lunes, 18 de julio de 2011
PASADENA, Calif. -- NASA's Dawn spacecraft has returned the first close-up image after beginning its orbit around the giant asteroid Vesta. On Friday, July 15, Dawn became the first probe to enter orbit around an object in the main asteroid belt between Mars and Jupiter.
The image taken for navigation purposes shows Vesta in greater detail than ever before. When Vesta captured Dawn into its orbit, there were approximately 9,900 miles (16,000 kilometers) between the spacecraft and asteroid. Engineers estimate the orbit capture took place at 10 p.m. PDT Friday, July 15 (1 a.m. EDT Saturday, July 16).
Vesta is 330 miles (530 kilometers) in diameter and the second most massive object in the asteroid belt. Ground- and space-based telescopes have obtained images of Vesta for about two centuries, but they have not been able to see much detail on its surface. "We are beginning the study of arguably the oldest extant primordial surface in the solar system," said Dawn principal investigator Christopher Russell from the University of California, Los Angeles. "This region of space has been ignored for far too long. So far, the images received to date reveal a complex surface that seems to have preserved some of the earliest events in Vesta's history, as well as logging the onslaught that Vesta has suffered in the intervening eons."
Vesta is thought to be the source of a large number of meteorites that fall to Earth. Vesta and its new NASA neighbor, Dawn, are currently approximately 117 million miles (188 million kilometers) away from Earth. The Dawn team will begin gathering science data in August. Observations will provide unprecedented data to help scientists understand the earliest chapter of our solar system. The data also will help pave the way for future human space missions.
After traveling nearly four years and 1.7 billion miles (2.8 billion kilometers), Dawn also accomplished the largest propulsive acceleration of any spacecraft, with a change in velocity of more than 4.2 miles per second (6.7 kilometers per second), due to its ion engines. The engines expel ions to create thrust and provide higher spacecraft speeds than any other technology currently available. "Dawn slipped gently into orbit with the same grace it has displayed during its years of ion thrusting through interplanetary space," said Marc Rayman, Dawn chief engineer and mission manager at NASA's Jet Propulsion Laboratory in Pasadena, Calif. "It is fantastically exciting that we will begin providing humankind its first detailed views of one of the last unexplored worlds in the inner solar system."
Although orbit capture is complete, the approach phase will continue for about three weeks. During approach, the Dawn team will continue a search for possible moons around the asteroid; obtain more images for navigation; observe Vesta's physical properties; and obtain calibration data.
In addition, navigators will measure the strength of Vesta's gravitational tug on the spacecraft to compute the asteroid's mass with much greater accuracy than has been previously available. That will allow them to refine the time of orbit insertion.
Dawn will spend one year orbiting Vesta, then travel to a second destination, the dwarf planet Ceres, arriving in February 2015. The mission to Vesta and Ceres is managed by JPL for the agency's Science Mission Directorate in Washington. Dawn is a project of the directorate's Discovery Program, which is managed by NASA's Marshall Space Flight Center in Huntsville, Ala.
UCLA is responsible for Dawn mission science. Orbital Sciences Corp. of Dulles, Va., designed and built the spacecraft. The German Aerospace Center, the Max Planck Institute for Solar System Research, the Italian Space Agency and the Italian National Astrophysical Institute are part of the mission's team.
jueves, 14 de julio de 2011
NASA chief Charlie Bolden went to bat for the agency's imperiled next-generation space telescope Tuesday (July 12), telling members of Congress that the instrument has greater potential for discovery than the iconic Hubble Space Telescope.
A proposed congressional budget bill announced last week would terminate NASA's James Webb Space Telescope (JWST), an ambitious instrument with a history of delays and cost overruns. But NASA can deliver JWST to space for about the same price as Hubble, Bolden said — and the science returns would be even greater.
"I have tried to explain what I think is the importance of James Webb, in terms of opening new horizons far greater than we got from Hubble," Bolden told members of the House Science, Space and Technology committee Tuesday. "I would only say that for about the same cost as Hubble in real-year dollars, we'll bring James Webb into operation. [Spectacular Hubble Telescope Photos]
A rocky history
The $6.5 billion JWST, named after a former NASA administrator, is billed as the agency's muscular successor to Hubble, which launched back in 1990 and is still going strong. JWST is an infrared observatory designed to peer further back into the universe's early days than ever before.
While many researchers have enthusiastically touted the telescope's potential, its development has been plagued by problems. Last November, an independent review panel found that JWST will cost at least $6.5 billion and could launch no earlier than September 2015, putting it $1.5 billion over budget and more than a year behind schedule.
The telescope's issues are primarily the result of poor management practices, the panel further concluded.
A recent budget and technology plan painted an even more pessimistic picture, estimating that JWST could launch by 2018 at the earliest.
And last week, the telescope's future came into even more serious question. The House Appropriations subcommittee that oversees NASA proposed a 2012 spending bill that would give the space agency just $16.8 billion, $1.6 billion less than last year. JWST's funding would be pulled completely.
miércoles, 13 de julio de 2011
Next week, Neptune will complete its first full orbit of the Sun since it was discovered in 1846.
The blue planet, the farthest out in the Solar System, remains one of Earth's most mysterious neighbours, but scientists now know one thing that they hadn't for the past 165 years: the precise length of its day.
Earlier estimates had set that figure at about 16 hours and 6 minutes. But, in a paper in Icarus1, Erich Karkoschka, a planetary scientist at the University of Arizona in Tucson, now pegs it at 15 hours, 57 minutes and 59 seconds.
Determining the day length of rocky bodies such as Mars or Mercury is easy, because scientists can look at their surfaces, in photos or radar images, and track the motion of easily identifiable features.
But Neptune is made mostly of thick clouds of gas, so it has no visible surface. The only visible features are storms, the apparent motion of which results from a mixture of the planet's rotation and shifting weather fronts. Until now, the best estimate of the planet's day length came from radio signals measured during a 1989 flyby by the NASA spacecraft Voyager 2. But studies of Saturn have since indicated that such signals are not as clearly tied to the planet's rotation as was once thought.
Karkoschka went back to basics. Poring over archived images from the Hubble Space Telescope, he found that Neptune has two cloud disturbances, dubbed the South Polar Feature and the South Polar Wave, that seem to be linked to surface features deep beneath the clouds, probably a hot spot on the planet's solid core.
"The best analogue is clouds moving over a mountain," says Karkoschka. "Each cloud moves, so if you track them you don't get the rotation. But the feature as a whole remains stable."
By painstakingly plotting the positions of the two features in 500 images taken by the Hubble Space Telescope over the course of two decades, Karkoschka was able to pin down the planet's day length to an accuracy of 0.0002 of an hour. To put this into perspective, even with the Cassini probe in orbit around Saturn, the planet's day length is known only with an order of magnitude less certainty, he says.
Knowing a planet's rotation rate to such high precision isn't just interesting information — it has practical applications, too. "It constrains models of Neptune's interior," says Karkoschka. "If you know how fast the planet rotates, you can determine the mass distribution inside."
It is also intriguing that Neptune has atmospheric features so strongly linked to its solid interior. "Nothing similar has been seen before on any of the four giant planets," says Karkoschka.
Neptune still holds many mysteries. In fact, Karkoschka's finding itself raises a new one: the source of the heat that produces the recurring cloud disturbances.
Another, says Sushil Atreya, a planetary scientist at the University of Michigan, Ann Arbor, is the source of Neptune's magnetic field. Such fields on Jupiter and Saturn are believed to result from the movement of metallic hydrogen, produced inside the gassy planets at pressures of several million times that of Earth's atmosphere. But Neptune is smaller than Jupiter or Saturn, with lower internal pressures. "Metallic-hydrogen formation in Neptune is unlikely," says Atreya.
There is also the question of why Neptune exists at all.
Nature Chemistry Guide to Authors
The planet is 30 times further from the Sun than is Earth. That far out, planet-formation models suggest that the solar nebula, from which planets condensed during the formation of the Solar System, should have been very diffuse, says Francis Nimmo, a planetary scientist at the University of California, Santa Cruz. So scientists believe that Neptune formed closer to the Sun, where the nebula was denser, then moved outwards. But many Neptune-mass planets in other planetary systems seem to have migrated inwards, rather than outwards, says Nimmo.
Atreya says that understanding these "hot Neptunes" elsewhere in the Universe requires a better understanding of why our own large planets have ended up so far from the Sun.
"The mystery of Neptune transfers to hundreds of exoplanets that superficially seem similar to Neptune," says Geoffrey Marcy, an astronomer at the University of California, Berkeley, who hunts for planets outside the Solar System.
Atreya adds that the solution to these mysteries will be best obtained by sending entry probes and orbiters to Neptune at some point in the future.
Preferably before the planet makes another full orbit of the Sun
martes, 12 de julio de 2011
Averiguar cuál es nuestra posición en el interior de una nube de miles de millones de estrellas (sin salir del planeta que nos acoge) es como descubrir la geografía de un bosque mientras pertenecemos amarrados a uno de los arboles. La idea somera que tenemos sobre la forma de la Galaxia solo se basa en la contemplación de lo que nos rodea, una banda irregular y borrosa que traza un circulo de luz en el cielo. Mide unos 15 grados de ancho y las estrellas se concentran de un modo bastante regular a lo largo de esa franja, la Vía Láctea. Esta observación revela que la Galaxia es un disco de estrellas y que nos encontramos en algún lugar próximo al plano del disco. Si no se tratara de un disco plano, la Vía Láctea ofrecería otro aspecto. Por ejemplo, si se tratara de una esfera de estrellas, veríamos su fulgor en todo el cielo, y no solo e una banda estrecha. Y si nos halláramos muy por encima o muy por debajo del plano del disco, no la divisaríamos partiendo el cielo en dos mitades, sino que el fulgor de la Vía Láctea seria mas intenso a un lado del cielo que al otro.
La posición del Sol dentro de la Galaxia se puede precisar, además, si se mide la distancia que lo separa del resto de estrellas que se divisan. A finales del siglo XVIII, el astrónomo Wilhelm Herschel llevó a cabo el experimento y llegó a la conclusión de que la Tierra se halla en el centro de una nube de estrellas en forma de piedra de afilar. Pero Herschel no tuvo en cuenta las pequeñas partículas de polvo interestelar que ocultan la luz de las estrellas más distantes de la Galaxia. Le pareció que ocupáramos el centro de la nube porque no podía ver más allá en todas direcciones. Una persona amarrada a un árbol en un bosque con niebla tiene la sensación de que el conjunto se extiende por igual en todas direcciones, con independencia de donde se encuentre.
Durante las primeras décadas del siglo XX se produjo un logro crucial para trasladar la Tierra del centro de la Galaxia a un punto que dista alrededor de tres quintas partes del borde: el astrónomo Harlow Shapley midió la distancia que nos separa de los grandes cúmulos de estrellas que denominamos globulares. Descubrió que estas concentraciones estelares se distribuyen formando una esfera de unos 100.000 años-luz de diámetro y centrada en un lugar que cae en la constelación de Sagitario. Shapley concluyó (y otros astrónomos lo han constatado desde entonces) que el centro de ka distribución de los cúmulos globulares también es el centro de la Galaxia, de modo que su aspecto se asemeja al de un disco aplanado de estrellas inmerso en una nube esférica, o halo, de cúmulos globulares.
Durante los últimos 75 años los astrónomos han refinado esta imagen recurriendo a diversas técnicas visuales, de radio, de infrarrojos y hasta de rayos X para completar los detalles: la ubicación de brazos espirales, nubes de gas y polvo, concentraciones moleculares, etc. La imagen básica actual revela que el sistema solar se halla en el borde interior de una brazo espiral a unos 25.000 años-luz del centro de la Galaxia, el cual cae en dirección a la constelación de Sagitario.