Spotlight: Tiny Measurement Gives Big Boost to Planet Hunt

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

Even though astronomers have discovered more than 100 planets around
stars other than the Sun in recent years, the "holy grail" of the
search -- an Earth-sized planet capable of supporting life -- remains
elusive. The main problem is that an Earth-like planet would be much
smaller than any of the gas giants detected so far (see illustration
at right).

Planets orbiting other stars are too dim to be observed directly, but
scientists infer their presence by the tiny gravitational "wobble"
they induce in their parent stars. Observed from tens of light years
away (one light-year is 5.88 trillion miles), this movement becomes
very tiny indeed. The smaller the planet, the less the star parent
wobbles.

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. That's about 1 millionth of the width of the thickest human
hair.

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.

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.

By doing this, the engineers proved they can measure the movements of
stars with an astonishing degree of accuracy never before achieved in
human history.

The testbed, which resembles a shiny silver submarine, is jammed with
mirrors, lasers, lenses and other optical components. Because even
small air movements can interfere with the measurements, all air is
pumped out of the chamber before each experiment is run. Laser beams,
moving mirrors and a camera are used to help detect movements of an
artificial star, which simulates the light that would be emitted by a
real star.

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."

NASA recently gave the go-ahead for the second stage of development
for the mission, which will not only be able to search for Earth-like
planets around other stars, but will also measure cosmic distances
several hundred times more accurately than currently possible.
Scheduled to launch in 2009, it will scan the heavens for five years
and provide astronomers with the first truly accurate road map of our
Milky Way galaxy.

"This is a historical time that we're intimately involved with,"
Watterson said. "Unlike any other culture in history, we have the
technological means, the budget, and the will to determine the
occurrence of Earth-like planets orbiting other stars. Everyone on the
team is aware of their role in this pivotal stage in the search for
life elsewhere in the universe."

The Space Interferometry Mission is managed by JPL as part of NASA's
Origins program.

Written by Randal Jackson/Planet Quest
Jet Propulsion Laboratory, Pasadena, Calif.

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

"SL" == Stuart Levy writes:

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

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

No, an "Earth-trailing solar orbit" is a heliocentric orbit in which
the Earth-spacecraft increases gradually. As seen from the Earth, the
spacecraft will appear to fall behind us. SIRTF is expected to be in
such an orbit. There was some discussion of SIRTF's orbit here
recently, you might check Google. Alternately, see the SIRTF Web
site.

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