While the Curiosity Mars Rover was cruising for eight months between Earth and Mars, one of the instruments aboard was measuring the radiation levels encountered along the way. The Radiation Assessment Detector—RAD—was inside the spacecraft, shielded in much the same way as astronauts would be on an interplanetary mission. Now the same instrument is continuing to monitor those radiation levels on the Martian surface. The results from the interplanetary cruise phase of the mission were recently released, and while they don’t rule out interplanetary voyages by humans, they do highlight a significant risk.

Let’s do a Q & A to address some background material and some of the issues involved in order to better understand this.

Where does radiation come from?
On the surface of the Earth, it comes mostly from…the Earth. There is natural radioactivity in soil and rocks and radioactive gas emitted by them. The Earth’s atmosphere shields us from most of the harmful radiation coming from off the Earth. Extraterrestrial sources of radiation start with the Sun, but some of the radiation zipping through space likely comes from beyond our solar system or even from beyond our galaxy.

What does this radiation consist of?
We can most conveniently separate harmful radiation into two categories: electromagnetic radiation and energetic particles. As the wavelength of electromagnetic radiation shortens, its energy increases. Earth’s atmosphere is transparent to visible light, but it blocks the very high-energy radiation given off by the Sun in the form of X-rays and gamma rays.

The Sun also emits energetic particles that are (mostly) blocked by the atmosphere, which also blocks even more energetic particles (rather confusingly called cosmic rays) that can come from extrasolar or even extragalactic sources. When these particles collide with the nuclei of atmospheric gases, they can interact to create a shower of secondary particles that reach the ground even when the original particle does not. As you might intuit, highly energetic particles can do a lot of damage when they encounter frail human flesh.

Why isn’t this a problem for the astronauts aboard the International Space Station?
They are clearly well above the protection of the atmosphere. But they are in an orbit that is still inside the protective bubble provided by the Earth’s magnetic field. Particle radiation (mostly protons—hydrogen nuclei—and electrons, both from the Sun) is deflected around the Earth by this field, and some of it is trapped in the famous Van Allen radiation belts. Both of these belts (inner and outer) are well above the altitude of the International Space Station. Even so, the ISS astronauts’ lack of atmospheric protection increases their exposure. Your exposure is also increased every time you fly in an airplane (less atmosphere between you and the Sun), and people who live at higher altitudes receive more exposure.

When the Sun cuts loose with a solar flare (a sudden brightening and consequent energy release of X-rays and gamma rays), it often follows that up with a coronal mass ejection, a massive “burp” of solar plasma, a dangerous brew of charged and energetic particles. When those particles hit the Earth’s magnetosphere, we are generally well protected, although they may generate some spectacular aurorae near our polar regions.

The only humans ever to travel beyond the protection of this magnetic “bubble” are the 24 Apollo astronauts who flew to the moon and back, three of them twice. They reported seeing light flashes which scientists concluded were probably caused by cosmic rays passing through their eyes.

So how much radiation did the Curiosity instrument measure between here and Mars?
So stated, the question is easy to answer, although I will spare you the details. We know the energies and types of each incident of radiation encountered during that interplanetary cruise. The trickier part is translating that information into how much potential damage it could inflict on human beings.

Turning a measurement of absorbed radiation dose (a clear and unambiguous physical measurement) into a value of effective dose (which measures biological effects) is not straightforward and necessarily involves some assumptions and resulting uncertainty. Effective dose is measured in sieverts (Sv), where one Sv corresponds to a 5.5% risk of eventually developing cancer. A more convenient measure is a millisievert (mSv), one thousandth of a sievert. For some perspective:

• Dental X-ray: 0.01 mSv
• Two-view mammogram: 0.5 mSv
• U.S. annual average from all sources: 3.6 mSv
• Abdominal CT scan: 8 mSv
• Radiation worker annual limit: 20 mSv
• Average for 6 months on the International Space Station: 75 mSv
• National space agencies limit on astronaut career exposure: 1000 mSv

In a paper published in the May 31, 2013 issue of Science, the authors convert the RAD measurements into a dose of 662 mSv for a hypothetical 360-day round trip to Mars. This does not include whatever radiation exposure astronauts might experience on the Martian surface.

Is that significant? Yeah, it is. It’s not just the amount of radiation absorbed that matters, it’s how quickly it is absorbed: whether the exposure is spread out over a decade or more or whether it all occurs in a relatively short period such as the 18 months or so of a round-trip mission to the Martian surface. 1000 mSv over a lifetime represents a slightly increased risk of cancer. After all, an average 80-year-old has gotten 288 mSv over her lifetime. 1000 mSv received over an 18-month period would probably result in clinically detectable symptoms that could impair performance.

The problem is that we know a whole lot more about the effects of some types of radiation than we do about others. In particular, we don’t know much about the effects of massive and energetic cosmic rays, simply because we aren’t exposed to them except in travel beyond low Earth orbit. It seems reasonable to assume, however, that they would not be less damaging than their lighter and less energetic cousins.

Does this rule out human interplanetary travel?
My own take is that it probably does not, but it certainly means that this risk has to be taken into account and dealt with to a greater degree than it currently has. Curiosity flew to Mars during a quiet phase of the Sun’s activity cycle. Future astronauts could well encounter bigger events. Curiosity’s radiation shield was pretty much what is being designed into NASA’s Crew Exploration Vehicle for use beyond low Earth orbit. Increased shielding means increased weight, which means less room for everything else. If I were an astronaut angling for a ride, I would be pushing for reconsideration of this issue.

Mars has neither the Earth’s thick atmosphere nor its strong magnetic field for protection, so more than a spacesuit is needed to keep an astronaut safe from overexposure on the surface. But once you are there, you can take advantage of the natural environment. A shelter constructed from available materials (stone, bricks made from Martian soil) would still require tools for that job. In some locations, however, you might be able to go underground. Several possible openings to caves have been observed from Mars orbit, and these could provide more than adequate protection from radiation.

Space is a hostile environment, and space beyond low Earth orbit is a bit more hostile than we knew. But human beings have a remarkable ability to use their technology to adapt to harsh environments. It just may take us a little longer to surmount the hurdles.

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In each image, the large blue dot is Venus (the brightest of the three), the large yellow dot is Jupiter (next brightest) and the smaller pink dot is Mercury.

Venus and Mercury are closer to the sun that is the Earth, and Jupiter is much farther away. The apparent proximity of these planets is a matter of perspective, of course. Here is a view from above the solar system that shows their positions relative to the Earth. You can see how they will appear close to each other in our sky even when they are millions of miles apart.

May 26,2013

Mercury and Venus will continue to climb in the sky in the days to come while Jupiter sinks into the sun’s glare, so enjoy this while you can!

Have you wondered about how these stars are named? You might guess that some names are ancient, bestowed upon the brightest stars visible throughout human history. But what about something like SAO 101729, or HD 2341? Did you know that Betelgeuse is also known as Alpha Orionis, HD 39801, SAO 113271 AND GSC 129:1873? Let’s back up a little and see if we can make some sense out of this scrambled mess.

The brightest stars were known and named by many ancient civilizations, but the names most familiar to us today are largely from medieval Islamic astronomers and are Arabic in origin. Aldebaran, Rigel, Betelgeuse, Altair, Caph: all of these derive from Arabic, even if they are sometimes mistranslations or mistransliterations. Some names are obviously Greek or Latin: Arcturus, Bellatrix and Polaris betray their naming origins to anyone even vaguely familiar with classical languages.

The first attempts to turn this into something systematic came with Johann Bayer’s Uranometria star catalog of 1603. This grouped stars by constellation, and named the stars from the brightest (alpha) in decreasing order of brightness, with the Greek letter followed by the genitive case of the constellation. (I know that last clause will send shivers of pleasure up the spines of my English major readers.) The brightest star in the constellation of Centaurus is therefore Alpha Centauri, the next brightest is Beta Centauri, etc. The problem with this is that there are only 24 letters in the Greek alphabet, and most constellations have more than 24 stars, even without the aid of a telescope.

John Flamsteed (England’s first Astronomer Royal) to the rescue! Replace the Greek letter with a number, and you are no longer bound by the limits of an alphabet. But, just to keep this interesting, Flamsteed listed stars by their position rather than their brightness. So while Arcturus is easily the brightest star in the constellation of Boötes (pronounced “boo-OH-tees”, not ‘booties”) and therefore earns the designation of Alpha Boötis, its position in the sky means the Flamsteed designation is 16 Boötis. Confused yet?

With the invention of the telescope, the impossibility of naming every single star we can see became apparent. Catalogs only attempted to name all the stars brighter than some minimum value; there are always more stars visible with larger telescopes. Very bright stars are first magnitude, and the dimmest stars visible to a naked-eye observer under a dark sky are sixth magnitude. We’ll limit ourselves here to just the more common designations still is use today, and use Betelgeuse as our example for multiple designations of a single star.

Henry Draper (HD): Covering the entire sky down to ninth or tenth magnitude, this was compiled in the early 1920s by astronomers at the Harvard College Observatory. It numbers stars in order of increasing right ascension (the celestial equivalent of terrestrial longitude; see more here). Those right ascension values were standardized to the 1900 epoch since stars do gradually move relative to each other; the catalog was named for an astronomer whose widow funded the work. Betelgeuse, with a right ascension of 5 hours 55 minutes, is designated as HD 39801.

Smithsonian Astrophysical Observatory (SAO): Published in 1966, this overlapped considerably with the Henry Draper catalog but only contained stars for which the proper motion (apparent motion relative to the more distant background) was known. The sky is divided into 18 ten-degree bands of declination (celestial equivalent of terrestrial latitude), then numbered in increasing order of right ascension within each band. Betelgeuse is SAO 113271.

Guide Star Catalog (GSC): This was first published in 1989 (with subsequent revisions) for use with the Hubble Space Telescope. The intent was to guide the Hubble in accurate pointing by marking the positions of stars down to magnitude 15. The latest version is still being used for that purpose and contains nearly a billion stars. The numbering system is too esoteric to be worth describing unless you plan to apply for observing time on the Hubble. Betelgeuse is GSC 129:1873.

The U.S. Naval Observatory is the source of today’s most accurate and detailed star catalogs. The current standard-bearer is UCAC-3 (the third U.S. Naval Observatory CCD Astrograph Catalog), with compiled information on around 100 million stars. A list of catalogs (somewhat outdated; it lists the four-year-old UCAC-3 as “forthcoming”) is here.

As a practical matter, even the most dedicated backyard astronomer seldom knows and can point out more than a hundred or so of the brightest or most interesting stars. A good way to begin to learn the night sky is to learn the names and locations of the one or two brightest stars visible in each constellation visible from your location. Right now (mid-May), some of the brighter stars visible from Lynchburg around 11 p.m. EDT are Castor and Pollux in the constellation of Gemini in the west; Regulus in Leo; the stars of the Big Dipper (Ursa Major): the “pointer stars” of Dubhe and Merak, the double stars Alcor and Mizar occupying the middle position in the dipper’s tail; Arcturus in Boötes nearly overhead; Spica in Virgo a little below it in the south; Vega (Lyra) and Deneb (Cygnus) rising in the northeast. Start with those, add a few each month, and you are well on your way to learning your way around sky.

A postscript: if you are ever tempted to pay money to name a star, one word of advice.  DON’T!!  Here is why not.

Even if we had not seen the lights, it would have been a wonderful trip. Iceland is an other-worldly place of great beauty. It truly feels at times as though you are on another planet. The mid-Atlantic rift valley that separates the Eurasian and North American tectonic plates runs right across the island.

This isn’t like a state border where you can stand in Texas and Arkansas at the same time; the whole valley is part of the rift.  This is just one of many fissures.

Volcanic craters are a relatively common sight. Not impact craters as are almost all of those on the moon, formed from something smacking into the ground, these are formed by magma erupting from under the ground. This one erupted about 6500 years ago and is filled with ice at the bottom.

You may recall that in 2010 a volcano erupted in Iceland that curtailed air travel to Europe for some weeks. This was Eyjafjallajökull, a name I could hear 100 times and still be unable to pronounce.  This image is from the 2010 eruption; it was quiet during our visit.

Iceland uses the geothermal energy that lies just below the surface for almost all of its power generation and space heating. We were never cold when we were inside. When we were outside it was a different story!

Glaciers and volcanoes, land of fire and ice, very tourist-friendly, almost everyone speaks English, and (pleasant surprise) excellent restaurants in Reykjavik. I highly recommend it!

Why Iceland? And why now? I posted earlier about aurorae, and here I want to give a basic primer in the form of a question-and-answer session.

What are aurorae?
These are glowing lights in the sky caused by charged particles colliding with atmospheric atoms. These particles come mostly from the sun and are trapped in the Earth’s magnetic field. They impart energy to the atoms; when the atoms return to a lower-energy state, they emit light. The aurorae can take the form of a generalized sky glow or of sharply defined features that look like waving curtains. The most we ever see at the latitude of Lynchburg (37.5° N) is this generalized glow when the sun is especially active and the region in which aurorae are most active shifts farther south. When I have seen it, it looks like a late sunset or a fire on the northern horizon.

Why Iceland?
Aurorae are best seen at high latitudes, either very far south or very far north. For someone living in the United States, Iceland is a lot more accessible than the southern equivalent, which is the Antarctic Peninsula. Aurorae mostly occur in a band called the auroral zone, a ring centered on the Earth’s magnetic (not geographic) pole. Here is a “weather forecast” of the ring from last year, courtesy of NOAA (National Oceanic and Atmospheric Administration—your tax dollars at work again).

The ring’s location is not absolutely fixed, but varies between 10° and 20° from the magnetic pole. You can see that Iceland (just to the right of Greenland in this view) is well within this range. For the current forecast, click here.  The northern magnetic pole does not stay in a fixed location, and is currently in the Arctic Ocean as seen in this image generated in Google Earth.

Why do aurorae occur in a ring?
I hope that at some time in your life, perhaps in school, you sprinkled iron filings around a magnet and saw them arrange themselves along the invisible lines of the magnetic field.

The Earth’s magnetic field also exhibits these lines of force.

You can picture the Earth as having a big bar magnet whose poles are deep beneath the surface. The lines of magnetic force dive into the surface and converge at a point far underground. The points on the surface, or in the atmosphere above it, describe a ring rather than a point.

Why 2013?
Solar activity goes through an 11-year cycle, and aurorae are most active when the sun is most active, sending more charged particles our way. One measure of this solar activity is the number of sunspots.

Having gone through a minimum in 2009, these data (current through 2011) show that we are due for a livelier sun, and indeed those predictions have been borne out.

Why April, 2013?
Auroral activity is greatest near the spring and autumn equinoxes. No one is entirely sure why, but part of the reason is thought to be the interaction of the sun’s magnetic field with that of Earth, opening a path for particles to more easily enter the Earth’s atmosphere.

Why the week of April 7th-14th, 2013?
Does this lunar phase calendar give you a clue?

We hope to capture some good images and videos while we are there. Failing that, we hope to make friends with fellow travelers who do so!

It reaches its closest approach to the sun today (March 10), and moves farther away after that. If you look at the diagram, you can see that it also moves farther north each day. PANSTARRS has been visible in the southern hemisphere for some time now, but is only coming into view for those us north of the equator in recent days. This image may help explain why.

The comet is moving from bottom to top (south to north) in this view, which is along the plane of the solar system.

Finally, what’s up with that weird name? Comets are named for their discoverers, so we have perfectly understandable names like Comet West, Comet Hale-Bopp (joint discovery) or even Comet Shoemaker-Levy 9 (the ninth joint discovery by these folks). Increasingly, however, comets are being discovered by automated surveys dedicated to the task. Pan-Starrs stands for Panoramic Survey Telescope and Rapid Response System. Based in Hawaii, where about 75% of the entire sky is visible, its cameras can image all of that in about a week. These images can be compared to the previous ones, and any change will reveal a solar system object (close enough so that its motion is detectable) such as an asteroid or comet.  It’s operated by the Air Force, and its main purpose is to detect possibly hazardous Near-Earth Objects. Every once in a while we get a nice bonus!

Near Earth Objects are strictly defined as anything whose closest approach to the Sun is less than 1.3 astronomical units (AU), one AU being the average distance of the Earth from the Sun. NASA has a page full of information here and you can plot orbit diagrams for them if you like. This application lets you take a 3D look at the orbit, always helpful in really understanding why orbits don’t always really intersect when you think they do. Type in 2012 DA14 to see the orbit of the near-miss asteroid of February 15, 2013.  This nice FAQ page tells how they are found and how the danger of an impact is assessed.

In what truly was a cosmic coincidence, the largest meteor to strike the Earth in more than a century streaked through the Russian skies a mere thirty hours before 2012 DA14’s closest approach. The inimitable Emily Lakdawalla rounds up some informative links, including this great illustration of why the two events are unrelated.

What if a really big one was headed our way? How would we quantify the danger? The Torino scale attempts to do so. For specialists in the field, the eminently geeky Palermo scale is preferred.

More than thirty years ago a team of scientists found evidence of a massive asteroid impact coinciding with the mass extinction that killed off the dinosaurs 65 million years ago. Lingering skepticism about this proposal has pretty much been laid to rest by recent results.

Russia has been the scene of the two biggest meteor strikes of the last 105 years. The Tunguska event of 1908 took place over largely uninhabited territory, which was a very good thing. Slate has collected some historical documents, some (but by no means all) of which require fluency in Russian.

NASA is so cash-strapped that most of the search for potentially hazardous objects is farmed out to private organizations. Preeminent among those is the B612 Foundation. Extra points to anyone who knows the origin of the name without clicking on this link!

NASA sort of has a plan to go to an asteroid. Sort of. Maybe. If they can raise the cash. If the stars align.

Despite Armageddon and Bruce Willis and Ben Affleck, nuclear weapons are not the first choice for saving the Earth from an impending asteroid impact. Phil Plait discusses the general topic, and an MIT student’s clever method involves paintball!

Most asteroids are just rock, interesting to scientists but not of any immediate economic value. A few, however, are metallic, remnants of the cores of small planetesimals that shattered early in the solar system’s history. The value of a million tons of high purity metal should be obvious. Get on board with Planetary Resouces if you want to be an asteroid miner. Check out a paper on just how feasible and profitable this might be before you sign up.

That depends on your time frame. In the short term: absolutely not. There is NO CHANCE that this particular object will impact the earth. Will there be a major asteroid impact in the next century? Very probably not. None of the thousands of asteroids discovered in the last few years, when means of detecting them have greatly improved, show any danger of hitting us in that time period. Will such an object strike Earth eventually? Absolutely yes, it will, unless we do something about it.

For the immediate future, there is asteroid 2012 DA14, which will whiz past us tomorrow more closely than the communications satellites that ring the Earth’s equator. I wasn’t able to find (or to create) an animation of the closest approach, so here are two images from different angles.

This views the Earth from above the North Pole and shows the asteroid approaching from the bottom left. The times are UTC (Universal Coordinated Time) on February 15. Subtract five hours for Eastern Standard Time. Taken by itself, this view is deceptive, because it looks as though the asteroid will come crashing through the ring of geosynchronous satellites! (These satellites orbit at a distance where their orbital motion matches the Earth’s rotation. They consequently remain stationary above a particular spot on the surface.) But take a look at a different view looking at the Earth’s equator instead of the North Pole—from the side, if you will.

This lets us see that the asteroid approaches Earth mostly from the south and moves somewhat in the direction of our point of view as shown here.

Can those of us in the U.S. see it? Only if we plan a very quick trip to the other side of the world! The best visibility will be from Eastern Europe, Asia and Australia. A live webcast is planned from Israel (link is here) starting at 12:30 p.m. EST. My prediction, based on past experience? They will be absolutely slammed with people all over the world trying to link, and you won’t be able to get through. But it’s worth a try.

Here is an earlier post  about asteroid impact dangers. Don’t worry—at least not for another few centuries!

• The angle of the sun, which is related to latitude and the time of solar day (morning, noon, etc.)
• The topography of the surface (an area might be shaded from the sun most of the time)

But while these are certainly the most important, are they really the only influences? Take a look at the figure below, showing the average temperature just below the surface over a two-Mercury-year period.

(From Paige, D.A., Siegler, M.A., Harmon, J.K., Neumann, G.A., Mazarico, E.M., Smith, D.E., Zuber, M.T., Harju, E., Delitsky, M.L., and Solomon, S.C., 2013, Thermal Stability of Volatiles in the North Polar Region of Mercury, Science, vol. 339, p. 300-303. Reprinted with permission from AAAS.)

See anything odd about this? The topographical variations that you would expect are there, and indeed they are the reason that ice can survive on this scorched planet. See if you can find something unexpected, though. (Explanation after the jump.)

See how those cooler temperatures (the green colors that indicate average temperatures around 250 K [-10° F]) extend to lower latitudes around longitudes 90° and -90°? Or conversely, how it gets hotter even at high latitudes around longitudes 0° and 180°? To quote from the paper:

The latitudinal and longitudinal symmetries in…near-surface temperatures result from Mercury’s near-zero obliquity, eccentric orbit, and 3:2 spin-orbit resonance.

Let’s break this down into each of the three influences cited.

Obliquity—This refers to the planet’s axial tilt, how much its spin axis is tilted relative to the plane of its orbit. If the axis is normal (tilted at 90°) to the orbital plane, the obliquity is said to be zero. Mercury’s obliquity is essentially zero.

That doesn’t explain the pattern, however, because with zero obliquity you would expect the temperature distribution to be completely symmetrical around the pole. In other words, the color bands should be in concentric circles.

Eccentric Orbit—Now we’re beginning to get somewhere. Eccentricity is an astronomical term that has nothing to do with the uncle you don’t let out of the house; it refers to the non-circularity of an orbit. An eccentric orbit is an ellipse, and an eccentric solar orbit has a point of closest approach to the sun (perihelion) and one of greatest distance from it (aphelion).

The perihelion distance for Mercury is 46 million km (29 million miles); aphelion is 70 million km (43 million miles). Quite a difference, and quite a difference in the intensity of sunlight hitting the surface. Solar heating would be more than twice as strong at perihelion as at aphelion! But this still raises the question of why heating would be more intense at some longitudes than at others.

3:2 spin-orbit resonance—Here is the answer to the puzzle. What this term means is that Mercury spins on its axis three times in every two trips around the sun. Let’s walk through the diagram below.

Mercury is not oblong of course, nor does it have a big red dot on the surface. The diagram uses these artifacts to make it clear how the surface is oriented in space at each of the numbered locations. Notice that one of two exactly opposite sides of the planet will be always be directly facing the sun when Mercury is at perihelion. And the two sides that are at a 90° angle to those two will always be directly facing the sun when Mercury is at aphelion.  The big red dot is at 0° longitude, exactly opposite it is 180° longitude. When it’s high noon at one or the other of these latitudes, Mercury is at perihelion. When it is high noon at either 90° longitude or -90° longitude (this is designated as 270° longitude by some), Mercury is at aphelion. It gets hotter at higher latitudes around 0° and 180° because when it hits high noon at these latitudes, Mercury is as close to the sun as it ever gets.

Here is a good web site to further illustrate the point. Cool (OK, let’s say interesting) stuff!