Friday, April 24, 2020

Hubble's 30th Birthday: A Personal Memory from Ms. FHST

Has it really been 30 years?  I can think of nothing better than to reprint below my 25th anniversary reminiscence published in my companion journal Alice in and out of State in 2015.  
In honor of the day, I'm wearing my Space Turkey T-shirt.  What I failed to mention in my 25th anniversary article is that this was our informal PASS project T-shirt dating from before Hubble's launch.  It was an inside joke in that one of the engineers in our project loved to call out requirements, designs, and anything else that was not up to snuff a turkey.  During the Hubble Trouble period of spherical aberration following launch, we put those T-shirts away for a several years, but a number of them are still carefully preserved, brought out for special days like this.

What a different world we lived in 30 years ago.  It is a tribute to all in the Hubble project that the mission continues to this day.  My own involvement is one of the proudest episodes in my life.

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30-летие запуска телескопа им. Хаббла:
Воспоминания госпожи астроориентатор

Под этим названием в 2015 былы опубликованы мои воспоминания о том, как я участвовала в запуске телескопа им. Хаббла в 1990.  Не стоит нажать на ссылку.  (
Увы, статью давно стёрли, и я сама потеряла текст на русском.  Даже в машине WayBack не сохранялась.  Приношу свои извинения, что внизу я перепечатаю только текст на английском.

Есть повод снова опубликовать эти воспоминания:  сегодня, 24ое апреля 2020, мы отмечаем 30-летие запуска.  Фотография показывает, как я надела особенную футболку в честь этого дня.  Футболка Космическая индейка являлась шуточным символом проекта, в котором я работала специалистом по системам ориентации.  Не знаю, сколько таких футболок сохранились до сегодняшнего дня.  Их было не больше нескольких дюжин и тогда, но некоторые из нас их нежно сохраняют и осторожно надевают в круглые даты.  Принимать участие в проекте Хаббл -- один из наилучших этапов в моей 40-летнем карьере. 

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Hubble's 25th Birthday: A Personal Memory from Ms. FHST

Hubble Space Telescope was launched by the shuttle Discovery (STS-31) on 24 April 1990 at 12:34 UTC.  For those of us who worked on the project, the inside joke that the “Hubble Constant is 2 years until launch” had been broken.  No longer was this a mission that we were working towards but, rather, a mission that was about to become reality.  The question in all our minds was, “Will it work?  Will all the years of hard work and planning pay off?”

I first joined the Hubble project in 1982.  It hadn't even been named for American astronomer Edwin Hubble yet.  That was to come a year later in 1983.  When I started, it was simply ST, Space Telescope.  I was a comparative latecomer to the project.  For those who had been there at the beginning in the 1970s, it had been the Large Space Telescope, the Large being dropped as budgets and the realities of operating a telescope in space began to settle in.  Still, it was to be Big with a capital B, a 2.4-meter optical telescope that would operate above the distorting layers of the Earth's atmosphere.  It would be controlled remotely from a control center at Goddard Space Flight Center (GSFC) in Maryland with all science planning done at the newly established Space Telescope Science Institute on the campus of Johns Hopkins University in Baltimore.  To those of us who worked there, ST ScI would become known simply as the 'tute, truly an internationally-run observatory whose telescope just happened to be in orbit.

My own role on Hubble was a modest one.  I had an MS degree in astronomy with specializations in celestial mechanics and in astrometry, the science of positional astronomy that compiles positions of stars and other celestial objects.  Thus it was no surprise that my first assignment was to work on the attitude determination system that would use data from spacecraft sensors to determine Hubble's pointing to an accuracy of better than an arcsecond.  The name of the project was PASS, an acronym for POCC Applications Software Support, with POCC itself being an acronymn for Payload Operations Control Center.  Our often repeated inside joke was that we had to be an acronym within an acronym in order not to be a somewhat impolite-sounding POCCASS.

Hubble was to be controlled to an accuracy of 3 milli-arcseconds, finer than any pointing control that had been attempted until that time.  It was to be done using data from gyroscopes, sun sensors, and star trackers.  In 1978, on an earlier mission, I had already made my acquaintance with the Fixed Head Star Tracker (FHST).  Hubble was equipped with three of them, and they would be used to update gyro-based attitudes after every spacecraft slew to a new target.  An FHST had a field-of-view (FOV) of 8-deg by 8-deg and could measure a star's position to 20 arcseconds, good enough to go to the next step of determining what stars were in the field-of-view of the telescope's main optics and use them to determine Hubble's pointing to the sub-arcsecond level.  Little did I know when I joined the project that I was to become the most knowledgeable person on FHSTs, eventually becoming known to many as Ms. FHST.

Cutaway Diagram of a Fixed Head Star Tracker

Hubble was scheduled to be launched by the Space Shuttle in October 1986, and we were all under pressure to complete the ground control systems on time.  The pace was frenetic, and from one system audit review to the next, it was becoming clearer that we would not be ready.  But a shuttle launch could not be changed without upsetting all of NASA's mission schedules.  Senior managers began to think that we would launch Hubble and let it sit in safe mode in orbit, a sort of minimum energy cocoon mode, until the ground systems could be finished and tested.

That went out the window on January 28, 1986, when the Space Shuttle Challenger exploded 73-seconds after launch, killing all on board in the most tragic space accident experienced by the US until that time.  After the tragedy of the loss of all the astronauts on board Challenger had sunk in, we began to realize that our own problem now was not whether we would be ready for a launch in October 1986 but, rather, whether Hubble would be launched at all.  Would the Shuttle ever fly again?  After a few months we were assured that Hubble's launch would take place in 1988.  That launch date soon began to slip, however, leading to our inside joke that we knew the true Hubble Constant[1] to be “two years until launch.”

For me the morning of 24 April 1990 was one of sitting in front of the television and watching the launch and feeling the same thrill I had felt at every launch since the early days of the space program.  This time, however, the thrill was even greater, for Shuttle Discovery was carrying a mission that I had a direct role in.

My own launch excitement in the sense of work, however, began two days later on the morning of April 26.  That afternoon the Canadian-built manipulator arm was to remove Hubble from the shuttle bay and release it into space.  Hubble's systems were being turned on one-by-one and tested before the release.  I had just arrived at my office a short distance from GSFC when a PASS friend and colleague called.  I don't remember his precise words, but they were something like, “Robyn, get out here.  We can't identify what stars the FHSTs are seeing.”  A chill went down my spine.  If Hubble were to be released without the FHSTs being able to identify star patterns, Hubble would be literally lost in space, locked into its cocoon-like safe mode until engineers like me could figure out what had gone wrong.

An hour later I was sitting in front of a terminal in the Space Telescope Operations Control Center (STOCC) at GSFC.   My colleague and friend explained, “We've been trying ever since the FHSTs were turned on, but no matter what we try, the algorithms can't identify the star patterns.”  As calmly as I could, I asked, “Can you get me all the FHST telemetry since the trackers were turned on?  Let's start reprocessing from scratch, taking it step-by-step and paying close attention to detail.”

From my experience on an earlier mission, I already knew just how temperamental FHSTs could be.  These were instruments from before the days of charged couple devices (CCDs).  They used simple optics and an image dissector tube, and they could observe only one star at a time.  A controllable magnetic field was used to cause the dissector tube's photomultiplier and photocathode to scan the FOV in a serpentine pattern and lock onto any object brighter than a threshold magnitude for 20 seconds before breaking track and continuing the scan.  FHSTs had been known to track not just stars but the Moon, planets, nebulae, other satellites, space debris, and even bright cities on the Earth's limb.  The trick was to edit out all the junk so that only star tracks remained and then massage those tracks into point images using gyroscope rate data that measured moment-to-moment spacecraft motion.  Finally, these FHST-measured star positions would be passed into a pattern match algorithm that would take the measured positions and compare them with positions in a star catalog.  That pattern match algorithm required fine tuning in order to work reliably.  All-in-all we had just a few hours to get it right before Hubble would be released into orbit on its own.

Slowly, as calmly as we could, we began reprocessing telemetry from the start.  We edited out spurious objects.  We adjusted the editing parameters to get star images with the smallest possible clump size.  As we worked, I became dimly aware of the big screen that hung at the front of the STOCC.  There was Hubble, perched on the manipulator arm, as the solar arrays began to unfurl, unrolling from their containers and glistening like ever-lengthening, golden sails in the bright sun.  Just as the second solar array finished unfurling, we did it.  We identified the stars that were being seen by the FHSTs.  We did some hand calculation sanity checks to make sure we had identified the right stars.  We had.  “Now let's do it again with another data set,” I said. 

Hubble on its Own, Released from the Maniupator Arm

One data set after another, we repeated the process, making further adjustments until we could identify stars correctly without further intervention from us.  The algorithms we had designed were working.  A higher level mission manager approached and asked, “Are we GO with the FHSTs?”  We nodded yes.  Shortly after we watched in real time as Hubble drifted away from the arm and from the shuttle.  We had done our part.  Hubble would not be lost in space.

That was my role 25-years ago.  My day in the STOCC as the solar arrays unfurled is one of those images frozen in my long-term memory.  Hubble didn't have an easy start.  Soon the newspapers were joking about Hubble Trouble when it turned out that the telescope's main mirror had been ground to the wrong figure and suffered from spherical aberration that was giving blurry images.  My FHSTs were not out of the woods yet either.  Another part of PASS, the Mission Scheduling System, was attempting to use the FHSTs in a way they had never been used before by commanding them to lock on to preplanned reference stars after each telescope slew to a new target.  The FHSTs were failing to find the right stars one time out of three, each failure resulting in the loss of science observations for a good part of an orbit.  It was the second largest problem in Hubble's early operations right behind the flawed mirror.

As they say, however, the rest is history.  Once the mirror's spherical aberration was understood, it was possible to grind corrective lenses that were installed by astronauts on the first servicing mission to Hubble in December 1993.  Those corrective lenses were known by the name of COSTAR, Corrective Optics Space Telescope Axial Replacement, and they silenced the cries of Hubble Trouble, enabling Hubble to give the crisp images that have become part of both our scientific and cultural lives.

For my part, I was brought onto a team whose mandate was to reengineer the Mission Scheduling System.  We were known as MSRE, the Mission Scheduler Re-engineering team.  We pronounced MSRE like ms'ry, and thus our inside gallows humor was “MSRE loves company.”  My part of the mandate was the Pointing Control Subsystem.  Over the next several years, working as a team, we improved the FHST reference star success rate to better than 99%. 

The last effort I had a small hand in before leaving HST and PASS in 2005 was the design of what became known as the Two-Gyro Science Mode that would radically change the pointing control algorithms in a way that had never been attempted before.  A gyro gives information in one dimension, and thus three gyros are needed to know a spacecraft's orientation in three dimensions.  Six gyros were installed on Hubble for redundancy and in the knowledge that gyros are mechanical devices that eventually wear out and fail.  Hubble's gyros began to fail within a few years after launch, but they were replaced during servicing missions.  After the Space Shuttle Columbia disaster in 2003, however, all future servicing missions to Hubble were canceled.  Of the six gyros on Hubble, three had already failed.  It was only a matter of time before yet another would fail and force Hubble into permanent safe mode, ending its mission of scientific discovery.

The idea behind this last effort on Hubble was to take me back to my FHSTs.  Gyroscopes give rate information, whereas FHSTs give position information.  But could we watch stars as they moved in an FHST FOV?  Could those position measurements be used to compute a rate, effectively allowing the FHSTs to take the place of one of the gyros?  The answer was yes, they could.  The newly designed control algorithms were so successful that NASA shut down the third of the three remaining operational gyros in August 2005, keeping it in reserve and thereby extending Hubble's operational life.  Even after a final servicing mission to Hubble was reinstated and six new gyros were installed in 2009, Two-Gyro Science Mode has remained the primary control algorithm for Hubble.

How long will Hubble continue to provide us with the beautiful photos and ground-breaking science for which it has no equal?  Current estimates are that Hubble will continue to operate at least until 2018, when the next generation James Webb Space Telescope is scheduled for launch.  It may continue in operation well beyond that as long as budgets allow and spacecraft systems continue to function.  Not bad for a telescope that was designed and built with 1970s and 80s technology and that many thought would not last for its original projected lifetime of 15 years.

If you're wondering by now how it was that this engineer left the Hubble project to start a diplomatic career with the U.S. State Department, the answer is that even in those days, I had something of a double life.  Outside of my day job on the Hubble project, I was known as a historian of Soviet science.  In the summer after Hubble's launch, I published perhaps my most important history work on Soviet astronomy in 1936-37 during the height of Stalin's Great Purges.  When I left the Hubble project in 2005, in a sense I exchanged my hobby for my career, my career for my hobby.

But on this April 24th, on the 25th anniversary of Hubble's launch, my mind will be back there, reliving the moments of frustration and exhilaration and recalling the faces and names of so many colleagues and friends from the PASS project who were there at the beginning.  And Ms. FHST will smile and feel an inner warmth to know that her children-in-engineering, those three Fixed Head Star Trackers on Hubble, have not missed a beat and continue to guide Hubble on to discoveries that take us back ever further towards the dawn of our Universe.

2014 Reunion Picnic with PASS Friends and Families

[1] The actual Hubble Constant is a measure describing the expansion of the Universe.  The current best estimates are in the vicinity of 71 km/s/Mpc, where Mpc is a megaparsec, a distance of approximately 3.3 million light years.

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