6.5 Observations outside Earth’s Atmosphere

Advantages of Space-Based Observations and Major Space Telescopes

Space telescopes solve a fundamental problem: Earth's atmosphere blocks, absorbs, or distorts most of the electromagnetic spectrum. By placing instruments above the atmosphere, astronomers gain access to wavelengths they'd never see from the ground and get far sharper images at the wavelengths they can.

Space vs. Ground Observations

Space-based observations:

• No atmospheric distortion, which means clearer images and higher resolution
• Access to the entire electromagnetic spectrum, including infrared, ultraviolet, X-rays, and gamma rays that the atmosphere absorbs
• Continuous observations uninterrupted by the day/night cycle or weather

Ground-based observations:

• Much lower cost to build, launch, and operate
• Easier to maintain and upgrade (you can physically access the telescope)
• Larger mirror sizes are possible since there are no rocket payload constraints
• Limited mostly to visible light and radio waves, the two main "windows" where Earth's atmosphere is transparent
• Adaptive optics can partially correct for atmospheric distortion, but can't eliminate it entirely

Contributions of Major Space Telescopes

Hubble Space Telescope (HST):

• Helped refine the age of the universe to approximately 13.8 billion years by measuring the distances to faraway galaxies more precisely
• Observed galaxies at various distances (and therefore various ages), revealing how galaxies evolve over cosmic time
• Studied exoplanet atmospheres to identify their chemical composition
• Captured iconic detailed images of objects like Jupiter's storms, the Crab Nebula, and the Pillars of Creation

James Webb Space Telescope (JWST):

• Optimized for infrared observations, letting it peer into the early universe and detect light from the first galaxies and stars, redshifted into infrared wavelengths
• Analyzes exoplanet atmospheres with enough sensitivity to search for potential biosignatures like methane and water vapor
• Observes star and planet formation inside dusty stellar nurseries (like the Orion Nebula) that visible-light telescopes can't see through
• Detects faint, distant objects too dim for Hubble because of its larger 6.5-meter mirror and infrared sensitivity

Types of Space-Based Observatories

Each part of the electromagnetic spectrum reveals different physical processes. That's why astronomers build specialized telescopes for different wavelength ranges.

Optical telescopes (Hubble, Kepler):

• Observe in visible light to image planets, stars, galaxies, and other celestial objects
• Kepler specifically monitored stars for tiny brightness dips caused by transiting exoplanets
• Measure galaxy distances using redshift, which helps track the expansion of the universe

Infrared telescopes (Spitzer, James Webb):

• Detect cooler objects like planets, dust clouds, and molecular clouds that don't emit much visible light
• See through cosmic dust to reveal hidden stars and structures that optical telescopes miss
• Capture light from the early universe, which has been redshifted from visible into infrared wavelengths due to the expansion of space

Ultraviolet telescopes (GALEX):

• Observe hot, young stars (spectral types O and B) that emit strongly in UV
• Map star formation activity in nearby galaxies like Andromeda
• Study the composition of the interstellar medium, the gas and dust between stars

X-ray telescopes (Chandra, XMM-Newton):

• Observe high-energy phenomena: material falling into black holes, supernova remnants, and hot gas in galaxy clusters
• Chandra's observations of the Bullet Cluster provided some of the strongest evidence for dark matter by showing that the mass of the cluster is separated from the visible hot gas

Gamma-ray telescopes (Fermi):

• Detect the most energetic events in the universe, including gamma-ray bursts and pulsars
• Study cosmic rays and trace them back to their origins in supernova remnants
• Search for gamma-ray signals that could be linked to dark matter annihilation

Gravitational wave observatories (LISA, planned for space):

• Designed to detect ripples in spacetime caused by massive accelerating objects. Ground-based detectors like LIGO have already confirmed these waves from:
  1. Merging black holes (the first detection, GW150914, in 2015)
  2. Colliding neutron stars (GW170817, in 2017)
• LISA, a future space-based mission, will detect lower-frequency gravitational waves that ground-based detectors can't pick up

Advanced Space-Based Observation Techniques

Space-based interferometry:

• Combines light collected by multiple telescopes separated by large distances to achieve much higher resolution than any single telescope could
• Could eventually enable direct imaging of Earth-like exoplanets and analysis of their atmospheres

Cosmic microwave background (CMB) observations:

• The CMB is the oldest light in the universe, released about 380,000 years after the Big Bang when the universe cooled enough for atoms to form
• Space missions like COBE, WMAP, and Planck mapped tiny temperature variations in the CMB, providing strong evidence for the Big Bang theory and cosmic inflation

Exoplanet transit spectroscopy:

• When an exoplanet passes in front of its host star, some starlight filters through the planet's atmosphere
• By analyzing which wavelengths are absorbed, astronomers can identify molecules in the atmosphere (water, methane, carbon dioxide) and assess potential habitability