It all started so simply, those fourteen years ago. . . .
"No doubt about it," Jill Chambers said, rereading a column of data on her screen just to be sure. "Our little helium source is slowing down."
Sammy nodded. They'd been squirreled away in the observatory here in Cape Town for three straight days and nights, tracking the Centaurus Helium Anomaly ever since it first appeared. It had been so dim, so tiny, that they'd barely noticed it at all against the glare of Alpha Centauri. How long had it been there before they spotted it?
They might never have seen it at all, in fact, had they not just by chance been doing a southern sky survey for emission lines. You could use a prism to split up the light coming from a star, or a distant galaxy, and see the whole rainbow of colors it produced. Normally, this spectrum would be peppered by a few sharp, dark lines where hydrogen or other elements absorbed some of the light. But occasionally, in the places where you expected the dark lines to be, you instead saw bright lines, brighter even than the rainbow around them. This only happened in the rare cases when an element was in an excited state, when its electrons had been kicked up by some wonderfully energetic process just begging for you to study it.
Most emission lines, when they did occur, happened at the frequency spacing produced by hydrogen. Helium emission lines were rarer, always meriting attention — and they were always, without exception, mixed in with emission lines from hydrogen. No known natural process could produce pure helium emission lines with no other lines mixed in. That was what made the Centaurus Helium Anomaly so exciting. It was only helium emission lines — no hydrogen lines, no lines from other elements, not even a reasonable blackbody spectrum in the background.
And most incredibly of all, it was blue shifted part way into the ultraviolet. That softly glowing helium was headed toward Earth at breakneck speed: 45 000 kilometers per second, a full fifteen percent of the speed of light.
"Well, I'll be damned," Jill said, "There is a blackbody spectrum here, underneath the helium emission lines. But . . . damn, is it faint!"
"Yeah," Sammy said, looking over her shoulder. "That's, what, a full ten magnitudes dimmer?"
"Twelve magnitudes dimmer," Jill said. "No wonder we missed it before."
"That's not just spillover from Alpha Centauri's spectrum, is it?"
Jill shot him a look. "Come on. This is the same glare mask they've been using at Palomar and on the space telescopes for three decades. And if you're trying to say you don't trust the atmospheric correction —"
Sammy held up his hands defensively. "No no. You're right. If either the mask or the air path laser had a glitch we'd be getting a lot worse that some super-weak blackbody background noise." He looked more closely. "Whoa. Are those absorption lines?"
Jill squinted at the dim rainbow of light in front of her, trying to make out what Sammy had seen. Then, she saw them. Lines, ever-so-slightly darker than the already dim blackbody spectrum, peppering it at regular intervals. "Yeah! There's one at 560 nanometers. And another one at 478. Let's see if the algorithm can pick 'em all out."
It didn't take long for the spectrum analysis algorithm to arrive at its answer.
"They're helium lines," Jill said. "Helium absorption lines. Just like the main emission lines, the ones that are so much brighter. The absorption lines are blue shifted, too, only . . . is that right?" Jill ran the data again. "Well, according to our best-fit matching algorithm here, these lines are only blue shifted by five percent."
"But the main emission lines are blue shifted fifteen percent," Sammy said.
Jill nodded. "We've got helium emission lines coming toward us at fifteen percent of the speed of light, and a blackbody spectrum with helium absorption lines coming toward us at five percent of the speed of light."
"From the same point in our field," Sammy confirmed.
Jill shook her head in amazement. "What the hell could produce this combination?"
Sammy rubbed his chin. "We're sure the two sources are exactly coincident, right? I mean, this new faint blackbody spectrum isn't from some totally different object that's almost along the same line of sight?"
"Down to the milliarcsecond in both coordinates," Jill said, pointing at the RA and DEC displays on the spectrograph.
Sammy frowned. "The two might still be unrelated. We could have some nearby helium emission source sitting right in front of a distant blackbody, or vice-versa." He caught himself. "No, forget the vice-versa. A blackbody would totally block our line of sight to any objects on the far side of it. Unless it's a tiny blackbody and the Helium Anomaly is some huge object much farther away."
"Well," Jill began, reluctantly, "Let's go with the assumption that both the emission lines and the blackbody spectrum are being produced by the same object, and see where that leads us."
"Okay," Sammy said, "It leads us to crazy land. Helium emission lines coming at us a full tenth of the speed of light faster than the blackbody that produced them? It would have to be a monumentally energetic process that could spit out helium at that speed."
Jill offered, "Cosmic rays go a lot faster than that."
"But cosmic rays aren't pure helium," Sammy countered. "I'd guess that they were alpha emissions from nuclear decay, only I don't know of any radioisotope that ejects alpha particles going that fast, do you?"
"It's been a little while since I've had to work with radioactive decay," Jill admitted.
Second Contact is continued in chapter 2.
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