The realities of a Lunar base haven't changed at all. It's still a null or losing proposition -whatever proposition is being proposed, it's got nothing we need first, right now.
Space is a better place. If you want an off-Earth base, then high orbit is better. No blocking of sunlight, no extra delta-V needed to get there & back.
Above all, better access to better resources in the NEAs.
All the Moon is good for is science and tourism (whenever people can be supported there).

It's hard to argue with the logic in this one, and it concentrates on gaining better access to the Moon and near-Earth space first.
Building up a transport infrastructure before going to the Moon for large projects (which have no real purpose) is a far better thing than crowing about another expendable rocket program.

Bootstrapping Space Communities with Micro Rovers and High Tensile Boot Laces (Tethers).
www.marshome.org/files2/BruceBootstrap.pdf

I think this is definitely worth reading:


http://www.thespacereview.com/article/838/1  

Asteroid missions: be patient, or bring lotsa gas

by Tom Hill

Monday, March 26, 2007

Theres been an upsurge in interest in crewed missions to visit a near Earth asteroid. The prospects of new spacecraft, along with the more distant, yet still possible, larger rockets to push the new craft to the Moon, Mars, and beyond have fired the imaginations of scientists and laymen alike. Many say that a near Earth asteroid is the next logical step after the United States return to the Moon, sometime before 2020. Others say that an asteroid mission could be taken even before the return to the Moon. They say such a mission could alleviate the "been there, done that" feeling which some detractors love to bring up any time the Vision for Space Exploration is mentioned.

The interest has led to a number of articles in space trade news (such as this SPACE.com article) and even in mainstream newspapers such as USA Today, where the classic but overstated "huge asteroid impact" graphic was placed above the fold on page 1, and missions to asteroids were part of the discussion within. Crewed asteroid missions also received a short mention in NASAs report to Congress on the NEO threat.

An asteroid mission is an exciting prospect. Its allure includes the possibility of using less propellants than a lunar landing mission and not requiring the development a separate landing vehicle. The idea of exploring new territory is always enticing and cannot be overlooked, while mission timelines are possible that are on the order of an extended lunar stay, serving as stepping-stones to much longer Mars missions.

It turns out that two of the criteria used to argue for an asteroid mission-low propellant use and short timelines-are linked to each other through the mathematical dance of orbit mechanics and the rocket equation. Unfortunately, asteroids that have the potential for short, low-fuel missions are extremely rare. In an ironic twist, the same attributes that make them good candidates for such a mission contribute to the rarity of such an opportunity.

Mission overview

Recent articles have focused on asteroid missions where the explorers dont have to travel any farther than a few times father from the Earth than the Moon, so those missions are the focus of this study. Other mission scenarios are possible, such as the Gaiashield mission proposed by Zubrin in Entering Space, but that mission in particular does not claim to hold journey durations or distances low.

Missions to near Earth asteroids working to take advantage of a pass close to Earth essentially meet an asteroid at the fringe of Earths gravitational influence. The craft travels out to the rendezvous point, taking between days and weeks depending on the propellant budget and trajectory chosen, and then adjusts its path to drift with the space rock within Earths gravitational well. This period of time is called the proximity operations period. When the asteroid reaches the opposite side of Earths sphere of influence, the craft fires its engines again to return to Earth.

Orbit mechanics

The mission sounds easy, right? Theoretically, as far as space missions go, it is easy. Finding a candidate asteroid that supports such a mission is not. Asteroids, like the planets or any other object traveling through space, follow the laws of orbital mechanics in their paths, and those laws complicate mission planning.

Six terms are necessary to define an orbit and an objects place within it. When describing an orbit using classic Keplerian elements, there are three major terms that affect the shape of the orbit: inclination, eccentricity, and semi-major axis. Another consideration is the phasing of the Earth in its orbit with the asteroid in question.

Inclination describes the angle that the target orbital plane makes in comparison to another plane and is typically measured in degrees. For planetary bodies, inclination is defined as the angle between the orbit in question and Earths orbital plane. Earths orbital plane is also called the ecliptic. Asteroids that pass near Earth are inclined to its orbital plane by some amount, and that amount varies greatly. Objects in the initial data gathered for this survey had inclination values between 0.1 and 63 degrees. While an orbits inclination is not related to the size of that orbit, inclination plays a large role in suitability for a mission profile described here. An asteroid in an orbit with any measurable inclination compared to the ecliptic will only be capable of a truly close approach to Earth when it crosses the ecliptic plane, a point also known as the nodal crossing. Even on these close approaches, the differential speed of the asteroid compared to Earth is approximately 500 meters per second for each degree of inclination of the asteroids orbit.

Eccentricity describes the shape of an orbit, and it is a dimensionless quantity. At the theoretical yet never achieved eccentricity of zero, an object is in a perfectly circular orbit around its parent body. Increasing eccentricity describes a more elliptical orbit, with the near point of the orbit growing closer to the parent body and the far point growing more distant, up through an eccentricity value of 1. An eccentricity of 1, another theoretical value, describes an infinite ellipse also called a parabola. The infinite varieties of ellipses available create some interesting situations, although very few produce orbits that are compatible with a low delta-v mission to an asteroid. All but the lowest eccentricities can create a situation where the orbit of the target asteroid crosses Earths orbit at an angle that drives delta-vs to an unacceptably large value. The missions that are the focus of this paper require the right balance of eccentricity and the next term, semi-major axis.

While eccentricity specifies the shape of an orbit, its semi-major axis, expressed in units of length, relates to the amount of time it takes for an object to orbit its parent. The orbits of two objects with the same semi-major axis but different eccentricities can look very different, but take the same amount of time to make one circuit around a parent body. Objects with a semi-major axis much smaller or larger than that of Earth can cross Earths orbit, but doing so requires a relatively high eccentricity and these objects rapidly fall out of consideration for low delta-v missions.

Phasing is not an orbital parameter per se, but it requires mention here. Any asteroid that makes a close approach to Earth will, in all likelihood, make another pass at some time in the future. The closer the orbital period is to Earths, the more time between close approaches there will be. The same effect can be seen in the launch windows that allow missions to other planets. The outer planets, having orbital periods much greater than Earths, regularly align for a minimum-energy launch window approximately once each Earth year. Mars, with an orbital period much closer to that of Earth, aligns for a mission only once every 26 months. Near Earth asteroids, many with periods even closer to Earths than Mars, can go years between close approaches that would allow the kind of missions discussed here.

Methodology

The research for this article is easy to duplicate for anyone interested. The list of near earth asteroids and their orbital elements (a potentially large web page) was downloaded in January of 2007 and saved as a text file. The list was converted into spreadsheet format, and only those asteroids with semi-major axes between the arbitrarily-chosen values of .9 and 1.1 astronomical units (AU) were used for further study. This smaller list (still containing over 300 objects) was then sorted by inclination followed by eccentricity, based on an initial assumption that inclination would be a larger delta-v cost than inclination for a mission to that asteroid.

The top candidates from the list were then examined using the orbital viewer from the JPL NEO office. The viewer contains a disclaimer that it is for visualization only, and experience shows that this warning should be taken seriously. Due to the basic nature of this research, however, the viewer was deemed acceptable.

Each candidate asteroid was observed in the viewer to find the closest approach that was less than 0.02 AU between now and 2100. The date and distance were recorded, and this information could be used as a starting point for further analysis.

Early frontrunners

Three asteroids jumped out as candidates to show the complexities of a crewed mission to each. The first one, 1991 VG, requires a relatively low delta-v to enter and exit proximity operations (833 m/s each) and comes fairly close to Earth at five lunar radii. The one problem with this asteroid is that its close approach doesnt take place until the year 2068. If doubling the proximity delta-v is an option, then theres another mission opportunity with the asteroid 2000 SG344, which comes within three lunar radii in 2028. The tradeoff is a required delta-v of 1686 m/s to both enter and exit proximity operations. Nearly doubling delta-v needs again opens an opportunity with the asteroid 2001 GP2 in the year 2020.

Hopefully, new discoveries will provide a larger selection of asteroids and mission dates that require less energy to visit using the mission profile described here. The particular class of asteroids best suited for this type of exploration is underrepresented in the NEO list, because of their difficulty to discover. Some of the detection methods described in NASAs recent NEO report to Congress would increase the number of such asteroids in the database. It is also possible that other candidates will make themselves obvious using more exact orbital determination methods on the list we currently have. This will remain unknown until someone doing research within the area using better tools makes their study public.

Conclusion

Asteroid missions are exciting for their daring, their potential for scientific return, their ability to help protect the planet, and their meaning in humankinds growth into a spacefaring species. Opportunities to carry them out while keeping people within Earths "neighborhood" are not common, however, and many of those instances require a lot of propellant in order to make the mission happen. This is not necessarily a bad thing. The fact remains that it requires a lot of propellant to get anywhere interesting in the solar system, and perhaps an asteroid mission will help kick-start architectures that will take us to those other destinations.


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Tom Hill is an aerospace engineer and author based in the Washington DC area. He worked on this paper during time that he should have spent doing his taxes. An expanded version of the work can be found here http://spacewhatnow.com/id37.html , and it includes a deeper description of the methodology, other asteroids researched, illustrations of key points, and discussion of a new article released after the final version of this summary was complete. He can be reached at tom[at]spacewhatnow.com.

-- Edited by 10kBq Jaro at 02:58, 2007-04-07

-- Edited by 10kBq Jaro at 03:01, 2007-04-07

I think the point is that, by contrast, the moon is always there, at the same distance.
And if NASA's two LCROSS impactors prove once & for all that there are large quantities of volatiles in permanently shadowed polar craters, who needs asteroids ?
Also, from what we've seen in recent years, it looks like the dust problem is an order of magnitude worse on asteroids & comet nuclei than on the moon: a landing could well mean instant burial, with engines so crudded up, that you couldn't take off again.
On the other hand, the dust could work in our favour for NEO deflection, as reaction mass (blast deflection strategy).

On the asteroid thing, what do you think the chances are that we might find large quantities of uranium? 

I think that's agood summary, Ty.
Uranium oxidizes very easily, if there is oxygen available, which is how a large fraction ended up in the Earth's crust.
The formation of concentrated ore bodies however, requires processes not likely to have been available on the protoplanetary precursors of asteroids.

On the other hand, there is also solid evidence that a sizeable fraction of planets formed from oxygen-poor material (some of which has been found as meteorites, and also deducing from the overall composition of the Earth), so significant quantities of U should be expected to concentrate at the center of the core.
Marvin Herndon ( www.nuclearplanet.com ) has attempted to estimate the fraction in the core vs. the surface, concluding that there was likely enough to form a U ball several miles in diameter at the center of the Earth.
The protoplanet ancestor(s) of asteroids were likely much smaller than the Earth, but its still possible that somewhere out there is a mighty big chunk of solid U.
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