(This post has been edited slightly from the original to include the MESSENGER mission to Mercury and to correct a misstatement about galactic black holes.)
As the end of calendar year 2011 approaches, I thought my readers might enjoy a look back at some of the key events of the year in astronomy. The usual caveats apply: this is by no means an exhaustive list, nor is it meant to be. These are the items that drew my interest, and that I consider significant. I suspect another astronomy enthusiast’s list would overlap mine in some respects, but surely not in all. Feel free to send me the item you thought most significant that I left out!
This was the year that the number of suspected and confirmed planets orbiting other stars doubled, and doubled, then doubled again, due almost exclusively to the Kepler mission. This space-borne telescope has zeroed in on more than 100,000 stars in a patch of sky near the constellation of Cygnus, the Northern Cross. A very sensitive light meter looks for periodic dips in a star’s brightness caused by a planet moving in front of the star and blocking a portion of its light-emitting surface from us. From the characteristics of the resulting light curve, the orbital and physical characteristics of the planet can be deduced.
Along with learning more about just how common planets are (very) and how similar other planetary systems are to our own (not very), there is understandably intense interest in finding Earth-like planets: small rocky planets at a distance from their stars that allows liquid water to exist on their surfaces. Such planets might—just might—be abodes of life.
Earlier this month, the first planet detected by Kepler that is small enough and at the right distance from its star to qualify as “Earth-like” was announced. Kepler 22-b is larger than Earth (but not so large that it must be a gaseous rather than a rocky planet), its star is quite Sun-like (a little smaller and cooler than the Sun, but not by much), and its orbit puts it well within the “habitable zone” (neither so close to the star that water boils, nor so far away that it freezes). The thing we don’t know is its mass, which is determined by other means that are difficult to apply in this case.
So the illustration below is speculative, but definitely not outside the realm of possibility!
This illustration shows the habitable zones of Kepler 22 and of our own solar system as the same scale:
There are so many robotic spacecraft active in the solar system that it is difficult to keep track of them all. I think the best way to do so is through my favorite astronomy blogger of all, Emily Lakdawalla of the Planetary Society. She posts a monthly “What’s Up In the Solar System” update of planetary missions; the December 2011 post is here. Highlights from 2011:
- In March, the MESSENGER probe settled into orbit around Mercury, the first spacecraft ever to do so. An earlier post is here, and the mission home page is here.
- The Dawn mission arrived at Vesta, the second largest asteroid in July. More about this mission here and here.
- The Curiosity Mars rover was successfully launched on its way at the end of November. Scheduled to land on Mars next August, this rover is the largest and most capable ever to explore the surface of the red planet.
- Another ambitious mission to Mars that hoped to return a sample from its tiny moon Phobos did not fare so well. The Phobos-Grunt (Russian for “ground” or “soil”) launched into low Earth orbit from the famous Baikonur Cosmodrome in early November. When firing commands were sent to launch to Mars, however, they failed. The craft is presumed dead, and will re-enter the atmosphere sometime in January.
In this loosely defined category, we have the following:
- A supernova observed in M101 (the Pinwheel Galaxy) in August is one of the nearest observed in quite some time. M101 is 21 million light years from Earth. What is most notable about this particular stellar explosion is how soon after the event it was detected on Earth, a consequence of increasingly automated nightly telescopic surveys of the sky. In the old days (20 years ago!), amateurs scanning the heavens with their 10-inch scopes discovered the great majority of supernova explosions. They have been outpaced by large-aperture automatic surveys such as the one that found this supernova. This was a Type Ia event, the complete thermonuclear destruction of white dwarf star. These are crucial to astronomers as so-called “standard candles”, objects of known luminosity. If we know how luminous (inherently bright) an object is, and can measure how bright it appears to be, we can determine its distance. While Type Ia supernovae are believed to have very similar luminosities, there can be small variations depending on the compositions of their precursor stars. Seeing this supernova so early allowed astronomers to refine their models of these explosions, and thereby more finely calibrate their cosmic distance measurements. More from an earlier Star Struck post.
- Newly discovered black holes set records for size at both extremes of size—the largest and the smallest. These cosmic oddities result from the ultimate victory of gravity over the other forces of nature. All stars involve a balancing act between gravity (which seeks to collapse them) and other forces which prevent that collapse. In our own Sun, nuclear reactions in its core produce energy that maintains the Sun at a constant size. Five billion years from now, when it runs out of fuel, the Sun will go through a series of stages that will culminate in its becoming a white dwarf, an Earth-sized sphere of incredibly dense matter. Its further collapse is halted by “electron degeneracy”. You just can’t pack atoms any closer (their outer electrons are pushing back) without piling on more mass than the Sun possesses.
Black holes result when there is so much mass that nothing can resist the ultimate collapse. The density zooms to infinity if we are to believe Einstein’s Theory of General Relativity (and there is as yet no good reason not to), and within the black hole’s event horizon, nothing—not even light—can escape gravity’s grip.
If you carefully review the preceding description, you may infer that a certain minimum amount of mass is necessary to create a black hole, but that there is no necessary upper limit to their size. You would be correct.
An X-ray satellite has detected periodic variations in X-rays probably being emitted from a flat disk of hot gas spiraling into the black hole. The rapidity of these variations indicates that the black hole responsible for them is less than three times the mass of our Sun. Any smaller and there wouldn’t be enough mass to generate the black hole. The animation below depicts the pulsations arising from instabilities in the black hole’s accretion disk:
- At the other end of the spectrum, a 21 billion solar mass monster lurks at the center of the galaxy NGC 4889. Supermassive black holes seem to reside at the center of
allmost galaxies, including our own. The larger the galaxy, the larger is the central black hole. A massive galaxy with a correspondingly massive black hole is imagined below:
- Lastly, let us mention a curiosity: four very red galaxies discovered with the Spitzer Space Telescope. These are invisible to the Hubble Space Telescope, which doesn’t see as deeply into the infrared wavelengths as the Spitzer, so they are “bright” only at these longer wavelengths, not in visible light. Galaxies are red for any number of reasons, and astronomers concluded that three of these reasons apply to all of these galaxies.
o They are very distant and their light is substantially red-shifted as a result. This means they represent galaxies at a very early epoch of the universe, only about a billion years after the Big Bang.
o They have significant ongoing star formation, which heats up surrounding dust and makes a galaxy luminous in the infrared.
o They have old stars that have evolved to become red, like Betelgeuse in Orion. This implies that these stars must have formed very early in the universe’s history.
If the stars in these galaxies are old enough for some of them to have evolved this far, that implies that star formation (and indeed galaxy formation) began earlier than we had thought. All of this together means that we have a lot to learn about the early universe. The more powerful instrumentation planned for the future will see farther out into the cosmos, and therefore farther back into time. Looking out (into space) is looking back (into time)! The science fiction dream of time travel is something astronomers do–and must account for–routinely.
HUMAN SPACE TRAVEL
- The last flight of the Space Shuttle took place in July as Atlantis returned from the International Space Station. While the shuttle is an impressive vehicle in many respects, it is also impressively flawed, as two disastrous losses of crew killing a total of fourteen persons demonstrate. Hanging on the side—rather than on the top—of a booster rocket is flaw number one. Witness the Columbia disaster, where falling debris fatally damaged the leading edge of a wing. Providing no means for the crew to escape a failing booster is flaw number two. Witness the Challenger disaster, where a 60s-style rocket escape tower could have saved the crew. Those engineers in the 60s got it right the first time.
- Space tourism crept ever closer in 2011. Immediately after the 1969 moon landing, I sent away for my reservation on Pan American Airways’ first flight to the moon. I don’t remember the exact number—it was in the 5000s, I believe—and of course neither the reservation card nor Pan Am any longer exist. But some adventurers will soon take a short hop into space on Virgin Galactic’s SpaceShip Two at $200,000 a pop, probably in 2013. The unofficial and arbitrary definition of the boundary of space is at 50 miles (80 kilometers) or 62 miles (100 kilometers) altitude, depending on whom you ask. SpaceShip Two will peak at 68 miles (110 kilometers) so there should be no question—you will be a space man or woman! The Virgin Galactic site has more on SpaceShip Two.
- Finally, where is NASA going with human spaceflight systems? After much confusion and controversy, the latest plans involve crewed vehicles that look like (and are) larger updated versions of the old Apollo-era capsules, and booster systems that look like an old Saturn V with Space Shuttle solid rocket boosters strapped to the side. The crew compartment rests at the top of the booster, and includes an escape system to pull the crew away from a failing booster.Where will the astronauts go? The proposed destination—a near-Earth asteroid—actually makes sense in several ways. It would be a long-duration (several months) mission that would rehearse the techniques needed for a later mission to Mars. It would not require a dangerous and fuel-hungry landing on and launch from the surface of a planet, since the asteroid’s gravity is so low that we simply need to rendezvous with it–not land on it–in deep space. Perhaps most importantly, it would let us test out techniques for nudging such bodies into different paths. While there are no large asteroids on a collision course with Earth for the next century, such will not always be the case. Right now, if a mile-wide body were aimed at the Earth, there is not a single thing we could to prevent it. I for one would like to develop the capability to do more than hope that the impact point was on the other side of the world.If you have read this far, you deserve either my congratulations or my condolences, and perhaps both! My next post will look forward to what we might anticipate in 2012. Happy Holidays to everyone!