Why do we have to observe ultraviolet radiation using rockets or satellites?

[kay89]

Why do we have to observe ultraviolet radiation using rockets or satellites, where as balloons are sufficient for obervations?

 

[Wallace]

Good question. Remember a few years ago that CFCs (a particular type of gas used as a propellant in aerosols and as a coolant in fridges) were banned globally due to the fact that when they get into the upper atmosphere they break down the ozone layer. This is a layer of ozone gas which are oxygen molecules with three rather than the usual two oxygen atoms. The vital importance of the ozone layer is that is absorbs much of the UV radiation coming from the Sun that would otherwise be very harmful to life on Earth.

The problem then for astronomers is that UV radiation coming from any distant stars or galaxies is also blocked by the ozone layer and can't reach their telescopes on the ground. The only solution then is to go above the ozone layer into space. That is why UV astronomy is almost entirely performed from space rather than ground based telescopes.

 

[turbo]

Gamma-ray observations (direct observations, that is) have to be made from space, as well. As gamma rays enter the atmosphere, they collide with components of the atmosphere, creating particle cascades. These cascades can be (and are) observed by special telescopes like the one currently employed by the MAGIC consortium, but these observations are necessarily indirect. When GLAST launches (hopefully by this summer), we will be able to observe gamma-ray sources directly.

 

[D H]

We don't use balloons for doing the kinds of observations that wallace and turbo discussed because balloons are not sufficient for those tasks. A very stable platform with a very well-known and attitude is absolutely essential when looking at the stars. Balloons are not stable. Since those UV and gamma ray instruments are quite expensive, we also need to platform to stay up for a long time. Balloons come down, sooner rather than later.

We do use balloons for a lot of measurements of the atmosphere. Weather balloons and Earth-sensing satellites perform complementary roles. Satellites that look at the Earth have a broad view of the Earth, while balloons only sense conditions in their immediate surroundings. However, balloons measure things like ozone concentration directly. The observation is much less direct with satellites. In fact, there is another name for data collected by satellites: "remote sensing". The science behind remote sensing is always viewed as suspect initially. Remote measurements need to be verified against some trusted measurement. Direct measurements such as those made by radiosondes, dropsondes, and rocketsondes provide the sanity check needed by remote sensors on satellites.

 

[Laura1013]

Why do we have to observe ultraviolet radiation using rockets or satellites, where as balloons are sufficient for obervations?

High-energy astronomy observations can and are observed by detectors flown with high-altitude balloons. Some small university and government programs still use high-altitude balloons for high-energy astronomy.

It's much cheaper than launching a space-based observatory, but the balloon can only observe for a few hours or days at the most before it sinks back down. The launch and landing requirements are very strict, and therefore launches can only occur a couple of times a year at the most. Detectors can be re-used, but the potential for damage upon landing is high.

High-altitude balloon astronomy was more common a few decades ago. I'm not sure why the interest has died down. Perhaps with the launch of better space-based observatories, the balloon programs were seen as unnecessary.

Edit: D H posted while I was typing. Yes, stability is a major draw-back. But balloon observations are still done. There's a small project run out of my workplace that launches an X-ray detector once or twice a year to do real science.

 

[Wallace]

As has been suggested, there have been many very good astronomical observations with balloon borne telescopes, though not of UV radiation to my knowledge. The best known one I can think of was the BOOMERANG project that got some very good measurements of the cosmic microwave background. See http://cmb.phys.cwru.edu/boomerang/press_images/cmbfacts/cmbfacts.html" for some info.

 

[turbo]

DH and Laura have given you good explanations why balloon astronomy is not appropriate for high-res observations. They require stable platforms. In the early 1970's I took a great course on photogrametry and remote sensing. I'd love to be able to go back to school, and see how that field has matured in the intervening ~35 years.