GPS Location Data for Four Million Commutes Reveal New U.S. 'Megaregions'

As economic centers grow in size and importance, determining their boundaries has become more crucial.

An ever increasing share of the world’s population is living in what are known as megaregions—clusters of interconnected cities. The concept of the megaregion is decades old and fairly easy to grasp, but geographically defining them has turned out to be rather tricky.

Now, researchers have attempted to map the megaregions of the contiguous United States by studying the commutes of American workers.

As megaregions grow in size and importance, economists, lawmakers, and urban planners need to work on coordinating policy at this new scale. But when it comes to defining the extent of a megaregion, they find themselves running into the same problems geographers and cartographers have always had when trying to delineate conceptual areas. Because megaregions are defined by connections—things like interlocking economies, transportation links, shared topography, or a common culture—it’s tough to know where their boundaries lie.

To try to solve this geographical problem, Garrett Nelson of Dartmouth College and Alasdair Rae of the University of Sheffield used census data on more than four million commuter paths and applied two different analyses, one based on a visual interpretation and the other rooted in an algorithm developed at MIT. Their results and maps appear today in the open-access journal PLOS ONE.

The map (left side) shows all the commutes of 50 miles or less (represented by straight lines between the start and end points) surrounding the San Francisco Bay Area, one of the more iconic megaregions in the country. Shorter, higher volume commutes are in yellow, with longer, lower volume paths in red. In this image it’s easy to see that the main centers of work are in cities, including San Francisco, Oakland, San Jose, and Sacramento, and that they are highly connected. (See "A Commuter Revolution: How Cities Are Collaborating to Solve the Challenges of Sustainable Urban Transport")

But where should planners draw the edges of a megaregion encompassing this activity? Which connections are statistically significant? Which are important for regional transit planning? Should they focus on the cities surrounding the bay, or is Sacramento just as important to the Bay Area economy?

For answers to these questions, Nelson and Rae turned to an algorithm-based tool designed by MIT’s Senseable City Lab to mathematically recognize communities. The algorithm only considers the strength of connections between nodes (more than 70,000 census tracts in this case), ignoring physical locations. This made for a nice test of Waldo Tobler’s “first law of geography”: that things that are near each other are more related than those that are farther apart.

The results of the algorithmic analysis took some cleanup and iterations—such as eliminating superlong commutes between places like New York City and Los Angeles and excluding nodes with only very weak connections—to produce a coherent map of plausible megaregions. The difference between the visual and mathematical approaches can be seen in the map above of the Minneapolis-St. Paul area.

 In the visual analysis on the left, activity clearly centers on the twin cities and extends outward concentrically, with weaker connections to surrounding cities. But where does the twin-cities megaregion end? Should St. Cloud be included? What about Rochester? In the smaller-scale map on the right, the algorithm assigned a broad swath of smaller surrounding cities, including Fargo, North Dakota, to the megaregion based on commuting paths. Here, the twin cities area shows up as the largest of multiple centers of activity. (See "The City Solution: Why Cities Are the Best Cure for Our Planet’s Growing Pains")

One of the decisions the researchers made was to limit the algorithm to 50 megaregions, which can be seen in the map below, where every node is colored according to the region it belongs to. This made the map more plausible visually. While 50 may sound like an arbitrary number, it makes sense mathematically because a very high percentage of commutes lie entirely within a megaregion relative to paths that cross boundaries between regions.

These algorithmic megaregions are easier to interpret on the map at the top of the post, which shows the connections. There are a few seemingly odd results, such as the sharp boundary that follows the New York-Connecticut state line or the small, splotchy green megaregion that floats between Birmingham and Dallas. Clearly, by ignoring geographical information and having no understanding of the country’s cultural character, the statistical method didn’t get everything right.

So Nelson and Rae combined the two methods to draw final boundaries around the country’s megaregions. They started by drawing lines around the dots on the map above. They then overlaid those shapes on the flow map at the top of the post and reinterpreted the boundaries to eliminate outliers and emphasize geographic continuity. The result is the map below, which eliminates some visual oddities. For example, the splotchy green area has been absorbed by the New Orleans-Delta megaregion, and a big swath of the west with relatively low population isn’t included in any megaregion.

The researchers hope their approach is a first step to a better grasp of the economic geography of the country. The map is clearly a work in progress, and some areas still don’t look right—the division of the New York City tri-state area into two megaregions, for example. And I’m not convinced the Bay Area-Sacramento megaregion where I live should extend all the way to Nevada. Does the map assign your home and workplace to a megaregion that makes sense to you?

Nikola Unveils its Hydrogen-Powered Semi-Truck

As reported by Engadget: It's not just passenger vehicles that are moving away from gasoline, big rigs are also saying goodbye to fossil fuels. In Salt Lake City today the hydrogen-powered Nikola One long haul truck was unveiled. According to the truck maker, the semi will be in production and ready to transport goods in 2020.  

The class 8 truck (the giant ones that transport goods) will have a range of 800 to 1,200 miles between refueling. If the company delivers on that range, the One -- if running at peak efficiency -- could get from San Francisco to Cheyenne, Wyoming on one tank of gas. And the fuel needed for that trip will be included in the 72-month leasing program that company is offering.

To solve the limited supply of hydrogen along the highways of the United States and Canada, Nikola also announced its plans to build stations to refuel its new trucks in both countries. The company will start breaking ground on the refueling stops in 2018 and they will open in 2019.

Without those hydrogen stations, it won't matter how impressive the range of Nikola One is if it can't be counted on to transport goods everywhere because it might run out of juice. Yet when it is on the road, the semi's tech will be making the most of its trip.

According to CEO Trevor Milton, the truck's navigation system will determine the most lucrative route between destinations. The dash will have a large display sort of like the one found in a Tesla.

Nikola is still determining where it'll actually build its new trucks and says it will figure that out sometime during the first half of 2017. Once those semis are built they will be sold, serviced and warrantied by trucking company Ryder's over 800 locations thanks to a agreement announced today.

The company also introduced the Nikola Two with the same range performance as the One but a smaller cab and more maneuverability. Like the One, it'll be available in 2020. Both trucks will have an electric motor connected to each wheel which should help with take off thanks to torque vectoring and braking.

No word on pricing on either truck. But Milton said that information will be shared soon.

Long-Range Anti-Drone Gun can Secure the Entire Neighborhood

Whether you're concerned about them snooping or indulging in something much more sinister like firing weapons, there are now a few precautions you can take when it comes to errant drones. If you don't have access to a flock of drone-hunting eagles, a new weapon could be the next-best option, with the ability to ground unmanned aircraft from up to 1.3 mi (2 km) away.

The Dronegun takes the same approach as a number of drone-jamming systems that fire radio waves into the distance to confuse the aircraft's control channels, such as the shoulder-mounted Battelle DroneDefender and the larger Anti-UAV Defence System.

The latter is capable of picking out drones as far away at 8 km (5 mi), but is a serious set of equipment that comes mounted on a vehicle. The Dronegun from Virginia-based Droneshield promises a more portable option that can quickly be whipped out in the case of an incoming threat.

Shaped like an extravagant laser-tag rifle, the Dronegun hooks up to a backpack and when adequately aimed, blasts the drone with electromagnetic noise at the same frequencies it uses for video transmission and control communications. This will typically cause the drone to return to its take-off point, though the Dronegun can block GPS too, in which case the drone will likely land on the spot.

The similarly designed Battelle DroneDefender can disable drones up to 400 m (1,300 ft) away, so the Dronegun promises a decidedly larger drone-free radius. It is not, however, intended for civilian use, at least not to begin with. The company is not offering it in the US, other than to government agencies, in the hope that they deem it a safe and appropriate anti-drone tool.

You can check out the Dronegun in action, in the promo video below.

MIT's New Method of Radio Transmission Could Make Wireless VR a Reality

If you want to use one of today's major VR headsets, whether the Oculus Rift, the HTC Vive, or the PS VR, you have to accept the fact that there will be an illusion-shattering cable that tethers you to the small supercomputer that's powering your virtual world.

But researchers from MIT’s Computer Science and Artificial Intelligence Laboratory (CSAIL) may have a solution in MoVr, a wireless virtual reality system. Instead of using Wi-Fi or Bluetooth to transmit data, the research team’s MoVR system uses high-frequency millimeter wave radio to stream data from a computer to a headset wirelessly at dramatically faster speeds than traditional technology.

There have been a variety of approaches to solving this problem already. Smartphone-based headsets such as Google's Daydream View and Samsung's Gear VR allow for untethered VR by simply offloading the computational work directly to a phone inside the headset. Or the entire idea of VR backpacks, which allow for a more mobile VR experience by building a computer that's more easily carried. But there are still a lot of limitations to either of these solutions.

Latency is the whole reason a wireless solution hasn't worked so far. VR is especially latency-sensitive, along with the huge bandwidth requirements that VR needs to display the level of high-resolution video required for virtual reality to work. But the MIT team claims that the millimeter wave signals can transmit fast enough to make a wireless VR headset feasible.

The issue with using millimeter wave technology is that the signal needs a direct line of sight, and fares poorly when it encounters any obstacles. MoVR gets around this by working as a programmable mirror that can direct the direction of the signal to the headset even while it’s moving to always make sure the signal is transmitting directly to the headset's receivers.

For now, the MoVR is simply a prototype, with the team hoping to further shrink down the system to allow for multiple wireless headsets in one room without encountering signal interference. But even as a proof-of-concept, it's an interesting perspective on how virtual reality could one day work.

How Otto's Self Driving Trucks Defied Nevada and Scored a $680 Million Payout from Uber

As reported by Backchannel:  It could be the plot of an Ocean’s Eleven movie: A team of daring engineers heads to the wilds of Nevada to create a dazzling spectacle, defy officials, and walk off with a $680 million jackpot.

In May of this year, Otto unveiled a video of an 18-wheeler thundering down a freeway with no one at the wheel. The San Francisco-based startup had retrofitted a freight truck with lidar, video cameras, radar, and other sensors, enabling it to operate “completely driverless without a driver in the backseat,” according to co-founder Lior Ron at the time.

Even Google, which had been developing self-driving cars for eight years, still insists on having a human in the driving seat should anything go wrong. That a tiny startup emerging from stealth mode could confidently demonstrate an autonomous big rig on a public interstate with no one in the front seats suggested Otto’s technology was really special.

Within just a couple of months, ride-sharing unicorn Uber had acquired the fledgling Otto in exchange for 1 percent of the company’s stock (representing $680 million) and 20 percent of any future self-driving trucking profits — a payout that could exceed a billion dollars for Otto’s founders.

So far, just another Silicon Valley fairytale. Yet Otto’s rapid ascent involved a high-stakes gamble that could have landed the startup in legal hot water, rather than the arms of the world’s biggest transportation network company.

When Otto performed its test drive — the one shown in the May video — it did so despite a clear warning from Nevada’s Department of Motor Vehicles (DMV) that it would be violating the state’s autonomous vehicle regulations. When the DMV realized that Otto had gone ahead anyway, one official called the drive “illegal” and even threatened to shut down the agency’s autonomous vehicle program.

The story of why Otto flouted Nevada’s directive, how it got away with it, and what this means for the future of self-driving regulation stretches back to the dawn of driverless cars.

Otto co-founder Anthony Levandowski

On May 1, 2012, the world’s first government-run self-driving testtook place on Las Vegas’s famous Strip. In the driver’s seat of a heavily modified 2008 Toyota Prius was Chris Urmson, then the head of Google’s self-driving car project. Next to him sat Anthony Levandowski, the brilliant engineer whose startup 510 Systems had been acquired by Google the previous year, after it had built the tech giant’s first autonomous vehicles.

As the Prius nosed its way past casinos and tourists, its performance was being watched from the back seat by a driving examiner and by Bruce Breslow, then the head of the Nevada DMV. The test was a quick zip around Las Vegas’s surface streets and freeways on a route chosen by Google itself. Despite Urmson having to take control of the vehicle on two occasions, the Prius passed the test with flying colors.

That was hardly surprising, as Nevada’s first autonomous vehicle rules had been proposed and partly shaped by Levandowski himself, with the help of David Goldwater, a lobbyist for Google. The company could now test its self-driving cars anywhere in the state. It was a publicity coup for Nevada, and the first official validation of Google’s vision to replace fallible human drivers with flawless robots.

Over the next four years, however, Google never actually tested its driverless cars in Nevada. And despite welcoming Daimler’s semi-autonomous trucks to the state with a glitzy ceremony in 2015, Nevada’s reputation as a pioneer in autonomous driving ebbed away.

Otto co-founder Lior Ron

The year 2016 brought some big changes: The energetic Levandowski decided to strike out on his own. “I’m excited about bringing robots into the market, about having the most effect in the world,” he told me. He gathered a handful of colleagues from Google, including Lior Ron, and founded Otto, a startup dedicated to “reimagining transportation—not just improving it.”

Initially, that meant a fully self-driving taxi rather than driverless trucks. In April of this year, Steve Hill, executive director of the Nevada Governor’s Office of Economic Development (GOED), wrote an email to Jude Hurin, the administrator in charge of autonomous vehicle (AV) regulation at the DMV. Backchannel obtained access to this and many more emails and documents under public records legislation.

“Just wanted to let you know that a company is interested in operating completely autonomously, shuttling passengers on a fixed route in Vegas somewhere around the end of this year,” Hill wrote. “David Goldwater represents them and will be contacting your office soon. The company is headed by Anthony Levandowski. All this is confidential.”

The company was Otto, and its plan was to offer driverless rides from Las Vegas’s McCarran airport to the heart of the Strip. Breslow, now director of the state’s Department of Business and Industry, chimed in. “Sounds like the Google project Anthony has wanted to set up for the last four years,” he wrote to Hurin on April 29. “[…] Should be relatively simple to deploy but political backlash from cab industry will occur.”


Photo credit: Otto

Less than two weeks later, Goldwater announced a surprising shift in direction. “Otto will have their company launch on Tuesday of next week,” he wrote to Hill on May 12. “[…] They are filming some video of self-driving trucks in Nevada tomorrow for the launch on Tuesday (there is someone in the truck [with] a kill switch.) The filming is being done up in Reno.”

Hurin’s response to Goldwater was swift. “I am concerned….Every company we have dealt with in the past has obtained a test license in our state prior to the media event….Your video is going to produce questions directly to your company and Nevada DMV as to why they do not have a red test plate on the vehicle. I am also going to have Daimler Trucking and others calling me to ask why Otto is allowed to do this when they couldn’t.”

Nevada regulations are clear on this point. Before an autonomous vehicle can be operated on the state’s roads, it must be issued a testing license and special red license plates. It has to be able to capture driving data in case of crashes, have switches to engage and disengage the autonomous systems, and have a way to alert the human operator if it fails.

To obtain a license, Otto would have had to produce evidence of 10,000 miles of previous autonomous operation and submit a truck for a self-driving test, such as the one completed by Google in 2012. It would also need to post a $5 million bond and file reams of paperwork. Even with all those requirements fulfilled, Otto’s demo would need two people seated up front, one of them poised to take over in the event of a failure.

“Without that license you do not have authorization to drive this on our highway in a semi or fully autonomous mode,” wrote Hurin to Goldwater. “Having just a ‘kill switch’ is only part of the safety requirements…but regardless a license is required.”

Goldwater had a different take on the issue, replying, “The truck does have a DOT [Department of Transportation] number, but is not part of a specific trucking company. Therefore, I think the easy answer is it’s not a ‘test truck’ yet. The efforts of Nevada’s leaders are to try to get Otto and similar companies to move here. Most certainly, when the trucks are testing, all laws and regulations will be followed.”

But Hurin was adamant. In an email sent early the next morning, May 13, he warned Goldwater: “It does not matter if you have another State’s license plate or US DOT sticker on it…if they show the vehicle engaging the technology and it implies or is obvious that Nevada is the site that they have engaged the system for the video, then it is a violation of our current AV testing laws.”

In an email to Hurin, Breslow agreed: “I told him [Goldwater] the same thing you did and that he couldn’t just do this.” When Hill, the GOED official, asked whether Otto might be quickly issued a testing permit, Hurin pointed out the impossibility of achieving that literally overnight.

Otto’s founders were faced with a stark choice. They could submit to the DMV and undertake the laborious process of modifying, testing, and licensing their truck. This would likely take a month or more, and could risk their first-mover advantage in driverless trucking. Or the engineers could continue with their test as planned.

They decided to go ahead with the filming, and over the weekend Otto started to emerge from stealth mode. Backchannel was one of the media outlets offered interviews with Otto on Monday, May 16. I heard from Ron about how Otto was going to revolutionize trucking, improve safety, and help the environment.

When I asked Ron where and how the truck had been tested, he said, “We’ve driven a couple of miles completely driverless without a driver in the backseat — not in California, in some other state. We did that on Saturday and will have video assets to show you later today.” Otto’s launch post on Medium confirmed this, saying, “We recently completed an autonomous demo of the technology on a public highway.”


Photo credit: Otto

When I watched Otto’s video, I could see its truck, without anyone in the front seats, passing roadside signs on Interstate 80 near Reno. I included that information in my story but wondered how Otto had obtained permission to operate in the state without a license. I subsequently filed a public records request to find out more.

In the meantime, Otto’s launch went smoothly. Terri Albertson, director of the Nevada DMV, noticed the Nevada reference in Backchannel’s story and suggested to Hurin that the department post it on their website. His reply was nothing short of explosive: “Nope!!…They have no license, they are testing no matter what anyone tries to tell me and they have no passengers in the vehicle when current law requires two people….This could very well damage Nevada’s Autonomous ‘Pioneer’ image and our relationships with current OEMs and automated companies who already have a license. Otto is driving these vehicles illegally and without the required $5 million bond that is needed to protect our citizens.”

Hurin even suggested the DMV might walk away from autonomous vehicles altogether. “If push comes to shove we may need to propose that GOED…transfer the AV testing and consumer responsibilities to them and no longer have this under the DMV,” he wrote to Albertson. “I will continue to defend this program and its integrity but if the pressures increase, then it needs to be placed under GOED.” GOED is a pro-growth department focused on attracting and stimulating new business in the state. It has not typically been tasked with overseeing or regulating the industries it serves. But the move did not happen, and no legal action was taken against Otto.

Since then, Otto’s trucks have carried a commercial cargo of beer in Colorado and are due to start testing this week in Ohio, all with the cooperation of state officials.

When I contacted Otto for comment, the company declined to be quoted. It walked back from its claims at the Nevada launch, when it had described its truck as operating “completely driverless without a safety driver in the backseat,” and its drive as “an autonomous demo.” Otto now says its truck did in fact have someone in the cab at the time. Otto also says that the self-driving truck is not an autonomous vehicle at all, at least on Nevada’s roads.

Nevada’s regulations define autonomous technology as “technology which is installed on a motor vehicle and which has the capability to drive the motor vehicle without the active control or monitoring of a human operator.” Because Otto’s truck had an engineer watching from further back in the cab, says the company, it does not count as an autonomous vehicle and thus does not need a permit.

Otto’s argument might not hold up if challenged, says Ryan Calo, a law professor who teaches a class on robotics law and policy at the University of Washington: “One question is whether or not monitoring counts if you’re not in the driver’s seat. Often customs wind up informing the law and the custom here is that other testers, like Google and Tesla, actually have a person sitting in front of the steering wheel.”

Bryant Walker Smith, a professor at the University of South Carolina, points out another difficulty: “If the autonomous law does not apply, what does? If the driver is using a laptop or cellphone, that might run afoul of driver distraction laws. And the highway patrol…could very readily conclude that a big rig without someone at the steering wheel is reckless.”

The DMV’s position remains that Otto’s truck is an autonomous vehicle under Nevada regulations, and that the company violated them. But it doesn’t really matter either way. Bizarrely, the autonomous driving regulations come with no penalties for breaking them.

“The autonomous regulations and policies are living, breathing pieces of this cutting-edge technology,” Hurin told me. “The Department will be submitting another set of AV regulatory amendments in 2017. One of the areas we have already earmarked for these regulations is penalties for this type of violation.”

Over the summer, Hurin kept pressuring Otto to apply for a testing license. Finally, he wrote directly to Levandowski: “We need to find out a specific time when you will be submitting your AV Test Application packet….It has been conveyed to me many times in the past that you are working on this, but to date have received nothing…We look forward to our partnership with Otto, which, as you would agree, must be built on honesty and integrity.”

Photo credit: Otto

But Otto was about to pivot once more. The company no longer wanted to test its trucks in Nevada, as it had previously indicated. Instead, in July it applied for a license to operate the state’s first Autonomous Technology Certification Facility (ATCF).

Before any self-driving tech can be sold for use in Nevada, such a facility must issue a certificate stating that it complies with safety requirements, including capturing data prior to collisions, complying with traffic laws, and having the right controls and indicators. The idea is that third-party organizations will take pressure off the state to assess autonomous technologies, providing a smooth route to market.

While Otto was in the process of putting its application together, the company was snapped up by Uber. Levandowski became Uber’s head of self-driving technologies, although Otto would continue to lead the ATCF program in Nevada. Otto had now come full circle, from a tiny startup intent on disrupting taxis in Las Vegas, to maverick autonomous truckers, to a key part of the biggest ride-sharing company on the planet.

The testing facility, a wholly owned subsidiary called Nevada ATCF LLC, received its license from the DMV at the end of September, and is currently hiring staff and remodeling a building in southwest Las Vegas. Otto says that the ATCF will only be used to certify vehicles developed by Otto and Uber.

It is another peculiarity of Nevada’s regulations that a company is not required to test its autonomous vehicles in the state prior to deploying them. The moment Uber licenses its own self-driving trucks or taxis at the ATCF, the vehicles will be ready for public use. “When they decide to sell their technology, they will already be licensed to do so,” Hurin said.

Not making an issue of Otto’s unlicensed demo certainly helped Nevada in the short term. It avoided alienating a leading player in driverless technology and probably helped secure its first autonomous testing facility. But this also came at a cost. Until Nevada clarifies its autonomous vehicle laws and introduces penalties for breaking them, it is difficult to see why any self-driving company should bother with the hassle and expense of certification.

The crumbling of Nevada’s regulatory credibility leaves only California with tough, consumer-focused laws around self-driving cars. In much of the rest of the US, autonomous driving is a free-for-all. Uber taxis are transporting passengers in Pittsburgh, Google’s self-driving prototypes are criss-crossing Texas, and Tesla’s cars are taking over the wheel nationwide, with little official testing or licensing of the technology beforehand.

That might accelerate innovation today, but it means there will be few regulatory tools in place to cope with the many ethical, logistical, and safety challenges that lie further down the self-driving road. And if industry experts can decide to simply skirt the requests of state regulators, the prospects for future regulation look dim.

Otto was nothing less than the perfect Vegas spectacle, equal parts technology, audacity, and showmanship, culminating in an overnight wedding to Uber. As Otto’s founders count their whirlwind winnings, Nevada has shown that well-meaning attempts at keeping tabs on new technology can evaporate just as fast.

Tesla Self-Driving Demo Shows You What the Car Sees

As reported by Engadget: Sure, Tesla's first demo of full self-driving features was intriguing. But did you wonder what it was like from the car's point of view? You're about to find out. Tesla has posted another demo video that shows what an autonomous EV sees as it navigates local roads. As the clip illustrates, the cameras and sensors have to detect many, many different objects at any given moment: road lines, signs, lights, pedestrians and cars are among the many examples. The car even captures the "motion flow" of the environment to get a sense of where it's going.

This isn't a flawless test run. You see the vehicle slow down unnecessarily for people running by the roadside (about 1:30 in), for example. It's also uncertain how well this sort of demo would work in less than favorable conditions, such as snowy weather or nighttime drives on poorly-lit roads. However, it's still impressive. The car is sufficiently aware of its environment that the driver doesn't have to touch the wheel once, even in relatively complex situations.

SpaceX Finds Rocket Explosion 'Smoking Gun'

As reported by Seeker: Investigators probing why a SpaceX Falcon 9 rocket burst into flames on its launch pad two months ago have found the smoking gun. They discovered that the rocket's liquid oxygen accidentally got so cold it became solid. That transformation, in turn, triggered a chemical reaction with a carbon composite container holding liquid helium that is located inside the oxygen tank.

The helium vessel's failure set off a series of explosions that destroyed the rocket and its payload, a $200 million communications satellite that it was due to carry into orbit two days later. The accident occurred as the rocket was being fueled for a routine, pre-launch engine test on Sept. 1.

SpaceX chief Elon Musk called the problem surprising and said it had never been encountered in the history of rocketry.

But it's not completely unexpected. A panel of highly respected aerospace experts told NASA eight months before the accident that the lack of a re-circulation pump aboard the Falcon rocket could be a safety problem.

"We are concerned that there may be insufficient pre-cooling of the tank and plumbing with (SpaceX's) current planned oxidizer fill scenario," former astronaut Thomas Stafford and members of the International Space Station Advisory Committee wrote in a December 2015 letter to NASA.

The letter was released on Friday.

"Without re-circulation there may be stratification of oxidizer temperature that will cause a variation in the input conditions to the oxidizer pump," the letter said.

NASA's space shuttles, for example, used a re-circulation system and pressure lines to help keep liquid oxygen temperatures consistent, Mike Leinbach, former shuttle launch director, told Seeker.

"That's key," said Leinbach.

NASA also has a bit of history with liquid oxygen behaving badly with a composite fuel tank, which was developed as part of the experimental X-33 spaceship in the 1990s.

"The composite tank experienced cracks when fueled in development tests. Composite was used to save weight. Never could overcome it technically, and contributed a lot to the program demise," Leinbach wrote in an email.

Camera view of the inside of the second stage's LOX tank with Helium COPVs.

The Stafford letter raised the lack of a re-circulation pump as part of a broader safety concern about SpaceX's plan to fuel its rockets with astronauts aboard. All previous rockets used to launch humans have been fueled before a crew arrived at the launch pad.

SpaceX is one of two companies NASA has hired to fly astronauts to the space station beginning in 2018.

During a committee meeting last week, Stafford also questioned SpaceX's unorthodox practice of putting helium canisters inside the oxygen tank. The helium, which is part of pressurization system, typically is located outside of the oxygen tank.

SpaceX has not yet said what it will do to fix the fueling system issue, but Musk said Friday said he is aiming to return the Falcon 9 to flight in mid-December.