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He shook his wraith-head. "Not exactly. It's one part of our overall need to know what the hell's gone
on in the Solar System during all those centuries incommunicado. But, true, it does appear important. I
do want to hear more about it."
"You shall, you shall."
"What say you explain the situation as you see it?" he proposed. "Never mind if you repeat what I've
al-ready heard. That can help put it in context for me, like scattered bricks getting picked up and
mortared together to make a nouse."
"Will you understand why that house bears the shape it does?"
"I'll try. Uh, you needn't go into the theory, of course, nor the layout, unless it's changed since I got my
briefing at Alpha Centauri." With electrophotonic speed, his mind reviewed them.
Einstein first found it, in the mathematics of his general relativity: mass determines the metric of
space-time, and thereby arises the phenomenon we call gravitation. This curves the trajectories not only
of matter but of radiation. The confirmation was triumphant when astronomers mea-sured a predicted
slight displacement of stars in the field of view near the eclipsed sun. By the mid-twentieth cen-tury they
were observing how galaxies act as enormous, irregular lenses to produce multiple images, distorted but
enhanced, of objects far behind them. Not long afterward, they were detecting dark bodies within our
own galaxy by the effects of these on the stellar background.
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But the sun is also a gravitational lens! Nearly spher-ical, it has little aberration. From this fact and
certain calculations sprang a wonderful dream. The closest focal points are five hundred and fifty
astronomical units out-ward, a fraction over three light-days however, well within the Solar System. Let
us send spacecraft there. We can do it by gravity assists, solar sails,magnetic sails. The journey will take
about fifty years, but along the way the instruments will gather a rich harvest of data on solar wind,
parallaxes, and who can foretell what else? Indeed, it should be worth going farther than the minimum
distance, both to continue the secondary investigations and to gain added image size while reducing
interference by the solar corona.
Finally the craft will reach their destinations. The or-bits they take up will be comparable to those of
comets in the Kuiper Belt, although not necessarily in the same plane as any. The observatories will
deploy their equip-ment to peer in the direction of the sun and on into the universe beyond, transmitting
the data home to Earth.
What knowledge they may win is incredible. Assum-ing the minimum distance or a trifle more, and a
modest 12-meter antenna, the angular resolution at the 1,420-megahertz frequency of neutral hydrogen is
on the order of several microarcseconds. Translated into spatial terms, this means that the observatory
can resolve distinguish from its surroundings an object at Alpha Centauri as small as 1,250 kilometers
across. At ten parsecs, 32.6 light-years, it can pick out 9,580-kildmeter sizes. Ten ki-loparsecs away, at
the galactic nucleus, it can see something less than 10 million kilometers wide.
This improves proportionately as frequency increases. At optical wavelengths, it could theoretically
detect a hu-man body near Alpha Centauri, a fifteen-kilometer aster-oid or island near galactic center,
individual continents on planets in the Andromeda galaxy.
No one gave serious thought to such extravagances. They went beyond any feasibility, and perhaps
beyond what natural law might allow. For a single example out of many, consider that the observatory is
in orbit. Slowly though it moves, it is never at rest, and so neither is its line of sight. The farther it is
looking, the fainter are the signals it receives, and the faster they sweep through its field of view which
becomes ever narrower as resolu-tion increases. There will simply not be enough time to catch enough
.photons to identify anything too small or too remote at frequencies too high.
The dreamers would be satisfied with radio waves. Those were already opening fabulous vistas to them;
the shape of the galaxy, the births and lives and deaths of stars, the titanic clouds and the ghost-winds
between, pul-sars and quasars and aliennesses untold out to the rim of observable space and the dawn
of observable time. To those among them who listened for word of sentience elsewhere, radio waves
seemed most likely to bear it, someday, somehow. Send forth the observers whose in-strument would be
the sun!
In particular, one that established itself in the sky as seen from Earth between the horns of the Bull, a
little south of Elnath, would be on a line between Sol and the center of the galaxy. It would be looking at
hordes of stars. We live in the outskirts, where they are thinning away into emptiness. The galactic heart
is enormous. This observatory could scan it for a long time indeed before orbital motion, very slow at that
distance from the sun, carried it out of the field. Surely its laser sendings back to Earth would tell of new
marvel after marvel, maybe even, at last, signs of other minds than ours.
Let the thing be done.
In the event, it did not happen soon, nor all at once. The human space endeavor came near dying soon
after it was born. Crewed missions grew scanty, and support was slight for a project whose time until
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fruition reached beyond the life expectancy of influential scientists. When Fireball Enterprises kindled
fresh vitality, there was at first too much else to do. But later, Juliana Guthrie took up the idea. She had
no difficulty in persuading her hus-band.
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