When one wishes to learn about a specific program, the best way to proceed is to contact the subject-matter experts who are responsible for much of the work surrounding the project. This certainly holds true for NASA’s HETE mission.

Dr. Villasenor is a Research Scientist at the Kavli Institute for Astrophysics and Space Research (KIASR) of the Massachusetts Institute of Technology. He joined the KIASR as a postdoctoral fellow in 1996 in the search for Gamma Ray Bursts with the High Energy Transient Explorer (HETE-1 and HETE-2) program. He was the Instrument Scientist who led the development of the Soft X-ray Camera flown on HETE-2. Currently, Dr. Villasenor is a Research Scientist at the Massachusetts Institute of Technology (MIT) and is the SXC Instrument Scientist, NASA HETE mission.

Luke Volpe is the Director of Engineering, Metrigraphics Division, Dynamics Research Corporation (DRC and has worked with that Company for more than 25 years, serving in positions such as the Research & Development Manager. Prior to joining DRC, Luke held the position of Manager of Photolithography at Analog Devices Semiconductor and at Transitron

When one wishes to learn about a specific program, the best way to proceed is to contact the subject-matter experts who are responsible for much of the work surrounding the project. This certainly holds true for NASA’s HETE mission.

Dr. Villasenor is a Research Scientist at the Kavli Institute for Astrophysics and Space Research (KIASR) of the Massachusetts Institute of Technology. He joined the KIASR as a postdoctoral fellow in 1996 in the search for Gamma Ray Bursts with the High Energy Transient Explorer (HETE-1 and HETE-2) program. He was the Instrument Scientist who led the development of the Soft X-ray Camera flown on HETE-2. Currently, Dr. Villasenor is a Research Scientist at the Massachusetts Institute of Technology (MIT) and is the SXC Instrument Scientist, NASA HETE mission.

Luke Volpe is the Director of Engineering, Metrigraphics Division, Dynamics Research Corporation (DRC) and has worked with that Company for more than 25 years, serving in positions such as the Research & Development Manager. Prior to joining DRC, Luke held the position of Manager of Photolithography at Analog Devices Semiconductor and at Transitron Electronic Corporation. Luke has a Bachelor of Science degree in Industrial Engineering from Boston University and is a field expert in semiconductor photolithography.

SatMagazine
What, exactly, is the purpose of the HETE-2 satellite?

Dr. Villasenor
HETE was designed to solve, what was until a few years ago, one of nature’s most enigmatic puzzles, the Gamma-Ray Burst (GRB).

SatMagazine
When and how were GBRs first discovered?

Dr. Villasenor
First discovered in the late 60’s by the Vela satellites which were designed to detect nuclear explosions, GRBs are short, intense bursts of gamma ray radiation emitted randomly anywhere in the sky. They eluded any explanation because they never repeated, and there were no preferred places to look. Follow up observations taken hours later did not reveal any trace of this phenomenon. Thus, decades after they were first detected no one was even sure whether they originated from our solar system, our galaxy or from distant galaxies.

SatMagazine
Would you describe the problem solving approach?

Dr. Villasenor
HETE’s approach to the problem was to detect and localize GRB occurrences to high precision, so that deep looking telescopes could be brought to bear on the search. Of equal importance, HETE also promptly disseminated these positions to all observatories within seconds. Prior to HETE, follow up observations could only be carried out hours after the GRB, and the searches were in relatively large areas of sky because the positions were not as accurate. Consequently, as it was later discovered, the GRBs optical afterglow had faded by orders of magnitude before any telescope could undertake any search.

SatMagazine
Would you please tell us what were the critical components of the HETE satellite system?

Dr. Villasenor
An important part of the HETE mission was the SXC (Soft X-Ray Camera), which would pin point the locations of GRBs to within an arc minute, as well as send out its position almost instantly to observation centers around the globe.

SatMagazine
How does the Soft X-ray Camera work?

Dr. Villasenor
The Soft X-ray Camera works both as an X-ray detector, and imager. It uses as detectors CCDs (charge coupled devices), the same ones you find in digital cameras but modified for sensitivity in the 1-10 keV X-ray band. To reveal spatial locations in the sky, a fine-coded mask (FCM) is positioned above the CCDs. X-rays from point sources cast shadows on the CCD, which can be checked against the mask pattern to reveal the angle of arrival. Our mask pattern only gives a one-dimensional direction, hence there are two modules which comprise the complete Soft X-ray Camera unit to give a singular point in the sky.

This was done to take advantage of the significantly lower computational load of 2, 1-D systems versus a 2-D system. The strength of this instrument lies in the very small pixel size of the CCDs, along with the fine features of the mask, and the precise alignment of the two, to yield accurate locations in the sky with as few as a dozen X-ray photons!

SatMagazine
How did Metrigraphics and MIT collaborate to create this satellite?

Luke Volpe
DRC‘s Metrigraphics was selected to build the fine-coded mask (FCM) elements due to their experience in forming precise apertures in metal foils. The HETE application in particular had several critical requirements that exceeded the technology’s state of the art at the time of manufacture. These requirements included slit width uniformity, sidewall perpendicularity, foil thickness control, and foil flatness control after being removed from the ultra flat glass base.

The FCMs are 127mm square and contain 10,250 precisely positioned rectangular slits. The slit widths, FCM thickness, and material were designed and specified by the MIT HETE team to filter out all but the GRB soft X-ray bandwidth. Gold was selected for the material because it is opaque to x-rays.

For the SXC to function properly, slit widths had to be uniform to within +/-0.001mm and perpendicular to the plane of the foil to within +/-1 degree. Any variation from the design specifications over the entire active area could reduce accuracy of the SXC and consequently reduce the system’s ability to yield accurate locations in the sky.

SatMagazine
Can you describe, in general terms, the FCM manufacturing process?

Luke Volpe
The manufacturing process is based on semiconductor level photolithography, thin film processing (sputter deposition/ion milling) and electrochemical metal deposition, (electroplating). The FCM foil structure is built on an ultra flat glass substrate that has been sputter coated with a 3000-angstrom layer of conductive seed metal. A precise plating mold is formed over the seed metal in UV sensitive photoresist. Using the pre-deposited, electrically conductive seed metal buss, gold is electroplated into the photoresist mold. Once the electroplating process is completed, the photoresist mold is dissolved away and the FCM is separated from the glass carrier.

SatMagazine
Please describe the successes of the HETE-2 satellite.

Dr. Villasenor
HETE-2 contributed greatly to solving the GRB mystery. It was the first satellite to rapidly disseminate an accurate GRB position within seconds of its actual detection, and subsequently allowed telescopes to observe the afterglow of GRBs, which among other things revealed the distance of these objects. They come from distant galaxies, and are now known to be very powerful occurrences.

On March 29, 2003, it detected a “nearby” GRB, which allowed us to conclusively tie GRBs with supernova explosions, or the death of massive stars. Three years later, on July 9, 2005, it detected a fainter type of short GRB, for which the first optical afterglow was found thus establishing cosmic origin as well.

This class of GRBs is now thought to occur from the merger of two neutron stars. Between these two discoveries, the range of GRBs was spanned, which led NASA to declare the mystery “solved.” Of course, GRB research is continuing and much more needs to be learned and discovered.

SatMagazine
Thank you for your work and the information you’ve given us regarding the NASA HETE mission.