Astronomy Picture of the Day
Discover the cosmos! Each day a different image or photograph of our fascinating universe is featured, along with a brief explanation written by a professional astronomer.
2004 August 31 
Posted on 08/31/2004 5:16:43 AM PDT by petuniasevan
Discover the cosmos! Each day a different image or photograph of our fascinating universe is featured, along with a brief explanation written by a professional astronomer.
2004 August 31
Explanation: What causes the black dots on dunes on Mars? As spring dawns on the Northern Hemisphere of Mars, dunes of sand near the poles begin to defrost. Thinner regions of ice typically thaw first revealing sand whose darkness soaks in sunlight and accelerates the thaw. By summer, the spots will have expanded to encompass the entire dunes that will then be completely thawed and dark. The carbon dioxide and water ice actually sublimes in the thin atmosphere directly to gas. Taken in mid-July, the above image shows a field of spotted polar dunes spanning about 3 kilometers near the Martian North Pole. Meanwhile, in the Southern Hemisphere of Mars, the Earth-sent robot Martian rovers will try to survive through Martian winter, which peaks in mid-September.
A European team of astronomers has discovered the lightest known planet orbiting a star other than the sun (an "exoplanet").
The new exoplanet orbits the bright star mu Arae located in the southern constellation of the Altar. It is the second planet discovered around this star and completes a full revolution in 9.5 days.
With a mass of only 14 times the mass of the Earth, the new planet lies at the threshold of the largest possible rocky planets, making it a possible super Earth-like object. Uranus, the smallest of the giant planets of the Solar System has a similar mass. However Uranus and the new exoplanet differ so much by their distance from the host star that their formation and structure are likely to be very different.
This discovery was made possible by the unprecedented accuracy of the HARPS spectrograph on ESO's 3.6-m telescope at La Silla, which allows radial velocities to be measured with a precision better than 1 m/s. It is another clear demonstration of the European leadership in the field of exoplanet research.
A unique planet hunting machine
Since the first detection in 1995 of a planet around the star 51 Peg by Michel Mayor and Didier Queloz from the Geneva Observatory (Switzerland), astronomers have learned that our Solar System is not unique, as more than 120 giant planets orbiting other stars were discovered mostly by radial-velocity surveys.
This fundamental observational method is based on the detection of variations in the velocity of the central star, due to the changing direction of the gravitational pull from an (unseen) exoplanet as it orbits the star. The evaluation of the measured velocity variations allows to deduce the planet's orbit, in particular the period and the distance from the star, as well as a minimum mass.
The continued quest for exoplanets requires better and better instrumentation. In this context, ESO undoubtedly took the leadership with the new HARPS spectrograph (High Accuracy Radial Velocity Planet Searcher) of the 3.6-m telescope at the ESO La Silla Observatory. Offered in October 2003 to the research community in the ESO member countries, this unique instrument is optimized to detect planets in orbit around other stars ("exoplanets") by means of accurate (radial) velocity measurements with an unequalled precision of 1 metre per second.
HARPS was built by a European Consortium in collaboration with ESO. Already from the beginning of its operation, it has demonstrated its very high efficiency. By comparison with CORALIE, another well known planet-hunting optimized spectrograph installed on the Swiss-Euler 1.2-m telescope at La Silla, the typical observation times have been reduced by a factor one hundred and the accuracy of the measurements has been increased by a factor ten.
These improvements have opened new perspectives in the search for extra-solar planets and have set new standards in terms of instrumental precision.
The planetary system around mu Arae
The star mu Arae is about 50 light years away. This solar-like star is located in the southern constellation Ara (the Altar) and is bright enough (5th magnitude) to be observed with the unaided eye.
Mu Arae was already known to harbour a Jupiter-sized planet with a 650 days orbital period. Previous observations also hinted at the presence of another companion (a planet or a star) much further away.
The new measurements obtained by the astronomers on this object, combined with data from other teams confirm this picture. But as François Bouchy, member of the team, states: "Not only did the new HARPS measurements confirm what we previously believed to know about this star but they also showed that an additional planet on short orbit was present. And this new planet appears to be the smallest yet discovered around a star other than the sun. This makes mu Arae a very exciting planetary system."
"Listening" to the star
During 8 nights in June 2004, mu Arae was repeatedly observed and its radial velocity measured by HARPS to obtain information on the interior of the star. This so-called astero-seismology technique studies the small acoustic waves which make the surface of the star periodically pulsate in and out. By knowing the internal structure of the star, the astronomers aimed at understanding the origin of the unusual amount of heavy elements observed in its stellar atmosphere. This unusual chemical composition could provide unique information to the planet formation history.
Says Nuno Santos, another member of the team: "To our surprise, the analysis of the new measurements revealed a radial velocity variation with a period of 9.5 days on top of the acoustic oscillation signal!"
This discovery has been made possible thanks to the large number of measurements obtained during the astero-seimology campaign.
From this date, the star, that was also part of the HARPS consortium survey programme, was regularly monitored with a careful observation strategy to reduce the "seismic noise" of the star.
These new data confirmed both the amplitude and the periodicity of the radial velocity variations found during the 8 nights in June. The astronomers were left with only one convincing explanation to this periodic signal: a second planet orbits mu Arae and accomplishes a full revolution in 9.5 days.
But this was not the only surprise: from the radial velocity amplitude, that is the size of the wobble induced by the gravitational pull of the planet on the star, the astronomers derived a mass for the planet of only 14 times the mass of the Earth! This is about the mass of Uranus, the smallest of the giant planets in the solar system.
The newly found exoplanet therefore sets a new record in the smallest planet discovered around a solar type star.
At the boundary
The mass of this planet places it at the boundary between the very large earth-like (rocky) planets and giant planets.
As current planetary formation models are still far from being able to account for all the amazing diversity observed amongst the extrasolar planets discovered, astronomers can only speculate on the true nature of the present object. In the current paradigm of giant planet formation, a core is formed first through the accretion of solid "planetesimals". Once this core reaches a critical mass, gas accumulates in a "runaway" fashion and the mass of the planet increases rapidly. In the present case, this later phase is unlikely to have happened for otherwise the planet would have become much more massive. Furthermore, recent models having shown that migration shortens the formation time, it is unlikely that the present object has migrated over large distances and remained of such small mass.
This object is therefore likely to be a planet with a rocky (not an icy) core surrounded by a small (of the order of a tenth of the total mass) gaseous envelope and would therefore qualify as a "super-Earth".
Further Prospects
The HARPS consortium, led by Michel Mayor (Geneva Observatory, Switzerland), has been granted 100 observing nights per year during a 5-year period at the ESO 3.6-m telescope to perform one of the most ambitious systematic searches for exoplanets so far implemented worldwide. To this aim, the consortium repeatedly measures velocities of hundreds of stars that may harbour planetary systems.
The detection of this new light planet after less than 1 year of operation demonstrates the outstanding potential of HARPS for detecting rocky planets on short orbits. Further analysis shows that performances achieved with HARPS make possible the detection of big "telluric" planets with only a few times the mass of the Earth. Such a capability is a major improvement compared to past planet surveys. Detection of such rocky objects strengthens the interest of future transit detections from space with missions like COROT, Eddington and KEPLER that shall be able to measure their radius.
Guinness recognizes X-43A world speed record
NASA NEWS RELEASE
Posted: August 29, 2004
Guinness World Records has recognized the world speed record set by NASA's hypersonic X-43A aircraft in an experimental flight over the Pacific Ocean earlier this year. The unpiloted, 12- foot-long aircraft achieved Mach 6.83 -- almost seven times the speed of sound -- or nearly 5,000 mph, while its supersonic-combustion ramjet (scramjet) engine propelled the craft for 11 seconds during the flight on March 27.
 The Pegasus booster fires into the sky with X-43A. Credit: NASA/Jim Ross |
"On 27 March 2004, NASA's unmanned Hyper-X (X-43A) airplane reached Mach 6.83, almost seven times the speed of sound. The X-43A was boosted to an altitude of 29,000 m (95,000 ft) by a Pegasus rocket launched from beneath a B52-B aircraft. The revolutionary 'scramjet' aircraft then burned its engine for around 11 seconds during flight over the Pacific Ocean."
If NASA researchers have their way, the record won't stand long. The final flight in the Hyper-X program is scheduled to take place in October, when another X-43A aircraft will attempt to fly at Mach 10 -- ten times the speed of sound -- or 7,200 mph.
The flight was part of NASA's Hyper-X program, designed to demonstrate advanced high-speed propulsion system concepts to overcome one of the greatest aeronautical research challenges - air-breathing hypersonic flight. The advantage of air-breathing flight is that the vehicle -- whether it is aircraft or spacecraft -scoops the air its engines need from the atmosphere rather than carrying heavy, bulky tanks, as rockets do.
The challenge is to introduce fuel, ignite it and produce positive thrust while highly compressed air rushes through the engine in mere milliseconds -- roughly analogous to lighting a match and keeping it burning in a hurricane-force wind.
Compared to rocket-powered vehicles like the Space Shuttle, scramjets promise more airplane-like operations for increased affordability, flexibility and safety for ultra high-speed flights within the atmosphere and into Earth orbit.
The X-43A flight easily set a world speed record for an air-breathing engine aircraft. The previous known record was held by a ramjet-powered missile, which achieved slightly more than Mach 5. A ramjet operates by subsonic combustion of fuel in a stream of air compressed by the forward speed of the aircraft itself, as opposed to a normal jet engine, in which the compressor section (the fan blades) compresses the air. A scramjet (supersonic-combustion ramjet) is a ramjet engine in which the airflow through the whole engine remains supersonic.
The highest speed attained by a rocket-powered airplane, NASA's X-15 aircraft, was Mach 6.7.The fastest air-breathing, manned vehicle, the SR-71, achieved slightly more than Mach 3.2. The X-43A more than doubled the top speed of the jet-powered SR-71.
Guinness World Records' science editor David Hawksett has already expressed an interest in attending the fall flight.
"Operating an atmospheric vehicle at almost Mach 7 is impressive enough, but to be able to use oxygen from the air, instead of a fuel tank, as it screams into the engine intakes at 5,000 mph is a mind-boggling technical achievement. It's wonderful to see scramjet technology finally begin to take off," said Hawksett.
The Hyper-X program is conducted by NASA's Aeronautics Research Mission Directorate with the NASA Langley Research Center, Hampton, Va., as lead center with responsibility for hypersonic technology development and the NASA Dryden Flight Research Center, Edwards, Calif., responsible for flight research and testing.
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Looks like something is growing.
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The late Arthur C. Clarke thought this stuff is vegetation. If it is analogous to Earth vegetation -- IMHO, analogies are the only kind of relationship we'll likely be able to draw when life is found on other worlds to the satisfaction of everyone -- it needs very little from its environment.Defrosting Martian sand dunes look like vegetationDark spots and streaks on defrosting sand dunes were first observed by MOC in the northern hemisphere in 1998. Similar dark-spotted dunes in the southern hemisphere were described at a NASA/Mars Global Surveyor Space Science Update media briefing in 1999. Despite the "sensation" one gets when looking at pictures of spotted, defrosting Martian dunes (i.e., the sensation that these images show some form of life, like vegetation, growing on Mars) these features are a normal, common manifestation of the springtime defrosting process on Mars. The ices involved -- because of the low temperatures at these locations -- are probably both frozen water and carbon dioxide, though it is unclear as to whether one type of ice dominates over the other in controlling the appearance and coalescence of the dark spots. It is known from the first Martian year of MOC operations that by summer all of the frost -- and thus all of the spots -- on the dunes will be gone.
June 19, 2001
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