PUBLIC INFORMATION OFFICE 
JET PROPULSION LABORATORY 
CALIFORNIA INSTITUTE OF TECHNOLOGY 
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PASADENA, CALIF. 91109. TELEPHONE (818) 354-5011
http://www.jpl.nasa.gov

CONTACT: Jim Doyle

FOR IMMEDIATE RELEASE                             May 16, 1996

ULYSSES MEASUREMENTS GIVE NEW CLUES TO DARK MATTER

     Measurements from the joint NASA/European Space Agency 
Ulysses mission found a surprisingly small increase in the amount 
of helium-3 since the formation of the solar system, allowing a 
more precise estimate of the amount of dark matter in the 
universe, two scientists announced today.  Their findings, from 
measurements of the Solar Wind Ion Composition Experiment aboard 
Ulysses, were published today in Nature magazine

     Drs. George Gloeckler, professor of physics at the 
University of Maryland, and Johannes Geiss, director at the 
International Space Sciences Institute in Bern, Switzerland, 
reported the first measurements of helium-3 in the interstellar 
cloud surrounding our solar system.  They said their findings of 
this lighter isotope of helium give additional clues to the 
amount of dark, or invisible matter, that was produced at the 
beginning of the universe -- the Big Bang -- 15 billion years 
ago.

     The exact nature of dark matter is one of the most 
intriguing mysteries in astronomy.  Although scientists do not 
know what it is, their best estimates indicate most of the 
universe -- perhaps as much as 90 percent -- is composed of dark 
matter.  This estimate is based partly on observations showing 
stronger gravitational attraction between galaxies than should be 
the case with the amount of matter they can see.  The dark matter 
may be "ordinary" matter such as planets and burned-out stars too 
dim to detect, or perhaps exotic objects such as black holes or 
as-yet-undetected particles which pervade the universe.

     "Basically, our measurements indicate the amount of dark 
matter in the early universe was fairly high," Gloeckler said.  
"Visible matter is a small fraction of the total.  By measuring 
the relative portions of the lightest elements and their 
isotopes, one can infer the amount of ordinary matter in the 
universe."

     "Since current theories of the earliest stages of the 
universe predict a much larger amount of matter coming out of the 
Big Bang, the difference is surmised to consist of an entirely 
different and unfamiliar kind of matter, commonly called dark 
matter," Geiss said.  "The precise nature of this dark matter is 
not known at this time, because it has eluded so far all efforts 
of detecting it directly.  It could very well have survived to 
this day, filling the whole universe and dominating its dynamics, 
yet be invisible to us."

     The light elements of helium-3, hydrogen, deuterium -- a 
heavy isotope of hydrogen -- and helium-4 were created in the Big 
Bang, the scientists said.  The abundance of helium-3 has been 
changing ever since because it is both produced and destroyed in 
stars.

     The direction and amount of this change have until now 
remained undetermined, they said.  They found, however, that the 
reported amount of helium-3 indicates a surprisingly small 
increase -- about 50 percent -- since the time of the formation 
of the solar system 4.6 billion years ago.

     Since the change was smaller than earlier models indicated, 
Gloeckler and Geiss said, the density of dark, or unseen, matter 
relative to that matter which we can see must be greater.

     "What is so exciting is that we have good numbers for the 
helium-3 abundance during three time epochs spanning the life of 
the universe," Gloeckler said.  "This gives a firm handle on how 
matter is processed or 'cooked' in stars."

     He said current models that predict large amounts of helium-
3 are not consistent with the new findings.

     The measurements were made over the south pole of the Sun at 
distances several times the distance of the Sun to Earth.  They 
detected the interstellar gas that made its way deep into the 
inner solar system and determined the composition of helium-3 in 
that gas.

     The Ulysses mission is managed jointly by NASA and the 
European Space Agency (ESA).  The Jet Propulsion Laboratory 
manages the U.S. portion of the mission for NASA's Office of 
Space Science, Washington, DC.

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5/14/96 JJD
#9634