Apr08T
SpaceWalk
For April 2008
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John Pazmino
National Space Society
New York City Chapter
Spacefarers had several bonus events in March 2008. There were three new bright planetary stars were announced. Several asteroids whizzed by Earth closer than the Moon. Daylight Savings Time started. There was a gamma-ray burst that could have been a naked eye 'nova' in the sky. And a new spaceprobe is being exploited by folk like you to discover comets.
Daylight Savings Time is a phony scheme with no use for either astronomers or spacefarers. It came on March 9 at 02h EST, which instantly became 03h EDST. I heard of some people suffering two sets of clocks. The one set changed properly to EDST on March 9th. The other stayed on EST, probably to change to EDST on the old date of April 6th. Astronomers and spacefarers stay on EST year-round for their work and use EDST only for terrestrial purposes like meeting notices.
The other celestial events steeped in spacefaring interest. Why? All, the asteroids, planetary stars, gamma-ray burst, comets were studied by spacecraft of the very kind we need and want in the ongoing theme of space exploration.
The planetary stars were in recent months. Thy were found from ground facilities and then examined by spaceprobes. In addition, there are two NASA projects in the works specially to look for planetary stars, the Kepler and Space Interferometer Mission. We learned more about these projects at a talk given at Columbia University on March 14th on 'Finding exoplanets'..
The three new stars all happen to be in our spring to summer sky. You can start showing them to friends now in April. Besides the new stars, I list below two older planetary stars in the spring sky.
Columns 'star' thru 'ly' give the usual specs for the star and are essentially fixed data. The HD number comes from the Henry Draper catalog, a prime source for candidate planetary stars and is the usual way to cite them.
'Sun' is the apparent magnitude of Sun as seen from the star. Many of the brighter planetary stars are solar-class stars. We from them look about as bright as they from us.
'Mjup' is the planet's mass in Jupiter units. The mass is a lower limit, (mass)*(sin(orbit inclination)). The planet could be far more massive. Jupiter is 318 Earth masses.
'SMA' is the semimajor axis of the planet's orbit in AU. This handles the frequent inquiry about the distance of the planet from its star compared to Earth's distance from Sun. In general, the planets run in strongly excentric orbits, not closely circular ones like the planets of our solar system.
'days' is the planet's orbital period, its year, in Earth days.
'year' is when the planet was announced. This typicly is the end
of a multiyear study of the planet to establish its properties.
~See Top Table to the Left~
70 Virginis (VIRR-jih-niss), tau Bootis (tao boh-OH-tis), and rho Coronae Borealis (roh ko-ROH-nigh bo-reh-A-liss) were known since the mid 1990s and are in the evening sky from April thru the rest of spring and early summer.
The new stars are kappa Coronae Borealis (KA-pa ...), si Aquilae (zee A-kwih-ligh), and 18 Delphini (dell-FEE-nee). I'll highlight them in mid summer when they are in evening high sky.
There is already one spaceprobe hunting for planets, France's CoRoT. Little about CoRoT, the first satellite dedicated to the search for extrasolar planets, turns up in American spacefaring news.
It is similar to the forthcoming Kepler mission in that it scours a limited zone of space to catch the minuscule dimming of a star when an opaque planet crosses over it. CoRoT discovered two planetary stars so far. Please note that when you keep up with this mission, much of its news is in French or Italian.
You can't actually see the planets around these stars. So far, as at March 2008, no one has seen such a planet. The race is on to be first to capture an optical image of an extrasolar planet, but not quite yet is success in hand.
One team in this endeavor is led by Dr Ben Oppenheimer at the Hayden Planetarium. He gives talks on his project about a special coronagraph camera attached to huge telescopes with adaptive optics. The name comes from the original camera of the early 20th century invented to study the Sun's corona outside of a solar eclipse. It has filters,, masks, baffles to dissipate and disperse the light of the Sun and daylight so that the feeble glow of the corona shows thru.
Oppenheimer and his crew hope a similar method will work to see the evanescent glint of a planet next to its parent star. The idea is plausible and we may any day hear of a exoplanet discovery from him.
While you will not see planets hovering next to these stars, it's fun to point out the stars to friends and wonder, 'Are those stars looking back at us?'.
In the detail chart here I label a few extra stars to help you starhop from the more obvious stars to the planetary stars. The same names are in the panorama chart, making it a bit crowded. Mind that the detail chart has north up. You must rotate it to line up the 'Y' figure of Arcturus (ark-TAW-russ), Seginus (SEH-jih-nuss), Alphecca (al-FEH-ka), and Izar (EE-zarr) with the stars in the sky.
It helps to wait until 22h or later to let this part of the sky rise higher above the skyline into clearer darker air. Keep the detail chart to hand thruout the spring and summer, as long as Arcturus is above the skyline.
Asteroids buzz Earth all the time, now that we have matured our skill and practice to detect them. It seems that the smaller the asteroid, becoming in the extremely large meteors, the closer they pass by Earth! A tiny asteroid of many hundreds of meter diameter crashing on us will alter human civilization overnight.
What most lay folk miss is the humongous speed of collision, no less than 11KPS! That's crossing the whole length of Manhattan, from the Battery to Sputen Duyvil, in TWO SECONDS. That's the MINIMUM closing speed for a asteroid pacing Earth and dropping straight down.
The greatest closing speed is 71KPS for an asteroid in solar orbit at Earth distance but in opposite direction. That would carry the rock over the whole length of Long Island, Fulton Ferry to Montauk Point, in TWO SECONDS.
The speed of the collision matters because it determines the kinetic energy transferred to heat and blast. For a small rock of a hundred meter diameter, the equivalent blast and heat is that of a small H-bomb. We are spared only the intense radioactivity.
Here's a list of closely-missing asteroids from January 1st thru March 15th in 2008. Since then, you can sleep soundly that several more stones whizzed by our ears.
For comparison, the radius of the Moon's orbit is 0.0026 AU = 384,400km. The reference is an electronic news bulletin of the Minor Planet Center, one of the astronomy centers that tracks asteroids and certain other celestial objects. An other is NASA's Jet Propulsion Laboratory, with its own bulletin service.
~See Bottom Table to the left~
In just two and a half months we had EIGHT close calls, all within the radius of the Moon's orbit! In a couple cases, we found the rock HOURS before its closest approach. If any one of these stones was on collision course, there would be NOTHING AT ALL we could do to divert it, even if we had a diversion system ready. The warning would be wholly too brief.
No spacecraft for detecting Earth-threat asteroids are under construction. There are many proposals. The usual one is to land a craft on the asteroid and nudge it off course by firing thruster rockets. For this to work, the threat must be confirmed many years or decades in advance.
Tracking asteroids is one example of spacefarers and astronomers working together on a joint purpose. Minor Planet Center is a glatt astronomy facility. NASA JPL is a glatt spacefaring outfit. The two cooperate to provide prompt, accurate, reputable reports and news of Earth-threat asteroids.
In order to plan for an intercept, we must monitor the stone's position and motion awfully accurately. That's why a preliminary response would be to send a probe to study the asteroid first and this was exacta mente a contest set up in early 2007 by Planetary Society.
In March it awarded 1st prize to a small beacon satellite to orbit asteroid Apophis and track it by relaying radio signals with Earth. It would also map Apophis by laser altimetry. The plan is only a paper one, but it, by contest rules, is feasible if funded and sponsored.
The gamma-ray burst is an event normally outside the spacefaring news. Astronomers are plugged into a network of alarms and alerts so they can rapidly respond to the explosion. The blast endures for only minutes, so a rapid response is crucial. Now we are summoned by email, texting, pager. Astronomy centers put up notices on their websites.
As gamma-ray astronomy progressed over the last decade, gamma-ray burst alarm networks improved. One major new facility is NASA's Swift probe. It patrols the sky for the telltale signature of X-rays emitted by a gamma-ray burst and relays s description to subscribers at ground observatories. These are fitted with motorized automatic telescopes that within seconds of Swift's trip zero in one the burst's location in the sky and start capturing images or photometry.
On March 19th a burst tripped Swift. It, named GRB080319B, was successfully tracked by the automatic telescopes and, within a few more minutes, by crewed observatories also. In faded to obscurity in about ten minutes leaving no observable trace.
When the day was over, four separate gamma-ray bursts were found by Swift, named in sequence GRB080319A, ...B, ...C, and ...D. GRBs A, C, and D were garden-variety bursts elsewhere in the heavens. The 'B' event was extra special, punching into the public news media.
Spectra of its decaying radiation were obtained. They showed a redshift, due to cosmic expansion, indicative of a 7-1/2 BILLION lightyear distance! To be seen at such a huge distance, the energy poured out exceded that of hundreds of galaxies combined!
As the robot scopes collected photometry an incredible fact emerged. This mother could have been seen BY HUMAN EYE, if any one was looking in the right place and time. Oh, not so casually as looking up nad, 'Yowsa! What the hell is that thing?!'. It would take an ideal night sky and an ideal good eye. IF there was any one examining the sky for novae ro comets, he COULD have seen it easily in binoculars. As at end March, no human observer is verified to have visually seen the explosion.
We do not understand what a gamma-ray burst is. It appears as a sudden expulsion of an immense block of radiation that rapidly dies out. During this decay, the radiation steadily increases om wavelength. The initial outburst is the upper high energy gamma-ray part of the spectrum. Within seconds it drifts to lower energy radiation of longer wavelength, passing thru X-ray (when Swift caught it), ultraviolet, visual, infrared, microwave, and so on. The fall off is precipitous, snuffing out in a few minutes. That's why astronomers are thankful for the rapid response capability, based largely on space-based facilities.
In the starcharts here this GRB is marked, about a degree south of Seginus. In theory it should have been easily captured as part of a sky patrol. Nova watches are concentrated along the Milky Way, where there are more stars to make a nova more likely. The area around Seginus is well away from the Milky Way, close to the north pole of the Milky Way. There is no routine nova patrols there.
Similarly for a comet patrol. When the GRB erupted, it was the owl hours in the City and in high sky in the east. Most comet searches are made in evening twilight and near the Sun in the west. A secondary area is during morning twilight near the Sun in the east.
Hence, it is not surprising that neither nova nor comet hunters spotted the gamma-ray burst. Even if a person did see the faint glimmer, he would have had only some 20 seconds to record it. It's entirely probably that when he tried to verify his sighting, the GRB faded beyond recoverable range and he would dismiss the observation as a spurious one.
Comets are the collateral target of STEREO, a pair of Sun-watching satellites that capture 3D data. They were launched last year into solar orbit close to earth's distance. They observe the Sun together from the two stations to assemble stereo pictures. The mission name is a acronym forced to spell 'stereo'.
The two graphics here show where STEREO is relative to Earth and Sun at the end of March 2008 and its camera coverage of the Sun. They are, with my annotatons, from the STEREO website. The two probes are not quite in the same orbit as Earth. They gradually migrate around, now receding from Earth by about 1 degree per month. Now they are 23 degrees ahead and behind Earth as seen from the Sun.
The 'A' probe is the 'Ahead' one; 'B', the 'Behind' one. Easy to remember which is which, no? In the sky, both are on the ecliptic with A about where the 3rd quarter Moon would be and 'B' about about where the 1st quarter Moon stands.
The two craft overlap their views of the Sun to cover quite more than ½ of his disc. The overlap zone is where the 3D pictures are composed. The sample images here are from one of the ultraviolet cameras, there being a battery of different cameras on STEREO.
STEREO images are salted away on the website, like those of SOHO, and are now 'mined' for other astronomy benefit. Specificly, the circle that scrutinizes SOHO pictures for comets is now turning up comets in STEREO plates. During February 2008 the first four STEREO-comets were found, with promise of a new steady stream of discoveries in the months and years to come.
So far these new bodies are sungrazers. They rush almost straight toward the Sun from infinity, whip around in a hairpin turn, and (if they survive the solar heating) fly back to the outer limits of the solar system. These are, for the most part, members of the Kreutz family of comets, an anigmatic species that puzzled us astronomers since the late 1800s.
Who's finding these comets? You. Campus astronomers are busy with the solar data from STEREO. They leave, as they do for SOHO, the comet hunting to the home astronomer and spacefarer. There's no reason why spacefaring fans can not avail of observations collected by spaceprobes. I do stress that it is sweaty work.
The dar and away most suspects in the pictures will be false alarms, duds, missed discoveries, returns of known comets. But, once in a while, with diligence, patience, attention, care, you may turn up an allnew member of the solar system, never before seen by humans.
An other outcome is to recover a comet formerly thought to be dead and gone, as happened many times with SOHO comets hunters. The 'new' comet is matched to one from many years ago.
Our panorama chart is plotted for April 18th at 21h EDST. That's on the night between the two days of the Earth Day fair. NYSkies has a booth there, on Vanderbilt Avenue next to Grand Central Terminal. NSS should have litterature and a crew to handle spacefaring inquiries.
We look east with zenith at the top. The scene is similar to that in the March 2008 Spacewalk, except that I labeled more stars to help key in with the detail chart of planetary stars. One extra star you should ask your astronomy buddy to show you is Porrima (PO-rih-ma), or gamma Virginis (GA-ma VIRR-jih-niss). Either name is valid, plus a couple other proper names in lesser circulation. . You recall Porrima from SpaceWalks in 2004 thru 2005. This is the amazing pair of star that in about 1836 SWALLOWED UP into a single perfectly round star! At that time, when the nature of stars was unknown, the coalescing of two whole stars into one was a freakout.
What happened is the two stars are in orbit so excentric that when closest together in 1836 blended their images. Scopes of the era were too small to show them as two distinct points. It's like looking at Mizar (MEE-zarr) in the Big Dipper with binoculars and seeing it as one star. A telescope breaks it into two separate stars. Your telescope friend can demonstrate this by pretending that your binoculars are the best telescope we got.
In a few years the stars span apart to become two again. Since then, until about 2003, Porrima was a favorite target for small scopes as well as one for the Moon to occult from time to time. In 2003 the two stars were spiraling together gain to ultimately merge into one single point in 2005-2006.
This was only humankind's second chance to see such a marvelous apparition. For spacefarers, and astronomers!, gamma Virginis is the only easy example of seeing within months genuine space motion of stars. Without this specimen, you would have to wait decades and generations to realize that stars have their own motions thru space.
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Chapter of the National Space Society