Smith lecture notes, Physics 10, 10/26/04

NASA space science themes:
   Origins 
       of stars, planets...life in the Universe?
   Structure and Evolution of the Universe:
       Cosmology, black holes, cosmic rays, gravitational waves, dark matter
   Solar System Exploration: 
       Planets, moons, comets, asteroids (Mars program split off)
   Sun-Earth Connection: 
       solar physics, space physics ["go & taste"], Earth's magnetosphere
   Roadmaps: http://science.hq.nasa.gov/strategy/roadmaps/index.html

Reorganization, Science Mission Directorate
Discussion of politics.....

Platforms:
   High-altitude aircraft (IR astronomy - 14 km)
   Balloons -- He, polyethylene, 40km, 0.3% atm, durations hours to weeks, ~2 tons
   Sounding rockets: 100km+, minutes, like ICBMs, < 100 lb
   Low-Earth orbit (400-800 km, 95 min, Shuttle, rocket launches)
   High-Earth orbit (continuous viewing, radiation belts, more cosmic rays)
   Geosynchronous orbit (36000 km)
   Lagrange point orbits
   Interplanetary probes (rocket launches)

Technologies:
 Science instruments:
   Photon detectors for astronomy (microwave through gamma-ray)
   "Optics" -- collimators, telescopes, masks/grids, gratings for spectroscopy
   In situ devices -- particles, fields, waves -- for space physics
   Dust and solar wind (particle) collectors (e.g. Genesis)
 Spacecraft/support:
   Data storage and telemetry (strong function of distance!)
   Thermal (blanketing, radiators, hot/cold!)
   Power (solar, RTG)
   Propulsion: chemical (solar/ion, nuclear/ion, solar sail)
   Pointing (gas, gyros, magnetic torquing)
   
NASA mission classes:
   Great Observatories ($ few billion) (Hubble, Compton, Chandra, SIRTF)
      Planetary equiv: (Galileo ($1.4B))
   Roadmap Missions (planned top-down, about $ 500 M)
   Explorer missions (planned bottom-up, $100-200M):
       "PI-class" (proposed by any science team and competed against others)
         Equivalent program is "Discovery" for S.S.E. theme
       (Endangered)
   Suborbital (rockets, balloons) (a few $M)
       (Endangered)

NASA missions currently operating: 
  58 listed including participation in primarily non-US missions. See:
      http://science.hq.nasa.gov/missions/phase.html

Other US agencies/organizations launching space science satellites: 
  Air Force
  NOAA [Nat. Ocean & Atmospheric Administration]
  DOE
  Lockheed, etc.
  NSF (balloons only) (NSF does most of the US *ground-based* astronomy)

Other major national/international agencies:
  ESA (Europe)
  JAXA (Japan)
  RKA (Russia)
  (China, Taiwan, India, Brazil ...)
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Physical mechanisms of gamma-ray production

   Bremsstrahlung (B)
   Synchrotron    (S)
   Nuclear decay  (N1)
   Nuclear de-excitation (N2)
   Positron annihilation (PA)
   Inverse Comptonization (IC)

Sources of gamma rays in the solar system and Universe:

   Earth's atmosphere: B, N2
     (radiation belt precipitation;
      Cosmic ray and flare protons causing de-excitation)
   Rocky planets: N1, N2
   Solar flares: B, N2
   Galactic supernova remnants: N1, S
   Black-hole jets: S, IC
   Black-hole accretion disks: B, IC
   Pulsars (magnetized neutron stars): S

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RHESSI: a Small Explorer to study solar flares
  Motivations: Solar flares as particle accelerators:
              Magnetic reconnection/particle acceleration as a 
                    unifying theme in astrophysics
              Space weather, Solar Energetic Particles (SEP)
              Plasma physics for fusion power?

What is a solar flare?
     Electric currents in Sun generate magnetic fields
     Motions of solar material (differential rotation, convection)
       "twist up" the coronal magnetic fields from the base
     Twisted fields emerge into the corona and reconnect
     Particles are accelerated downwards along closed field lines
       to make the flare, upwards along open field lines into
       interplanetary space (SEPs)
     Downward electrons collide with nuclei making "bremsstrahlung" (braking)
       radiation
     Downward protons and nuclei collide with other nuclei making excited
       nuclear states, emitting gamma-ray lines.
 
Trials and tribulations of a mission: RHESSI

  Idea for this type of mission discussed since the mid 80s among some
       interested scientists

  First promoted as a roadmap mission; eventually scaled down to a MIDEX,
       then a SMEX, finally accepted the third time it was proposed as
       a SMEX, in 1997.  Acceptance involves peer review against SMEXes
       in all science fields, followed by technical and programmatic
       reviews.

  Solar flares go in an 11-year cycle (along with sunspots).  This cycle
       peaked in mid-2000. Fast schedule: one year for design, one for
       building, one for testing, launch in mid-2000.

  Launch delays:  Vibration-test disaster at JPL, Pegasus launch-vehicle 
       doubts.

  Instrument discussion:
       Germanium detectors (high energy resolution, see x-rays through gamma-rays)
       Imaging by rotating grids

  One early result: flare of July 23, 2002: the gamma-ray lines come from a different
       place than the bremsstrahlung!

  A result from RHESSI in the field of non-solar astrophysics (cross-theme!):
       Use the Earth as a blocking disk;
       Subtract spectra at times when the Galactic center is not seen from
          times it is not seen;
       The spectrum of the Galaxy shows a gamma-ray line from decay of radioactive
          Aluminum.  This is made in Supernovae and other places. Half-life 1 Myr.
       Earlier results found this line was broad (presumably due to Doppler-shifting,
          i.e. the aluminum atoms must have high velocities).
       But how can the aluminum atoms wander around the Galaxy at high speed for
          a million years?  Big mystery....
       RHESSI data (the best on this line so far) show that it doesn't look so
          broad after all.
           
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Gamma-rays from rocky bodies:  Mercury Messenger GeD.

   Nuclear lines like those from the Sun are created in any rocky body exposed
   to cosmic rays (the Earth's atmosphere and magnetosphere protect it, 
   and us, mostly).  By looking at the lines coming off a bare planet (like 
   Mercury) or almost bare (like Mars), you can see what it's made of.
   Great success so far at Moon and Mars (including water!)

   Mercury probe called Messenger will launch next year, 2 Venus flybys, 
   2 Mercury flybys, then Mercury orbit insertion.  A single small 
   germanium detector (about 1/3 the size of one of RHESSI's) will measure
   gamma-rays from the planet, both those induced by cosmic rays and those
   from naturally radioactive elements (uranium, thorium, etc.)  Maybe
   even ice in craters at the poles!

   Questions:  Is there ice in craters on Mercury's poles?
               Were volatile and refractory elements separated in the early 
                      Solar nebula?
               Why is Mercury so dense?  What does its surface comp. say about 
                      its interior?
   
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