skyandtelescope.com • AUGUST 2019 15Turns out, Sco X-1 is the brightest endur-
ing X-ray source outside the solar system.
But the rocket launched by Giacconi’s team
might easily have missed it: A big chunk of
sky was hidden behind Earth during the
nearly six minutes the rocket was aloft. The
discovery was fortunate indeed for Giacconi,
as it played a key role in winning him the
2002 Nobel Prize in Physics. But it was even
more fortunate for X-ray astronomy, galva-
nizing the fi eld as well as the imagination.
“It was pretty hard to explain [Sco X-1] by
anything except something that you didn’t
expect to fi nd,” says former Chandra X-ray
Center director Harvey Tananbaum (Center
for Astrophysics, Harvard & Smithsonian).
Over the next six decades, X-ray astron-
omy kept growing, albeit sometimes in fi ts
and starts. As more instruments took to
space, the data that came back helped con-
fi rm theorists’ wildest imaginings — X-rays
provided crucial evidence for the existence
and physics of neutron stars, black holes, dark matter, and
more. “There’s a whole range of activity in the universe that
would have been completely unknown to us without X-ray
astronomy,” says Peter Kretschmar (European Space Agency).
Yet even as 2019 sees the triumphant 20th anniversaries
of the Chandra X-ray Observatory and the XMM-Newton
satellite, astronomers are still struggling to maintain their
space-based window on the X-ray universe.Freedom: Satellites Take to the Sky
Not long after the fi rst discoveries, scientists began craving
more than the few minutes of observing time that a rocket
could grant them. Giacconi led the team at AS&E as they
began to build a dedicated X-ray satellite — this time with
NASA funding.
The project, offi cially known as Small Astronomical Satel-
lite A, would contain two sets of proportional counters to scan
the sky and pinpoint X-ray sources to within a few arc-
minutes. These proportional counters consisted of beryllium
casings surrounding interiors that were fi lled with argon gas.
An X-ray photon that penetrated the tube’s thin beryllium
windows would collide with an argon atom and initiate an
avalanche of ion-electron pairs proportional to the photon’s
energy. Combined with a star tracker that told scientists
which way the satellite was pointing, the proportional coun-
ter could record X-rays by their position on the sky.pDISCOVERY Riccardo Giacconi and his team
launched an Aerobee rocket and, in addition to solar
fl uorescence off the surface of the Moon, the second
proportional counter onboard detected X-rays
streaming from a source identifi ed as Scorpius X-1.
The third counter, which was pointing in another
direction, saw a pervasive background of X-ray pho-
tons. (The fi rst counter failed and isn’t shown.)uX-RAY VISION Rather than using wavelengths, astronomers typi-
cally refer to X-rays in terms of their energy, given in units of electron
volts (eV) or, more often, kiloelectron volts (1,000 eV, or 1 keV). Uhuru, for
example, used proportional counters to detect X-rays in the range from
2 to 20 keV. Nowadays, XMM-Newton sees X-rays between 0.1 and 10
keV. Some instruments, like Integral, go to even higher energies, catch-
ing both X-rays and gamma rays (3 to 10,000 keV).The just-graduated Tananbaum joined
the team in 1968, when the fi eld of X-ray
astronomy was only six years old. He accom-
panied equipment to the launch site off the
coast of Kenya, an equatorial location where
Earth’s rotation boosts rocket speed. With
the rocket due to launch shortly after midnight, Tananbaum
followed the crew’s progress via a computer on the mainland,
six miles from the rocket platform. He was waiting to perform
a last equipment test about half an hour before launch.
“They’re counting down, and they go into a hold, they’re
checking something on the rocket,” he recalls. “There’s limited
information fl owing, and the Sun comes up and it’s starting
to get warm.... Temperatures inside the payload are going up
and up.” High heat and humidity were threatening the mission
before it could make it into space. The platform crew had to
decide: scrub or launch.
The decision was quick — the count jumped from T minus
hours to T minus minutes. “And bingo, it launches,” Tanan-
baum chuckles. That day, December 12, 1970, happened to
fall on Kenya’s independence day, and Giacconi renamed the
mission Uhuru (Swahili for freedom) in the country’s honor.
Before long, computer screens showed so-called “X-ray
stars” seething on the sky. Some varied predictably in theirRevealing
the X-ray SkyYear Number of
X-ray sources
1962 1*
1965 10
1970 60
1974 160
1980 680
1984 840
1990 8,
2000 340,
2010 780,
2017 1,250,
*Excluding the Sun.
Data source: NASA’s High Energy
Astrophysics Science Archive
Research Center0°Num
ber^ of^ counts60°50150Emission from Moon Peak of X-ray emission250350450120° 180° 240° 300° 360°Counter 2
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62 ;ELECTROMAGNETICSPECTRUM:^ GRE
GGDINDIERMANTemperature of objects at which this radiation
is the most intense wavelength emitted.100K
–173°C10,000K
9,727°C10,000,000K
~10,000,000°CInfrared Visible UV X-ray Gamma ray10,10,000 100,000 1,000,1,1,1001001010110.0.0.01 0.
Wavelength (nanometers)Energy (electron volts)