Adaptive Optics in my Basement

So recently I’ve been putting a bunch of thought into some interesting things to do with the Junior/Senior Optics course this Spring,  particularly the labs.  Given my own areas of interest and research, Astronomy, I was drumming my brain about how to incorporate that into the lab experience.  We’ll be going to the Belk Observatory to do some observing under the guise of telescopes are optics, but quite honestly I don’t want to turn the course into “how to build a telescope.”  Ok, I’ve joked with folks that the course’s labs will be one single project – get the new spectrograph working by semester’s end and they pass, but in seriousness I wanted something neat to do.

Absolutely one of the coolest bits of optical wizardry that has come down the pipe for astronomy is the idea of adaptive optics (AO); by making some form of real-time measurement of how the atmosphere distorts the light from a point source you (well computers) can send commands to movable and deformable mirrors and actually correct for the atmosphere.  This is a hugely great thing since the atmosphere (while nice for keeping us alive) is terrible for astronomy.  The sensors, computers, and deformable optical elements of an AO system, while expensive, are vastly cheaper than the “easy” solution – putting your telescope in space.  That’s one of the reasons why the Hubble Space Telescope (HST)  takes such amazing images with a modest 2.4-meter primary mirror, outperforming much larger aperture ground-based telescopes.  With AO systems the largest class of ground-based telescope can make observations that are much more “HST-like” in their angular resolution.

While I said these systems are vastly cheaper than going to space, they are still really pricey.  Even simple bench-top set-ups are costly, particularly since I’m not looking to even put it on a telescope, but rather have something for my students to poke at.  That’s when I got to thinking, and asked myself “is there a way to make a working model of this?”  And I stumbled upon something.

The heart of a particular class of AO systems is the ability to make a measurement of the wavefront of light that is being observed.  Without any sort of distortion this wavefront could be though of as “flat.”  When the light has to go through some kind of inhomogeneous medium, like our atmosphere, it gets distorted, with “bumps” getting added it.  A classic way of observing this wavefront distortion is through the use of a Shack-Hartmann Wavefront Sensor (SHWS).  The crude explanation of how it work is this:  The deformed light wavefront passes through a very fine grid of small lenses, or lenslets.  Each of these lenslets focuses the light independently of the others (they are, after all, separate lenses).  As long as these lenslets are small compared to the distortions they will make a sharp, in-focus, image.  The image from each of the lenslets will however be at a slightly different position with respect to the lens’ center because they are each “seeing” a slightly different wavefront since they each sample a very small portion of the overall “bumpy” wavefront.  If the distortions change with time (like the atmosphere) the images from each of the lenslets will drift around with respect to each other.  Instruments employing this method not only are used in AO telescope systems, but also in checking the quality of optics, and even examining the human eye!

So what?  These lenslet arrays are also really expensive, but what if I could be a cheap “macro-sized” version of it?  For a demonstration wouldn’t a grid of 9-16 small lenses be able to show this?  If I have an object to image far away, and let that light pass through such an array of lenses I should be able to produce 9-16 small images of the object.  By sticking something that disturbs the light between the object and the lenses I should be able to make the images wander around with respect to each other, just as in a SHWS!

Well, I tried it out.  I found and cleaned 9 small lenses in the physics lab, and brought them home to make a test run.

One of the lenses.

I then found a thick piece of styrofoam that I could make a crude lens holder out of.  The first hole I drilled in ended up being too big, even though the 1.5” bit should have made a hole slightly smaller than the lens itself (~39mm) - too much styrofoam chipped out.  I ended up drilling holes with a 1.25” bit that chipped out enough styrofoam in the end that the lenses could fit in snugly.

The Lens Holder Mark I is ready for action.

I stuck the lenses in, and holy moly, it worked!  I could make a 3x3 array of small images of an object on the other side of the room, and by moving a large, glass, floating candle holder between the object and the lenses, see that the crude wavefront sampling I was trying to get was actually, to a degree, happening!

An LED Christmas tree as seen through the lens array (left) and projected onto the wall (right)

This is way cool – my proof of concept works.  Now I need to make something a bit better to mount the array and I’ll be in business.  In it’s final form for the lab-demo I’m thinking of mounting it parallel to the floor, with the source object high above it and the lenses creating their images on a screen on the floor.  That way I can have an intermediate tray which I could put an irregular medium that the light would have to go through, a tray of water or other transparent fluid, or if it is sensitive enough we might only need a hairdryer or blow torch to stir up the air a bit.

A Comet Kerfuffle

On September 24th a very faint comet was discovered in the constellation of Cancer at a distance from Earth of about one billion kilometers.  Now named C/2012 S1, or colloquially “Comet ISON” it has picked up quite a bit of press in the past weeks.  Looking at the orbit of this comet it certainly looks like it has the potential to be “spectacular” as seen from Earth.  It will pass close (within 1.2 million kilometers) to the Sun in late November 2013, and then pass by the Earth at about 40% of the Earth-Sun distance (~ 65 million kilometers).  The comet already seems to be active, that is showing some fuzziness as a result of gas and dust being liberated from its surface, even though it is still out beyond the orbit of Jupiter.  These two things have a whole lot of folks talking about this being a great comet, one that will put on a heck of a visible show in the night sky.  The idea being that this comet will produce a tremendous cloud and tail of gas and dust as it passes by the Sun, and will shine brightly in the night sky.  Some have even proclaimed that Comet ISON could put on a show similar to that of the Great Comet of 1680. Or, you know, it could be a total dud, viewable by almost none <cough Kohoutek cough>

The Great Comet of 1680 by Lieve Verschuier

Now, how can people on one had be comparing this comet to very visible, spectacular looking comets like the Great Comet of 1680, or even Comet Hale-Bopp, yet at almost the same time caution that we may end up seeing nothing?  It all has to do with how a comet behaves, and how unpredictable that behavior actually is.

What we see of the comet in the sky is not the object itself: The actual comet is a small, dark, potato-shaped object made up of rocks, carbon, and ices.  In 1950 Fred Whipple described comets as “Dirty Snowballs,” and that’s how we still see them, although recent observations, and fly-bys of several comets have me leaning toward calling them“Icy-Dirtballs” to better describe them.  As these objects (the comet nucleus) nears the sun, many of the ices near the surface will vaporize, producing a “coma” or cloud of gas and dust around the comet.  This is what we see when we look at the head of a comet.  This gas and dust interacts with the solar wind and gravity and eventually produces tails for the comet, completing our mental image of what a comet is.

That said, the exact processes and amounts of gas and dust to be liberated are unknown quantities on a comet-by-comet basis.  How much ice is left near the surface from previous passages through the solar system?  How deep can the Sun actually warm the comet?  Will the comet crack, or break apart under the gravitational and thermal stresses as it passes by the Sun?  Will a large amount of dust be liberated with the vaporizing gasses?  All of these questions play a vital role in determining how a comet will look in the sky.  A very gassy, dusty comet that dredges up material from deep within could put on quite a show.  At the same time, one that only vaporizes a thin surface layer of ice could certainly be a “dud” for the eyes (although it could still have quite a bit of scientific value!).

So what will happen in late November 2013 when Comet C/2012 S1 comes by?  No one has any clue, but we may get lucky and have a really nice comet hanging in the sky at the end of that year.  Or not.

Yes Virginia, you can see the flags on the Moon!

One of the all-time questions that people ask about any big telescope is “can you see the flags on the Moon?”  The answer for all ground based, and Earth orbiting (e.g the. Hubble Space Telescope) is no for a variety of reasons: too small at that distance, on too bright of a surface, etc.  In fact the Hubble website has this question (with answer)  in their FAQ!  With the fantastic images from the Lunar Reconnaissance Orbiter Camera (LROC) in Lunar orbit however, we can indeed see many of the objects left behind by the Apollo astronauts.  The landers, footpaths, rovers, and science experiments are all visible in amazing detail.  For example, below is a recent LROC image of the Apollo 11 landing site at the Sea of Tranquility.

LROC image credit NASA / GSFC / ASU 

But what about the flags?  Now in many of the images there seems to be something around where the flags were planted, but it’s really tough to tell anything about them from a single image.  Even with shadows, it is hard to make out if the shadow is from the flag, or from the flagpole!  Quite a number of people, myself included, have postulated that the flags on the Moon have deteriorated away during the 4+ decades that these flags have been there.  Harsh UV, charged particle, and micrometeorite bombardment – all things that our atmosphere and magnetic field protect us from – might have easily destroyed a standard nylon flag, which indeed was all that the Apollo flags were.

That said, you can indeed make out the “flag” site of most of the Apollo landings in the LROC images, and something is there!  But are they the flags, or just the poles with a pile of nylon dust?  A recent round of LROC observations has answered that question:  Despite my pessimism on their survivability, the flags are still there.  Having now observed the landing sites at many angles, the LROC team has been able to look at the shadows cast by the flags, and those shadows not only match those expected by a flag+pole, but change orientation with the sun from image to image!  Below is a recent image of the Apollo 17 site, along with a blow-up of the portion with the descent stage and flag – that shadow is certainly more than just the pole!

LROC image credit NASA / GSFC / ASU

The LROC team has also made an animation out of the still images of the Apollo 12 site, you can watch the shadows move around during a “lunar day” reconstructed from the LROC observations:

Ok, to be fair, the flags might not be intact.  While we can now see that they are still standing, the 40+ years they’ve spent on the Moon may have “bleached” out their colors, but it’s still pretty flipping cool that 5 out of 6 Apollo Flags have been found.  Buzz Aldrin mentioned that he thought he saw the flag get knocked over as he and Neil Armstrong took off from the Moon, and what do you know, he was right.  The only Apollo flag not identified in the LROC images is Apollo 11’s.

Short Break!

Lots of stuff going on - I've got blog snippets about images of Earth from other Worlds, cool results from LRO about the Apollo flags still standing on the moon, and of course MSL Curiosity landing on Mars, including the NASAsocial (tweetup) at the NASA Langly Research Center prior to the entry, descent, and landing portion of the MSL mission.  It is also almost time for courses to begin and I'm putting the wraps on my Fall Solar System course, so that's been where most of my writing time has gone.  I'll have some official babbling on stuff again real soon.

Home library organization

Well, the moving truck finally brought our stuff to us in Virginia, and that meant it was time to organize and shelve all of our books.  How to do it?  By subject?  By author?  By color?  By sum of all ISBN digits? By how loud the book is when dropped from 4.5 feet?  Well being the giant academic nerd that I am, I suggested “why not just use the Library of Congress?”  We did.

Now really, even though my family has a lot of books, we don’t have nearly enough to call what we have “a library,” and so the Libraray of Congress system isn’t exactly a perfect fit, but it made the decision for us, and it was both a little fun, and turned up a couple of surprises along the way.  Here are a couple that stood out while we were sorting and shelving:

Anthony Bourdain, the opinionated trash-talking chef and travel guide of No Reservations (among other shows) has written a number of books about his career and experiences as a chef and world traveler.  Where does the LoC put his books? Why TX – Home Economics of course!

The Physics of Christmas by Roger Highfield gets put into GT - Manners and Customs, as the subject has been determined to be 1. Christmas and then 2. Science.  Also in the G section (subsection GF - Human ecology. Anthropogeography) are the The Worst Case Scenario Survival Handbook line of books.  Except, oddly enough, The Worst Case Scenario Survival Handbook: Work  Which ends up in PN - Literature (General).

Historical creative non-fiction also end up in weird places.  The creative non-fiction The Perfect Storm by Sebastian Junger gets sorted into QC – Physics, and Seabiscuit: An American Legend by Laura Hillenbrand goes into SF - Animal husbandry, Animal science.

Now I’m not a librarian, nor do I have any training in library science.  I’m not knocking the classification system here at all – in fact I may have even been poorly informed by the various search engines I used to look up the LoC classifications for my books that didn’t have them printed on the copyright page.  I’m just an astronomer with a couple bookcases of books who is rather amused by where this classification scheme puts some of them.  With that disclaimed I’ll sign off with one last placement oddity: The Pursuit of Happyness by Chris Gardner (now a movie starring Will Smith) gets filled under HG – Finance.

And then there were five…

134340 Pluto has a new moon, bringing the distant dwarf planet’s collection of satellites to 5.  No official name for the little guy yet, but this 10 to 25 km piece of (more than likely) ice takes about 20 days to orbit Pluto at a distance of about 42,000 km, placing it between Charon (the largest and innermost know moon) and Nix.  With the New Horizons probe on the way to Pluto, the little world’s family of moons continues to grow.

Pluto’s system of five moons.  Pluto and Charon are added into this composite image from a different source – the light from them needs to be blocked in order to make out the much fainter satellites.

Pluto’s first satellite, Charon was discovered by James Christy in 1978.  He noticed a “bump” in the images of Pluto that changed position (and even disappeared) from image to image.  Since then studies of Charon has allowed for much better mass measurements of Pluto, as well as revealing information about the moon itself.  Charon is, in relation to its parent, the largest object we consider a “moon.”   Charon’s diameter is about half that of Pluto, with 12% the dwarf planet’s mass.  Compare that to the Earth-Moon system: our Moon is about a quarter the diameter of the Earth in size with only 1.2% of the mass of the Earth, and our Moon is abnormally large compared to most planetary satellites (e.g. Saturn’s largest moon Titan is about than .02% the mass of Saturn!).  Charon is so massive compared to Pluto that it causes Pluto to actually orbit a point ouside of itself in space.  Really Pluto-Charon could be considered a double dwarf planet (or a binary Kuiper Belt Object) as they both orbit around a point partway between each other.

Now you see me now you don’t: The “bump” that would become known as Charon is visible to the upper right of Pluto in the first image, but not in the second.

Charon and Pluto are also tidally locked to one another, Pluto’s rotation, the rotation of Charon and the the orbit of Charon all take the same amount of time, roughly 6 days, 9 hours.  This situation results Pluto and Charon always “facing” each other.  Charon will always be in the same place in the sky for an observer on Pluto (and the other way around too!).


The rest of Pluto’s family of satellites are more recent discoveries.  The dwarf planet had been under detailed study to prepare for the New Horizon’s mission, which was launched in 2006, and is scheduled to fly-by Pluto and its moons in 2015.   By blocking the light from the bright sources of Pluto and Charon, the region near Pluto may be searched for additional, small and faint bodies.  In 2005 a team conduced a search for companions of Pluto using the Hubble Space Telescope and discovered two new satellites of Pluto, later officially designated Nix and Hydra.  In 2011 a 4th moon of Pluto was discovered, “P4”  Which brings us up to today, with the recent announcement that a 5th moon of Pluto, “P5” had been identified through HST images.  All four of these newer moons are pretty small, with the largest one, Hydra, between 60 to 170 km across, while the smallest moon,  the newly discovered “P5,” being only 10-25 km in diameter.  Quite a bit of the uncertainty in size comes from not knowing how reflective these moons are.  If they have very dark surfaces, they will be larger than if they had very reflective surfaces since these size estimates are based on how bright the sunlight is that has reflected off of their surface and been collected by our telescopes.


Of interest is the relationship that the orbits of Pluto’s moons have with each other.  They are all very close to mean motion resonances with the Pluto-Charon system.  From closest P5, Nix, P4, and Hydra are almost in a 1:3:4:5:6 resonance with the Pluto-Charon.  That means that every 6th time Charon orbits Pluto, P5 will have completed 5 orbits, Nix will complete 4, P4 will have completed 3 and Hydra will have finished one orbit of Pluto.  Details are being studied right now, but it seems as though none of them are in a “perfect” resonance – but the orbital dynamics of the Pluto system are getting very interesting indeed.


In fact a colleague of mine, Dr. Alex Parker at the Harvard–Smithsonian Center for Astrophysics, has made a wonderful demonstration of how close to resonance these moons are.  By translating their orbital frequency into sound, and boosting it by 29 octaves (to be in the auditory range) Dr. Parker has turned the Plutonian orbits into “music”.  One can visit his SoundCloud page: http://soundcloud.com/alexhp-1/plutos-five-moons and hear the slight difference between a perfect resonance, and what we have measured the Plutonian system to be in.  Seriously, check it out – it is super cool.

Off the air for a week or so.

A Sky Full of Rocks will resume its normally sporadic schedule later this month.  I’m moving across the country and will be at the whim of when the cable folks show up to the new place.  That said, cool stuff is happening, from the CERN / Higgs Boson stuff on the 4th of July, to the MSL Curiosity Rover’s landing on Mars on August 6th, and bunch of things between then too.

Flag Day - The Updating!

Turns out I’ve already got an update to my Flag Day! post from a little bit ago.  I had speculated that like Pathfinder, the flag decals on the Mars Exploration Rovers, Spirit and Opportunity, were under the camera-mast, and thus not imaged by the rovers.  Boy was I wrong!

Here’s the flag on the instrument deployment device (IDD), the rover’s “arm” on Opportunity.  All that dust on the instrument is left over from using its rock abrasion tool (sort of a grinder/drill) on the exposed rocks during Opportunity’s 31^st Martial day.

I also missed an obvious “2-fer” on Spirit!  Beyond the decal on the IDD, Spirit also carried with it a memorial to the crew of the Space Shuttle Columbia, STS-107. 

The above memorial plaque carries the US flag, along with the names of the Astronauts who were lost on the Columbia.  If you look closely you can see an additional Israeli flag next to the name of Ilan Ramon, Israel’s first astronaut.

As a bonus here’s a shot of the first “nationally branded” object deployed to the surface of the Moon

These two steel spheres (diameters 7.5 and 12 cm respectively) were carried to the Moon by the Soviet Union’s Luna-2 spacecraft.  Each of these spheres was filled with a an explosive designed to fragment them like a very large grenade, showering the Lunar surface with the little pentagonal pennants that the spheres were crafted out of.  It is really unlikely that any of these little medals survived.  On September 13, 1959 Luna-2 didn’t land gently on the Moon, but rather plowed into it at over 3 km/s.  The energy generated by the impact of a 400kg spacecraft at the speed would generate enough heat to vaporize steel.  One of the ideas behind the explosives inside the spheres was to try and remove some of the impact velocity, and thus allow at least some of them to survive.  It’s possible but, in my opinion, unlikely that they made it through the impact intact.

The first Soviet Moon probe, Luna-1, also carried a similar sphere, but missed the moon (Luna-1 passed within 6,000 km of the Moon on January 4, 1959), and is now in a 450 day orbit about the Sun.

New stuff from really old rocks.

A recent article in caught my (and several other people’s) eye. Chi Ma, et al., “Panguite, (Ti⁴⁺,Sc,Al,Mg,Zr,Ca)_1.8 O₃, a new ultra-refractory titania mineral from the Allende meteorite: Synchrotron micro-diffraction and EBSD,” American Mineralogist , July 2012, v. 97, no. 7, p. 1219-1225.  Now I’m not a geologist.  Most of the “meteorites” that I study are still in space, and I don’t know the author at all.  Why am I excited about it?  Therin lies the story…

On February 8, 1969 thousands of rocks fell from the sky over an area some 300 km² is size near the village of Pueblito de Allende in Chihuahua Mexico.  Now known as the Allende meteorite, it stands as one of the most famous and important meteorites in modern times.  Why is that?  Well Allende’s main claim to fame is that it extremely primitive, or in other words, it is really honking old, and is basically unchanged from the earliest times of the Solar System.  It is known as a carbonaceous chondrite, a class of meteorite that are very dark and rich in volatiles like water and (sometimes) organics.  Some carbonaceous chondrites have been known to sweat water when heated.

The fact that these meteorites exhibit that trait indicates something striking – they were never part of a very large parent body (asteroid).  If that was the case the heat generated by the formation of slamming all these small rocks together, and the mutual heat generated by the natural decay of radioactive nucleotides would have radically changed the composition of these rocks.

Allende in particular is know to have in it many, tiny, little white bits trapped within the generally dark matrix that makes up the bulk of Allende meteorites.  These little white bits are called calcium-aluminium inclusions, or CAIs. 

Above is a slice of Allende with a couple prominent CAIs visible.  It is these CAIs that hold the Solar System’s clock.  They were the (some of) the very first high temperature solids to form in the Solar System.  When someone says that the Solar System is 4.57 billion years old, they are really saying that these first solids (the CAIs) formed that long ago (age “zero” for a rock is when it crystalizes/becomes solid).  Essentially, meteorites like Allende are the left over building blocks of the Solar System.  Put enough together you get Mercury.  Put enough together you get Mars.  Put enough together and you get the Earth.  They are the most primitive bits of Solar System solids, older than any rock we can find on large bodies like the Earth, Mars, or even the Moon.  These CAIs have basically the same elemental composition as the early Sun (excluding gasses of course), and CAI bearing meteorites like Allende preserve these pre-Solar System grains - samples of what the Solar System itself was like almost 4.6 billion years ago!

Now, back to the Chi Ma, et al., 2012 paper.  Take apart the title and you have the story: they found a brand new high temperature mineral, now officially named “Panguite,” in samples of Allende.  The same Allende that fell in Mexico in 1969, was collected and has been studied for over 40 years!  Allende, one of the most famous and well studied meteorites in history still has many secrets to reveal – and that’s one of the things that makes planetary science, astronomy, and science in general, great to me:  It always has some new way of surprising you, whether it is bizarre, unexpected features on the first images of a new world or brand new minerals being found in rocks that have been continually studied for half a century.

Science just plain rocks.  Sorry, had to make the obligatory geology joke there.

Panguite is ready for its close up as seen in Figure 2 from Chi Ma, et al., 2012.

A Black Hole Made of Water?

So while reading through twitter the other day I noticed a few folks posting that if you took the mass of a Black Hole and spread it out through its entire volume you would find that the density of this smeared-out black hole would have the same density of water.  This tripped my astronomer-senses right away.  A 1 solar mass black hole (that is a black hole with the same mass as our sun) has a classical, non-rotating radius of about 3 kilometers.  A 3 km ball of water won’t spontaneously collapse into a black hole, as evident by the the fact that our oceans and Jupiter’s moon Europa are not black holes right now.  While this statement was wrong, it got me thinking – “Could a Black Hole be massive enough for it to be true?”

In a classical sense this problem isn’t too tough to work out, I just need to find how the volume of a black hole scales with its mass and figure out where that gives a gross density of about 1000 kg/m³, the density of water (1 g/cm³ for those using cgs units!).

To make things easy on myself I’m taking a classic, non-rotating black hole, and assuming that the radius in question is the Event Horizon, where the local escape velocity equals the speed of light.   Take the equation of escape velocity v_e:

 

with G being Newton’s Gravitational Constant (6.67×10⁻¹¹ m³ kg⁻¹ s⁻²), M being the mass of the world one is trying to escape, and r the radius of that place.

Setting v_e equal to the speed of light c (3×10⁵ km/s) and solve for r one finds the radius for which the mass is enough to keep even light from escaping.  This is the Schwarzschild Radius.

 

Now this is a pretty small radius for most masses - as it should be since we don’t see ordinary things collapsing into black holes all around us!  A Black Hole with the mass of the Sun ends up with a radius of about 3 km.  An Earth-mass black hole would be about the size of a single playing die, and a person-mass Black Hole would be much smaller than a proton!

Now I can use that equation to find the radius of a Black Hole of a given mass.  To find the gross density rho I need to divide the Mass M by the volume of a sphere of radius r_s. 

Plugging in the Schwarzschild Radius we get

And finally using the the numerical values for all these constants we get the fairly simple

So the gross density enclosed within the event horizon of a black hole scales as 1/M².  Solving for the density of water (1000 kg/m³) I find that a black hole with a mass of about 2.7 x 10³⁸ kg would do it at a radius of about 4 x 10¹¹ km.  This is over the mass of 100 million Suns in a sphere about 90 times larger than the orbit of Neptune.  Huge, but there are Super Massive Black Holes that dwell at the center of galaxies that can achieve this mass.  While own Milky Way Galaxy has a central black hole with the mass of about 4.5 million Suns, too small for the water-density hypothesis, the most massive Galactic Black holes that we have detected check in at thousands of millions of solar masses!  These monsters (under this simple analysis) would indeed have a gross density that would be much less than water.  To visualize this I made up a quick and dirty plot showing Black Hole density on the y-axis and the Black Hole mass on the x-axis.  My simple density equation is the diagonal red line, while the strait line shows the density of water for comparison.  They cross right at a Black Hole mass of 2.7 x 10³⁸ kg. 

So while my astronomer-sense did kick in to point out that the statement about the gross density of a Black Hole being about the same as water is incorrect in general, there are known Black Holes that are massive enough for this to be true!  While the Black Holes left over after the death of a super-massive star or the Black Holes in the center of the Milky Way and Andromeda galaxies are far more dense than water, the super massive Black Holes found at the heart of giant elliptical galaxies, for example NGC 4889 located in the Coma Cluster end up being much less dense than water.

Flag Day!

For Flag Day I pulled out a few extraterrestrial flags that humans have placed on other worlds.

On The Moon: 

Have to start off with the big one, Apollo 11’s flag.  The flags placed by the Apollo astronauts remain the only flags ever planted on the surface of another world by a human being.  Also they are the only ones actually on flag poles!  Whether these flags are still around today is debatable – a number of them were knocked over during lift-off of the Lunar Excursion Module’s (LEM) Ascent stage, as they were placed too close to the LEM.  Buzz Aldrin has mentioned that he saw the flag get knocked over when he and Neil Armstrong left the Moon.

This blurry image is a screen capture I made from of video footage from the video camera left behind on the Moon after Apollo 17 left.  In this case the flag remained standing, and can be seen as the blurry rectangle on the right of the image.  Even that flag however may not really be in that great of shape.  For 40 years they have been exposed to the extreme day/night heating cycles of the Moon, vicious UV light from the Sun, and potentially micrometeoritic bombardment, if the fabric is still there at all it may very well be essentially bleached white!

More flags than just the US flag are on the Moon.  In the above image from the Soviet Union’s Luna 17 Lander (bringing with it the Lunokhod Rover).  The Soviet Union’s flag can bee seen on the right hand side of the image.

While not a soft landing, India’s flag arrived on the moon on the side of the Moon Impact Probe, from the Chandrayaan-1 orbiter.  Japan and China both have also had “hard” landings on the Moon – but I haven’t been able to track down a clear image of national flags on either Japan’s Hiten and Selene/Kaguya probes, or on China’s Chang'e 1.

On Mars

Moving even further away from Earth, here is the flag carried on the body of the Viking 2 Lander on the surface of Mars.  The Viking probes were a pair of orbiter and lander probes that pretty much were the backbone of Mars data until the 1990’s.

In the mid 1990’s, Mars Pathfinder landed on Mars with the flag decal seen above in this pre launch image, but to my knowledge there are no images of the flag on Mars since the camera mast was above the decal and couldn’t see it.  I think the same is true for the Mars Expedition Rovers, Spirit and Opportunity

The Phoenix Mars Lander was able to include this flag here while taking images of the polar regions of Mars where it landed in 2008.

On Venus:

No images of the Soviet Union's flag itself from the surface of Venus, but here it is painted on the side of the Venera 13 Lander, which would eventually land on Venus in 1982, and managed to operate for 127 minutes (about 4 times longer than planned!) in the harsh (460C/900F and 90ATM) condition on the surface of Venus.

And beyond…

Finally, while this flag did not end up on the surface of any world, I think it deserves a bit of special mention here.  This is John Casani, Voyager Project Manager, holding a small flag that was to be folded and sewn into the thermal blankets of the Voyager spacecraft in 1977.  The Voyagers are the two most distant objects from Earth ever made by humans (not counting radio signals!) and are now at 18 and 14.7 billion kilometers from Earth.  You can even follow them on twitter – @NASAVoyager2 updates it’s distance and engineering tasks!

I’m still putting together a final collection, a few other probes which may have placed flags on other worlds that I’m interested in finding out about are NEAR Shoemaker (soft-crashed onto asteroid 433 Eros in 2001), Galileo (burned up in Jupiter’s atmosphere in 2003).  I don’t think there was a flag on the Deep Impact impactor that collided with comet 9P/Tempel, and my understanding is that ESA doesn’t include national flags on their missions (the Huygens lander on Saturn’s moon Titan for example).  If you’ve got info on these or any others hit me up on twitter @rocksinspace.

Killer rocks from space!

I had an excellent topic raised today at the Adler Planetarium's Space Visualization Lab about everyone's favorite killer asteroid, 99942 Apophis (the asteroid formerly known as 2004 MN₄). Indeed there is a small chance of Apophis having a rather intimate encounter with our home planet on April 13, 2036.  By slim, I really mean slim.  As of earlier this year the "odds" of Apophis hitting us were being cited at about 1:250,000.  For perspective, that's about the same odds as rolling all 6's on 7 playing dice.

It all comes down to how Apophis passes the Earth on it’s next close passage (no real chance of impact associated with this one!) in 2029.  If Apophis misses the Earth just right it could pass through an exceedingly narrow gravitation keyhole, which would alter the orbit of the asteroid just enough to make a possible impact in 2036.  That said, it isn’t anything to get worked up about, the odds of the asteroid missing us are probably even better than what are being cited due to how incredibly hard it is to pin down the exact positions of the Earth and Apophis with enough accuracy to really predict the impact.

Now how can that be?  I mean it is just gravity after all.  It is true that just about any second year physics major in the country could take on the problem of the Sun and Apophis, and solve the system such that they could predict at all times where it would be in its orbit.  It is also true that adding just a single additional body to that system (say Jupiter, or Earth) makes it such that no single person can exactly solve the resulting system!  This “n-body” problem is at the heart of the very complex calculations needed to predict if an impact can take place.  Since it can’t be worked out analytically, the orbits of asteroids in the solar system must be solved numerically, through simulating the orbits.  Work out all the forces involved, let the solar system move for a very short period of time, work out the changed forces, evolve the system some more, and on and on.  The resulting solution is only as good as the computer and code that is trying to solve it!

What type of accuracy is needed?  A whole lot.  It takes just 7 minutes for the Earth to travel a distance equal to its diameter.  Every 7 minutes that go by Earth is in a completely new portion of space.  One needs to find out if the asteroid shares that space within that 7 minute window.  To complicate matters even more, to get that level of precision, much more than just point-particle Newtonian gravity must be taken into account.  For example, the Sun isn’t a perfect sphere, and is slowly losing mass.  The masses of the Planets and other asteroids are only known to a finite amount, and our own Milky Way Galaxy exerts a tidal force on the solar system.  Largest of all the uncertainties is the way in which the asteroid itself absorbs and reradiates sunlight (the Yarkovsky effect).  Depending on the thermal make-up of the asteroid and how it is spinning this force can significantly alter its orbit, even of fairly short time scales.

In short, it is really hard to nail down exactly where an asteroid will be at a particular time, which creates enough uncertainty to be unable to rule out an impact for objects like Apophis.  As time goes on (and the projected time errors grow smaller) I fully expect the impact chance to continue to drop, although you’ll never see that on cover of the New York Times, as “Asteroid to miss the Earth – everything is fine” probably won’t move many copies.

Worse case, Apophis is only about 270 meters across, enough to cause some significant damage, but not any sort of Doomsday Scenario.

This figure by J. Giorgini (JPL) shows the results of evolving the orbit of Apophis under dynamical conditions, and assumed properties of the asteroid.  The "Nominal" solution shown in red is the "most probable" position of the asteroid on April 13, 2036 - about 0.3 AU from the Earth.  The blue shows the 3-sigma range of the position uncertainty which does encounter the Earth, indicating the simulation produces a small possibility of impact on this date.  The unknown physical properties of the asteroid introduce even more uncertainty, sliding the whole line of probabilities along the asteroid's orbit.  For more details on this dynamical model check out JPL's Apophis Webpage.