Frickin’ Laser Beams: Fact vs Fiction

Last month I spent a week out at Los Alamos National Laboratory vaporizing things with a high powered laser. Now, as I drown in data that I collected out there, I thought I’d take a moment to talk about lasers.

When I tell people that I zap things with lasers, I can almost see the mental images flickering behind their eyes. They tend to look something like this:

Man, I wish.

I hate to burst your bubble, but working with lasers, although very cool, is not as showy as most sci-fi depictions. To help understand why, let’s first talk about how lasers work.

The word laser is actually an acronym for Light Amplification by Stimulated Emission of Radiation, and that actually sums up how they work quite well. There are lots of different types of lasers these days but they all share a few common characteristics. First, you need the “lasing medium” – that is, the stuff that will give off the light. The first lasers used artificial ruby crystals, but now there are lasers that are based on everything from CO2 gas to organic dyes to various semiconductors. The laser I use for my research is a Nd: YAG which stands for Neodymium-doped Yttrium Aluminum Garnet crystal.

Ok, so we have a “lasing medium”, now we need to make it shine. Things give off light when they have electrons in high energy levels jumping back down to lower energies and getting rid of the excess energy as photons. In a laser, the goal is to get something called “population inversion”, meaning that there are more electrons in excited energy levels than there are in the ground state. This is typically done with a flash lamp in a process called “pumping“. By shining very intense light on the lasing medium, the electrons all get excited and the laser is ready to, well, lase.

Diagram of a ruby laser from HowStuffWorks.

Of course, the goal of a laser is to have a nice narrow beam, but if you just have a lump of stuff with excited electrons, the light will be given off in all directions. A fluorescent bulb is a good example of this. A lasing medium acts in much the same way, shining a diffuse light in all directions, unless we do something to it. The secret is to place it between two mirrors, one which reflects all light, and one which reflects only some of the light that hits it.

Initially, the atoms in the lasing medium give off light in all directions, but some of those photons will end up traveling along the laser, bouncing back and forth between the two mirrors. Here is where the laser really starts working. It turns out that when you have photons of a certain energy traveling along through a bunch of atoms with excited electrons that have the same energy, you get “stimulated emission“. The first photons cause the electrons to jump down and emit identical photons. And I do mean identical. Yes they have the same energy (and therefore the same frequency/wavelength/color), but the new photons also have the same phase, polarization and direction as the initial ones. They are completely indistinguishable at the quantum level.

As you might expect, this stimulated emission leads to a chain reaction. Each photon of laser light can stimulate new photons to join it. Since one end of the laser is partially transparent, the result is a narrow beam of light made up of identical photons: a frickin’ laser beam!

Wonderful. Now that we understand how they work, I want to address a few misconceptions about lasers in science fiction and popular culture in general.

1. Laser beams are visible.

With a laser, the idea is to have all of the light going in the same direction, right? That means that if you can see the laser beam from the side, as shown in this picture from Star Trek, and in pretty much every depiction of lasers ever, then something isn’t right! The light is being scattered out of the beam. If you’ve ever used a laser pointer you know that even though it gives off visible (usually red or green) light, you just see a dot where it is pointing. Now, if you shine it at someone who is smoking, or if you use it outside in the fog,  or in a dusty room, you can see the beam because the light is reflecting off of particles in the air (smoke or water droplets or dust).

So, yes sometimes visible lasers in air are plausible because there could be stuff in the way, but visible lasers in space? No way!

There are some other caveats to this also. Not all lasers use visible light! The Nd:YAG that I use for my research and the similar laser used by ChemCam emit infrared light. It is completely invisible, no matter what. This makes it incredibly dangerous to work with lasers like this, especially when first lining up the optics, because you can’t tell if the laser is being reflected around the room! Just because these lasers are not visible doesn’t mean they can’t destroy your retina in a millisecond, so we wear special protective goggles designed for the specific wavelength that the laser emits at all times when the laser is on.

Also: you can’t see the laser beam traveling from the source to the target. It’s going at the speed of light. So all those sci-fi depictions of laser blasts whizzing by the hero’s head like tracer bullets: wrong.*

*Yes, I know, some sci-fi explains this by invoking pulses of plasma and not actual lasers. That’s a whole different can of worms with its own issues. Suffice it to say that most people *think* those blasters, phasers, etc. are supposed to be lasers, so I’m debunking that misconception.

2. Pew pew pew!

That’s not what they sound like. I know. I’m sorry.

Low powered lasers don’t really sound like anything. And can you imagine how annoying it would be if they did? At the grocery store checkout: pew pew pew! Using a CD or DVD player: pew pew pew! Laser pointer: pew pew!

Yes, but the “pew pew” sound really comes from things like Star Wars, depicting lasers used as weapons. So what about big lasers, capable of vaporizing things? Nope. With higher powered lasers, at least the kind I work with, the main sound comes from the flash lamp. It’s sort of a ticking noise, one tick per flash, one flash per laser pulse. Now, when we crank up the power or use something called a “q-switch” to make each pulse shorter and more intense, you get another noise that comes from the laser actually vaporizing things. That noise is more of a “crack” or “pop” noise. In fact, I once popped some bubble wrap in the laser lab while my collaborators were aligning the laser and totally freaked them out because they thought it was the laser. Oops…

The popping noise is essentially the same thing as thunder: a rapidly expanding ball of plasma causes the air to be compressed in a shockwave. Our laser plasmas are tiny, so they just make a little noise. Lightning bolts (plasma formed by electrical discharge) are rather larger, and so is their noise. Many of my experiments are done zapping rocks inside a vacuum chamber, and it’s always fun to hear the noise fade away as we decrease the air pressure in the chamber.

3. Lasers as weapons.

They’re really not that great.

There are a lot of issues with using lasers as weapons. First of all: the optics. For a laser to be useful as a weapon, you would have to focus the light as tightly as possible on the target. De-focus at all, and you might still blind them, but there won’t be much vaporization going on. The precision required for the optics to do this makes a hand-held laser really impractical. The slightest bump or wiggle and all of a sudden your gun is a high-powered flashlight. There’s also the issue of air. Anyone who has looked through a telescope or out over a parking lot on a hot day has seen the shimmering mess that the air can make of an otherwise clear image. Now imagine trying to shine a tightly focused beam of light through that mess and hitting a target. Not an easy task. The military has worked on this to some extent with adaptive optics used for giant plane-mounted anti-missile laser, but it is a significant problem.

The air poses another problem: it absorbs light. In fact, a high enough powered laser can cause the air itself to break down into a ragged line of plasma. I’ve seen this in the lab and it is awesome. The problem is that plasma is full of free-flying electrons, so it absorbs light. A laser strong enough to use as a weapon would also be strong enough to turn the air to a plasma, which would then block the laser from hitting its target. One way around the plasma problem is to use a pulsed laser. As long as the pulses are timed so that the plasma has dissipated before the next pulse is fired, the plasma is not as much of a problem.

I mentioned lightning earlier and that’s relevant here. There is a way to make use of the “plasma issue”, because plasmas conduct electricity. So in theory it would be possible to use a laser as a long-distance taser! The laser would first create a conduit of plasma out of the air, and then with a high enough voltage, an electric shock could be send down the plasma to the target. This would not be a subtle weapon: at this point the lightning analogy is not really an analogy anymore. It would basically be a lightning gun, and would make a noise to match. I thought I was being really clever when I thought of this, but it turns out I’m not the first: the US military has experimented with them.

Another problem with lasers as weapons is the power source. It takes quite a lot of power to make a laser capable of doing damage, and it would probably not be practical for a person to carry such a power source around. I’m playing the video game “Fallout 3″ right now, and the energy weapons use things called “microfusion cells” for ammunition. Right now, we don’t even have power-positive macro-fusion cells, so bullet-sized fusion powerplants are not available yet.

Finally, there is the issue of collateral damage. The thing with light is that it tends to reflect off of things. This means that anyone using a laser weapon better be wearing the appropriate protective eyewear or else their own target is going to blind them. Aside from the practical issues with blindness, the Geneva conventions also specifically forbid laser weapons that cause blindness (in other words, all of them).

In my opinion, I highly doubt that lasers will ever be practical as pistols or rifles. Maybe as large mounted guns on tanks or something. But really, the most likely place for lasers as a viable weapon is space. Without air, the difficulties with plasma creation and turbulence are removed. The issue of power and optics remain, but I could plausibly see a satellite or space station with the stability and power to use a laser as a weapon. It might still be difficult to focus on a distant target, just due to the physical limits on the optics, but the advantage of near-instant travel-time might be of benefit when you’re aiming at a target thousands of km away, traveling at thousands of km per hour.

NASA’s New Budget

The internet has been a whirlwind of wailing and gnashing of teeth, interspersed with the occasional optimistic or guarded response, as space advocates respond to Obama’s fiscal year 2011 budget request for NASA. In case you haven’t heard, the main points of the FY2011 budget are nicely summarized in this overview document:

Increase of $6.0 billion over 5-years (FY 2011-15) compared to the FY 2010 Budget, for a total of $100 billion over five years.

Significant and sustained investments in:

• Transformative technology development and flagship technology demonstrations to pursue new approaches to space exploration
• Robotic precursor missions to multiple destinations in the solar system
• Research and development on heavy-lift and propulsion technologies
• U.S. commercial spaceflight capabilities
• Future launch capabilities, including work on modernizing Kennedy Space Center after the
  retirement of the Shuttle
• Extension and increased utilization of the International Space Station
• Cross-cutting technology development aimed at improving NASA, other government, and
  commercial space capabilities
• Accelerating the next wave of Climate change research and observations spacecraft
• NextGen and green aviation
• Education, including focus on STEM

Cancellation of the Constellation program; and $600 million in FY 2011 to ensure the safe
retirement of the Space Shuttle upon completion of the current manifest.

It’s that last point that has many people upset. Constellation was the ongoing program to build the huge Ares 1 and Ares V rockets to replace the shuttle and return humans to the moon. The program was initiated by the previous administration, but then consistently underfunded. Last year, a blue-ribbon panel of aerospace experts – the “Augustine Commission” -  was called in to assess the direction of NASA’s human spaceflight program, and they found that the Constellation program was “on an unsustainable trajectory” and that NASA was “pursuing goals that do not match the allocated resources”.

Given the Augustine Commission’s report, it’s not surprising that Constellation was canceled, but plenty of people are not happy about it. Unsurprisingly, particularly angry are those who were directly involved in the program and their representatives in congress. I don’t blame them for being upset, and they have every right to complain, but I think that the decision to cancel Constellation was probably the right one.

Don’t get me wrong, I liked the Constellation program. The test-launch last year of the Ares-1X dummy rocket was spectacular, and when I was in NASA Academy in 2006, I got to see some of the early behind-the-scenes work being done. It would have been great to see towering NASA rockets sending our astronauts to the space station and back to the moon. But between the inevitable delays in such a massive project, and the funds falling short of those needed to stay on target, the program really was becoming unsustainable. And worse than that, the delays compounded a serious problem in public interest. It’s hard enough to get people interested in a program designed to repeat what was done 40 years ago with Apollo. Good luck maintaining interest if that program gets drawn out indefinitely due to delays.

The new budget places a strong emphasis on commercial spaceflight, relying on launch vehicles developed by private companies to send US astronauts to the space station. No doubt about it, this is a risky move. No private space company currently has a rocket or spacecraft capable of doing this. But they’re getting close. Space-X said today that they will be capable of sending astronauts to the ISS two to three years after receiving a NASA contract to do so, and for a price of ~$20 million per seat – significantly cheaper than the $50 million price tag of a flight on a Russian Soyuz rocket.

The Space-X "Dragon" capsule could be how US astronauts get to the space station in the near future.

Even if these estimates are somewhat optimistic, it seems likely that commercial providers will be able to send astronauts to the ISS far sooner than Ares 1 would have been able to, and they’ll do it much cheaper. Yes, their cargo capacity will be much smaller, but cheaper launches could lead to more frequent launches, and that leads to a healthy commercial space industry. This change in the way of doing things, although painful for many right now, could have huge positive implications for the future of space exploration if commercial space “takes off”. Bigelow Aerospace and Space X have both mentioned lunar or even Mars missions on the horizon. Healthy commercial space (and therefore lower launch costs) could also lead to more-practical space-based solar power.

Some people have complained that the jobs created by commercial space companies would be nothing compared to those lost due to the cancellation of Constellation, but I think this is a case of short-term thinking. Yes, right now probably more people will lose jobs from constellation than will be able to gain jobs from space companies, but what we’re witnessing might well be the creation of a new industry. In the long run, the job growth could be huge.

Most of the discussion today has been about the Constellation cancellation, but the rest of the budget is extremely exciting. I’m very happy to see that more money will be spent on developing game-changing technologies, such as the VASIMR engine which could reduce the duration of a crewed mission to Mars from years to months. I’m also really excited about the proposed “precursor missions”. These would be missions similar to the Lunar Reconnaissance Orbiter designed specifically to lead the way for human missions to the moon, asteroids, Mars or elsewhere. I was also excited to see the provision for production of new plutonium, which is crucial to power missions to the outer solar system. Whether or not you agree with the decisions regarding human spaceflight, there’s no denying that this budget is great for science.

Research into advanced technologies such as the VASIMR engine could pay off for future human missions.

My main complaint about this budget is that it is somewhat vague on the development of heavy-lift capabilites, and that it does not spell out what the new destinations for human spaceflight will be. It’s clear that the plans presented are based heavily upon the “flexible path” option described by the Augustine Commission, but I’d like to see a series of destinations spelled out if that is the case. It’s probably premature for that, but concrete goals and deadlines would make a lot of people more comfortable.

I was skeptical of this budget at first but the more I think about it the more it makes sense. And even if you don’t like it, what were the alternatives? It was clear that even with increased funding, Constellation would leave a huge gap in access to the space station. And with the current budget crisis, it would have been hard to justify $3 billion per year for a program that wouldn’t accomplish what we wanted very quickly. So the administration took a different approach, increasing NASA’s budget modestly and redirecting human spaceflight funds to commercial providers. This could provide cheaper access to space sooner than Constellation, and meanwhile NASA’s great engineers and scientists can focus on R&D for the next-generation technologies that will lead beyond low Earth orbit. Meanwhile, robotic science will be extremely strong under this new budget, teaching us amazing things about the solar system and the universe.

I’m not the only one who is optimistic about the budget. The Planetary Society has weighed in and they are thrilled with it. So is Buzz Aldrin. Norm Augustine is also supportive, and Phil Plait weighed in in favor of the budget and particularly its emphasis on science. Of course, the real question is what will happen in congress. As I said, many people involved in Constellation are furious about the decision, and their representatives in congress will put up one heck of a fight to keep things from changing.

In the end though, I suspect something very similar to the proposed budget will be passed, and despite the naysayers, I think that’s going to be a good thing for NASA and a great thing for science and space exploration.

Update: NASA administrator Charlie Bolden’s remarks from today are available here. He spells out the changes being made and makes a compelling case for them.

How to Cook Primordial Soup, with Julia Child

(Courtesy of Amanda Bauer’s blog, astropixie)

xkcd Spirit

xkcd (a comic which you should all be reading if you aren’t already) has a nice comic up today about Spirit. Click the image to see the whole thing.

Awesome new Mars flyovers

Check out these awesome flyovers of Mars, generated by Doug Ellison of UnmannedSpaceflight! These are based on digital elevation models from HiRISE, draped with the HiRISE images, so it’s about as close as we can get to actually flying above the surface of Mars. I particularly like the Gale crater one, but I may be slightly biased, having stared at Gale for the past year or so…

So we’ll go no more a roving…

With yesterday’s news of Spirit’s defeat at the hands of the sulfury sands of Mars, I was reminded of this poem. It is by Lord Byron, but I first encountered it in one of my favorite short stories in this blog’s namesake, Ray Bradbury’s The Martian Chronicles. The story is entitled “And the Moon be Still as Bright”, and the poem is “So we’ll go no more a-roving”:

So, we’ll go no more a-roving
So late into the night,
Though the heart be still as loving,
And the moon be still as bright.

For the sword outwears its sheath,
And the soul wears out the breast,
And the heart must pause to breathe,
And love itself have rest.

Though the night was made for loving,
And the day returns too soon,
Yet we’ll go no more a-roving
By the light of the moon.

Lord Byron (1788-1824)

Spirit is no longer a Rover

An animation of Spirit's final attempts to adjust its position in the soft soil of "Troy". Image credit: NASA/JPL-Caltech (click to view if the image is not animating)

In a news conference yesterday, NASA announced that Spirit’s driving days are likely over, but by virtue of remaining stationary, new science possibilities are opened up. Here’s the text from the press release:

After six years of unprecedented exploration of the Red Planet, NASA’s Mars Exploration Rover Spirit no longer will be a fully mobile robot. NASA has designated the once-roving scientific explorer a stationary science platform after efforts during the past several months to free it from a sand trap have been unsuccessful.

The venerable robot’s primary task in the next few weeks will be to position itself to combat the severe Martian winter. If Spirit survives, it will continue conducting significant new science from its final location. The rover’s mission could continue for several months to years.

“Spirit is not dead; it has just entered another phase of its long life,” said Doug McCuistion, director of the Mars Exploration Program at NASA Headquarters in Washington. “We told the world last year that attempts to set the beloved robot free may not be successful. It looks like Spirit’s current location on Mars will be its final resting place.”

Ten months ago, as Spirit was driving south beside the western edge of a low plateau called Home Plate, its wheels broke through a crusty surface and churned into soft sand hidden underneath.

After Spirit became embedded, the rover team crafted plans for trying to get the six-wheeled vehicle free using its five functioning wheels – the sixth wheel quit working in 2006, limiting Spirit’s mobility. The planning included experiments with a test rover in a sandbox at NASA’s Jet Propulsion Laboratory in Pasadena, Calif., plus analysis, modeling and reviews. In November, another wheel quit working, making a difficult situation even worse.

Recent drives have yielded the best results since Spirit became embedded. However, the coming winter mandates a change in strategy. It is mid-autumn at the solar-powered robot’s home on Mars. Winter will begin in May. Solar energy is declining and expected to become insufficient to power further driving by mid-February. The rover team plans to use those remaining potential drives for improving the rover’s tilt. Spirit currently tilts slightly toward the south. The winter sun stays in the northern sky, so decreasing the southward tilt would boost the amount of sunshine on the rover’s solar panels.

“We need to lift the rear of the rover, or the left side of the rover, or both,” said Ashley Stroupe, a rover driver at JPL. “Lifting the rear wheels out of their ruts by driving backward and slightly uphill will help. If necessary, we can try to lower the front right of the rover by attempting to drop the right-front wheel into a rut or dig it into a hole.”

At its current angle, Spirit probably would not have enough power to keep communicating with Earth through the Martian winter. Even a few degrees of improvement in tilt might make enough difference to enable communication every few days.

“Getting through the winter will all come down to temperature and how cold the rover electronics will get,” said John Callas, project manager at JPL for Spirit and its twin rover, Opportunity. “Every bit of energy produced by Spirit’s solar arrays will go into keeping the rover’s critical electronics warm, either by having the electronics on or by turning on essential heaters.”

Even in a stationary state, Spirit continues scientific research.

“There’s a class of science we can do only with a stationary vehicle that we had put off during the years of driving,” said Steve Squyres, a researcher at Cornell University and principal investigator for Spirit and Opportunity. “Degraded mobility does not mean the mission ends abruptly. Instead, it lets us transition to stationary science.”

One stationary experiment Spirit has begun studies tiny wobbles in the rotation of Mars to gain insight about the planet’s core. This requires months of radio-tracking the motion of a point on the surface of Mars to calculate long-term motion with an accuracy of a few inches.

“If the final scientific feather in Spirit’s cap is determining whether the core of Mars is liquid or solid, that would be wonderful — it’s so different from the other knowledge we’ve gained from Spirit,” said Squyres.

Tools on Spirit’s robotic arm can study variations in the composition of nearby soil, which has been affected by water. Stationary science also includes watching how wind moves soil particles and monitoring the Martian atmosphere.

Spirit and Opportunity landed on Mars in January 2004. They have been exploring for six years, far surpassing their original 90-day mission. Opportunity currently is driving toward a large crater called Endeavor and continues to make scientific discoveries. It has driven approximately 12 miles and returned more than 133,000 images.

Model Mars Landscapes!

Check out these spectacular new photos of Mars! It certainly looks like the rovers have stumbled upon some more interesting terrain! The only catch is, these aren’t pictures of Mars at all, they are photographs of models made of, among other things, paprika, chili powder, and charcoal. They are the work of Matthew Albanese, and you need to go check out some of his other photographs. There are steel-wool tornadoes, faux-fur fields, and this spectacular glowing volcano:

(Hat tip to Ann Martin, fellow Cornell Astronomer and blogger at the ALFALFA blog for sharing the link to these pictures!)

Book Review: Revelation Space

I just finished reading Revelation Space, a hard sci-fi space opera written by Alastair Reynolds.

The premise of the story is that in the distant future, when humans have spread into deep space, they discover the remains of the Amarantin civilization that was wiped out just as it discovered spaceflight. The main character, Dan Sylveste is a scientist studying that civilization, driven by an unstoppable compulsion to solve the mystery of what happened to the Amarantin. Meanwhile, Ana Khouri is an assassin who has been hired by the mysterious “Mademoiselle” to kill Sylveste and prevent him from discovering the answer. A third main character, Volyova, is one of only a handful of surviving crewmembers on a starship that holds some of the most powerful weapons ever concieved. The ship is gradually being consumed by a virus emanating from their captain, who is kept frozen in stasis to slow the spread of the infection. Volyova hires Khouri as gunnery officer on the ship, but things get interesting when it turns out that Sylveste is the only person who might have a chance of stopping the virus.

Sound complicated? It is. And that’s basically just the set-up. That little summary leaves out many of the subplots and several other characters. This book is intricate and it takes concentration to understand what is going on. In fact, I really didn’t know what was happening for large portions of the first half of the novel. I am glad I pressed on though, because a lot of the pieces introduced in the first half end up fitting together into a very bizarre but somewhat more comprehensible whole by the end.

One of the main problems is that none of the characters are particularly sympathetic or even likeable. Dramatic events occurred throughout the novel and I never really felt any emotional connection to the characters involved. Heck, two of the characters fall in love and get married and, at least to me, there was no hint of real affection between them at any point. This lack of character connection makes events that already are a bit fuzzy even harder to follow.

I think part of the bewilderment that I experienced was deliberate on the part of the author. The real driving force behind this book is not the characters but the detailed vision of the future that it portrays. Reynolds has created a future full of fantastically detailed technological marvels and he wants you to be a little bit confused but impressed by them. The line between organic and inorganic, computer and brain in this vision of the future is very blurry, and Reynolds makes good use of this. For example, at one point, competing entities do battle with one another after finding themselves both downloaded into one of the main characters’ brains. Ships are capable of manufacturing their own components, or weapons, or other ships, using that universal magic of modern sci-fi: nanotechnology. Spacesuits are no longer bulky bags of breathable air, they are shape-shifting, heavily armed, antimatter-powered intelligent spacecraft in their own right.

What I really enjoyed about this book was that the technology described was always well grounded. Reynolds in an astrophysicist, and it shows. Most of the technology described is so far beyond our current capabilities that it might as well be magic, but Reynolds has enough scientific knowledge to spin some extremely convincing technobabble. He has clearly put enough thought into the ideas to understand some of the more subtle implications. Nothing goes faster than light, suits need to refuel their thrusters, relativistic effects play an important role, etc.

As a scientist myself, the tech-talk wasn’t what got me confused. The politics and the backstory were what left me scratching my head. I wish Reynolds had spent less time on the tech-talk and a little more time fleshing out the cultures and geography and political landscape, especially as they related to the main characters and main events. It doesn’t give away too much to say that there is a political assassination at some point in the book, and I had only the faintest understanding of why it occurred and what it meant. Sylveste starts off as a political figure of some power, but it was never really clear to me why or how he ended up that way.

Complaints about confusion and characterization notwithstanding, I liked the ending. A lot of the seemingly disparate pieces finally do come together, and they form a bizarre and mind-bending conclusion that was nonetheless pretty satisfying.

Bottom line, I would recommend this book to people who like their sci-fi mind-bending and “hard”. For deep emotional connections and characterization, look elsewhere. But if you want a nice long immersion in an awesomely imagined intricate and strange future that still manages to be surprisingly convincing, I’d recommend Revelation Space.

Time is Running out for Spirit Rover

JPL just released this update on Spirit’s status and it doesn’t look good:

The list of remaining maneuvers being considered for extricating Spirit is becoming shorter. Results are being analyzed Wednesday, Jan. 13, from a drive on Sol 2143 (Jan. 12, 2010) using intentionally very slow rotation of the wheels. Earlier drives in the past two weeks using wheel wiggles and slow wheel rotation produced only negligible progress toward extricating Spirit.

The right-front wheel has not rotated usefully since Sol 2117 (Dec. 16, 2009). With the right-rear wheel also inoperable since Sol 2099 (Nov. 28, 2009), Spirit now drives with only four wheels.

Pending results of the latest drive, the rover team is developing plans for their final few attempts, such as driving backwards and using Spirit’s robotic arm to sculpt the ground directly in front of the left-front wheel, the only working wheel the arm can reach. Such activities may take several sols to implement, but time is getting short as winter approaches and the team needs to focus on Spirit’s winter survival.

The amount of energy that Spirit has each day is declining as autumn days shorten on southern Mars. If NASA does determine that the rover will not be able to get away from its current location, some maneuvers to improve the tilt toward the winter sun might be attempted.

I’m on downlink duty for Pancam this week, and I can say that watching each day tick by with, often, just fractions of a millimeter of progress is painful. The team is generally upbeat in the meetings, but there’s a sense of urgency and all eyes are on the calendar as we inch closer to dark days on Mars. Spirit has survived previous Martian winters, but that was with the rover tilted toward the sun to maximize the power available. Right now, Spirit’s tilt is not so good, and I don’t know if we’ll be able to fix it in time.

For more thoughts on the current predicament, head over to the Planetary Society blog.