Relativity Gravity Probe Press Kit

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rvii~s\ News76-/0110lNational Aeronautics andSpace Administration

Washington, D.C. 20546AC 202 755-8370

For Release IMMEDIATE

Press Kit Project Relativity GravityProbe

RELEASE NO: 76-106

Contents

GENERAL RELEASE.................................. 1-7

COMPONENTS OF EXPERIMENT ............ .8-9

SCOUT D LAUNCH VEHICLE DESCRIPTION................ 10

LAUNCH SEQUENCE. ................................. 10-11

GR-.-YVITWTION RESEARCH USING ATOMIC CLOCKS

IN SPACE............................, ... 12-17

GP-A/SCOUT D PROGRAM/PROJECT MANAGEMENT........... 18-19

., ......



I

National Aeror ia l tics and

Space AdministrationWashington, D.C. 20546AC 202 755-8370

A I

For Release:

IMMEDIATE

Nicholas PanagakosHeadquarters, Washington, D.C.

(Phone: 202/755-3680)

Don Worrell

Marshall Space Flight Center, Huntsville, Ala.(Phone: 205/453-0035)

l LJJEASE NO : 76-106

SPACE PROBE TO TEST EINSTEIN'S 'SPACE-TIME WARP' THEORY

A clock-carrying space probe will be launched by NASA

in late June to test an important part of Einstein's general

theory of relativity.

Known as Gravity Probe-A (GP-A) or the Red Shift Experi-

tent, the probe will test Einstein's "equivalence principle",

which is the foundation of almost every theory of relativity.

The experiment will be a sivaificant step toward a bet-

ter understanding of gravitational effects, since GP-A is

expected to be about 500 times more accurate than any pre-

vious measurement using ground-based instruments.

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Einstein's 70-year-old relativity theory is of parti-

cularwinterest currently because of its significance to

scientists studying astronomical concepts, such as "black

holes", which have to do with gravitational:phenomena.

('Black holes" are believed to be bodies of such tremendous

gravitational magnitude that even light cannot escape them.)

According to the "equivalence principle", within a

limited region of space, every form of acceleration is

indistinguishable from a gravitational field. Newton

showed that, in the absence of an acceleration, every

object moves in a straight line at a constant speed. The

theory of relativity generalizes this by stating that every

body which is effected only by a gravitational field moves

along a a path in four-dimensional space-time--

which is analogous to the moti n along a straight line with

constant speed in the absence of the field. -Because (as

viewed in three dimensions) the path of a body in the neigh-

borhood of a large mass is not a straight line but is deviated

towards this mass, physicists say that space-time is "warped"

in the presence of a massive body.

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In the GP-A experiment, managed by NASA's Marshall

Space Flight Center in Huntsville, Ala., a scientific pay-

load-.which includes an extremely accurate clock--will be

launched by a four-stage Scout D rocket into a two-hour

elliptical flight trajectory over the Atlantic Ocean.

Launch will be from NASA's Wallops Flight Center in Virginia.

During the flight, the probe clock will always be in

a weaker gravitational field than an identical clock on

Earth. Hence, the frequency of the clock in the probe, as

observed by telemetry, will always appear to be greater than

that of the clock on the ground. Moreover, as the clock

rises from the Earth through the increasingly weaker field

to its maximum altitude of 10,000 kilometers (6,200 miles),

it will appear to run increasingly faster.

Its rate will then progressively decrease as it re-

turns to the stronger field at lower altitudes. During the

flight, the difference between the clock rate in the probe

as indicated by the telemetry signal and that on the ground

will be compared with the difference predicted by Einstein's

theory.

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At GP-A's maximum altitude, Einstein's theory pre-

dicts that the red shift effect should be about seven parts

in 10 billion (7 x 1 10

To measure this small--but significant--effect, the

Smithsonian Astrophysical Observatory (SAO), Cambridge, Mass.,

has developed atomic hydrogen MASERs that function as clocks

of extraordinary stability, or accuracy. (MASER is an

acronym for Microwave Amplification by Stimulated Emission

of Radiation.) The interaction of the electron and proton

in the hydrogen atom generates a microwave signal (1.42 bil-

lion cycles per second) stable to one part in a quadrillion

(1 x 10 15)--or the equivalent of a clock that loses le3s

than two seconds every 100 million years.

For the GP-A mission, the reference clock on the

ground will be located at the Merritt Island Launch Aret

(MILA), Kennedy Space Center, Fla.

Comparison of therelative rates of the probe anc.

ground clocks will be made by telemetry for the duratic.

of the flight. The rocket-borne clock will return to Erth

and impact somewhere between thle South American contine it

and Africa. The clock will not be recovered.

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The clocks are expected to provide a measurement

accuracy within five thousandths of one per cent (5 x 10 )

of the predicted effect.

In addition to the experiment design and the construc-

tion of the two prime clocks, SAO provided the control and

signal process ng equipment. The Marshall Center built the

payload support system and had responsibility for payload

integration and testing.

The GP-A experiment payload is 114 centimeters (45

inches) long, 96 cm (38 in.) in diameter and weighs 102

kilograms (225 pounds).

Within the NASA Office of Space Science, the GP-A

program is assigned to the Astrophysics Program Office for

overall program management. Project management is assigned

to MSFC.

The Scout-D launch vehicle, Scout trajectories, launch

operations personnel and equipment will be provided by

NASA's Langley Research Center, Hampton, Va.

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The experiment package will be launched from the

Wallops Center located off Virginia's eastern shore,

using Scout launch personnel, existing equipment and faci-

lities. The launch will be in an easterly direction from

Launch Area No. 3.

Tracking and data acqnisition will be accomplished

by existing U.S. ground stations provided and operated by

NASA's Goddard Space Flight Center, Greenbelt, Md. The primary

ground station at MILA will be supplemented with experi-

ment-peculiar equipment and ground-based comparator MASERs.

Three other stations will be used in receiving only--Bermuda,

Wallops and Network Test and Training Facility at Goddard.

Principal investigator (PI) for GP-A is Dr. R. F. C.

Vessot and the co-investigator is Dr. Martin Levine, both of SAO.

They will interpret experiment data and prepare and publish

the final report. Frederick E. Vruels of the Marshill

Center's Space Science Project Office is project ma.iager.

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The cost for providing the GP-A payload with support

equipment and performing required data reduction, analysis

and completion of a final report, is $4.5 million. The

Scout vehicle launch operation and the tracking and data

acquisition facilities and operation are funded separately.

Cost of the Scout vehicle is $1.5 million.

(END OF GENERAL RELEASE. BACKGROUND INFORMATION FOLLOWS.)



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COMPONENTSOF EXPERIMENT

(See photo on following page)

Major components of the GP-A experiment package include:

the "E section", a ring which adapts the fourth stage of the

Scout D launch vehicle to the GP-A payload; the battery, a

silver-zinc unit which provides a minimum of 300 watt-hours

of energy; the translator, which converts the MASER signal

to the proper transmit frequency, the transponder, which

converts the up-link signal to the proper down-link signal

by changing it to a different frequency; the dissociator cool-

ing loop, a device for keeping the dissociator (which breaks

down molecular hydrogen.into atomic hydrogen for use in t the

hydrogen MASER) within operating temperatures; the distributor,

the central distribution point for all payload electrical re-

quirements; and the hydrogen MASER, a quantum mechanical

oscillator. The acronym MASER describes a class of devices

which amplify a particular frequency of radiation because of

their ability to extract energy from molecular or atomic

transitions.

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SCOUT D LAUNCH VEHICLE DESCRIPTION

The Scout D launch vehicle is a four-stage, solid-fueledrocket system. Scout S-193 and the GP-A spacecraft will be launchedon an initial azimuth Of 85 degrees.

The four Scout D motors -- Algol III, Castor IIA, AntaresHA and Altair HILA-- are interlocked with transition sections thatcontain guidance control, ignition, instrumentation system,separation mechanics and the spin motors required to stabilize thefourth stage.

Guidance for Scout D is provided by a programmed guidance system,is achieved by a combination of aerodynamic surfaces, jet vanes andhydrogen peroxide jets. The vehicle is approximately 22. 25 meters(73 feet) long and weighs about 21,545 kilograms (47,400 pounds)liftoff.

LAUNCH SEQUENCE

EventTime (Min-Sec)

Liftoff 00:00

Second Stage Ignition 01:14. 60Secorid Stage Burnout 01:52. 95Payload Heat Shield Separation 01:56. 25Third Stage Ignition 01:58.45Third Stage Burnout 02:31. 42Spin Motor Ignition 02:41.42Third Stage Separation 02:42. 92Fourth Stage Ignition 02:47. 77Fourth Stage Burnout 03:21. 93Payload Separation

05:42. 92

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GRAVITATION RESEARCH USING ATOMIC CLOCKS IN SPACE

By Dr. R. F. C. Vessot

Smithsonian Astrophysical Observatory

Principal Investigator, GP-A

Atomic clocks with a stability of one part in a hundred million

million (1014) have been developed and adapted for space. With recent

advances in space technology, we can now expand our laboratory to

span the entire solar system- and use massive b6ders and large distances

to measure directly the changes caused by gravitation on time and

dimensions. Communication by phase-coherent microwave systems is

now possible over enormous distances, and we can realistically consider

performing the "gedanken a or thought experiments described in the

literature on gravity and relativity.

Traditionally, relativity has been described in terms of systems

moving with respect to one another, each containing rods ar.d clocks.

Pulsed-light signals connect the systems observationally and provide the

basis for comparisons. To make experimental measurements, we can,

in fact, use rods and clocks. However, the rod lengths arc related to

the clocks by the velocity of light, and we can describe distances in

terms of wavelengths of the clock frequency if we postulate that the velocity

of light is constant in space-time. Thus, we can design relativity

experiments that require clocks only.

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Most theories of relativity describe space and time by four-

dimensional geometry--the three dimensions of space as we- perceive

them--and tine. The presence of accelerations, and in particular

acceleration due to gravity, affects the shape of the geometrical

lattice work, or coordinate trames used to describe physical phenomena.

These frames are said to be warped by the presence of massive bodies

and the warping affects both the spatial and the temporal dimensions.

GP-A, an experiment using an atomic clock aboard a space

vehicle, will determine directly the effect of gravitation on time by

comparing the rate of the rocket-borne clock with another on Earth.

Our new "laboratory" has extended into space and may well be the

forerunner of other direct measurements of relativistic and gravitational

effects probing even as far as the Sun itself.

Our objective is to test the validity of our geometrical picture

of relativity. In particular, we will test the principle of equivalence,

the cornerstone of Einstein's GoneralTheory of Relativrity. F s -enunciated

in 1907, this principle asserts that there is no way of distinguis' ing

locally between the field effects of gravity and those generated b an

oppositely applied acceleration. It is a logical extensionof the *

bserved

proportionality between gravitational and inertial mass that has een tested

by Newton, by Eotvgs, and, more recently, by both Dicke and B *aginskii

to an accuracy of one part in 10l2.

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A further and more crucial test of the equivalence principle

is to see if light waves are also affected equally by glavity and

mechanical acceleration. In the latter, light waves traversing our

laboratory in the- diiection of its mechanical acceleration will be

received at a slower rate than they were transmitted. This results

from the finite transit time between the transmitter and the receiver;

the receiver (still connected to the transmitter) will have gained

velocity, and the arriving wave crests will encounter the receiver

at a slower rate. This shift in the received frequency of the waves

is, of course, the familiar Doppler effect. We c')uld describe the

"red shift" of our mechanically accelerated laboratory as the Doppler

effect due to the velocity gained by the receiver during the transit time of

the signals. Our goal is to see if the signals will behave the same way when

our laboratory is on the Earth's surface and experiences the pull of

gravity.

To date, the best test has been performed by -. V. Pound A

and his co-workers at Harvard. They have shown, using gamma

rays from iron (Fe 57), that the equivalence principle is

valid within one per cent (1 x 10 2) for a vertical distance

of 75 feet at the Earth's surface. The forthcoming test in

space will extend the distance to 6,200 miles and

could be as accurate as 50 parts per million (5 x 10 5)

Furthcrmore, the test will be performed continuously as a

function of altitude and will establish the behavior of the

shift over distances comparable to the Earth's diameter.

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We plan to use a clock in a spacecraft and compare it by

microwave signals to a clock on the ground. To overcome possible

errors due to slow drifts in the clock rates, the clock should be

moved into space and back to Earth in a reasonably short time.

We must obtain as large a span of gravitational potential as possible,

consistent with acquiring a sufficient number of measurements at

high and low altitudes and making the best use of the clocks' stabilities.

This suggests a space probe that attains a very great altitude and falls

back to Earth. Since the measurements near the Earth are as important

as those far from it, and because in such a trajectory the velocity near

the Earth is very high, not much time is available near Earth for making

measurements. This puts a high premium on clocks (or oscillators)

that can deliver high stability over short time intervals. A further

requirement is that the precision of the measurements must also be

maintained over the entire experiment so that frequency measurements

at both high and low altitudes can be compared.

We have chosen the atomic hydrogen-MASER oscillator as an

embodiment of a "proper" clock since it is stable to better than oi e part

in 1014 over 100-second intervals and up to periods extendingto nmany

hours. The total predicted red shift due to the Earth is about

seven parts in 1010. If we take our clock to infinity, this

enables us, in principle, to measure the effect with an accura-

cy of 14 parts per million (14 x 10 5). By going to a distance

cf about two Earth radii from Earth's center, we obtain a

test only slightly inferior--and definitely more feasible.

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The test will consist of a simple, one-shot, up-down

experiment in which we can overcome about 90% of the Earth's

gravitation and still have reasonable time to perform the experiment.

But what of the communications problem? Surely, if we wish to

see frequency changes as small as one part in 1014 in a rapidly moving

oscillator, we must learn to cope with very large Doppler shifts in the

frequency of the oscillator signals. Furthermore, the rapid motion of

the clock causes a kinematic (or second-order Doppler) frequency shift,

described by Einstein's SpecialTheory of Relativity. (This effect has

been well tested by other experiments. ) It is here that our space

technology in microwave communications comes to bear. To account for

the second-order shift at any given time, we will use data available to

us from our knowledge of the probe's trajectory and obtain the velocity

of the probe at all times.

Far more serious is the problem of removing from the clock

signal the first-order Doppler shift--about three parts in 105 --in order

to see the 6-part-in- 1010 shift with reasonable accuracy. Ln addition to

the Doppler shifts due to the probe's motion, there are also frequency

shifts due to changes in the electrical path through the Earth's atmosphere

and ionosphere. We must account for the trtal shifts on a real

time, point-by-point basis during the flight of the probe.

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Fortunately, all these shifts of the probe signal can be directly

measured by using a second signal transmitted from the Earth and

received and retransmitted by the probe back to Earth. The frequency

of this re-transmitted signal received at the Earth is Compared with

the frequency of the original signal from the Earth. The frequency

difference is twice the one-way Doppler shift associated with the probe

clock signal as received on Earth.

This correction signal (divided by two) is combined with the

signal received from the probe so as to eliminate the propagation

effects, and we are left with a signal whose frequency contains the

information we seek.

At low altitudes, where the probe moves rapidly, the frequency

of the probe clock will appear to be retarded by about two cycles per

second due to the second-order Doppler effect. As the probe gains

altitude and slows down, this effect diminishes and will be offset by the

gravitational shift, which makes the probe clock appear to run faster,

eventually reaching one cycle per second at apogee.

It will certainly be surprising if these shifts are not as predicted

by Einstein's general theory. No doubt, our first question will bo "How

well did the experiment really work? " However, we expect the e-aperi-

ment will indeed confirm the postulates of relativity.

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GP-A/SCOUT D PROGRAM/PROJECT MANAGEMENT

NASA Headquarters

Dr. Noel 11inners Associate Administrator for the

Office of Space Science

T. B. Norris Director, Astrophysics

Program Office

Hubert D. Calahan Program Manager, GP-A

Dr. Nancy G. Roman Program Scientist, CP-A

Paul E. GoozhManager, Scout Launch Vehicie

Marshall Space Flight Center

Dr. William R. Lucas Director

Dr. Fred A. Speer Manager, Space Science Projects Office

Frederick E. Vreuls Manager, GP-A Project

Dr. Rudolph Decher Project Scientist, GP-A

Dave Gardiner Chief Engineer, GP-A Project

Langley Research Center

Donald P. Hearth Director

Howard T. Wright Head, Projects Group

Donald E. Forney Head, Langley Mission Support

Office, Western Test Range

Roland D: EnglishHead, Scout Project Office

Joseph B. Talbot Scout Payload Integrator

Lee R. Foster, Jr. Scout Project Operations Head

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Wallops Flight Center

Robert L. Krieger Director

William T. Burns GP-A Project Engineer

William L. Lord GP-A Test Director

Goddard Space Flight Center

Dr. John F. Clark Director

Tecwyn Roberts Director, Networks Directorate

Kenneth D. McDonald Tracking and Data Acquisition Manager

Kermit Blaney GP-A Project Engineer

Contractors

Smithsonian Astrophysical MASER Experiment System

)bs ervatory

Camibridge, Mass.

Texas Instruments Translator

£i lla', Tex.

Motorola Corp. Transponder

Scottsdale, Ariz.

Vought Corp. Launch Vehicle

Dallas, Tex.

Jun8.g9lt7

June~ 8, 1976

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Relativity Gravity Probe Press Kit

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