WISE witnesses Trojan war

Brand new results from the WISE telescope, presented today at the 216th American Astronomical Society meeting in Miami, Florida, have revealed a stunning new look at the star-forming Heart and Soul nebulae, plus indications that smaller asteroids shadowing Jupiter are undergoing frequent collisions.

WISE, the Wide-field Infrared Survey Explorer launched by NASA in December 2009, is designed to observe anything in the Universe that is “old, cold and dusty”. The latest spectacular imagery from the orbiting telescope’s 40cm mirror provides a unique look at the Heart and Soul nebulae (IC 1805 and IC 1848) unfamiliar to those acquainted with its appearance in visible light. The picture combines 1,147 individual frames, just a small fraction of the million images WISE has taken of the infrared sky thus far, covering 30,000 angular degrees.

The Heart and Soul nebulae, imaged by WISE. Image: NASA/JPL�Caltech/UCLA.

The Heart and Soul nebulae are located about 6,000 light years away in Cassiopeia, in the direction of the Perseus spiral arm. The dusty nebulae, which span 580 light years in diameter, hide newborn stars blowing bubbles in the gas with their hot ultraviolet radiation. At top right in the image is a particularly brilliant star-forming region designated W3.

“Many of the dust features glowing green in the infrared composite image just don’t show up in visible light, in particular much of the dust in W3,” says Edward ‘Ned’ Wright of the University of California, Los Angeles, who is the principal investigator on the WISE mission. The colours in the image are false colour, designed to be representative of the infrared light that our eyes cannot detect. Blue and cyan represent light mainly from stars at 3.4 and 4.6 microns, while green (12 microns) and red (22 microns) indicate warm dust.

WISE’s mission isn’t just to take pretty pictures; a large part of its portfolio is to study and discover dark objects in the Solar System, such as asteroids and comets. “We are getting Solar System data out to the astronomical community every day, because it needs to be fresh or else you lose those asteroids,” says Wright. To date, WISE has observed 201 Near Earth Objects (NEOs) discovering 55 of them, approximately 61,000 Main Belt asteroids finding about 11,000 new ones, has spotted 51 short period and 22 long period comets, finding four and eight new ones respectively, and also 700 Trojan asteroids, of which about 50 are new.

The Trojans hang out in the stable gravitational traps of the Lagrange 4 and 5 points, sixty degrees in front and behind Jupiter in its orbit. Following up on previous surveys of the Trojans by the Infrared Astronomical Satellite (IRAS) in the 1980s, and the Spitzer Space Telescope, WISE has been able to confirm that the smaller Trojans with low inclination orbits are actually the brightest. The reasoning behind this is that in the plane of Jupiter’s orbit, there are more Trojans, increasing the chances of collisions that smash them into smaller pieces.

“The reason they are brighter lies in the fact that asteroids are subject to space weathering,” explains Tommy Grav of Johns Hopkins University in Baltimore. “Cosmic rays and dust impact on them, evolving a tar-like substance that explains why asteroids don’t reflect much light. But a collision can excavate fresh, bright material from below the surface.”

NEO studies are of more immediate relevancy to Earth. WISE’s infrared eyes are capable of measuring the sizes of asteroids more efficiently than visible light observations can. “We want to know with great accuracy the size of NEOs so that we know how much damage they could do if they were to hit Earth,” says Grav. WISE can also accurately measure the rotation rates of asteroids, which tell us partly about their structure and composition.

WISE has enough cryogen to keep its infrared cameras cool until November 2010 (the spacecraft has to be chilled to avoid its own thermal emissions interfering with the observations), but it has already far surpassed previous infrared survey missions. “We’ve done in six months what it took 100 years to do in the optical,” says Grav, referring to WISE’s Solar System work. The first ever infrared survey, the Two Micron Sky Survey in 1969, catalogued 5,000 infrared sources. IRAS identified 250,000 sources after it launched in 1983, but by the end of its mission WISE is expected to have spotted half a billion infrared objects. Such a colossal survey will keep astronomers finding new surprises in the data long after WISE has been decommissioned. As Eric Young, a scientist at the Universities Space Research Association, comments, “WISE is a mission that will continue to give and give for years to come.”