Balloon-borne telescope

A balloon-borne telescope is a type of airborne telescope, a sub-orbital astronomical telescope that is suspended below one or more stratospheric balloons, allowing it to be lifted above the lower, dense part of the Earth's atmosphere. This has the advantage of improving the resolution limit of the telescope at a much lower cost than for a space telescope. It also allows observation of frequency bands that are blocked by the atmosphere.

History
Balloon-borne telescopes have been used for observation from the stratosphere since the Stratoscope I was launched in 1957. A number of different instruments have since been carried aloft by balloons for observation in the infrared, microwave, X-ray and gamma ray bands. The BOOMERanG experiment, flown between 1997–2003, and the MAXIMA, which made flights in 1998 and 1999, were used to map the Cosmic Microwave Background Radiation.

Disadvantages
Balloon-borne telescopes have the disadvantage of relatively low altitude and a flight time of only a few days. However, their maximum altitude of about 50 km is much higher than the limiting altitude of aircraft-borne telescopes such as the Kuiper Airborne Observatory and Stratospheric Observatory for Infrared Astronomy, which have a limiting altitude of 15 km. A few balloon-borne telescopes have crash landed, resulting in damage to, or destruction of the telescope.

The balloon obscures the zenith from the telescope, but a very long suspension can reduce this to a range of 2°. The telescope must be isolated from the induced motion of the stratospheric winds as well as the slow rotation and pendulum motion of the balloon. The azimuth stability can be maintained by a magnetometer, plus a gyroscope or star tracker for shorter term corrections. A three axis mount gives the best control over the tube motion, consisting of an azimuth, elevation and cross-elevation axis.

Future missions
NASA is planning to launch the largest ever balloon observatory on December 1, 2024 with a 400 foot balloon and 2.5 metre far-infrared telescope. ASTHROS (Astrophysics Stratospheric Telescope for High Spectral Resolution Observations at Submillimeter-wavelengths) will launch from the Antarctic and is envisioned to last for four weeks. Its primary mirror consists of nine panels and is 8.2-foot (2.5-meter) in diameter. Optics is produced by Italian manufacturer Media Lario. The balloon may reach an altitude of 130,000 feet (40 kilometers).

When fully inflated, the 40-million-cubic-feet helium balloon will be about 400 feet (150 meters) wide. The current best estimate for the weight of the observatory, including the gondola, solar panels, antenna, scientific instrument and communication systems, is about 5,500 pounds (2,500 kilograms). The telescope's detectors must be cooled down to 4 Kelvin using a cryocooler powered by electricity from its solar panels. One of ASTHROS' main science goals is to provide new information about stellar feedback in the Milky Way and other galaxies, a process in which stars either accelerate or decelerate the formation of new stars in their galaxy. ASTHROS will be the first mission to conduct high spectral resolution spectrometry in a few specific wavelengths of light, and identify two specific nitrogen ions that are formed by the processes that drive stellar feedback. As a target of opportunity, ASTHROS will observe TW Hydrae.