by Paul Gilster | Sep 30, 2005 | Exoplanetary Science, Missions |
Finding planets around other stars is hampered by a key fact: the light from the primary star effectively masks the far dimmer reflected light from any planets. But NASA engineers at the Keck Observatory (Mauna Kea, HI) have used the Keck Interferometer in conjunction with a light-blocking device to suppress the starlight around three stars, one of which is Vega. The procedure may be used to detect dust disks of planetary systems in formation.
“We have proven that the Keck Interferometer can block light from nearby stars, which will allow us to survey the amount of dust around them,” said Dr. James Fanson, project manager for the Keck Interferometer at NASA’s Jet Propulsion Laboratory.
Keck’s interferometer links its two 10-meter telescopes to provide the resolving power of a much larger instrument (in Keck’s case, one the size of a football field). Examining dust disks in greater and greater detail is crucial, because NASA needs to select targets for its Terrestrial Planet Finder mission, which will use both visible light and infrared to look for Earth-like worlds. Acquiring the right ‘signature’ for likely systems means studying their dust disks to determine which might have spawned such planets.
Image: Astronomers know that dust disks may breed planets. This artist’s conception is based on data from the Spitzer Space Telescope that reveals a gap in a protoplanetary, or planet-forming, disk surrounding a young star. The new work with the Keck Interferometer may provide parameters for judging which stars are the most likely candidates for terrestrial planets, based on much better imagery and analysis of such disks. Image credit: NASA/JPL-Caltech.
To put the search for terrestrial worlds in context, the discovery of six new worlds was announced in August, ranging from 20 to 289 light years from Earth. But the largest of these is 1.6 times as massive as Jupiter; the smallest is roughly the size of Uranus, some 14 times the mass of Earth. All six of the new planets orbit their stars much closer than the distance between Mercury and our Sun. It is clear that a new generation of instruments like those aboard the Terrestrial Planet Finder missions will be needed to uncover the kind of small, rocky world that could be the home to life.
Source: Jet Propulsion Laboratory news release.
by Paul Gilster | Sep 29, 2005 | Deep Sky Astronomy & Telescopes
Are elliptical galaxies influenced by a halo of dark matter? The theory has been accepted until recently through observation of the gravitational effects apparently caused by such matter. But 2003 findings (Romanowsky et al., Science 301, pp. 1696-1698) turned up little evidence for dark matter in such galaxies.
Now a different explanation for those observations has surfaced, one that seems to rescue the dark matter concept. That’s good news, because dark matter ought to be there. From a University of California at Santa Cruz press release:
“A dearth of dark matter in elliptical galaxies is especially puzzling in the context of the standard theory of galaxy formation, which assumes that ellipticals originate from mergers of disk galaxies,” added Avishai Dekel, professor of physics at the Hebrew University of Jerusalem and first author of the Nature paper. “Massive dark matter halos are clearly detected in disk galaxies, so where did they disappear to during the mergers?”
The dark matter effect has changed our view of galactic structure. Studies of the orbits of stars in spiral galaxies first revealed it — the rotational speed of stars on the outer edges of the galactic disk remained constant despite their distance from the core. What you would have expected, because most of the visible mass of such galaxies is concentrated in the central region, is that stars far from the center would move much more slowly.
That led to the assumption of a dark matter halo that exerted its own influence on these outer stars. But elliptical galaxies pose a different challenge. They’re thought to result from the collision and merger of two spiral galaxies, and they appear as round, smooth shapes rather than rotating disks. The 2003 study found a decrease in velocity of planetary nebulae with increasing distance from the center of the galaxy, a finding inconsistent with dark matter theory.
Image: A Hubble Space Telescope view of an elliptical galaxy, on the left, and a spiral galaxy to its right, both within the Coma Cluster. Similar dark matter haloes may be affecting the motion of stars within each. Credit: STScI/GSFC.
The new paper draws a different conclusion, noting that many of the stars studied are moving in elliptical orbits that mask their motion as seen from Earth. They may be moving quickly, in other words, but without as much motion along the line of sight. From the release:
To an observer outside the galaxy, a star on such an elongated orbit would only appear to be far from the galactic center if the long axis of its orbit is more or less perpendicular to the observer’s line of sight. If the long axis of the orbit is aligned with the line of sight, the star would always appear to be in the crowded center of the galaxy from the perspective of the observer.
Dark matter seems to be saved, at least in elliptical galaxies, and the earlier work proves even more useful. “”Our conclusion is that what they saw is exactly what the cold dark matter model would predict,” said Joel Primack, professor of physics at the University of California, Santa Cruz, and a coauthor of the Nature paper. “Their data are great, and this actually gives us more insight into how elliptical galaxies form.”
The paper is Dekel, Stoehr, Mamon et al., “Lost and found dark matter in elliptical galaxies,” Nature 437, 707-710 (29 September 2005), p.707.
by Paul Gilster | Sep 29, 2005 | Missions, Outer Solar System
The New Horizons spacecraft, slated for a January launch and a decade-long journey to Pluto and Charon, has arrived at Kennedy Space Center for final preparations and testing. This follows a four-month series of tests at Goddard Space Flight Center and the John Hopkins University Applied Physics Laboratory, where the craft was designed and built.
What’s in the immediate future for New Horizons? The October testing period includes readiness checks, tests of instrument functionality and checks on communications via NASA’s Deep Space Network. Hydrazine fuel for attitude control and course correction maneuvers will be loaded in November, and the craft will then undergo a final spin-balance test. A launch countdown rehearsal will be held in November, and in December the spacecraft will be loaded onto the Atlas V rocket that will carry it aloft. Launch is now scheduled for January 11, 2006, with later launch windows available daily between January 12 and February 14.
by Paul Gilster | Sep 28, 2005 | Communications and Navigation |
Celestial mechanics seems a long way from atomic physics, but new work by scientists and engineers suggests some remarkable parallels. In fact, the mathematics describing both have provided new designs for space missions, as witness the Genesis spacecraft, which returned particles from the solar wind to Earth. Genesis’ highly unstable orbit was controlled by the L1 Lagrange equilibrium point, a point between Earth and the Sun where the gravity of both bodies is balanced. The orbit is an example of a chaotic trajectory identical to those traversed on the atomic level by highly excited electrons.
Image: The extraordinary orbit of the Genesis spacecraft, a lesson in controlling chaos. Credit: Jet Propulsion Laboratory.
The linkage between orbits and atoms is discussed in an article running in an article called “Ground Control to Niels Bohr: Exploring Outer Space with Atomic Physics,” running in the October 2005 issue of Notices of the American Mathematical Society. The article is also available online through the AMS Web site (free registration required). Authors Mason A. Porter and Predrag Cvitanovic write that the “…almost perfect parallel between the governing equations of atomic physics and celestial mechanics implies that the transport mechanism for these two situations is virtually identical…”
This is provocative stuff, but with such insights, engineers, mathematicians and physicists were able to set Genesis on a trajectory that met its misson goals with a minimal use of fuel. From the article:
On the celestial scale, transport takes a spacecraft from one Lagrange point to another until it reaches its desired destination. On the atomic scale, the same type of trajectory transports an electron initially trapped near the atom across an escape threshold (in chemical parlance, across a “transition state”), never to return. The orbits used to design space missions thus also determine the ionization rates of atoms and chemical-reaction rates of molecules!
Centauri Dreams’ take: In addition to providing new tools for mission designers, this work is valuable because, like spacecraft, comets and asteroids can also behave in a chaotic way, making an accurate determination of some orbits difficult. Creating a statistically valid description of near-Earth asteroids demands tools that can make sense of chaotic systems. Clearly, the crossover work occurring between astronomers, mathematicians and chemists can provide unexpected insights toward this goal.
by Paul Gilster | Sep 27, 2005 | Asteroid and Comet Deflection, Outer Solar System
There aren’t many natural disasters we know how to prevent, as the recent sad events along the Gulf of Mexico demonstrate. But one thing we can manage with today’s technology is to deflect an incoming asteroid so that it doesn’t destroy a large chunk of the Earth. At least, we think we can manage it, but it will take technology testing like the European Space Agency’s Don Quijote mission to see whether asteroid deflection really is within our capabilities.
Don Quijote is envisioned by ESA’s Advanced Concepts Team as a two-part mission. One spacecraft, named Hidalgo, is to strike the asteroid; the other, named Sancho, is to orbit the asteroid months before Hidalgo’s advent, observing it before and after impact. ESA has now selected two target asteroids for this mission, designated 2002 AT4 and 1989 ML. Design options for the twin spacecraft are now under active consideration.
Image: An artist’s impression of an asteroid striking the Earth. Credit: ESA.
But wait — isn’t deflecting even a low-risk asteroid asking for trouble? This ESA news release says no:
Even a very dramatic impact of a heavy spacecraft on a small asteroid would only result in a minuscule modification of the object’s orbit. In fact the change would be so small that the Don Quijote mission requires two spacecraft – one to monitor the impact of the other. The second spacecraft measures the subtle variation of the object’s orbital parameters that would not be noticeable from Earth.
In other words, choosing an asteroid that is not an Earth-crosser will allow deflection testing with no risk to Earth. A final decision between the two ‘finalist’ asteroids for the Don Quijote mission will not be made until 2007.
And remember 2004 MN4, an asteroid some 400 meters in diameter once thought to pose a risk for impact in 2029? It has now been in the news enough to have been given an official name, (99942) Apophis. Recent observations at Arecibo have reduced the impact possibility sharply for 2029, although this is quite a near miss, a close approach of about 32,000 kilometers, which places the asteroid within the geosynchronous orbit used by telecommunications and weather satellites. And just because 2029 is unlikely to see a collision, we don’t know how the close pass will affect the future path of the asteroid.
All in all, Don Quijote is a necessary and worthwhile mission, and one of its significant effects should be to continue to raise public awareness of the threat of such impacts. Be aware as well of the active work of the B612 Foundation, whose goal is to …”significantly alter the orbit of an asteroid in a controlled manner by 2015.”
