In article , Ron Baalke wrote:

Spotlight: Tiny Measurement Gives Big Boost to Planet Hunt
Written by Randal Jackson/Planet Quest
Jet Propulsion Laboratory, Pasadena, Calif.
July 22, 2003

[...]
To detect the stellar wobble caused by a planet as small as Earth,
scientists need an instrument of almost unbelievable sensitivity --
one that could measure an angle just one-tenth the width of a hydrogen
atom.

How's that an angle? Something must have gotten lost here...

Or look at it this way: Let's say there's an astronaut standing on the
moon, wiggling her pinky. You'd need an instrument sensitive enough to
measure that movement from Earth, a quarter million miles away.

That sounds like about 10^-5 arc seconds. Wow.

If so then the 1/10 H atom would subtend that angle if seen 20 cm away,
e.g. comfortable reading distance. Wow, too. Atoms really are small.

But...

Is such precision possible? After a six-year struggle, engineers at
the Jet Propulsion Laboratory recently proved that the answer is yes.

Such sub-atomic measurements were conducted for the first time ever
within a vacuum-sealed chamber called the Microarcsecond Metrology
Testbed.

[...]
The instrument that engineers have demonstrated in the laboratory will
become the heart of a revolutionary new space telescope known as the
Space Interferometry Mission.

"Six-and-a-half years ago, this technology was unproven and
unsubstantiated," said Brett Watterson, the mission's deputy project
manager. "It was just a remote possibility that we could do it. It was
through ingenuity, insight, leadership and sheer perseverance that the
team was able to overcome these difficult technological challenges."

That's great news.

A few years ago someone from JPL came to UIUC to speak on SIM prospects.
I didn't get to ask the question that bothered me most:
he seemed to be suggesting it'd be in Earth orbit,
but surely that'd cause time- and direction-varying heating of the
spacecraft (by reflected light from the Earth, if not Earth's shadow!).
It seemed awfully hard to compensate for, given how stable
they'd need the platform to be.

I just now looked on the SIM web page, and see that it's to be in an
"Earth-trailing solar orbit". Does that mean it'd be placed at
a Lagrangian point 60 degrees behind the Earth? If so that'd make
good sense for keeping a stable environment.

Stuart Levy