Ultraviolet (UV) radiation is a segment of the electromagnetic spectrum characterized by wavelengths shorter than visible light, typically spanning from 10 to 400 nanometers. Observing this high-energy light is valuable for astronomers because it originates from the universe’s most energetic and hottest phenomena. UV telescopes are suited to study young, massive stars, quasars, and the superheated gas in galactic halos, sources that emit most of their light at these shorter wavelengths. However, almost all ultraviolet light from space is completely blocked before it can reach the ground.
The Atmospheric Barrier to Ultraviolet Light
Earth’s atmosphere acts as an effective, life-preserving shield against high-energy radiation, which is the primary reason UV observation must occur outside this protective layer. The blockage is not a simple filter but a complex absorption process involving two specific molecular components. Molecular oxygen (O2) absorbs the shortest, most energetic UV wavelengths, known as Extreme and Far UV (EUV and FUV, below 200 nm), high in the upper atmosphere. This absorption occurs well above the stratosphere and is what initially creates the conditions for the ozone layer.
Ozone (O3), a molecule formed from molecular oxygen, is the second major absorber, primarily operating in the stratosphere between 10 and 50 kilometers in altitude. This ozone layer is extremely efficient at absorbing the remaining short-wavelength radiation, including most of the UVC and highly damaging UVB bands (200-320 nm). The combined effect of O2 and O3 absorption ensures that virtually no radiation below approximately 300 nanometers can penetrate to the Earth’s surface.
Terrestrial Observation Limits
The atmosphere’s strong absorption profile means that ground-based observatories are limited to viewing only a narrow band of the ultraviolet spectrum. Astronomers often divide the UV range into three main bands: Extreme UV (EUV), Far UV (FUV), and Near UV (NUV). The most energetic EUV and FUV bands are completely inaccessible from the ground due to the dual blockage by molecular oxygen and ozone.
Only the longest wavelengths, the Near Ultraviolet (NUV) portion, which is closest to the visible spectrum (320–400 nm), manages to pass through the atmosphere with minimal attenuation. Even this small window requires specialized instruments and placement at very high-altitude ground observatories to maximize the signal.
Orbital Placement: Above the Veil
To gain an unimpeded view of the ultraviolet universe, the telescope must be physically placed above the absorbing atmosphere, making orbital placement the primary solution. This strategy allows for continuous, long-duration observation across the entire UV spectrum, including the scientifically rich FUV and EUV bands. The most common location for UV observatories is Low Earth Orbit (LEO), typically between 500 and 1,000 kilometers in altitude, a region exemplified by the Hubble Space Telescope and the Galaxy Evolution Explorer (GALEX).
While LEO offers the advantage of proximity for maintenance and high data rates, it presents engineering challenges. Satellites in LEO still encounter residual traces of atmosphere, resulting in atmospheric drag that causes their orbits to decay over time. Solar activity increases the sun’s UV and X-ray emission, causing the upper atmosphere to heat up and expand, intensifying the drag force. Consequently, LEO telescopes like Hubble require periodic orbital boosts to maintain altitude.
For maximum thermal stability and freedom from Earth’s influence, the most scientifically optimal location is the Sun-Earth Lagrange Point 2 (L2), located 1.5 million kilometers away from Earth on the side opposite the Sun. At L2, the gravitational forces of the Sun and Earth balance, allowing a spacecraft to remain in a stable position. This placement offers a consistently clear view of deep space, low thermal interference, and eliminates the need for frequent orbital maneuvers, maximizing the instrument’s operational time for sensitive UV measurements.
Specialized High-Altitude Missions
When the cost or long lead time of a dedicated orbital satellite is prohibitive, or for short-term scientific goals, two alternative high-altitude platforms are utilized. Sounding rockets offer a rapid and relatively inexpensive means of briefly carrying instruments above the bulk of the atmosphere. These suborbital missions follow a parabolic trajectory, typically reaching altitudes between 150 and 300 kilometers.
The primary limitation of sounding rockets is their brief observation time, usually only five to twenty minutes of viewing time above the atmosphere before falling back to Earth. This method is often employed for testing new instrument technology or observing extremely transient phenomena. Stratospheric balloons, the second alternative, float much lower than rockets, generally plateauing around 40 kilometers, which is still below the peak of the ozone layer. While balloons offer observation durations of days or weeks, they can only access the very upper end of the NUV band, serving a more limited scientific role than true space-based UV telescopes.