NASA's stratospheric balloon mission gets a telescope with a giant mirror

NASA’s stratospheric balloon mission gets a telescope with a giant mirror

NASA's stratospheric balloon mission gets a telescope with a giant mirror

The primary mirror of the ASTHROS mission is one of the largest ever flown on a high-altitude balloon. The light mirror is plated with gold and nickel to make it more reflective in far infrared wavelengths. Credit: Media Lario

Telescopes that are designed to work in space must be designed differently from those that are designed to work on the ground. But what about telescopes that work in between?

An upcoming NASA mission will use one balloon larger than a football field to send a telescope 130,000 feet (about 40,000 meters) over Antarctica. From that height, the telescope will study a phenomenon that is suffocating star formation in some galaxies, effectively killing them.

The mission, called the Astrophysics Stratospheric Telescope for High Spectral Resolution Observations at Submillimeter Wavelengths, or ASTHROS, will use a primary mirror (this telescope’s main light collection tool) that is bound to be the largest ever to fly on a high-altitude balloon. Construction of the 8.2-foot (2.5-meter) mirror was completed this month. Designing and building it proved to be challenging due to two key requirements: The mirror and its support structure must be exceptionally light to be able to travel with a balloon, but still strong enough to keep the traction of the earth’s gravity from deforming its almost perfect parabolic shape with more than about 0.0001 inch (2.5 micrometers) – a fraction of the width of a human hair.

ASTHROS, operated by NASA’s Jet Propulsion Laboratory in Southern California, is set to launch in December 2023 at the earliest and circulate the South Pole for up to four weeks. NASA’s scientific balloon program, run by the agency’s Wallops Flight Facility in Virginia, launches 10 to 15 balloon missions each year. These missions usually cost less than space missions and take less time to go from early planning to deployment, and they use new technology that can be used in future space missions.

NASA's stratospheric balloon mission gets a telescope with a giant mirror

The gold-colored ASTHROS mirror panels look blurred to the naked eye, as seen on the left. But when photographed with an infrared camera, as on the right, the panel reflects a technician’s image as clearly as if the person was looking in a mirror. Credit: Media Lario

High up in the stratosphere, ASTHROS will observe wavelengths of light blocked by the Earth’s atmosphere, in an area known as far infrared. Its large mirror will improve the telescope’s ability to observe weaker light sources and resolve finer details in these sources.

These abilities are essential to the mission’s approach to studying stellar feedback, the process by which clouds of gas and dust – the constituents of stars – disperse in galaxies, sometimes to the point where stellar formation is completely stopped. Many processes contribute to feedback, including eruptions from living stars and explosive deaths of massive stars such as supernovae. ASTHROS will look at several star-forming regions in our galaxy where these processes take place and create high-resolution 3D maps of gas distribution and motion. The mission will also look at distant galaxies containing millions of stars to see how feedback unfolds on a large scale and in different environments.

“It is difficult to explore feedback all the way from where it originated, on the scale of individual stars, to where it has an effect, on the scale of galaxies,” said Jorge Pineda, chief investigator for ASTHROS at JPL. “With a large mirror, we can connect these two.”

NASA's stratospheric balloon mission gets a telescope with a giant mirror

Raised by a stratospheric balloon, shown in this illustration, NASA’s ASTHROS mission can reach an altitude of 130,000 feet (40 kilometers). From that height, the telescope can detect wavelengths of light that are blocked by the Earth’s atmosphere and cannot be observed from the ground. Credit: NASA / JPL-Caltech

Meets the challenge

NASA hired Media Lario, an optics company in Italy, to design and manufacture ASTHROS ‘entire telescopic unit, including a primary mirror, secondary mirror and supporting structure (called a cradle). Media Lario had previously developed a unique method for manufacturing light infrared and optical telescopic mirrors, which the company used to manufacture many of the panels for the primary mirrors in the Atacama Large Millimeter Array, a group of 66 ground-based telescopes in Chile.

ASTHROS primary mirror has nine panels, which are much easier to manufacture than a one-piece mirror. The main part of the mirror panels consists of lightweight aluminum, formed into a honeycomb structure that reduces its total mass. The panel surfaces are made of nickel and coated with gold, which improves the mirror’s reflectivity at far infrared wavelengths.

Since the ASTHROS team will not be able to fine-tune the orientation of the panels when the telescope is lifted, the cradle that supports the mirror must be light but still exceptionally strong and rigid to prevent deformation. Carbon fiber would do the trick. So to build the cradle and other structural components, Media Lario turned to local companies in Italy that usually produce specialized structures for competitive racing boats and cars.

“I think this is probably the most complex telescope ever built for a high altitude balloon missions“said Jose Siles, the ASTHROS project manager at JPL.” We had specifications that were similar to a space telescope but with a tighter budget, schedule and mass. We had to combine techniques from ground-based telescopes that observe at similar wavelengths with advanced manufacturing technology used for professional racing sailboats. It’s quite unique. “

Media Lario will deliver the entire telescopic unit to NASA in late July. After that, the ASTHROS team will integrate it with the gondola (the structure that holds the entire payload and attaches to the balloon) and other key components. Then they will begin a series of tests to ensure that everything is ready for flight.

The NASA mission will study the cosmos with a stratospheric balloon

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