Europe's Arrows to the Sun

1
Europe's Arrows to the Sun

• 1919 Arthur Eddington confirms Einstein's
  theory of General Relativity (with 20 per cent
  uncertainty)
• 1960s Robert H. Dicke proposes an alternative
  theory of gravity
• 1969 More than a dozen alternative theories

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• Theories are defined by three Parametrized-Post
  Newtonian parameters
• a 1 means that gravity in first approximation
  is Newtonian
• b influences orbits ( 1 for Einstein)
• g influences time delay and light deflection (
  1)

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• In 1969 Jacques Blamont proposes to ESRO a solar
  orbit mission to measure b, g and J2 of the Sun
• g requires a precise measure of the time delay of
  a signal (radio or light) near the Sun. Requires
  a proper source (laser) and a clock or a 2-way
  system.
• b and J2 require a precise measure of the clean
  drag free motion of a proof mass. All spurious
  acceleration down to 0.1 trillionth of a gee
  must be measured and corrected.

4

• September 1969 An expert SOREL Mission
  Definition Group is formed within the Launching
  Programmes Advisory Committee
• The mission could be proposed as a joint project
  with NASA with the use of a Titan III and of the
  DSN

5

• MBB Six month orbit (perihelion _at_ 0.29 AU).
  Need for an orbit optimization effort to minimize
  planetary perturbations.
• Laboratoire Suisse de Recherches Horologères
  Recommended the use of cesium clock or a hydrogen
  maser (far heavier)
• ONERA Been studying the CACTUS for geodesy.
  CACTUS measures accelerations of 1 billionth of a
  gee (not enough!). A new optical system using a
  large cavity, spinning spacecraft and optical
  pickup.
• A 15 g mass 10 cm from the proof mass gives an
  acceleration of a trillionth of a gee.

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• Drag Free Systems
• The proof mass responds to gravity forces only.
  The spacecraft to gravity and surface forces
  (radiation pressure etc) and to perturbations.
  The accelerometer measures the displacement of
  the proof mass wrt the cavity and fires thrusters
  to re-center it.
• 1st Drag Free satellite TRIAD (US, 1972) achieved
  better than 1 billionth of a gee.

7

• 1971 Preliminary SOREL study at ERNO Bremen
• Envisages a Helios derivative with a 1 m
  spherical optical accelerometer 1-way laser
  telescope.

Telescope
IR Pickup
Optical Accelerometer Cavity
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• Helios-Like orbit with 6 month period
• perihelion at 0.285 AU

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• Statistical analysis of orbital accuracy (SEMA,
  ERNO)
• After 400 days of flight
• b, g, J2 known
• Time variation of G
• Mass of Earth and planets
• Mass and mass change of the Sun

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• 1971 SOREL proposed to NASA
• Involved in
• Gravity Probe A (suborbital relativity redshift
  experiment)
• Gravity Probe B (gyroscopic precession
  experiment)
• MM69 conjunction (g 1.02 0.04)
• Helios conjunction
• Viking conjunction
• Grand Tour tracking

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• 1972 ESRO and CNES study optical accelerometer
  for D5B/Castor (launched 1975)
• 1973 Final feasibility study
• No experiment on drag-free, clock, laser receiver
  etc.
• SOREL is terminated

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• April 1975 SSWG meeting on OOE
• Colombo proposes a second probe solar
  plunger/grazer with Jupiter flyby
• Determination of b, g, J2 (CACTUS accuracy plus
  2-way ranging)
• Solar plasma, particle and fields
• Coronal imager

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• Definition study on the Arrow to the Sun _at_ ESA
• In-house mini-study at JPL (spurred by Colombo)
• Thermal issues manageable with Sun shields
• Orbit optimization
• perihelion distance (no closer than 4 Rs)
• Inclination
• E-S-P angle at perihelion

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• ESA's Arrow to the Sun

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• SAC recommends that OOE and Solar Probe are not
  pursued together (unless OOE is slowed)
• No Solar Probe (yet)

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• Where are we now?
• g 1.000 0.002 Viking 1976
• g -1 (2.1 2.3)10-5 Cassini 2002
• b 1 within 100 ppm (LLR)
• J2 measured by helioseismology

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• At which level does g show deviation from
  unity? 10-6? 10-7?
• New missions proposed LATOR, ASTROD, ESA Cosmic
  Vision 2015-2025
• Pioneer Anomaly?
• And MOND?