Hubbub on Hubble
One of the tools used to forecast the health of the Hubble Space Telescope is called the ‘Reliability Model’. This engineering tool, developed by the Ball Aerospace Corporation and validated by the Hubble Independent Review Team, is considered extremely useful in planning servicing mission strategies, payload content and priorities for the world’s largest orbiting telescope, one of the four Great Observatories.
|Hourglass marking dawn since nebula, an exploded star peering back through time.|
The science lifetime of the Hubble Telescope is limited by four factors: its gyroscopes, instruments, orbital decay, and optics.
Among these, the health of the Hubble spacecraft itself is considered most important in achieving continued science operations through 2010.
Even with the servicing of Hubble in an expected 2005 service mission, the goal of meeting science operations through 2010 however will likely fall short, according to the recent findings of the Reliability Model.
In considering science operations beyond 2010, spacecraft health continues to be a major factor, and potentially orbital decay can affect the longevity of Hubble science. To overcome this, a servicing mission between 2009 and 2010 that includes a re-boost activity would be required. With such a servicing mission, the Hubble science lifetime could be expected to continue into 2015.
The reliability model forecasts that 5 years after Servicing Mission 4, the Hubble’s system-level probability for continued science operations is 30 percent. Thus, given the current launch date baseline of May 5, 2005, the formal probability of continuous science operations through May 2010 is 30 percent. Further, the likelihood of continuous science operations through May 2011 is formally only 18 percent. However, the situation may not be quite so dire.
The model predictions are conservative in two ways. First, an individual part failure is assumed to cause loss of function for the hardware component modeled. Components may still function at some level in spite of a single part failure. Secondly, loss of hardware functionality may be mitigated by workarounds, e.g., added flight software functionality to overcome the hardware problem. It is simply not possible in advance of a failure to evaluate how overly-pessimistic the model predictions might be.
Probability of Science
Prior to the next servicing mission, the model indicates that the reliability of the Hubble gyroscopes dominate overall system reliability.
Three gyroscopes are required to perform Hubble science operations. Of the six gyroscopes on board Hubble, four are currently functional (Gyroscopes 1, 2, 4, and 6) and two have failed (Gyroscopes 3 and 5). Given the current service mission launch date of May 5, 2005, the probability of 3 or more functional gyroscopes still available then is approximately 70 percent. However, the probability of 3 functional gyroscopes decreases rapidly for a launch date after May 2005.
From the predictions, the probability of 3 functional gyroscopes is approximately 50 percent as of December 1, 2005, and only 30 percent as of July 1, 2006.
It should be noted that the modeling of gyroscope failure probabilities is statistically stronger than has been possible for other spacecraft components. It is based on the performance histories, both in ground testing and in flight, of over 90 similar gyroscopes across many programs. The gyroscope analysis includes both random failures and physical wear-out.
It is of interest to consider the historical record of the durations between Hubble servicing missions, vis a vis cessation of science operations.
- Launch (4/90) to Service Mission 1 (12/93) – 44 months
- Service Mission 1 to Service Mission 2 (2/97) – 38 months
- Service Mission 2 to Service Mission 3A (12/99) – 34 months (loss of science operations 11/99)
- Service Mission 3A to Service Mission 3B (3/02) – 27 months
- Service Mission 3B to Service Mission 4 (~5/05) – >= 38 months
Only one episode of cessation of science operations has occurred, for approximately six weeks prior to the launch of SM3A, due to gyroscope failures. Currently the Pointing Control System requires 3 working gyroscopes. Less than 3 gyroscopes leads to Hubble entering ‘Zero-Gyro Safemode’, as happened just before SM3A. The Hubble Program is actively planning the potential implementation of a 2-gyro mode for science operations, that would enable continued science operations, with some loss or degradation of observing capabilities, for approximately 12 to 15 additional months.
Loss of Scientific Instrument Capabilities
Hubble’s scientific instruments are each designed for a five-year operational lifetime in orbit.
|Fragments of Comet P/Shoemaker-Levy 9 colliding with Jupiter (July 16-24, 1994).|
Historically, they have lasted considerably longer than that. Only one instrument, the Goddard High Resolution Spectrograph (GHRS), has suffered an electrical or mechanical failure that left it completely inoperable. That failure occurred in 1997, just a few weeks before the GHRS was to be removed from Hubble during the second servicing mission. The GHRS was one of the original five instruments launched on Hubble in April 1990. After about two years of operation, GHRS lost a portion of its observing capabilities, but a simple repair by the astronauts restored these during Servicing Mission 1 (SM1) in 1993.
The Near Infrared Camera and Multi-Object Spectrometer (NICMOS) was originally designed to be cooled by a block of solid nitrogen ice, with an expected lifetime of about four years. A thermal short in the instrument reduced its ‘first life’ to about 22 months. However, during Servicing Mission 3B (SM3B) in March 2002, astronauts installed a newly designed mechanical cooling system and the instrument’s science operations were resumed (with a 30-50% improvement in sensitivity).
The Space Telescope Imaging Spectrograph was launched along with NICMOS in Servicing Mission 2 (SM2), February 1997. In the summer of 2001 Side 1 of STIS’ electronics failed irretrievably and it is currently operating on its redundant Side 2. Thus, even with a significant electronics failure, STIS continues to operate nearly two years beyond its design lifetime.
The current record holder for longevity among Hubble’s instruments is the Wide Field and Planetary Camera 2 (WFPC2), which has operated without significant failures for nearly ten years, since its launch in December 2003. Scientifically, it has been superseded by the Advanced Camera for Surveys (ACS) and will be replaced by Wide Field Camera 3 (WFC3) during Servicing Mission 4 (SM4) in 2005.
|Spectacular gas remnants from exploding star.|
Image Credit: Hubble
At the conclusion of SM4, Hubble’s complement of scientific instruments will consist of two spectrographs and three cameras (as well as a Fine Guidance Sensor useful for astrometry). Although there is some overlap of function, as a general rule these instruments are complementary to each other in their designs. The new Cosmic Origins Spectrograph (COS) provides some backup to STIS. However, it does not replicate STIS’ capabilities for long-slit imaging spectroscopy, needed for the detection of massive black holes in galactic nuclei, nor for very high-resolution spectroscopy required for measurements of chemical abundances in the interstellar gas, for example.
The COS is purely an ultraviolet spectrograph and does not provide a backup to STIS‘ coverage from 300 to 1000 nm. The new WFC3 will supplant NICMOS for most near-infrared imaging, and will complement ACS by providing Hubble’s first wide field, high sensitivity imaging capability at near-ultraviolet wavelengths. The WFC3 provides some backup to ACS for visible light imaging, but is not as sensitive as ACS at the red wavelengths of interest for studies of galaxy evolution.
|Scientists would like to know the origin of the atmospheric patches imaged on Saturn’s moon, Titan, as imaged by Hubble. Image Credit: Hubble Space Telescope/UA Smith|
The lifetimes of WFC3 and COS should extend to 2010. Thus, in 2010 we would expect Hubble still to have powerful instrumentation both for ultraviolet spectroscopy to very deep levels of sensitivity and for high-resolution, wide-field imaging spanning the range 200 – 1700 nm (nanometers, wavelengths).
The ACS will be about two years beyond its design lifetime in 2010, but can reasonably be expected still to be in operation. Its CCD detectors will have suffered significant levels of degradation in both charge-transfer efficiency and growth of the population of unusable ‘hot pixels’ to about 6%. However, these signs of ‘aging’ potentially affect only a sub-set of ACS science and there likely will be operational approaches to mitigating them.
Mechanical cooling systems such as the one now cooling NICMOS have demonstrated lifetimes of over a decade in ground testing. The life-limiting element for NICMOS is likely to be the Power Conversion Electronics for the cryocooler that provides power to the microturbines. This subsystem is expected to have a 9% probability of failure after five years of operation. The near-infrared channel of WFC3 will supplant most of NICMOS’ science capabilities.
There is no way of predicting STIS’ longevity, as it operates on its remaining redundant electronics. It could fail tomorrow, or it could last many additional years.
Hubble’s science lifetime could potentially be limited by spacecraft orbital decay. Long-term orbit decay predictions are developed based on atmospheric models and solar flux predictions. All contributing combinations of solar flux strength and timing are run in order to bound the orbit decay predictions from a best case atmosphere to a worst case (‘unkind’) atmosphere. The predictions also consider the effects of Space Shuttle re-boost during Hubble Servicing Missions. A worst case is considered a high solar cycle (the eleven-year peak intensity in solar flares), followed by an early onset to another solar cycle of average intensity–both of which describe an ‘unkind’ atmosphere that can force an earlier than hoped-for orbital decay.
For the case of no further Hubble re-boost in any future servicing mission, the prediction is that Hubble will reenter the Earth’s atmosphere in late 2013 or early 2014. The Hubble science program would cease approximately one year prior to re-entry due to loss of the precise attitude control capability required for science observing, as the atmospheric drag increases. The earliest expected end of the Hubble science program due to orbital decay is thus late 2012.
Degradation of Primary Optics
Degradation of the Hubble’s primary optics could potentially degrade the scientific performance of the observatory.
|In a universe brimming with stars…|
Contaminants on the primary or secondary mirror, or physical degradation of the mirror surfaces can result in loss of sensitivity or changes in the properties of stellar or other point-source images. To date, there is no evidence of any loss of throughput of the telescope at the level of uncertainty of the measurements (3-5%), nor is there any evidence of changes from increased wide-angle scattering due to pitting of the mirror surfaces.
In mid-August, NASA will launch the Space Infrared Telescope Facility (SIRTF)– the fourth and final element in the family of Great Observatories. Each observatory examines the heavens in a different electromagnetic spectrum . Because they are space borne telescopes, orbiting above the distorting atmosphere of the Earth, they are able to gain unprecedented views of the universe. In addition to Hubble, the Compton Observatory , launched in 1991, examined gamma rays until its mission ended in 1999. The Chandra Observatory , launched in 1999, examines X-rays and is scheduled to operate through 2004.
Like the Hubble Space Telescope , SIRTF has large mirrors that will provide unprecedented views of the universe . But while Hubble is mainly a visible-light telescope, SIRTF will detect infrared light. In other words, SIRTF will be hunting for heat.
Also due for launch in 2009 is the NASA-ESA Next Generation Space Telescope, or NGST [James Webb Space Telescope], a near-infrared telescope that will succeed the Hubble Space Telescope. Like Hubble, NGST will be a general-purpose telescope with an emphasis on cosmology. But it will investigate stars with dusty disks – the early stage of planet formation – and may also be able to study Jupiter-size planets.