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Test of the Equivalence Principle on ISS. Ho Jung Paik, Krishna Venkateswara, M. Vol Moody Department of Physics, University of Maryland Collaborators: Inseob Hahn, Talso Chui, and Konstantin Penanen Jet Propulsion Laboratory NASA ISS Workshop on Fundamental Physics

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Test of the Equivalence Principle

on ISS

Ho Jung Paik, Krishna Venkateswara, M. Vol Moody

Department of Physics, University of Maryland


Inseob Hahn, Talso Chui, and Konstantin Penanen

Jet Propulsion Laboratory

NASA ISS Workshop on Fundamental Physics

October 13-15, 2010, Dana Point, CA


scientific value of ep tests
Scientific Value of EP Tests

In string theory, the 10-D tensor gravitational field G has two partners: scalar field  (dilaton) and antisymmetric tensor field B. They are coupled to the other fields in ways generally violating the EP.

Many scalar and pseudo-scalar partners of the graviton may survive as massless particles in the four-dimensional low energy world (dilatons, axions, moduli fields, etc.).

The observed accelerating expansion of the universe is consistent with a cosmological constant , which is 120 orders of magnitude smaller than the quantum corrections to the vacuum-energy density.

It is important to test the founding principles of GR, such as the EP, to the highest possible precision because the failure to quantize gravity and the  problem may be partly due to incompleteness of GR.


spherical outer test mass
Spherical Outer Test Mass
  • A spherical shell approximates a point mass more closely.

 Smaller moments for  3

  • Closure conditions satisfied:

SMART test mass pairs


STEP test mass pair


suspension and alignment
Suspension and Alignment
  • Suspension and alignment by current along asingle tube

 Axis alignment 10-5 rad

 CMRR 108

with error compensation

  • Centering by currents on 4 auxiliary tubes

Meander-pattern suspension coil for STEP


accelerometer orientation
Accelerometer Orientation




Orientations of the EP test masses with respect to the spacecraft spin axis (z)


technology heritage
Technology Heritage
  • Superconducting Gravity Gradiometer (SGG)
  • Wire-based S/C technology
  • SGG for airborne gravity
  • Diff angular acc.
  • CMRR = 109
  • SGG for 1-m 1/r2 law test
  • Diff linear accelerometer
  • CMRR  107
  • 41013g Hz1/2 noise
  • Best resolution (104) of 1/r2 law at 1 m
  • SGG for submillimeter 1/r2 law test
  • Differential linear accelerometer
  • Search for extra dimensions to 20 m


levitation on a single tube
Levitation on a Single Tube
  • This critical technology has been demonstrated. Test setup
  • Measured frequency squared versus current squared:

Sliding mode Vertical mode



S/C tube

p i ho jung paik jpl contact inseob hahn january 11 2008

P.I.: Ho Jung PaikJPL Contact: Inseob HahnJanuary 11, 2008

SMEX-ISS Concept Gate Review


(Standard Model And Relativity Test)

smart h j paik university of maryland
SMART: H.J. Paik, University of Maryland
  • Exp. module: JEM-EF, Site #9 preferred.
  • Goalminimum temperature: 2 K.
  • Science cold instrument mass: 10 kg.
  • Instrument power consumption:100 W.
  • Pointing:Rotation at 0.01 Hz about the ISS pitch (or roll) axis is required.
  • Science objectives: To test EP to 1017 at range  104 km. Most quantum gravity theories involve EP-violating forces. SMART tests GR and other theories beyond Einstein, and searches for new interactions and particles beyond the Standard Model.
  • Science team members:

PI: Ho Jung Paik, U. Maryland

Co-I: M.V. Moody, U. Maryland

JPL Project Scientist: TB

  • JPL roles:Project management/ system engineering, support science instrument team, flight engineering, I&T, ATLO, CTM

Science Objectives

  • Science goals and objectives:

To test EP and search for new interactions and particles beyond the Standard Model.

  • Relationship to the astronomy program objectives in NASA science plan:

SMART supports NASA’s strategic goal:“Discover the origin, structure, evolution, and destiny of the universe.”

  • Relationship to other investigations:

SMART will improve by 102 over Microscope mission, a factor of 10 short of STEP.

  • Justification for space:

In orbit, Earth’s gravity is fully modulated,gaining 103 in signal,and accelerometers can achieve higher sensitivity (by 102-103).

  • The current state of instrumentdevelopment:

SMART uses superconducting differential accelerometers, which are very similar to SGG, fully developed at UM.

  • Diagrams of the instrument:
    • Mass and power:The instrument weighs 10 kg, and requires 100 W.
    • Limits to the sensitivity:ISS dynamic and gravity noise will be dominant.
    • Heritage:GP-B, which utilizes similar technologies, has flown.

ISLES was supported by Microgravity Program (MP) in 2002-06.

      • Under NSF support, a ground ISL experiment is being performed.

Detection circuit

EP EP Accelerometer/ EP EP

Ti-PtPt-Nb Gradiometer Nb-TiNb-Nb

science traceability baseline investigation
Science Traceability: Baseline Investigation

Airborne SGG

with CMRR = 109

ISS vibration noise spectra

mission d esign
Mission Design
  • Level of microgravity required: 10–5g.
  • Temperature required: 2 K with stability to better than 0.1 K.
  • Servicing required after installation:None.
  • Uplink and downlink bandwidth required:<< 0.01 Mbps, 0.2 Mbps (ref: LTMPF PIA).
  • ISS payload accommodation location required:

The dewar needs to be rolled about the pitch or roll axis at ~10–2 Hz.

Site #9 of JEM-EF is a natural site that allows this rotation.

However, SMART can be accommodated at any other sites, as long as it is rotatable.

  • Mission time:Minimum 3-month space operation.

Launch Lock & Rotation


Grapple Fixture




Payload Interface Unit (ISS/JEM-EF Interface)

HTV Carrier Interface

Site #9


Management: Top-Level Schedule

  • Science instrument
    • 1.5 year for Phase A/B
    • 0.5 year between CDR and Del to system I&T
    • 0.5 year system I&T at JPL
    • 4 month ATLO
    • 4 month operation in space
    • 6 month data analysis
  • JEM-EF site #9 availability is a big assumption.
    • But we can occupy another site with more engineering and less science.








Ph F




Ph A/B

Yr #4

Yr #1

Yr #2

Yr #3