred”. Not on my street. I would have thought that if you wanted to “Golden Fleece” NSF, you might come up with a more compelling expenditure to point to, and I’ll bet dollars to donuts that in the Golden Fleece department, space exploration can hold its own. Yep, we’re spending an almost insignificant fraction of our GDP on space exploration. Why? Because no one has ever come up with a good answer to that mundane “why”.

  19. Mike Long says:

    Now I feel bad about that “pig kidney” comment, as well I should, seeing that my own father is currently on dialysis due to diabetes related kidney failure. I wasn’t trying to hijack this discussion, but maybe there is something here. Perhaps NASA really does need to put a few more “why” words in it’s mission statement, or maybe it should just be rolled into the NSF and re-focused on basic science (I meant to say that instead of basic exploration).

  20. 1. Even minimal protection from the potential threat of a solar event would require 0.5 to 1 meter of water shielding during a 90 day to 120 day trip to a NEO. That’s at least 50 to 100 tonnes added to the weight of an Orion capsule.

    2. The NASA radiation limits (50 rem/yr) for astronauts are 10 times higher than the legal limits for radiation exposure for nuclear workers on Earth (5 rem/yr). And we still don’t know how damaging long term exposure to galactic radiation is to human brain cells. Interplanetary flights would expose astronauts to about 25 rem/yr (no including the potential for lethal solar storms during such journeys).

    3. Long term exposure to radiation is already known to increase the frequency of cataracts in astronauts.

    The safest way for humans to travel through interplanetary space is on vehicles that are easily capable of transporting hundreds of tonnes of mass. Enormous space manufactured light sails weighing less than 50 tonnes each should be able to transport several hundred tonnes of payload relatively rapidly (less than six months) to Mars and a few thousands of tonnes to Mars in less than a year. Such reusable interplanetary space craft could completely open up the solar system for asteroid exploitation and human colonization.

  21. Pete says:

    The “why” question is intuitively obvious but incredibly hard to articulate, especially without prompting the giggle factor.

    If life is good then maximizing life is likely good. If maximizing life is good then expanding into space is probably the most critical thing that humanity currently needs to do beyond raw survival. Enabling the expansion, diversification and robustification of life ~trillions fold…

    If life, or even just human life, is not good, well that infers that we should phase it out… Many people kind of think this.

    To not advance into space is to effectively murder trillions of highly competent and free individuals who may one day do truly great things – before they even have a chance to be born. From a moral stand point, I do not wish to carry this on my conscience.

    To invoke population control and preemptively prevent expansion into space is to deny the people of the future the right to self determination. This dictates that they would not have the right to decide whether life was good or not, this decision having already been made for them by us, this seems a little presumptuous to me. It seems to me our first responsibility is, to the best of our abilities, to give them the choice, to give them freedom to live or not as they so choose.

    So the objective for NASA should probably be to explore and open up the new worlds of space. That means encouraging low cost commercial space transport, low cost space habitats, extra terrestrial resources, exploration, science, etc. NASA’s current manned space efforts in this regard are almost counter productive at this point in time. NASA should be doing the basic science and research, it should also be running the grants, prizes and regulatory encouragement necessary to help make this happen. It should be sponsoring the Columbuses, the Lewis and Clarks, the Amundsens, and so forth. But of course such NASA reform is not practically possible – the AC had to work with what NASA is, not what it should be. Taking what small steps it could and giving NASA every opportunity to save face and move forward.

  22. kert says:

    I have found the “why” really easy to answer :
    to gain a new continent, and a few more in the longer-term future.

    That simple.

  23. Pingback: Carnival of Space #126 (& Carnival of the Moon) - Out of the Cradle

  24. John Bossard says:

    Jon,
    The delta-v requirements to get to and from NEOs is indeed a compelling argument for using lunar resources. However, one issue that I think favors NEO resources is that of property rights. Rightly or wrongly, I believe that there will not be as much legislative and political furor over individuals claiming ownship over, for example, a 100 m dia. asteroid, as compared with making land claims on the lunar surface.
    The delta-v requirements, and their concomitant propellant cost, also argue for at least some amount of in-situ processing and beneficiation of NEO materials.
    Nice post.

  25. Mike Lorrey says:

    Landing on the moon requires significant sums of dV, but you need to go there anyways to make L2 depots worthwhile. Refuelling L2 from Earth is a huge waste of money.
    Better yet, loft nuclear powered electric propulsion.

  26. Krishna Kattula says:

    Solar events are of limited duration. On the order of an hour. So you don’t need full vehicle protection, just a small ‘storm shelter’. For two people, that should mass less than 5 tonnes for adequate protection. Much less if supplies & propellant are arranged around it for extra shielding.

    Astronauts can be encouraged to sleep and spend off duty time in the shelter as well, to reduce the background dose.

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