Astronomy may be revolutionized more than any other field of science by observations from above the atmosphere. Study of the planets, the Sun, the stars, and the rarefied matter in space should all be profoundly influenced by measurements from balloons, rockets, probes and satellites. In a new adventure of discovery no one can foretell what will be found, and it is probably safe to predict that the most important new discovery that will be made with flying telescopes will be quite unexpected and unforeseen.
– Lyman Spitzer, Jr., 1961
Let’s talk about birthdays! No, not mine – though today is my birthday – but, rather, the Spitzer Space Telescope’s. Yes, spacecraft do celebrate birthdays in a way: shortly after midnight on the morning of August 25, 2003, a Delta II rocket lifted from the launchpad at Cape Canaveral, lofting what was then called SIRTF – the Space Infrared Telescope Facility – into the sky and beginning its journey of the exploration of the universe. So, happy belated 13th birthday to you, Spitzer Space Telescope!
This year, for Spitzer’s birthday, we all received a present: the announcement that the observatory will soon begin the “Beyond” phase of its mission. What does that mean, exactly? It means that Spitzer will be providing us with two more years of great infrared imagery and science, bridging the time before the launch of NASA’s next infrared observatory, the James Webb Space Telescope. That might not seem all that significant, but underlying this announcement is a remarkable story of engineering ingenuity and resourcefulness.
You see, Spitzer was never meant to last this long. NASA’s spacecraft have gained a reputation for operating well beyond their expiration dates. One famous example is the Mars rover Opportunity, which is currently on sol (Mars day) 4476 of its originally-planned 90-sol mission. However, for Spitzer, operating this long isn’t just a simple matter of longevity. To keep on going, Spitzer has had to overcome a variety of engineering challenges along the way, adapting to new and innovative ways of operating.
These challenges stem from the telescope being designed to observe the infrared spectrum. In a sense, just like a pair of night-vision goggles, this means the telescope is essentially looking at heat, as hotter objects tend to emit more infrared radiation. This poses a unique and interesting challenge: in order to prevent the telescope from drowning itself out with its own infrared emissions, the telescope’s mirrors have to be kept extremely cold: 5 Kelvin, or about -450°F! To achieve this frigid temperature, the observatory utilized active cooling using liquid helium, and passive cooling by using its large solar panel as a Sun shield.
The active cooling is fairly simple to understand: the liquid helium absorbs heat and evaporates. The gas is then vented overboard, taking the excess thermal energy with it. Though simple and effective, the downside of this method should be immediately obvious: if you keep discarding your supply of helium, you’ll eventually run out. Spitzer was launched with enough helium to cool the telescope through 2009, the duration of its originally-planned mission. And on May 15, 2009 – right on schedule – the helium tank ran dry. Without the aid of refrigerant, the Sun shield on its own is only able to keep the telescope cooled down to a temperature of 28 Kelvin (roughly -400°F). While still quite chilly, this is warm enough to drown out two of Spitzer’s three instruments, rendering them unusable: the Infrared Spectrograph (IRS) and the Multiband Imaging Photometer for Spitzer (MIPS).
However, the Spitzer team made a wonderful and fortuitous discovery: two of the four wavelength bands of the Infrared Array Camera (IRAC) are still able to function at this warmer temperature with little impact to image quality. Thus began the “Warm Mission”: Spitzer’s first stay of execution.
Not too long after that, in January 2011, I joined the Spitzer Space Telescope team as a flight controller for the Pointing Control Subsystem (PCS) – the collection of sensors, actuators, and software responsible for keeping the telescope pointing in the right direction. After a whirlwind three months of training under the tutelage of veteran PCSer Ryan Olds, I was certified for on-console work. Interplanetary and astrophysics missions tend to have small mission control teams – much smaller than the Shuttle or ISS mission control rooms you frequently see on TV – so I was thrust immediately into a position of responsibility. In addition to maintaining the day-to-day operations of the telescope with Noel Hughes, fellow PCS and Mars Odyssey Attitude Control System (ACS) controller, I was also tasked with working on solution to another looming hurdle.
In order to get away from all the infrared noise emitted by the planet Earth, Spitzer chases our planet around the Sun in an Earth-trailing orbit, slowly falling further behind. To talk to mission control, the spacecraft tilts to point its high-gain antenna – the silvery dome fixed to the back end of the spacecraft in the artist’s impression above – back toward Earth. There’s a problem, though. As the telescope moves farther away, the angle it needs to tilt gets larger and larger, as illustrated by infographic to the left. The Sun shield does a good job of keeping the telescope cool, but it only works if it’s pointed in the direction the Sun. Eventually, in the Fall of 2013, it was expected that the tilt angle needed would exceed the maximum angle the Sun shield had been designed to cover. We expected the spacecraft “bus” – the section of the spacecraft containing all of the computers and supporting systems – might provide some measure of thermal protection, but we didn’t know for sure. Nobody had performed a thermal analysis of this particular scenario before – after all, the mission originally hadn’t been expected to continue past 2009. Without help from the liquid helium, it’d been estimated that the telescope could take up to a year to cool down again if the Sun were to light it up, so any slip-up had serious consequences. We were marching into uncharted territory!
As a PCS engineer, my job was to prepare the pointing system to reach angles it hadn’t been designed or programmed to reach. Building on work laid down by Ryan and previous PCSer Michael Epstein, we identified software – both on the spacecraft and in our ground software – that had been hard-coded to prevent the spacecraft from exceeding these limits. We ran multiple tests in our spacecraft hardware-in-the-loop simulators, breaking and reprogramming these virtual roadblocks, only to find more that we hadn’t considered – the original software programmers had been determined to ensure we never broke those angle limits! And we rewrote calibration sequences and simulator test routines to calibrate our pointing sensors for flight regimes they’d never before encountered.
Of course, software and programming is only part of the story. Eventually, you have to test for real.
On January 16, 2013, a flight sequence programmed by me and Systems engineer Paul Travis took Spitzer past an angle of 30° for the first time ever during the mission. The sequence took the telescope on a brief “toedip” outside the angle limit, confirming we’d found all of the software preventing us from passing that limit, and ensuring some nasty thermal surprise wasn’t waiting for us just on the other side. Over the following weeks, we performed more tests, taking the spacecraft out into the unknown for longer periods of time. This allowed us to gain confidence with the spacecraft and calibrate the pointing sensors using our new calibration routines. All this work culminated in October, when, for the first time, Spitzer spoke to us over its high-gain antenna while pointed past 30°. Spitzer had overcome yet another hurdle!
Spitzer has faced a number of other challenges as well. As Spitzer moves ever farther from Earth, the Deep Space Network dishes on the ground have a harder time “hearing” Spitzer. Not long after I started, the science and engineering teams implemented newer, advanced data compression algorithms to enable more science data to be transmitted over the limited bandwidth. And a couple of years ago, in 2014, this effect caused the normal signal from the spacecraft’s emergency low-gain antennas to become too weak to be able to be heard from the ground. To get around this, we updated the spacecraft’s automatic fault protection software with a new “Carrier-Only Safe Mode”. This allows the spacecraft to be safely commanded and recovered from problems by mission control while receiving only a data-less “carrier-only” radio signal from the spacecraft.
This most recent milestone – the beginning of the “Beyond” phase – represents the team overcoming even more challenges. As the spacecraft reaches a tilt angle of 45°, the spacecraft’s Sun sensors – essentially 45° pyramids of miniature solar panels – are unable to accurately measure the tilt of the spacecraft. Working without input from these Sun sensors while at high angle is one of the increased risks that the news from NASA is talking about. Additionally, as the tilt angle turns the solar panels further away from the Sun, recent additional programming and mission planning rules have implemented a new “cool and recharge” mode. In this mode, the spacecraft points the solar panels directly at the Sun and performs only low-angle science for a certain amount of time after talking with Earth to help the spacecraft bus to cool down, and to recharge the batteries. In the spirit of gaining lots of responsibility quickly, a good amount of the PCS portion of this work has been performed by Patrick Haas, who took over my place on the team just after graduating from the University of Colorado.
It’s been a great run so far! Time and time again, Spitzer has proven itself to be the little space telescope that could, handling whatever challenges came its way. In the meantime, it’s given us a great deal of science data, allowing us to better understand our universe. It’s helped us find and study exoplanets – planets orbiting distant stars – allowing us to characterize their sizes and orbits, determine the compositions of their atmospheres, and even measure their weather patterns (capabilities that weren’t designed into it or even imagined when it launched!). It’s also given us GLIMPSE – the Galactic Legacy Infrared Mid-Plane Survey Extraordinaire – an enormous, 20-gigapixel infrared mosaic of the entire Milky Way (“If we actually printed this out, we’d need a billboard as big as the Rose Bowl Stadium to display it,” imaging specialist Robert Hurt explains).
To everyone on what is perhaps the best engineering team I’ve ever worked on – the folks at JPL, Caltech, Lockheed Martin, and all the researchers around the world (sadly, way too many great people to list by name, but you all know who you are!) – a big congratulations on keeping Spitzer flying! Here’s to another two years of great infrared science!
For all you readers at home…If you’d like to learn more about the Spitzer Space Telescope, stop by the Spitzer website for news, spacecraft and mission information, and, of course, some great imagery of the universe!