Sky & Telescope - USA (2019-08)

X-ray Explorers

18 AUGUST 2019 • SKY & TELESCOPE

became clear that what appeared to be a diffuse X-ray back-

ground was actually emission from many individual active

galaxies that remained too faint to be seen.

Stars provided another, unexpected source of X-ray emis-

sion. “If it were up to me,” Tananbaum admits, “we might

never have looked at stars.” But a few astronomers insisted,

and Einstein ended up spotting emission from normal stars

far more powerful than that from the Sun. The fi nd estab-

lished an entirely new fi eld, the study of stellar atmospheres.

Then in 1980, disaster struck. The spacecraft temporar-

ily lost the use of a gyro used to help point the telescope on

the sky; multiple attempts to utilize a solar sensor to control

the spacecraft pointing consumed large

amounts of propellant. “It was pretty

desperate,” Elvis says. “[Einstein] lost a year

of its mission.” While the malfunctioning

gyro was eventually restored, the spacecraft

ran out of fuel a few months later.

Still, with 2½ years in space and more

than 5,000 targeted observations, the

observatory had imaged everything from

comets to quasars. After Einstein, Elvis

says, it became clear: “Everything emits

X-rays. You can’t do astronomy without

X-ray astronomy.”

Gearing Up

In 1976, before Einstein even fl ew, Giacconi

and Tananbaum had submitted a proposal

to NASA for a new X-ray telescope to suc-

ceed it, one that would be longer-lived and

higher-performing. NASA approved and the mission — ulti-

mately named the Chandra X-ray Observatory — was pro-

jected to launch in the 1980s. However, technological chal-

lenges, budget struggles, and political meanderings caused

more than a decade delay.

From a technical standpoint, cutting, grinding, and espe-

cially polishing the X-ray-refl ecting mirrors posed the biggest

challenge. The mirrors had to be extremely smooth, with

bumps no more than a few atoms high; otherwise, X-rays

would smash into the bumps instead of skipping off the

surface. (For comparison, a human hair is roughly 500,

atoms wide.) By the end of 1991, the team of scientists and

engineers had fi nally proven the technology they needed.

But two months after a successful demonstration of mir-

ror performance — and 16 years after the mission was fi rst

proposed — NASA’s Astrophysics Division Director Charlie

Pellerin called the science team together. “At the meeting he

tells us there’s not enough money to build [Chandra], and we

need to work with him to fi gure out how to downscope it,”

Tananbaum recalls. “We were... incredulous, is the kindest

word; ballistically angry is more accurate.”

Nevertheless, the team didn’t have much choice. So,

instead of nesting six mirror pairs, the scientists decided four

pairs would suffi ce. The heaviest, most expensive instru-

qRUSSIAN DOLLS Each of XMM-Newton’s three X-ray telescopes con-

tains 58 gold-coated mirror pairs nested inside each other.

pWOLTER DESIGN X-rays traveling through a Wolter mirror design are

twice refl ected at grazing-incidence angles, once off a parabolic mirror

and then off a hyperbolic mirror, before coming to a focus. Multiple mirror

pairs nested inside each other help capture more photons.

pARIEL V

LAUNCH The

Uhuru and Ariel V

satellites both

took off from the

Italian San Marco

launch platform,

located off the

coast of Kenya.

1980 1981 1982 1983 1984 1985 1986 1987 1988 1989 19

Ariel V

Einstein (HEAO 2)

Hakucho (CORSA B)

Salyut 7 space station

Tenma (Astro B) Ginga (Astro C)

Exosat Kvant

Spartan 1

Spacelab 2

Paraboloid

mirrors

Hyperboloid

mirrors

X-rays

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