Category Archives: Basics

Rebirth

The heart of NCT–its detectors and readout electronics–survived our launch mishap in Alice Springs.  They have been integrated into a new and improved instrument called COSI: the COmpton Spectrometer and Imager.

COSI

COSI during a rollout test in McMurdo.

The COSI team is in Antarctica now preparing for what is hoped to be the first ultra-long duration science flight on a superpressure balloon.  The current generation of students is chronicling their adventures on a new campaign blog

Goodbye, Alice

This afternoon I gave my (fairly limited) input to the NASA board investigating our launch mishap.  Fulfilling that last responsibility brings my balloon campaign here in Alice Springs to a close.  Tomorrow I start my long-awaited journey home!

Obviously this was not the outcome I–we–were working so hard towards.  Nevertheless, I believe the NCT team has reason to be very proud of our efforts.  It was a huge challenge to turn around and fly again on a new continent less than a year after our New Mexico campaign.  Ingenuity, perseverance, and effective teamwork were all crucial in overcoming the obstacles we encountered.  On the way, we had help from many people who proved uniformly capable, friendly, and supportive.  The instrument we rolled out to the launch pad two weeks ago was fully prepared to do excellent science.

I am saddened, though, by the reach of the media coverage of our launch failure.  Videos of accidents have a huge visceral impact, so these images will be more viewed and remembered than those from the many successful launches CSBF conducts in the USA, Sweden, Australia, and Antarctica each year.  The impression created–that NASA ballooning is unsafe or wasteful–could not be more false.  For many types of astronomy, stratospheric balloons provide the lowest-cost, most efficient access to space, costing ten or a hundred times less than satellites which can take a decade longer to prepare.  Balloon scientists build affordable telescopes which do cutting-edge science, enriching our understanding of the universe we live in.  The CSBF personnel perform inherently challenging balloon launches with extraordinary capability, exquisite professionalism, and remarkable good humor.

I started this blog two years ago as a way to share what I was learning about stratospheric ballooning with my friends and family.  After the roller coaster of emotions these campaigns have brought, I continue to be convinced of the compelling story presented by this remarkable scientific enterprise.  It has been a privilege to share its drama, excitement, and culture with a broader audience.

What We Came For

The news at today’s weather briefing was expected, but unwelcome:  The winds at float altitude continue to be as high as 55 knots (nautical miles per hour).  Those high winds will push us east rapidly, forcing us to cut down after less than 20 hours at float to avoid crossing a mountain range.  That’s not enough time make our observations, so: we wait.  The current forecast suggests lower float winds starting next Monday, so we might have a window then.[1]

Still, now seems like an appropriate time to discuss what exactly we’re trying to observe down here in Australia.  Why isn’t twenty hours enough?  What is it we’re looking for?  To my mind, the answer is both simple and complex.  We have specific targets we expect to detect at certain significance, but this individual balloon campaign is situated in a larger context of scientific exploration.

First, the specific targets.  The center of the Milky Way Galaxy is home to two specific sources of gamma-ray emission.  The first is a cloud of antimatter positrons, produced perhaps by black hole binary systems.  When these positrons encounter electrons (their oppositely-charged matter counterpart), they annihilate and produce gamma-rays of very specific energy, 511 keV.  The shape and intensity of the emission produced by this cloud should provide clues to its origin, and satellite experiments have made progress in this direction already.

INTEGRAL 511 keV map (Weidenspointner et al. 2008)

The second major source of diffuse gamma-ray line emission near the Galactic Center and in the Galactic Plane is due to an unstable form of aluminum.  Like all heavy elements (including all those found around us on Earth!), it is produced by massive stars.  Since the aluminum decays away fairly quickly after it is produced, mapping it can tell us about the life cycles of stars and the formation of elements (known as “nucleosynthesis.”)

COMPTEL 26Al map

With its fine energy resolution and broad field of view, NCT is designed to make  maps like these.  Still, one of the great parts of high-energy astrophysics is the wide range of exotic sources, and we hope to do interesting science with some of the other sources we see along the way.  These include supernova remnants and faraway active galaxies driven by accretion disks around supermassive black holes.

One of the most interesting possibilities is to extend our New Mexico observations of the Crab Pulsar (a rapidly rotating neutron star).  As a Compton telescope, NCT is capable of measuring gamma-ray polarization.  This is a technique I am particularly interested in and which is just starting to come into its own.  Polarization measurements of pulsars should help distinguish between models of the source of their gamma-ray emission.

Of course, the sky in gamma-rays is hardly stagnant, and so there’s also room for serendipity:  transients like gamma-ray bursts (my speciality), the more rare soft gamma-repeaters, and large solar flares would all provide great discovery potential if they were to occur while we were flying.

Let’s be clear, though.  Particularly with the forecast of an abbreviated flight–well below our hopes for this campaign–it is unlikely that NCT will make “best-ever” measurements of any source we’re targeting during this flight.  Why do it, then?  First, we hope to fly NCT again in the years to come, maybe even on an around-the-world flight.  The data we obtain now can be combined with that taken later for increased exposure, and we’ll learn more about the analysis methods in the process.  Additionally, our measurements will provide important verification of results obtained by other instruments, particularly those employing different technologies and imaging techniques.  These cross-checks are crucial to the scientific process.  Finally, our efforts help lay the groundwork for future satellite missions, whose higher cost and longer timelines require proven technologies.

The gamma-ray sky is full of fun, interesting things we’d like to learn more about–now if we can just convince the winds to let us take a look!

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Blowing in the Wind

With TIGRE nearing flight readiness–they are planning for compatibility tomorrow morning–today CSBF held the first of what will be ongoing daily weather briefings.  The news was that the upper atmosphere winds will begin turnaround by the end of the week.  Turnaround is the best time to fly balloons, as the prevailing winds are shifting from blowing west to blowing east (or vice versa).  The transition stage tends to have low overall wind speeds, so balloons can float for a long time without going very far.  With the size and emptiness of the Australian continent, catching the peak of turnaround is somewhat less crucial for flight duration than it is in New Mexico.  Nevertheless, we left the meeting motivated to get ready.

Thankfully, Jane and Steve have decreased the noise we were seeing through judicious rerouting of the cables, so tomorrow we will return to integrating the remaining gondola systems.

The regional news station did a report on the balloon launching station and the upcoming campaign recently.  See if you can spot the NCT team member who makes a cameo!

Dependencies

With our large project and small team, at any given time each of us has a to-do list longer than we can read in one sitting.  For the most part, we each choose tasks to work on based on mood or momentum.  In the last few days, though, I’ve been trying to determine which of our myriad of checkboxes is limiting our progress towards the distant but looming goal of flight readiness.

The two most immediate items on that prioritized path are finalizing the detector connections and installing the shields.  Our heavy bismuth germinate (BGO) shields surround the NCT detectors on the bottom and the sides, reducing background from the atmosphere.  Installing them is a risky and dicey procedure, though, so we want to make sure our detectors are behaving as they should and the connections are all good before we wall them off.  All this takes time, consultation, and reference to archival data.  We’re making progress, but it’s not as clear cut as determining that things turn on.

For their part, the shields turned on fine, but we found higher background rates than we expected.  With ground backgrounds poorly characterized and influenced by everything from the presence of smoke detectors to the composition of the hangar cement, sorting this issue out has been a headache.  So far, though, careful tests by Jane and Zach suggest the shields are working as they should.

Steve has been busy building an electronic isolator to report the cryostat temperature.  Alfred and Ming-Zhe are working on the test solar panels.  I’ve made a few minor changes to the flight software, and Alan continues to improve our ground support software.  Alan and I are also discussing our calibration plans–if and when the sources we need arrive!

Keeping it all straight is a task in itself, but we’re helped by great support from team members still at home.

Targets

This morning’s flight requirements meeting with CSBF was smooth.  Alice Springs looks like a great place to fly from–the size and emptiness of the continent allow for long flights before cities or the coast force a cutdown.  Unfortunately, we’re unlikely to be able to take full advantage of the space due to the timing of the campaign.  Our primary targets are up at night, when the balloon cools and drops lower in the atmosphere, making it harder to see.  After a couple of nights, we’ll exhaust our ballast and may not be high enough to continue productively.  We would (and hopefully someday will) get more data from a fall campaign.

This is probably a good point to revisit some projections I made after the last flight for the shape of this campaign.  We had planned for a long-duration, around the world flight departing from Alice Springs, but safety clearances restricted us to a conventional turnaround flight of 1-4 days.  As discussed above, the spring campaign makes a longer flight irrelevant anyway.

One simplification of the shorter flight is that we can power ourselves on batteries alone.  Since our solar panels were a source of major problems in the last flight, we feel a lot more comfortable.  We’ll fly a couple of small test panels to prepare for future flights, though; Alfred arrived today to start work on those.

Our New Mexico flight also featured pointing problems due to our rotor; after the flight, we tracked the problem down to a bubble under one of the strain gauges that measures the torque on the rotor.  Its irregular output kept the pointing feedback loop from working.  With a new gauge glued in, the rotor has been rock solid.

For me at least, the flight requirements meeting also was a reminder that our target readiness date–cued to the winds–is fast approaching.  We’ve got a lot of systems to check out and check off!  For the next few days our focus is on making sure the detectors are performing correctly; as more systems are added, making modifications becomes increasingly difficult.  We also started checking the large BGO shield pieces that help reduce background from outside the field of view.  Determining the dependencies for the tasks to be completed is challenging–maybe it’s time for a Gantt chart!

Tied Up

Sometimes the connections trip you up.  We’ve spent the last several days trying to get our unruly Gorgon of signal cables to lie straight and carry signals cleanly.  Each of our ten detectors has forty channels, so there are plenty of chances to get our wires crossed.  Our signal cables are great–long ribbon cables, each strand of which is actually a coaxial wire with two conductors.  The cables are accordingly much lighter and more compact than individual wires would be, but they are rather delicate.  Routing them safely through the twists and bends of the journey from the cryostat, around the cradle, through a hole into the electronics bay, and to the appropriate board in the right card cage is a challenge.  We’re also finding a few of the connectors are showing signs of fatigue, introducing flakiness into some channels.  Replacing bad cables with our few spares can aggravate that fatigue, though.  As usual there’s a balancing act between pushing for ideal performance and avoiding causing larger problems in solving smaller ones.

The connections in the back can be tricky.

Still, the system is taking shape.  The third payload, HERO, has arrived with their gear, so the hangar is now quite full.  We’re all sharing a single crane along the centerline of the building, so we put the gondola on our cart for now to allow them to use that space to build.  We also started up the flight computer and are now running the system through it.  Since programming the flight software was my main responsibility last flight, it was very satisfying for me to see the flight computer running nicely again.  I also connected our differential GPS antennas, which seem to be functioning just fine.

Flight computer is up!

Tomorrow, a rest day for most of us.  Soon we’ll be calibrating and testing the system with radioactive sources.  Monday also brings our Flight Requirements meeting with CSBF, where we’ll discuss our altitude and duration targets for the flight.

The CSBF personnel have been modifying this crane to use as a launch vehicle.