Wednesday, August 25, 2010

Rock Envy

With the exception of Hayabusa, all asteroid missions to-date have been to targets bigger than 1 km in "spherical radius".

DateEncounterAsteroidRadius (km)Spacecraft
1991Flyby951 Gaspra6.1Galileo
1993Flyby243 Ida15.7Galileo
1999Flyby9969 Braille~1Deep Space 1
2000Flyby2685 Masursky~8Cassini
2001Landing433 Eros8.42NEAR Shoemaker
2002Flyby5535 Annefrank2.4Stardust
2005Sample
Return
25143 Itokawa0.165Hayabusa
2006Flyby132524 APL~1.1New Horizons
2008Flyby2867 Šteins~2.8Rosetta
2010Flyby21 Lutetia95.8Rosetta

This has led a number of people to express dismay that all the asteroids which have been identified for human exploration missions have significantly smaller estimated sizes.

DateAsteroidRadius (m)
20162008 HU4~5
20171991 VG~45
20192008 EA9~6
20202007 UN12~4
20251999 AO10~35
20262008 JL24~2.5
20282006 RH120~2.5
20292000 SG344~22.5

Notice that the scale has changed from km to m. Of course, exactly why these rocks are small is most probably what makes them optimal for a human mission. Their close approaches to Earth most likely would have ended long ago if they were any bigger. Collecting samples from these asteroids will help us to understand why they are not in the main asteroid belt, which is very important to know as their larger cousins threaten the Earth.

Some mission planners at NASA and elsewhere have expressed a desire to exclude any target with a radius smaller than 25m, being referred to as "mere building sized" asteroids. This reduces the targets in the table above to just two, although the last typically scrapes by due to a lack of options. Exactly why this policy is being suggested is unknown. Some speculation includes:

  • the belief that more targets will become available with greater funding directed to finding them, so it's best to downplay the available targets.
  • the difficulty of approaching a small rapidly spinning body.
  • lack of mass for sample collection.
  • lack of surface area for exploration given extended mission duration.
  • overall "spectacle".
These are all terrible reasons, and I'd hate to be accused of putting up a "strawman" to knock down, but they are the only reasons that I have read about. If you've heard more, let me know.

The first reason is just bad politics, and it's completely unnecessary. No-one can reasonably say the asteroid surveys are getting "enough" money, but they're certainly getting some, and they're getting valuable priority time on telescopes, both optical and radio. Hopefully this is just an ugly rumor.

Approaching a spinning body is something astronauts have done before. Dale A. Gardner and others used the Manned Maneuvering Unit (aka, the astronaut jetpack) to match rotation with satellites in LEO to return them to Earth on the Space Shuttle. With this in mind, it would seem engineering and astronaut experience is more applicable to small spinning bodies than it is to large ones.

The lack of mass argument should be immediately recognized as a failure of imagination. Referring to these asteroids as "small" in the first place is suggestive of this. The smallest asteroid on the list above has an spherical radius of ~2.5m, about the size of this:


The biggest asteroid on the list above, 1991 VG, is about half the size of this:


And unlike these reference objects, asteroids are solid with few to no internal voids. So there's plenty of mass, and when it comes to surface area and mission duration, I actually think making the case for short mission duration makes a lot of sense, and besides, suggesting that just a few days is too much time to spend exploring one of those London bus sized objects is just silly. A meteorite of such a size would be investigated for years.

In regards to spectacle, I recommend watching the video I linked to above of Dale A. Gardner catching that satellite. Even with a London bus sized object you're in for one hell of a show.

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.

Thursday, August 19, 2010

Smacking Asteroids For Resources


Video by Eric Brueton


Summer studies of space settlements by Gerard O'Neill and NASA in the 70s and again in the early 90s both determined that significant amounts of mass is required for passive radiation shielding. Although structurally, most designs call for refined steel, it has been suggested that mass for the shielding could just be raw lunar regolith, left-over slag from future on-orbit industrial processing, or obtained from the asteroids. The asteroids are seen as preferable as, in terms of delta-v, they are most easily available. The typical argument is that a long duration mission to rendezvous with a near-Earth asteroid or comet (collectively, near earth objects, or NEOs) could skip a lot of launches from deep gravity wells, either digging into the NEO or
dismantling and processing it to make a nearby structure, or both.

The wrinkle, however, is in that "long duration" part. In terms of delta-v, there are NEOs which are easier to hit than the Moon, but all currently known NEOs require months to year long travel times. This rules out manned expeditions for the time being, as long duration human spaceflight beyond the Van Allen radiation belt requires significantly more shielding than Apollo style short term missions.
To break the stalemate, many have suggested the use of autonomous or teleoperated spacecraft to develop a soon-to-be Earth Hill-sphere crossing NEO into a life sustaining capable radiation bunker. As many appropriately sized NEOs cross within 2 or 3 lunar distances (LD) of the Earth, it's clear that only a short term mission is required to "jump on board" and bunker down. With adequate prediction of the NEOs orbit, resupply could be successfully staged ahead of time on slower, more energy efficient, trajectories.

When considering the risk involved in such an adventure, and the market opportunities of any such settlement, Earth or Moon orbiting alternatives start to sound a lot more attractive. So let's consider an orbital habitat that is closer to home.

To get mass from the surface of the Moon into an appropriate orbit, it is suggested, one can use a solar powered mass driver. The immense cost of building a lunar mass driver is quickly amortized over every kg of steel and regolith launched and the sheer quantities of mass required - 150,000 kg for the torus designs, 42,300,000 kg for the cylindrical designs, and that's just the structural mass (steel), for shielding it's 9.9 and 23.3 million tons of regolith - mass drivers beat rockets hands down. But building a mass driver of that scale, and all the infrastructure required to supply it with regolith and power is another roadblock. When people talk about it, I suggest that by the time those quantities of regolith are available from lunar industry there will be tract housing available there anyway.

Do we have to rule out NEOs for shielding of Earth or Moon orbiting space settlements?

Recently, the asteroid 2010 AL30 flew by Earth, closer than the Moon's orbit, traveling at a relative velocity of 9855 m/s. Approximately 10-15 meters in size, it is estimated to be one of nearly 2 million such objects in near-Earth space. On average, an asteroid of this size passes within a lunar distance once a week. Asteroids come in a variety of different types and densities, but on the lower end of the spectrum (2.6g/cm^3), 2010 AL30 would have a mass of at least 2,600,000 kg.

Capturing small asteroids like this in Earth or, better yet, the Moon's gravity is a difficult proposition. For the altitude that 2010 AL30 was traveling at, it would have to lose 8139 m/s of velocity to enter a circular orbit. Suppose we were to send a spacecraft to hit it head-on. The most recent example of an impactor in near-Earth space was the LCROSS mission, impacting a crater at the lunar south pole, it delivered a 2305 kg Centaur stage at 2700 m/s. If we take that velocity as representative, we would require our spacecraft to have a mass of 7,929,037 kg.. unsurprisingly, we require a bigger rock to slow down the smaller rock.

A better question might be, how big of an asteroid can we capture by head-on impact with a Centaur stage? Again using 2700 m/s as a reference, you'll find the asteroid can be no more than 756 kg in mass, which converts to an asteroid 66 cm on a side using the same density as before.



LCROSS was launched as a secondary payload on an Atlas V 401. The Centaur provided the entire burn, although there was a small amount of gravity assist provided by the Moon. The shepherding spacecraft followed the Centaur into Cabeus crater as the Moon is quite a hard target to miss at that range, but I can imagine a similar spacecraft avoiding the brunt of the impact of intercepting an asteroid. If the
spacecraft is equipped with a solar powered propulsion system, be it Hall-effect thruster or an electrostatic ion thruster (as used on the Deep Space 1 spacecraft that did a flyby of a comet) or even a mass driver, it is conceivable it could rendezvous with the various fragments, assemble them into a whole and shepherd them to another intercept.

As the shepherded mass grows, larger asteroids can be intercepted.

Such a spacecraft could also find purpose cleaning up more traditional
orbital debris.

Note: I wrote this a while ago, but forgot to post it, I'm not terribly sure of the math anymore. It seems about right, but I know now there are lower delta-v asteroids that wiz by the Earth within a lunar distance. But it has some good references.

Prospector's Skymap

There have been over 535,000 asteroids discovered to date and they're all different. Some 7,121 of them are known to cross the orbit of the Earth and so are referred to as the near-Earth asteroids. If you're interested in flying a robotic mission to an asteroid, you need some idea of how much delta-v your spacecraft is going to need. I've written on this before. However, if you're interested in flying humans to an asteroid, you also need to know how long the round trip is likely to be.

To answer this, one needs to know both how far away the asteroid is at closest approach to Earth, and for how many days it will remain that close. To the interested public, finding this information out using the available NASA tools is a slow process, and understanding what you've discovered is difficult without good visualization.

click for interactive skymap

Introducing the Prospector's Skymap, a tool for visualizing Earth-centric plots of asteroid trajectories over the next 20 years, in 3d. Included is ~500 asteroids that have low delta-v rendezvous. The blue part of the trajectory is "within range" of an average human mission duration of 6 months, which can be configured by setting the maximum lunar distance in the range box. See the online help.

In addition to visualization I've been doing some offline data processing. Probably the most interesting result is a list of targets that have low delta-v rendezvous requirements, close approach within 20 lunar distances, sorted by year, for the next 20 years. Here's the csv file.


* under 20 LD
AsteroidClose approachDistance (LD)#days*
(2009 SH2)2010 SEP 307.12333537
(2005 UN)2010 OCT 2411.01831426
(2006 JY26)2010 NOV 0414.30419131
(2010 JW34)2010 NOV 2119.35112616
(2008 KT)2010 NOV 235.56063661
(2009 BS5)2011 JAN 113.39771261
(2009 UK20)2011 MAY 028.56922830
(2009 BD)2011 JUN 020.900437127
(2007 RQ17)2011 JUL 2213.23054356
(2007 DD)2011 JUL 239.29576344
(2009 TM8)2011 OCT 171.10182922
(2009 WN6)2011 NOV 056.32427124
(2008 UR)2011 NOV 1317.55155519
(2008 EL68)2012 JAN 058.29607746
(2008 EJ85)2012 MAR 069.22909625
(2010 FR9)2012 MAR 2218.4059868
(2001 CQ36)2012 MAY 3010.00337228
(2009 BW2)2012 AUG 0913.12194325
(2010 JK1)2012 NOV 259.30796028
(2008 ON10)2013 AUG 1619.01999813
(2007 CN26)2013 AUG 2811.86088421
(2008 PW4)2013 AUG 3013.84872319
(2005 TG50)2013 NOV 1215.92644738
(2001 AV43)2013 NOV 182.889049103
(2004 FJ31)2014 APR 1011.56957118
(2007 HB15)2014 APR 287.40614228
(2010 KV7)2014 AUG 1517.87178217
(2006 WZ184)2014 NOV 0919.8203364
(2010 GH7)2015 MAR 207.38268635
(2006 HX30)2015 MAY 1611.01162832
(2007 BB)2016 JAN 1611.16409139
(1994 UG)2016 MAR 2517.27587312
(2008 HU4)2016 APR 164.882098136
(2009 DL46)2016 MAY 245.95465826
(2005 OH3)2016 AUG 046.46712048
(2007 RU19)2016 AUG 1113.42974322
(2004 KG17)2017 MAY 1610.74741430
(2006 SR131)2017 SEP 2712.11301815
(2001 WH49)2017 OCT 2615.11413823
(2006 XY)2017 DEC 206.14696635
(2008 WM61)2017 DEC 023.59290338
(1991 VG)2018 FEB 1118.36795347
(2010 AG3)2018 MAR 0415.75370025
(1999 FN19)2018 MAY 079.64622628
(2008 HS3)2019 MAY 1014.44907123
(2001 KW18)2019 MAY 2117.54578318
(2006 QV89)2019 SEP 2114.49611228
(2008 UD95)2019 OCT 0816.75618826
(2008 EA9)2019 NOV 2311.24244671
(2007 UN12)2020 JUL 0416.82181637
(2009 OS5)2020 JUL 1417.84387832
(2001 GP2)2020 OCT 033.12459267
(2008 TY9)2020 OCT 1017.51670915
(2007 TA23)2020 OCT 1216.49788921
(2008 GM2)2020 OCT 2515.82038030
(2005 AZ28)2020 NOV 1013.80427926
(1993 BX3)2021 JAN 1718.42055419
(2004 RW2)2021 SEP 0515.73470515
(1982 DB)2021 DEC 1110.23523523
(2009 BF58)2022 JAN 256.08112523
(2008 JP24)2022 JAN 2619.2299329
(2007 UY1)2022 FEB 0813.88990819
(2009 FX4)2022 AUG 059.98926435
(2003 YS70)2022 DEC 1310.01710938
(2004 XD51)2022 DEC 2711.59694325
(2006 EW)2023 FEB 1214.57483420
(2004 OW10)2023 MAR 2512.79139124
(2009 QR)2023 AUG 212.68601633
(2001 QQ142)2023 DEC 0614.37718919
(2005 FG)2024 APR 0715.85655919
(2009 FK)2024 MAY 2117.90962013
(1998 KY26)2024 JUN 0111.98590627
(2005 ND63)2024 JUL 1414.22454014
(2002 NV16)2024 OCT 2411.75661158
(2006 HU50)2025 MAY 029.26089223
(2008 ST)2025 MAY 2014.25303651
(2008 DG5)2025 JUN 069.08584125
(2008 CM74)2025 JUN 254.86452343
(1999 SF10)2025 DEC 187.27896964
(1999 AO10)2026 FEB 1310.43336156
(2000 SZ162)2026 OCT 1614.38559324
(2006 SF281)2026 OCT 186.03932842
(2009 SH1)2026 SEP 1710.78361319
(2009 HC)2027 APR 106.02300480
(2008 VC)2027 NOV 025.11435633
(2004 AD)2028 JAN 0618.2709469
(2005 ER95)2028 MAR 2313.02533046
(2009 FQ32)2028 MAR 313.80321238
(2009 WC106)2028 MAY 0216.25007621
(2000 SG344)2028 MAY 077.636435212
(2009 WR52)2028 MAY 202.46465131
(2006 RH120)2028 AUG 0811.213795188
(2008 EY84)2028 SEP 0911.88311431
(2007 DC)2028 NOV 055.71717550
(2006 SU49)2029 JAN 283.18766637
(2006 UQ216)2029 FEB 1913.49571745
(2009 CV)2029 MAR 038.61242277
(2007 WA)2029 MAY 136.03633634
(2000 SL10)2029 MAY 166.76024021
(2010 NN)2029 JUN 1412.25846121
(2009 DC12)2029 JUL 0618.68069815
(2008 TN9)2029 SEP 2918.17875710
(2006 HE2)2029 SEP 302.02912734
(2008 UA202)2029 OCT 205.29932660

From this data you can determine which target is most favorable in any given year. If you're after really low delta-v requirements, as well as short transit times, then the list gets smaller.


AsteroidClose approachDistance (LD)#days*delta-v
(2010 JW34)2010 NOV 2119.351126164.839129
(2008 KT)2010 NOV 235.560636615.800001
(2009 BD)2011 JUN 020.9004371274.793479
(2005 TG50)2013 NOV 1215.926447386.153250
(2008 HU4)2016 APR 164.8820981364.577387
(1991 VG)2018 FEB 1118.367953475.238135
(2008 EA9)2019 NOV 2311.242446715.392948
(2007 UN12)2020 JUL 0416.821816376.080512
(2009 OS5)2020 JUL 1417.843878325.764455
(2001 GP2)2020 OCT 033.124592675.595356
(2008 ST)2025 MAY 2014.253036515.637181
(1999 AO10)2026 FEB 1310.433361565.858708
(2005 ER95)2028 MAR 2313.025330465.814350
(2000 SG344)2028 MAY 077.6364352125.197308
(2006 RH120)2028 AUG 0811.2137951883.385696
(2006 UQ216)2029 FEB 1913.495717456.081869
(2008 UA202)2029 OCT 205.299326605.953219

And the list of known near-Earth asteroids keeps growing.

Saturday, August 14, 2010

Mission To Asteroid Using SpaceX Hardware - NASA's Target

As a target of study, NASA has identified the asteroid 1999AO10 as the 2025 destination for human exploration. We've heard that NASA plans to build a giant heavy lift vehicle to make the trip, but is it really necessary?

I previously described a human asteroid mission, but I assumed the logical choice of asteroid with the lowest known delta-v (and included analysis for the second lowest too), but for some reason this isn't as interesting to NASA, so let's consider how one might do the trip to their preferred target, using existing SpaceX hardware.


The reference numbers are: Earth departure stage 3291m/s, and storable propellant 3939m/s of total delta-v. We could improve this by carefully measuring the boiloff of LOX in Falcon 9 upper stages and analyzing the required insulation to do the arrival rendezvous using non-storable propellants, but I don't really have that information handy, so I'll just go with the storables.

Like last time, we'll use the Dragon spacecraft as our crew vehicle. Unlike last time we'll actually have a look at the thrusters, it uses 18 Draco thrusters which are similar to the Aerojet 445 in performance with 309s of ISP. The Dragon is carried to orbit on a Falcon 9 with 3000kg of payload in the "trunk", 3000kg of payload pressurized (that includes the astronauts), and 1290kg of storable propellant, giving a dry mass of around 1710kg. To provide 4710kg (dry mass + pressurized payload) with a total of 3939m/s of delta-v requires a gross mass of 19544kg, which means the external tank will be 13544kg when filled.

The Earth departure stage will be the Falcon 9 upper stage, with the Merlin 1C vacuum performance of 342s ISP. The total initial mass in low Earth orbit is therefore 56710kg. Meaning 37166kg of that is LOX/RP-1. With a mixture ratio of 2.56, that means 10440kg of RP-1 and 26726kg of LOX. (btw, if we had a Raptor stage the IMLEO would be 46328kg, and presumably mass-to-LEO of each flight would be bigger, but the boil-off analysis would be completely different, as you'll see).

Ok, so we have all the numbers we need, now we just have to decide what order to launch it all in to reduce the total mission risk. Storable propellants are called that because they can sit without being used for long periods of time and be ready to go when needed. Also, they don't suffer from boil-off when stored on-orbit, or at least not so much as we need to care in this kind of analysis when compared to cryogenics like LOX. As such, I'm of the opinion that the best strategy is to launch the storable propellants in the external tank, and the RP-1, first.

This is a total of 23984kg and includes some of the mass for the tanks to contain the propellants. So we're looking at two Falcon-9 flights with a shortfall of 3084kg. If an initial parking orbit is chosen well, these first two flights can be spread out over as long a time period as desired, limited only by orbital decay.

We now need to deliver 26726kg of LOX, along with the remaining 3084kg of non-cryogenic propellants, for a total of 29810kg. These three Falcon-9 flights will deliver 1540kg of excess LOX which should account for boil-off if the flights are launched without delays. At 0.1% per day boiloff, the launch campaign must be complete in 50 days. But we have an ace up our sleeve.

The final flight of the launch campaign is the manned Dragon. It will be carrying the crew, with all their provisions, on-board propellant, some of which will be used for rendezvous and orbital assembly activities, and 3000kg of LOX in the trunk. Carrying cryogenics in the trunk may seem risky, but it has such an awesome payoff on cryogenic boil-off that it's worth it.

As described in the NASA concept, the mission departs the low Earth parking orbit, and arrives at the asteroid 3.5 months later. The astronauts stay on the rock for 2 weeks, then return to Earth a month later. The entire trip is 5 months. Radiation exposure is similar to a year long stay on the ISS, about half of an astronaut's lifetime limit.

Because of the suboptimal choice of destination, the new mission has five tanking flights at $56M each, and the manned Dragon flight which I estimate at $150M, for a grand total of $430M. This is about 100 times less than what I expect NASA will spend trying to do the same thing.

Wednesday, August 11, 2010

The Asteroid Menace

Day one of the Exploration of Near Earth Objects Objectives Workshop saw the presentation of three key reasons to send humans to visit asteroids: science, mining and planetary protection. Of these, the last has has been shown to be an issue that attracts mainstream support, no doubt we have Bruce Willis to thank for this.

The workshop began with presentations of the robotic missions that have been flown to asteroids and comets. The recently returned sample return mission Hayabusa, taking pride of place. All the presenters had war stories of the operational difficulty of flying to these objects, and some expressed surprise that their missions succeeded at all. They also talked about the high cost of these missions in terms of remote sensing equipment and the lack of good ground truth information to calibrate these instruments.

Unsurprisingly then, they all support a human mission to an asteroid or a comet near Earth to more efficiently gain scientific results. However, when asked about planetary defense, the consensus opinion wasn't just that a human mission would be nice: it is absolutely necessary.


Here's some interesting audio from the workshop.

An asteroid is heading to Earth that will kill every one of us, someday. Hopefully we will discover it and track it for long enough to have 5 or 10 years prior notice. Now what? We'd love to send a robotic probe to get some idea of what the object looks like, what it is made of, how fast it is rotating, etc. Unfortunately, a robotic mission will take at least 5 years to go from concept to launch and it has a very low chance of success at even this precursor mission. Sending a robotic mission to deflect or otherwise mitigate the threat is simply unthinkable at this time.

That means we need to send humans, and it means we need to send them to meet the threat with no robotic precursor. The astronauts will do the scientific investigation to determine the composition of the object. This will most likely include planting seismic detectors, and launching one or more kinetic impactors into the surface. Relaying this data back to Earth, the astronauts would wait for ground control, probably with the support of the national laboratories, to decide on a mitigation plan. Obviously the plan will be limited by what the astronauts have with them, so they will need to carry an array of tools for the various types of threats that may be expected.

For example, the best strategy may be to install large motors which provide constant thrust for a period of years diverting the asteroid off course just enough to miss the Earth. Such a strategy would only be possible on objects where in-situ resource utilization could produce sufficient propellant. Another strategy may just be the placement of a station keeping spacecraft with a lot of mass, possibly removed from the surface of the object, to act as a "gravity tractor", again diverting the course away from Earth. More exotic strategies may include converting rotational energy into propulsive energy using long tethers or in the drastic use of nuclear bombs.

The workshop continues today and is being webcast. Ultimately, failure to send astronauts to visit near-Earth objects within the next few decades will be fatal to humanity, so tune in and participate.

Monday, August 09, 2010

Deep Fried Astronauts

Back in 1967 the Bellcomm put together a study for the then Manned now Marshall Spaceflight Center. The mission was a one year human flyby of Venus. The study included some innovative stuff, like using the Earth departure stage tanks as a living module after venting any remaining fuel into space, but it also contains a fair bit of misinformation about radiation exposure, advocating that no attempt be made to shield against galactic cosmic radiation. This is to be expected.

What isn't expected is that this is still the general consensus today, even though a more recent computational study has provided some interesting numbers for various shielding materials.

Shield Material (5g/cm^2)Annual radiation dose (mSv*)
Aluminum542
Polyethylene450
Iron581
* Quality factor recommended in ICRP-60 is assumed.

This looks pretty good when the astronaut lifetime radiation limits are considered.

Astronaut ageCareer effective dose limits
(mSv, average life loss)
MalesFemales
25520, 15.7370, 15.9
30620, 15.4470, 15.7
35720, 15.0550, 15.3
40800, 14.2620, 14.7

Polyethylene shielding works better than aluminum which works better than iron because it has more carbon atoms. Adding shielding to the spacecraft and only sending crew of the appropriate age can drastically reduce the amount of life you take from astronauts on one-year long missions. Of course, most Mars missions designs are much longer duration than this and so more heavy radiation mitigation is needed.

Before discussing those options, let's think about how heavy this "light shielding" actually is. A Saturn S-IVB provides a heck of a lot of space compared to an ISS module, and has proven sufficient for a one year journey by SkyLab. It was 17.8m long by 6.6m diameter. With flat ends, this is a surface area of 4,374,982 cm^2, and at 5g/cm^2 the polyethylene shielding would weigh 21,874 kg.

On a trip to Mars one does not only have to consider the fuel required to burn in LEO to get to escape velocity, one must also consider the fuel required to do course corrections to get to and maintain Mars transit and perform Mars orbit injection. One way to reduce this fuel is to carry a large heat shield and do aerobraking. All this fuel is mass, typically with a lot of carbon in it, and so can also be used to shield the crew.

It seems reasonable at this point to wonder exactly how much radiation protection you and I get here on the surface of the Earth. The answer "enough" is sufficient so perhaps a better question is, how? Go outside and look up, what do you see? Air. How much? The answer is 1,030g/cm^2. As such, the 22t of shielding we added is providing just 0.49% as much protection.

If we want to provide sufficient radiation protection for long duration spaceflight, it seems obvious that we need to make the spacecraft smaller. Shielding just a smaller part of the spacecraft would be pointless as the crew is required to stay in there for the majority of the trip anyway.

Let's consider a 8m long cylinder of 4m diameter. Internally, it would be about 80 m^3 of pressurized volume. The surface area is 1,256,637.06 cm^2. Here's the masses required for various levels of radiation protection.

Radiation protection (vs sea level)Mass (kg)
1%12,943
5%64,717
25%323,584
50%647,168
100%1,294,336

As you can see, the life expediency of the crew can be improved by almost 3.5 times by reducing the surface area of the living volume by just under half. An interesting rule of thumb: 7% of Earth normal radiation shielding takes one year of lifespan off men for every year spent on the mission.

Making the spacecraft even more cramped and arranging the storable propellant and supplies as additional shielding would permit the creation of 5% of Earth normal radiation shielding, meaning the crew could go on the long multiyear excursions required to explore Mars, but no conceivable technology today can practically provide passive shielding for 100% radiation protection.

Sunday, August 08, 2010

Throwing Stuff In Space

In my last post about the Russian space program I said that cosmonauts regularly throw stuff in space so it will burn up and not result in permanent space junk. A reader asks whether you can actually do this.. Man, way to ask a hard question.

Orbital mechanics says "if a space vehicle comes within 120 to 160 km of the Earth's surface, atmospheric drag will bring it down in a few days, with final disintegration occurring at an altitude of about 80 km", and we can work out how much delta-v a cosmonaut has to impart to get the semi-major axis of the orbit of the debris below 160 km.


dVA = sqrt(GM*(2.0/rA - 1.0/((rA + rB) / 2.0))) - sqrt(GM/rA)


where rB = 160km + roE

where rA = 278km + roE to 460km + roE

where roE = 6378.1km

where GM = 6.67300 * 5.9742 * 10^24 * 10^-11


with the ISS at 278km the delta-v retrograde is 34.684365m/s or 77.5867148 mph, which is major league baseball.

with the ISS at 428km the delta-v retrograde is 77.243278m/s or 172.788292 mph, with is space cannon territory.


Ok, let's work out how long it will take to degrade from 250km as a throw to that altitude from the ISS is pretty easy most the time.


We need to know how big the thing we're throwing is, let's say 1m x 1m and 100kg, with a drag coefficient of 2.67.

a = 250km + roE = 6628100

darev = (-2 * pi * Cd * A * p * a^2) / m
darev = (-2 * pi * 2.67 * 1 * 2.62 * 10^-12 * 6628100^2) / 100
darev = -19.3094776

L ~ -H / darev
L ~ -58200 / -19.3094776
L ~ 3014.06393 orbits

P = sqrt(4 * pi^2 * a^3 / GM) = 5369.860522 seconds

L ~ 3014.06393 * 5369.860522 seconds
L ~ 187.32758 days

So the answer is: it's unlikely. It depends how high up the ISS is and how long you're willing to wait for the debris to fall below the 160km altitude. Just letting the part go without a throw will cause it to reenter eventually because no low-Earth orbit is stable, but if you want it to come down in just months you've gotta throw it, and note that you don't throw it "down", you throw it retrograde, and preferably at periapsis, but that's any point on a circular orbit.

Thanks for the question Ian.

Saturday, August 07, 2010

A Disappointing End To The Russian Space Station Program

Thankfully no-one really cares about the Russian space program, or, ya know, they don't speak English, I guess. Everyone has heard of Mir but name any prior station. Go on, name one. Ok, that's easy, but how many space stations did the Russians have before Mir? I'll get back to you on that.

So what was the point of all these stations? We all know the reason why the US has the ISS, and anyone who watched the Augustine committee proceedings last year heard about why the "international community" is demanding it be extended until 2020 and beyond. Scientific research or something right? The 2005 NASA Authorization Act designated the ISS as a national laboratory. Oh sorry, the American segment of the ISS, because obviously the US can't designate the Russian segment as a national laboratory of the US, but that's what it is right? Well, no.

The Russians do very little scientific research on the ISS. They have only "mini-racks" on the Poisk module, and finding out what exactly they do is harder than finding out all the "world class research" that the US apparently does on their side of the station - and that's saying something. So what, maybe the Russians just don't care about scientific research. Ahh, no. (I should stop finishing paragraphs like that).

Starting before their first station the Russians flew a series of Bion satellites which contained a whole heck of a lot of biomedical experiments - most of them a lot more ambitious than what is apparently done on the ISS these days. In fact, people groan at me when I say they didn't go far enough with their rat life-cycle experiments because no-one has ever done anything like it since 1979. However I hear the upcoming Nauka module to the ISS will have rat experiments, maybe they'll continue that research.. and that has me worried. Although mammalian reproduction experiments in space are important - stop giggling children, we're never going to colonize space if we can't have babies out there - that's not what the Russian human space program is about.

Their first station was Salyut 1, and by all accounts it was an unmitigated failure. The crew actually died on the way back to Earth and no-one ever returned to the station, it was deorbited after just 6 months. This immediately set the tone for the Russian space program, if people had to die to do work in space then it better be important work. So the next station, although called Salyut 2 was actually a military station under the secret Almaz program. Many consider this a distinction without a difference, after all, everything was secret in the Soviet Union. To those people I say: the Almaz stations were armed with freakin' cannons! They actually shot down test satellites with it.

Of course, Almaz was also an unmitigated failure. No-one made it to the first one. Salyut 3 had only one crew, and although Salyut 5 had three crews, one of those crews couldn't actually get into the station and had to turn around and go home. But hey, space cannons!

Whereas, the "civilian" Salyut program was significantly more impressive, it had to be, the world was watching. The last, Salyut 7, was visited by 10 crews including French and Indian cosmonauts, and did 13 spacewalks. The experienced gained from in-space operations was considered the primary payoff of the program. They demonstrated the multi-module principle that led to the seven module Mir, with the goal of a permanently human occupied station.

Then the international cooperation started. Of course, the 30 years of experience the Russians had paid for, with both Rubles and human blood, wasn't worth squat to the Americans. Even now, 12 years into the ISS program, I'm regularly told that the US shoulda gone it alone with Space Station Freedom. Oh, and then there's the complaints that Russia hasn't spent as much building the station, because dollars are such a great metric of productivity. In my opinion, the ISS would be scattered over the Australian outback (or Canada) by now if it wasn't for the Russians.

Let's take a modern example. Recently, two Russian cosmonauts did a spacewalk to install a new homing beacon and throw away an old camera - the standard boring maintenance that you do on a space station. Watching the spacewalk on NASA tv was enthralling - yes, I just said that, NASA tv was enthralling. The cosmonauts were crackin' jokes, and making fun of the bad comms with mission control, oh, and making decisions. They were sent out the airlock with nothing more than the broad goals and a rough schedule, the rest was up to them. (oh, and they didn't "create space junk" by throwing away that camera, cosmonauts actually know how to throw stuff in space to put it into a degrading orbit which will burn up.)

Just three days later an ammonia coolant pump broke on the American side of the ISS. My first reaction was to wonder if they were going to scramble the Shuttle to fix it, or downplay the problem until the Shuttle gets there in November. But no, they're going to send two of the expedition crew out to replace the broken pump. What a unique opportunity to compare the two programs.

Whereas cosmonauts are both involved in the planning and have operational decision making responsibility of spacewalks, astronauts are not. Two days after the failure, ground control decided to cancel already planned spacewalks to focus on planning the "repairs". Five days after the failure, the spacewalk to replace the pump was delayed, "to give planners more time to fine-tune the required procedures." Yesterday, eight days after the failure, the plan for the spacewalk was released, tune in to NASA tv to watch the spacewalk, oh sorry, spacewalks and see how many jokes and decisions the astronauts make on their own.

Space is boring because there are no humans in space, they're just robots in human form. Humans make jokes and decisions, they get frustrated, they argue their opinions and have ambitions. Human space programs do too. Saying you want to go to Mars some day is visionary, but actually doing it involves cutting humans off from ground support. It involves trusting them to make the right decisions. It means actually fixing broken coolant pumps, not just swapping them out and sending the order in to BoeLockMart for a new one.

To the Russian people I say: keep demanding humans in space doing important work to prepare humanity to go out into the solar system, leave the "science" and "spinoffs" and all the other justifications to the US.

Friday, August 06, 2010

Nuclear Rockets In The Atmosphere?

In James Dewar's latest book he proposes the development of a new solid core highly enriched uranium rocket engine based on the B-4 core developed in the Rover/NERVA program, but unlike that program he recommends starting small, testing in a dedicated exhaust processing facility and building successive generations of engine to prove safety and gain operational experience.

The first engine to be put into operation would have 40,000lbf (800MW), an ISP of 1,000s and weigh 6,000lb. It would have a maximum burn time of 15 minutes. The gross mass for the stage would be 91,000lb with 45,000lb of LH2 fuel, and a 3,000lb cocoon to recover the engine, to deliver a 17,000lb payload to LEO*. The stage would be dropped from 50,000ft by a cargo plane (such as the C-5A), and solid rocket boosters would carry it to 100,000ft before the solid core engine engages. The deorbited engine in its cocoon would be recovered from a splashdown for processing, as the U-235 would only be ~1% spent in the short burn required for orbital insertion.

This is just the first operational vehicle, bigger payloads to justify the cost would follow - Dewar makes recommendations for other rocket engines in the book, but this is his pièce de résistance.

Obviously, there are a lot of political issues to overcome before this could ever be funded at a government level and, excluding Bond villains, it almost certainly would have to be a government program. But what are the technical arguments against this? What are the (non-nuclear-hysteria) safety arguments?

And, to counterbalance the risk, are there good arguments for such a program?

These are not rhetorical questions, I want your opinion.

* in metric: 177,929N (800MW), 2,722kg. Gross mass for the stage would be 41,277kg with 20,412kg of LH2 fuel, and a 1,361kg cocoon, to deliver a 7,711kg payload to LEO.

Thursday, August 05, 2010

Scaled Composites' Dirty Little Secret

The public is incredibly easy to fool. Way back in 2004, Mike Melvill made history by flying Burt Rutan's beautiful creation SpaceShipOne across the unofficial border to space, twice, and later that year Brian Binnie did it again, winning the Ansari X-Prize and raising the hopes of all that private access to space had finally arrived. But how did they do it?

There is no question that Burt Rutan is a natural genius at aircraft design. His true innovation on SpaceShipOne was the shuttlecock styled effortless reentry system, and in particular, the ease of replacing these two large booms after a few flights to mitigate wear. SpaceShipOne/Two is a glider, and just like the Space Shuttle the wings are "only" used on the way down. There are wings used on the way up, of course, they are on WhiteKnightOne/Two, but once separated from the carrier aircraft the only lift is generated from the rocket.

As smart as Burt Rutan is, he's not a rocket guy. For SpaceShipOne he turned to Tim Pickens, a gifted rocket experimenter who became Scaled Composites' Propulsion Lead Engineer in 2002. It didn't last long, he was only there for a year, but in that time he gave them a motor. Think about that for a minute - it's truly remarkable. Because of that incredible pace the motor design was as simple as possible but no simpler, as they say. Being a hybrid, it shakes like a solid, along with half the reusability of a liquid.. and for some reason it can't restart and had half the performance of both. But it did the job.

Even before winning the prize, Scaled Composites signed a deal with Richard Branson to supply Virgin Galactic with a design for a new set of vehicles and form The Spaceship Company to put them into production. I'm not sure anyone thought it would take as long as it has, but Rutan set to work and delivered the scaled up vehicle designs with the creative names of WhiteKnightTwo and SpaceShipTwo.

Unfortunately, those designs also included details for RocketMotorTwo, which Scaled Composites decided to do in-house this time. Three years passed. In 2007 there was an accident which killed three and injured three more. Although the investigation cleared the company of any wrong doing, it was apparent that they were in a hole that they weren't climbing out of quickly enough.

In mid-2008 WhiteKnightTwo was unveiled. The media ate it up, but some of the statements made by Virgin Galactic's Will Whitehorn to promote the utility of the vehicle were a bit concerning. Everyone wanted to know: where's SpaceShipTwo? And the response seemed to be "we don't need it."

Late last year SpaceShipTwo was unveiled and Virgin Galactic had a big party. Since then, there has been captive carry tests and we're told there may be drop tests by the end of the year. It's all getting very exciting!



Oh I'm sorry, I was talking about the rocket wasn't I? I guess I got distracted by all the shiny white aircraft that I forgot all about it. Well, it seems Virgin Galactic have too.

Sometime between the 2007 accident and today Scaled Composites figured out that they're an aircraft company, not a rocket company, and decided to call in the Sierra Nevada Corporation to get back on track. Since then they've done hot fire tests which, of course, have been complete successes. Oh, they're using ablative nozzles too? Eww.

I've asked a lot of rocket professionals about these scraps of information and, in private, they've all told me the same thing. In public, "how would Jeff Greason say this?" is the what-would-Jesus-do of the space community, and this is how he said it:

In a sane world, a company that has a vehicle without an engine would purchase one from a company like XCOR, which specializes in engines.

So why can't Scaled Composites just buy the best rocket engines in the world (and yes, XCOR's engines really are that good). It would seem to be the rational decision. Apparently, the answer is simple: Rutan sold Branson a hybrid, so he has to deliver a hybrid. It doesn't matter how much hard won experience tells him that hybrids are not safer than liquids (or solids!). It doesn't matter that there's now acceptable engines that you can buy off-the-shelf that just weren't available 10 years ago when Rutan made his decision to go it alone. To change the deal now would loose face and more than likely have contractual implications.

And that's Scaled Composites' dirty little secret.