Once beam has circulated stably in both rings, some time next week the LHC team will attempt to collide protons at the injection energy of 450 GeV (a total center of mass energy of 900 GeV). While this is much less than the Tevatron is colliding presently, it could provide some sorely needed initial data for the detectors to do timing and calibration of the various subsystems. There will even hopefully be a few collision events recorded with clear “dijet” structure – collisions where quarks and/or gluons inside the protons hit head on and effectively bounce sideways into the detector, giving two back-to-back collimated sprays of particles. Pictures of such events will be great to see, at long last!

You can follow progress live on twitter: http://twitter.com/cern and I will update this post as I learn more.

10:32 PST: The LHC has gotten beam around clockwise, to Point 6! Woo hoo!

10:45 PST: Magnet quench – should be recovered soon…

11:25 PST: Beam has reached Point 7!

11:30 PST: Point 8! Next beam will be sent past Point 1 where ATLAS is…

11:39 PST Beam all the way around the ring! WOO HOO!! It’s baaaaaack! The LHC Page 1 display shows that the injection probe beam made it more than once around the machine:

11:54 PST: Next goals: do the same with the counterclockwise beam. Will they attempt RF capture tonight? Trying to find out…

13:11 PST: Turns out (no pun intended) they decided to go for RF capture of the clockwise beam rather than probe counterclockwise. They are up to 10 million turns with the RF on! Fantastic!

13:30 PST: Having captured the beam for several minutes, the LHC will now switch to counterclockwise.

14:53 PST: About to go for a full orbit of the counterclockwise beam…done!! Now to RF capture!

15:30 PST: Counterclockwise beam is RF captured! The LHC is operational…colliding beams within a week? Stay tuned.

The LHC machine commissioning will pick up where it left off more than a year ago, and the plan is, if all goes well, to collide beams of protons in the experiments at a center of mass energy of 7 TeV (3.5 TeV per beam) before the end of the year. The luminosity will not be large at first, but should increase steadily with time until next fall, when the long shutdown to retrofit the remaining magnets with new quench detection and helium pressure relief systems begins. By that point the experiments hope to have accumulated upwards of 200 pb^-1 of integrated luminosity. This initial data sample is sorely needed to shake down the detectors and start tuning up the event reconstruction and analysis. And who knows, maybe we’ll see something totally unexpected. (Please, no black hole comments!)

The next main milestone will be beam circulating around the whole ring and captured by the RF system. That should happen by mid-November. Fingers crossed!

As this video shows, astronomical observatories are frequently at risk from wildfires, since both tend to occupy dry remote mountaintops. Indeed, close to seven years ago, one of Australia’s major observatories on Mt. Stromlo was nearly obliterated by the fires that raced through the area:

Thankfully, Mt. Wilson survived this round.

PS. You can find a bit more about some of the ground breaking work that was done at Mt. Wilson along with some terrific old Life magazine photos here.

Fancy titles notwithstanding, I’m by no means a true car nut, so I can’t offer the insider perspective of a real expert. My take is that of an ordinary person who just had a chance to drive an exotic car through the hills north of San Francisco. After considering the experience carefully, my considered judgment could be expressed as follows: pretty frikkin’ awesome.

Let’s get some basics out of the way: the Tesla, with a body based on the Lotus Elise, is a tiny car — a two-seater with a trunk that can at best be described as decorative. And it’s low to the ground; climbing inside is a bit of a process for the uninitiated. Inside, the electronics are all state-of-the-art (as one might expect), but the Roadster is not a cushy luxury car. It’s not uncomfortable, but you’re not being coddled by piles of plush leather. Removing the convertible soft top is a matter of unsnapping and stowing by hand; takes just a few seconds, but we’re not talking about a top-of-the-line Mercedes where there are separate buttons to stow the top, clean your sunglasses, and freshen your martini. The Tesla experience is about the driving; fripperies are for future incarnations.

So you sit down, turn the key to start the engine, and: nothing. That’s to be expected, and should be familiar to anyone who has driven a Prius or other hybrid. The electric motor doesn’t need to be turning when the car isn’t moving, so turning the vehicle on just means some lights come on. Spooky at first, but you get used to it.

Actually pulling out into the road and driving is a different story. There are basically three things that distinguish the Tesla driving experience from that of your typical Ford Taurus or what have you. First, as you may have heard, the Tesla doesn’t believe in a little thing called a “transmission.” Technically, there is a transmission, but really it’s just a reduction mechanism that translates a certain number of motor revolutions to a certain fixed number of tire revolutions — there are no gears, so there is no shifting, manual or otherwise. The original plans called for a two-speed transmission, but it proved unreliable, so they said screw it, let’s just have one gear. As a result, the rate at which the motor is turning is directly proportional to the rate at which your car is moving. That includes reverse; when you’re backing up, the motor is spinning in the opposite sense from when you’re moving forward. In a conventional car with an automatic transmission, there can be a bit of a delay between when you push down on the accelerator and when you actually accelerate, as the car tries to decide what gear it should be in. No such hesitation in the Tesla.

The second thing, which you may not have heard, is that there is no power steering. I don’t know whether that was a matter of cutting down on weight, or whether it was just thought that power steering wouldn’t be keeping it real. But despite its diminutive profile, the Tesla is not a light car, coming in at about 2,700 pounds — a third of that in the form of batteries. (The Elise, in comparison, is only about 2,000 pounds; but a Mazda Miata comes in at 2,500 pounds and a BMW Z4 at 3,200 pounds, so the Tesla isn’t unreasonable.) To those of us who have gotten used to having the car practically steer for us, the Tesla is a bit of an adjustment. But the adjustment happens quickly, and it’s very much in keeping with the sporty nature of the car — you’re here for performance, not coddling.

The single gear and the lack of power steering combine to create an effect I hadn’t really anticipated before the drive: a visceral connection between the driver and the ground. It’s hard to imagine a driving experience that is on the one hand that fast, and on the other hand features so little mediation between what you do at the controls and how the car responds. The engine turns, and the car zips along, at precisely the speed you tell it to, no more, no less; and the wheels turn at an angle precisely proportional to the attitude of the steering wheel in your hands. You are in control.

And — to come to the third crucial distinguishing feature — you’re in control of a lot. This puppy is fast. By which I do not mean, as physics training might lead you to suspect, that it travels at a high velocity. In fact the car is electronically regulated so that its maximum speed is 125 miles per hour (and I didn’t approach the limit, don’t worry). That’s fine, because despite the emphasis on sportiness, this is a car that is meant to be driven on actual roads with actual traffic laws. But because state legislatures aren’t required to pass any calculus exams, our rules of the road feature speed limits, but not acceleration limits. And it’s really acceleration that gives a car a feeling of being “fast”; when you push down on the accelerator, how quickly do you speed up?

In the Tesla, the answer is: as quickly as you could possibly want to accelerate outside a racetrack. The technical numbers tell us that the Roadster goes from 0 to 60 in 3.9 seconds. (A Porsche Boxter does 0-60 in about 5 seconds.) All I can say is, it’s incredibly, breathtakingly fast. Punch it, it’s gone. Only after driving this car did it occur to me that maybe there should be acceleration limits written into the traffic laws; being able to accelerate faster than this strikes me as very plausibly dangerous. Once you adjust to the parameters of the vehicle, the combination of the incredible power and the unmediated response to your actions yields a driving experience that is pretty darn breathtaking.

There are a couple of other idiosyncrasies to remind you that this is not your father’s Oldsmobile. Although the Tesla is utterly silent while standing still, it definitely does make noise while moving. Not very much noise, but what comes to mind is less a Ferrari and more a muffled jet engine. I presume this is because the engine is turning notably faster (perhaps 7,000 RPM at highway speeds, I didn’t check carefully) than in an ordinary car. The other thing is the regenerative powertrain. When you take your foot off the accelerator, the car slows down perceptibly — it’s taking some of your kinetic energy and using it to recharge the batteries. So you don’t need to put on the brakes while going downhill. (Sadi Carnot would have something to say about this, but don’t worry — you’re still creating some entropy, just achieving something closer to theoretically maximal efficiency.)

In other words: the Tesla Roadster is an extremely fun car. But is it practical?

Well, it’s not practical for most of us to actually buy — the sticker price is on the order of $120,000. And you’re not going to take four kids and a dog to the soccer game. Nor are you going to take a road trip across the country; under ordinary driving conditions, the Tesla gets about 200 miles between charges.

But all that is okay. The vast majority of driving is not done on long hauls or with a car packed full of people; it’s done by a single person on relatively short jaunts. For those purposes — commuting to work, running errands, going to meet friends — something like the Roadster is just about perfect. There’s no reason to lug around two tons of car with room for six when there’s only a driver inside. Very few people would want a Tesla as their only car, but if they had two cars, it would be the one they were driving most of the time. And if you can afford to buy the thing in the first place, you can afford another car.

More importantly, in its current incarnation the Tesla is not about practicality; it’s a proof of concept. Electric cars have long suffered under the image of being under-powered and super-short range, needing to return home every 50 miles for a lengthy recharge. The Tesla blows those stereotypes out of the water, and that was the idea. Here is a car that is environmentally conscious, but is no sacrifice once you’re behind the wheel. It proves that an electric car can have a decent range and be easily recharged. And let’s face it — it’s hot.

Not that it is quite plug-and-play. The Tesla is powered by an array of about 7,000 lithium-ion batteries, not too different from what you have in your laptop computer (but with special care taken to ensure long life, no overheating, and no explosions). You could, in principle, plug the recharger into an ordinary 110 volt outlet already in your home; problem is, a complete recharge would take about 30 hours. (If you’re only driving about 30 miles a day, you wouldn’t need anywhere near a complete recharge.) If you’ve gone this far, however, you probably want to install a 220-volt receptacle; most homes are already wired for the increased voltage, but you have to spend a hundred bucks to install the appropriate unit. Now the car can be fully recharged in about 3 1/2 hours. In other words: come home, plug it in overnight, drive away the next morning.

Of course, even if we all were driving Teslas, the world would not suddenly transform into a green utopia. That electricity has to come from somewhere, and right now it mostly comes from burning dirty fossil fuels like coal. I’ve read that, under the current setup, driving a mile in a Tesla is just a little bit better in terms of total carbon emissions than driving in an ordinary car; you’re using less energy, but it’s coming from a dirtier source.

The system is going to have to change. We can’t keep burning petroleum in our individual cars, nor can we keep burning coal to get our electricity. The point is that it’s fairly easy to see how to get electricity form sources other than coal — we’ll need a portfolio of nuclear, solar, wind, etc. But the cars are going to have to go electric, there’s little question about that. (Believe Steve Chu if you don’t believe me.) A major challenge is going to be upgrading the electrical-power transmission grid; T. Boone Pickens recently had to abandon an ambitious plan to build a giant wind farm in Texas, after he realized that he didn’t have the resources to carry the power to the people who actually wanted to use it. But doing that upgrade is not optional, and it’s a matter of willpower rather than technological breakthrough.

Tesla obviously isn’t the only company that’s caught on to the promise of electric cars, although the Roadster currently blows away the competition in terms of speed, acceleration, and range. The much-hyped Chevy Volt from GM is actually a plug-in hybrid, which includes an internal combustion engine to help the electric motor along when you want to go fast or far. That may be the wave of the near future, but I suspect that 100% electric is the medium-term solution. (Until we all have personal jetpacks, or the Singularity arrives.)

Still, it would be nice to have a car more people could afford, and which could hold a couple of friends and/or offspring as well as the driver and a single lucky passenger. Behold: the Tesla Model S. Scheduled for first delivery in late 2011, this will be a true four-dour sedan, with a range of up to 300 miles. Still not cheap; estimated cost is around $60,000. But that’s completely competitive with executive-class sedans from Mercedes, BMW, or Audi. The Model S won’t put an electric car in everyone’s garage, but it will help “normalize” the idea of owning one — you’ll start seeing them on the streets in increasing numbers. And after that, there are hopes to offer another model for less than $30,000. Still not cheap, but getting there.

The future belongs to electricity. The good news is, it’s a pretty sexy future.

Doesn’t that word conjure up the majesty of space exploration? The triumph of human drive and ingenuity?

Or perhaps, it makes you think of an automated laser-guided milking machine?

Seriously. Wrap your mind around that. “Automated laser-guided milking machine”.

Cow walks in when it decides it’s ready to be milked. Sensors read a tag around the cow’s neck to determine if the cow is indeed ready to be milked. If so, the machine launches a veritable Pink Floyd Lasarium around the udder, locating the teats, which are then cleaned and hooked up to the milking units. Sensors then disconnect when the milk flow drops, and the cow goes on its way.

Lasers and cows. Two fine things that I never thought I’d see together.

(and below, an informative video, if you really, really care)

Government is the appropriate agent for certain forms of collective action: roads, public schools, national defense. It’s also good for big-picture things without immediate financial payoff, like support for the arts or basic scientific research. It makes perfect sense for the government to shoulder the burden for developing the technologies to get us into space, and it will continue to make sense for them to play an active role in astronomical research in space. But for commercial purposes, like launching satellites, it ultimately makes a lot more sense for space travel to be a private-sector enterprise. We’re on the brink of seeing it happen.

SpaceX is a private company founded by Elon Musk, who previously co-founded PayPal and the electric car company Tesla Motors. For a while now, SpaceX has been developing reusable launch vehicles and space capsules. They’ve been awarded a contract from NASA to take over re-supplying the International Space Station after the Shuttle fleet is mothballed next year. And they’ve had one launch that reached orbit, but also a few failures; until yesterday, they hadn’t succeeded in putting a satellite into orbit.

But now they’ve done it. I was watching on live webcam last night as the Falcon 1 rocket launched a Malaysian satellite into orbit.

It’s incredibly exciting, but just the beginning. The idea behind the Shuttle was to make trips to orbit cheap, reliable, and routine; it failed spectacularly on all counts, and NASA’s capabilities and plans for space flight have become somewhat disjointed (while its science missions continue to have amazing success). Hopefully we’re moving past the point where we have to rely on the government to get us to space.

I was reminded that I still owe CV readers a discussion of gravitational waves, following-up from an ancient post on their convoluted theoretical history. While the theoretical community was arguing about the existence of gravitational waves, the observational community was essentially non-existent. The wave strengths expected at Earth are extraordinarily weak, with the most promising sources being the inspiral and merger of stellar-mass compact binary systems (e.g., neutron stars and/or black holes). It took great courage and vision to propose instruments to detect the waves at all. One of the pioneers of gravitational wave detection was Joe Weber, who first invented and built so-called Weber bar detectors. As the gravitational waves pass through a large cylindrical bar of material, they excite resonant modes of the bar, which might then be detectable. In 1969 Weber published an article in Physical Review Letters announcing the detection of gravitational waves by noting coincidences between two bar detectors separated by a thousand kilometers. These observations generated tremendous excitement, especially given that they suggested wave strengths greatly in excess of what was expected. Unfortunately, as other experimenters built ever more sophisticated follow-up detectors, they were unable to reproduce his findings. Over three decades later, and with many orders-of-magnitude improvement in detector sensitivity, gravitational waves have yet to be directly detected. An unfortunate side-note is that Weber continued to claim that he was seeing gravitational waves, even in the face of compelling counter evidence, right up until his death in 2000. Weber was an experimental master (having independently invented the maser and the laser), and is widely credited as the father of gravitational wave astronomy, but in his last decades he was an outcast of the very community he helped found.

A profound development in gravitational waves detection came in 1974 with the discovery by Hulse and Taylor of a binary pulsar. This system consists of a 59 millisecond pulsar in orbit with another star, with a period of 7.75 hours. The pulsar provides an exceedingly precise clock, allowing us to measure the spin-down of the binary system due to the emission of gravitational waves (using the same quadrupole formula mentioned in the previous post). Theory and observation agree spectacularly well, and Hulse and Taylor were awarded the Nobel prize for this indirect detection of gravitational waves. The community is now breathlessly awaiting the first direct detection of gravitational waves. So why am I nattering on about all this? Because of this:

This is a sensitivity plot of LIGO, the Laser Interferometer Gravitational Wave Observatory. LIGO is composed of two power-recycled Michelson interferometers with 4-km long Fabry-Perot arms, located in Hanford, Washington and Livingston, Louisiana. Each curve shows the noise floor, with LIGO sensitive to sources falling above the curve. The curves represent official science runs, and the steady progression shows the improvement since 2002 (1st science run). LIGO has been in development for decades, and has in the last few years (the red curve) reached its original design sensitivity (the solid black curve). LIGO has finished a year-long science run (S5), which constitutes by far the deepest look we’ve ever had at the gravitational wave Universe. It hasn’t seen anything yet; only upper limits. (At least, that is the public stance. Given the convoluted history detailed above, there will be a long period of double and triple checking before any sort of public announcement is made. Although it’s hard to imagine rumors of a first detection won’t leak out, and my rumor well is dry.) At the moment LIGO is being “Enhanced” (a factor of 2 improvement), with the installation of more powerful lasers. It should be back up and running within the year, will run for a year, and then will undergo a major upgrade. By 2014 LIGO will come back online at “Advanced” sensitivity (another factor of 5, which translates into a factor of 1000 in volume compared to today), at which point the first direct detection of gravitational waves is widely anticipated.

There is lots to say about LIGO, but I’d like to focus on one point: the scale on the y-axis of the plot above (which represents strain, the fractional change in the length of the LIGO arms). LIGO is sensitive, over a wide range of frequency, to a strain of better than 1 part in 10²². In other words, it measures changes in the relative length of its 4km arms to better than a thousandth of the size of a proton. This plot should absolutely blow your mind. If not, perhaps I’m being too abstract? This is the equivalent of monitoring changes in the distance between New York and San Francisco to better than one ten billionth the width of a human hair. LIGO is a technical tour-de-force. It is one of the most amazing instruments humankind has ever built.

N.B.: In the comments, Brian137 points out that LIGO will be back on starting tomorrow (7/7/09) for a month-long run! (From the LIGO blog.) And nicolas points out that I was remiss in neglecting to mention Virgo, which is a French/Italian GW detector currently operating in Italy. It is easily as impressive as LIGO, since it achieves similar sensitivity with 3km arms (instead of 4km for LIGO).

And, living here in California, which a friend of mine notes is a beautiful land with a decidedly savage side, it’s not bad to have a means of communication that doesn’t depend on the grid, be it the electric grid or the Internet/phone grid. My in-laws live in Pacifica, south of San Francisco, which is hemmed in on all sides: the ocean to the west, mountains to the north and south, and the San Andreas fault to the east. A big earthquake could easily isolate them from the rest of the peninsula. So my father in law (N6FG) helps run a 2-meter repeater on a nearby mountaintop; he and and my mother-in-law (K6IIP) participate in local emergency response groups.

A friend of mine joked that amateur radio is the original social networking tool. (Well, unless you count the postal service.) Early in the last century, when radio was young, the advent of high-power vacuum tubes made it possible for amateurs to build transmitters that allowed them to talk to other hams all over the country, and around the world, ionospheric conditions permitting. At night, when the lower layers generated by solar radiation dissipate, a vast electromagnetic mirror called the F layer forms several hundred miles up. Signals from the surface can bounce off this mirror essentially all the way around the planet. Hard-core DXers still go to great lengths with antennas and legal-limit (1500 watt) transmitters to make contacts with Morse code. (And then there are the truly crazy ones who go on expeditions to remote islands off Antarctica solely for the purpose of making nearly 100,000 ham radio contacts all over the world.)

With the advent of transistors, and, more recently, digital signal processing, modern ham radio rigs can range from the relatively simple and inexpensive (like mine) to unbelievably complicated units going for over $10,000. So it’s one of those hobbies that you can get into at a number of different levels, you might say.

Electromagnetic bandwidth is limited by its very nature. And of course the governments of the world have to regulate who gets to use what portion of the available spectrum for what. The International Telecommunication Union coordinates this across borders. It’s pretty fascinating to study the US band allocation chart. When you do you’ll see that amateur radio has been granted little slices in a lot of different ranges: 160/80/60/40/30/20/10 meters, (the HF bands) and 6/2/0.7 meters (the UHF/VHF bands). By far the most popular among hams is the 2 m band, 144-148 MHz. The reason is that with a low power mobile unit, it’s easy to make contacts on 2-meter (144 MHz) FM repeaters which are scattered all over the country.

Repeaters listen on one frequency, and re-transmit the signal on another. This means that someone with a signal too weak to pick up with his or her mobile unit directly can be heard by the repeater, and then their signal can be sent out at much higher power, covering a big range, typically 50 miles or more. Sort of like a big megaphone. Often repeaters are run by local amateur radio clubs, and access is left open to whoever wants to use it. The clubs run weekly “nets” where all the members check in, news and announcements are shared, and so on. Ham clubs provide comm support for events like bike races, festivals, and so on, and there are also more official emergency radio amateur emergency services (RACES) that some folks help with.

There’s no “broadcasting” on amateur bands, and only licensed folks can use them legally. (Though there are a lot of renegade truckers out there, sick of the CB chatter, who are rogue 10 meter amateur band interlopers.) On the amateur bands one endeavors to make two-way contacts, and you are required to identify your station in any such contact, periodically.

The problem with UHF/VHF, though, is that propagation is limited to about 100 km at the most, typically. For making more distant contacts, you need the longer wavelength HF bands. The longer the wavelength, though, the less available frequency bandwidth there is. Building an antenna for 80 m operation, for example, is non-trivial; even a half wave dipole is 40 meters in length!

But then there’s the 40 m band, long enough wavelength for distant contacts, and short enough that it’s not hard to build a simple dipole antenna. On my roof I’ve built a simple one from parts I got from Home Depot and some coax cable from eBay, and a little gizmo called a balun I got from the radio store. I got a HF transceiver from my colleague and friend Tony Tyson (KQ2I) who has been a ham since before I was born. I put it all together, and have pulled in signals from Georgia, Quebec, Indiana, and someone speaking Spanish (Mexico?). I also have a half wave dipole for 2 m, and check in on the local nets.

But I have made no contacts yet! Why? My license needed upgrading…so I just took the ham General Class exam last Sunday, passed, and now can legally transmit on that band, though my rig only goes to 100 W. (Anyone interested in making contact? I need to lose my 40 meter virginity.)

Actually I have not done all that much with ham radio yet, but in fact the scientist in me is intrigued by the possibility of working on a large radio array to measure the highest energy cosmic rays. The Europeans have an advanced concept for a large scale radio array, E-LOFAR, which can actually form an image of a particle shower produced by an incoming proton with an energy of 10²⁰ electron volts (that’s 10 million times the energy of an LHC proton). The large Pierre Auger air shower array, in Argentina, has been recording such events with water tanks scattered over a county-sized region, combined with atmospheric fluorescence (AF) telescopes (mirror arrays pointed at the sky). Radio arrays have the potential to cover a much broader area, for a lot less cost. It also works 24 hours a day, rain or shine, unlike AF. Other people are proposing to sink antenna arrays into the ice near the South Pole; my bet is that this will not yield as big a sensitive area, but who knows?

Anyway, as the hams say, “73″ to you all…

Could a meteor have brought down Air France 447? Today we are starting to see reports that there actually may have been a meteor:

However, both pilots of an Air Comet flight from Lima to Lisbon sent a written report on the bright flash they said they saw to Air France, Airbus and the Spanish civil aviation authority, the airline told CNN.

“Suddenly, we saw in the distance a strong and intense flash of white light, which followed a descending and vertical trajectory and which broke up in six seconds,” the captain wrote.

Obviously for any given flight the chances are very, very small that a meteor will bring down an airliner, but as Hailey and Helfand pointed out in a letter to the NYT in 1996, the correct question to ask is this: “What is the probability that, for all flights in history, one or more could have been downed by a meteor?” They concluded that there was a 1-in-10 chance that this could happen…let’s use their logic, brought up to date somewhat, for 2009, for Flight 447.

Helfand, an astronomer, is presumably the one who estimated that “approximately 3,000 meteors a day with the requisite mass strike Earth”. This is a difficult number to get. How much mass? How fast does it need to be moving? But let’s assume that this number is correct; it translates to 125 meteors per hour.

Next we need to know the total number of flight hours at altitude for all commercial planes. In 2000 there were about 18 million flights per year. Clearly in the past 20 years (which we’ll take as our reference, since it spans 1989-2009, with both flights 800 and 447) it was not always so…but let’s take a guess that the 18 million figure is roughly correct for that 20 year period. That would yield 360 million commercial airline flights from 1989-2000. Hailey and Helfand assumed that each flight was two hours in duration. Again, a tough number to find on line, so we’ll take it at face value, giving us 720 million flight hours in our reference period.

They also claim that if there were 3500 planes in the air at any time, this would correspond to covering two-billionths of Earth’s surface. Now the earth’s surface area is 5×10¹⁴ m². Using my trusty HP-15c, I get that this would imply an average target area for a commercial airliner of 291 m², which is reasonable. Each plane, that is, covers 5.7×10^-13 of Earth’s surface. If a meteor hits the earth it has that probability of hitting a given plane on average.

So, in our reference 20-year period we have 720 million hours of flight time, times 125 meteors per hour, times 5.7×10^-13 = 0.051, which we can take as the average number of airliners struck by meteors in the period 1989-2009. That’s a one-in-twenty chance of some plane going down for this reason in that 20 year period. Extrapolating to all flights ever would require a better estimate of total flight hours, but it’s not twenty times the number in the past 20 years, for sure – that is, it’s not yet close to one.

Obviously there are a lot of uncertainties in this estimate; perhaps a factor of two from the number of meteors of sufficient mass per day, the average flight duration and number of flights?

Anyway the meteor idea is not crazy, though not likely. The weather seems more likely to be at the root of the tragedy…but we may never know. One thing, though, is clear: if we keep flying big planes at high altitude, eventually one will get hit by a meteor.

The Atlantis mission is one of the finest examples of what excites people about manned space flight. It was an incredible success, from start to finish. And we can hope for a decade of mind-blowing science to prove it. It helps that Hubble is perhaps the most remarkable scientific instrument ever built. (Okay, the LHC is pretty cool too, but it doesn’t produce pretty pictures. And it isn’t working yet.) Without a doubt, the fact that Hubble was “serviceable” has played a large role in its success. With its initially flawed optics, Hubble would have been a catastrophic failure. It took a servicing mission to fix it, and provide us with an unparalleled scientific instrument. And now it is as if we have launched a brand new, state-of-the-art Hubble. These appear to be strong arguments in favor of manned maintenance of space telescopes.

But this mission is historic for another reason. It is likely to be the last time in our lives that human beings go to space to “fix” an orbiting scientific instrument. [Granted, prediction is very difficult, especially about the future.] It is incredibly expensive to send human beings up to fiddle on stuff in space. Furthermore, Hubble was placed in low-Earth orbit (600 kilometers up) to facilitate these sorts of repairs, which is a sub-optimal location for a telescope. The successor to Hubble, the James Webb Space Telescope (JWST), will be placed 1.5 million kilometers away at Lagrange point 2 (WMAP is already there, and Planck and Herschel are on their way). It will be a very long time before humans venture out that far from Earth. JWST is not being designed to be repaired or upgraded; it’s a one-shot deal. The additional costs of sending humans into space far outweigh the benefits. If you want to know whether there’s water on Mars, you send a rover. You do not spend many orders of magnitude more to send a human to dig with a shovel. Although the space shuttle is undeniably cool, and this latest Atlantis mission was astounding, manned space flight is not the economical way to do science.

With the successful landing of Atlantis we celebrate a transformed Hubble telescope. And we mark the end of the current era of human beings tinkering with telescopes in space.