While this latest close approach places Cassini more than 6,400 kilometers (3,970 miles) higher above Titan's surface than the Jan. 12 flyby, it should not considered of lesser scientific value. Instead, this high-altitude encounter will provide an opportunity for some of the spacecraft's instruments to gain another unique perspective on this crepuscular world.

During T-66, the Imaging Science Subsystem is set to acquire high-resolution observations during and after closest-approach, covering territory from the trailing hemisphere at high southern latitudes northeast to near-equatorial Adiri. On the inbound leg, the Visual and Infrared Mapping Spectrometer will have the opportunity to do one stellar occultation. (A stellar occultation occurs when an intervening body -- in this case Titan -- blocks the light from a star). Thursday's stellar occultation should allow the Cassini science team to further constrain the composition and the spectral properties of Titan's atmosphere.

Although this latest flyby is dubbed "T66," planning changes early in the orbital tour made this the 67th targeted flyby of Titan. T66 is the 22nd Titan encounter in Cassini's Solstice Mission.

The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency and the Italian Space Agency. JPL, a division of the California Institute of Technology in Pasadena, manages the Cassini mission for NASA's Science Mission Directorate, Washington, D.C. JPL designed, developed and assembled the Cassini orbiter. The Huygens probe, built and managed by the European Space Agency, was bolted to Cassini and rode along during its nearly seven-year journey to Saturn, before being released for its descent through Titan's atmosphere.

One member of Atlantis' crew, Leland Melvin, was drafted by the NFL's Detroit Lions in 1986. The crew also flew other NFL-related memorabilia, including jerseys and a football inscribed with the name of every member of the Hall of Fame.

From left: Astronauts Bobby Satcher, Randy Bresnik, and Charlie Hobaugh; Joe Horrigan, Vice President of Communications/Exhibits for the Pro Football Hall of Fame, Steve Perry, President/Executive Director of the Pro Football Hall of Fame; astronauts Berry Wilmore, Michael Foreman and Leland Melvin.

"The explosion dynamics in typical supernovae limit the speed of the expanding matter to about three percent the speed of light," explained Chryssa Kouveliotou, an astrophysicst at NASA's Marshall Space Flight Center in Huntsville, Ala., co-author of one of the new studies. "Yet, in these new objects, we're tracking gas moving some 20 times faster than this."

The new results, published in this week's edition of the journal Nature, used observations from several space and ground-based observatories, including NASA's SWIFT satellite.

The astronomers discovered the ultrafast debris by studying two supernovae at radio wavelengths using numerous facilities, including the National Science Foundation's Very Large Array in Socorro, N.M., and the Robert C. Byrd Green Bank Telescope in West Virginia. One team used the real-time operating mode of the European Very Long Baseline Interferometry Network, an international collaboration of radio telescopes, to rapidly analyze data.

"In every respect, these objects look like gamma-ray bursts -- except that they produced no gamma rays," said Alicia Soderberg at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass.

Soderberg led a team that studied SN 2009bb, a supernova discovered in March 2009. It exploded in the spiral galaxy NGC 3278, located about 130 million light-years away.

The other object is SN 2007gr, which was first detected in August 2007 in the spiral galaxy NGC 1058, some 35 million light-years away. The study team, which included Kouveliotou and Alexander van der Horst, a NASA Postdoctoral Program Fellow in Huntsville, was led by Zsolt Paragi at the Netherlands-based Joint Institute for Very Long Baseline Interferometry in Europe.

The researchers searched for gamma-ray signals associated with the supernovae using archived records in the Gamma-Ray Burst Coordination Network located at NASA's Goddard Space Flight Center in Greenbelt, Md. The project distributes and archives observations of gamma-ray bursts by NASA's Swift spacecraft, the Fermi Gamma-ray Space Telescope and many others. However, no bursts coincided with the supernovae.

Unlike typical core-collapse supernovae, the stars that produce gamma-ray bursts possess what astronomers call a "central engine" -- likely a nascent black hole -- that drives particle jets clocked at more than 99 percent the speed of light.

By contrast, the fastest outflows detected from SN 2009bb reached 85 percent the speed of light and SN 2007gr reached more than 60 percent of light speed.

"These observations are the first to show some supernovae are powered by a central engine," Soderberg said. "These new radio techniques now give us a way to find explosions that resemble gamma-ray bursts without relying on detections from gamma-ray satellites."

Perhaps as few as one out of every 10,000 supernovae produce gamma rays that we detect as a gamma-ray burst. In some cases, the star's jets may not be angled in a way to produce a detectable burst. In others, the energy of the jets may not be enough to allow them to overcome the overlying bulk of the star.

"We've now found evidence for the unsung crowd of supernovae -- those with relatively dim and mildly relativistic jets that only can be detected nearby," Kouveliotou said. "These likely represent most of the population."

For more information, images and animations about this discovery, visit: