Hubble Status Report - May 2007
HST entered inertial safe mode on Saturday, January 27, 2007, when a pressure sensor located in the section of the telescope that houses the science instruments detected a rise in pressure. At the same time, an electrical fuse blew up in the Advanced Camera for Surveys (ACS), probably as the result of a short circuit. An anomaly review board of engineers and scientists is still investigating the situation and will present its findings in April. The failure is severe and precludes ACS CCD observations with the Wide Field Channel and the High Resolution Channel. The Project and the STScI restored Solar Blind Channel (UV) in time for the joint HST and New Horizons observations of Jupiter, during the New Horizons' fly-by only two weeks later. Excellent scientific data were obtained.
The manifest for the Servicing Mission 4, currently planned for September 2008, includes two new instruments, the Wide-Field Camera 3 (WFC3) and the Cosmic Origins Spectrograph (COS), as well as several life-extending items such as gyroscopes and batteries. The astronauts will also attempt to repair the Space Telescope Imaging Spectrograph (STIS), which failed in August 2004. Work is under way to analyze possible scenarios for the partial repair of the ACS, however until the failure location and mechanisms are fully identified these plans are just preliminary and there is no strong expectation that they may even be feasible.
The HST spacecraft continues to operate nominally in the two-gyro mode; the STScI staff has succeed in minimizing the restrictions of this mode of operation and the efficiency rapidly returned to the earlier values (greater than 50%) when three gyros were being used.
ACS Proposals Review
A team of STScI scientists reviewed about 120 currently active Cycle 14 and 15 ACS proposals. In a two-stage process, the goal was to ascertain, first, if a proposal could be converted to other science instruments, particularly WFPC2, from the technical point of view, and second, whether the science goals of the program could be achieved with the new configuration. This process was completed at the end of February, with a number of programs recovered and are being implemented using WFPC2.
The formal Cycle 16 deadline occurred on Friday, January 26, the day before the ACS failure. A total of 747 proposals were received, including 498 to use ACS/WFC or ACS/HRC. Since the latter proposals were no longer viable and in order to ensure the accommodation of the science areas covered by those programs, the HST Cycle 16 deadline was extended to Friday, February 9. PIs who submitted proposals for ACS observations with either WFC or HRC were encouraged to consider whether those observations could be made with WFPC2. Contrary to expectations, the total number of new and revised proposals rose to over 850 and the request for observing time exceeds that available by a factor of 7! The HST TAC met during the week of March 19 and the results of their selection were announced one week later. European proposals did not do so well in this selection, only 13% of the selected proposals were from PIs of ESA member states. This probably reflects the better readiness of US scientists to react to the loss of ACS by submitting proposals to use other HST instruments.
On the other hand, in October of 2006 the STScI had invited the community to submit proposals to use during the current Cycle 15 should something happen to the ACS, as indeed has been the case. Of the six selected contingency programs, three are from PIs of ESA member states. These programs have now been fully activated.
Scientific Highlights
Scientific results continue to accrue even after the partial loss of ACS. Images were obtained using ACS' ultraviolet camera in support of the New Horizons mission en route to Pluto. New Horizons made its closest approach to Jupiter on February 28, 2007. The ultraviolet images show auroral emissions that are always present in the polar regions of Jupiter. They are typically 10-100 times brighter than the aurorae seen on the Earth. The equatorial regions of Jupiter were imaged in blue light on February 17, 2007 with the Wide Field Planetary Camera 2. This reveals cloud features in Jupiter's main atmosphere. In the ultraviolet views, the atmosphere looks hazier because sunlight is reflected from higher in the atmosphere. HST will continue to photograph Jupiter as well as its volcanically active moon, Io, over the next month as the New Horizons spacecraft flies past Jupiter.
A team of European and US scientists obtained an HST view of the nearby barred spiral galaxy NGC 1672, which unveils details in the galaxy's star-forming clouds and dark bands of interstellar dust. Some of the most striking features are the dust lanes that extend away from the nucleus and follow the inner edges of the galaxy's spiral arms. Clusters of hot young blue stars form along the spiral arms and ionize surrounding clouds of hydrogen gas that glow red. Curtains of dust partially obscure and redden the light of the stars behind them by scattering blue light. Galaxies lying behind NGC 1672 give the illusion they are embedded in the foreground galaxy, even though they are really much farther away. A few bright foreground stars inside our own Milky Way Galaxy appear in the image as bright objects. As a prototypical barred spiral galaxy, NGC 1672 differs from normal spiral galaxies, in that the arms do not twist all the way into the center. Instead, they are attached to the two ends of a straight bar of stars enclosing the nucleus. Viewed nearly face on, NGC 1672 shows intense star formation regions especially off in the ends of its central bar.