year from the nebula. Tommy's work was guiding the ship's course, now, but the work was done. During all the stay of the exploring ship in the nebula, Tommy Dort would loaf. But he'd more than paid his way so far.
He had just completed a quite unique first—a complete photographic record of the movement of a nebula during a period of four thousand years, taken by one individual with the same apparatus and with control exposures to detect and record any systematic errors. It was an achievement in itself worth the journey from Earth. But in addition, he had also recorded four thousand years of the history of a double star, and four thousand years of the history of a star in the act of degenerating into a white dwarf.
It was not that Tommy Dort was four thousand years old. He was, actually, in his twenties. But the Crab Nebula is four thousand light- years from Earth, and the last two pictures had been taken by light which would not reach Earth until the sixth millennium A.D. On the way here—at speeds incredible multiples of the speed of light— Tommy Dort had recorded each aspect of the nebula by the light which had left it from forty centuries since to a bare six months ago.
The Llanvabon bored on through space. Slowly, slowly, slowly, the incredible luminosity crept across the vision plates. It blotted out half the universe from view.
Before was glowing mist, and behind was a star-studded emptiness. The mist shut off three-fourths of all the stars. Some few of the brightest shone dimly through it near its edge, but only a few. Then there was only an irregularly shaped patch of darkness astern against which stars shone unwinking. The Llanvabon dived into the nebula, and it seemed as if it bored into a tunnel of Darkness with walls of shining fog.
Which was exactly what the spaceship was doing. The most distant Photographs of all had disclosed structural features in the nebula. It was not amorphous. It had form. As the Llanvabon drew nearer, indications of structure grew more distinct, and Tommy Dort had argued for a curved approach for photographic reasons. So the spaceship had come up to the nebula on a vast logarithmic curve, and Tommy had been able to take successive photographs from slightly different angles and get stereo-pairs which showed the nebula in three dimensions; which disclosed billowings and hollows and an actually complicated shape. In places, the nebula displayed convolutions like those of a human brain. It was into one of those hollows that the spaceship now plunged. They had been called 'deeps' by analogy with crevasses in the ocean floor. And they promised to be useful.
The skipper relaxed. One of a skipper's functions, nowadays, is to think of things to worry about, and then worry about them. The skipper of the Llanvabon was conscientious. Only after a certain instrument remained definitely nonregistering did he ease himself back in his seat.
'It was just barely possible,' he said heavily, 'that those deeps might be nonluminous gas. But they're empty. So we'll be able to use overdrive as long as we're in them.'
It was a light-year-and-a-half from the edge of the nebula to the neighborhood of the double star which was its heart. That was the problem. A nebula is a gas. It is so thin that a comet's tail is solid by comparison, but a ship traveling on overdrive—
above the speed of light—does not want to hit even a merely hard vacuum. It needs pure emptiness, such as exists between the stars. But the Llanvabon could not do much in this expanse of mist if it was limited to speeds a merely hard vacuum will permit.
The luminosity seemed to close in behind the spaceship, which slowed and slowed and slowed. The overdrive went off with the sudden pinging sensation which goes all over a person when the overdrive field is released.
Then, almost instantly, bells burst into clanging, strident uproar all through the ship. Tommy was almost deafened by the alarm bell which rang in the captain's room before the quartermaster shut it off with a flip of his hand. But other bells could be heard ringing throughout the rest of the ship, to be cut off as automatic doors closed one by one.
Tommy Dort stared at the skipper. The skipper's hands clenched. He was up and staring over the quartermaster's shoulder. One indicator was apparently having convulsions. Others strained to record their findings. A spot on the diffusedly bright mistiness of a bow-quartering visiplate grew brighter as the automatic scanner focused on it. That was the direction of the object which had sounded collision-alarm.
But the object locator itself—. According to its reading, there was one solid object some eighty thousand miles away—an object of no great size. But there was another object whose distance varied from extreme range to zero, and whose size shared its impossible advance and retreat.
'Step up the scanner,' snapped the skipper.
The extra-bright spot on the scanner rolled outward, obliterating the undifferentiated image behind it. Magnification increased. But nothing appeared.
Absolutely nothing. Yet the radio locator insisted that something monstrous and invisible made lunatic dashes toward the Llanva- bon, at speeds which inevitably implied collision, and then fled coyly away at the same rate.
The visiplate went up to maximum magnification. Still nothing. The skipper ground his teeth. Tommy Dort said meditatively:
'D'you know, sir, I saw something like this on a liner on the Earth- Mars run once, when we were being located by another ship. Their locator beam was the same frequency as ours, and every time it hit, it registered like something monstrous, and solid.'
'That,' said the skipper savagely, 'is just what's happening now. There's something like a locator beam on us. We're getting that beam and our own echo besides. But the other snip's invisible! Who is out here in an invisible ship with locator devices? Not men, certainly!'
He pressed the button in his sleeve communicator and snapped:
'Action stations! Man all weapons! Condition of extreme alert in all departments immediately!''
His hands closed and unclosed. He stared again at the visiplate which showed nothing but a formless brightness.
'Not men?' Tommy Dort straightened sharply. 'You mean—'
'How many solar systems in our galaxy?' demanded the skipper bitterly. 'How many planets fit for life? And how many kinds of life could there be? If this ship isn't from Earth—and it isn't—it has a crew that isn't human. And things that aren't human but are up to the level of deep-space travel in their civilization could mean anything!'
The skipper's hands were actually shaking. He would not have talked so freely before a member of his own crew, but Tommy Dort was of the observation staff. And even a skipper whose duties include worrying may sometimes need desperately to unload his worries. Sometimes, too, it helps to think aloud.
'Something like this has been talked about and speculated about for years,' he said softly. 'Mathematically, it's been an odds-on bet that somewhere in our galaxy there'd be another race with a civilization equal to or further advanced than ours.
Nobody could ever guess where °r when we'd meet them. But it looks like we've done it now!'
Tommy's eyes were very bright.
'D'you suppose they'll be friendly, sir?'
The skipper glanced at the distance indicator. The phantom object still made its insane, nonexistent swoops toward and away from the Llanvabon. The secondary indication of an object at eighty thousand miles stirred ever so slightly.
'It's moving,' he said curtly. 'Heading for us. Just what we'd do if a strange spaceship appeared in our hunting grounds! Friendly? Maybe! We're going to try to contact them. We have to. But I suspect this is the end of this expedition. Thank God for the blasters!'
The blasters are those beams of ravening destruction which take care of recalcitrant meteorites in a spaceship's course when the deflectors can't handle them.
They are not designed as weapons, but they can serve as pretty good ones. They can go into action at five thousand miles, and draw on the entire power output of a whole ship. With automatic aim and a traverse of five degrees, a ship like the Llanvabon can come very close to blasting a hole through a small-sized asteroid which gets in its way. But not on overdrive, of course.
Tommy Dort had approached the bow-quartering visiplate. Now he jerked his head around.
'Blasters, sir? What for?'
The skipper grimaced at the empty visiplate.
'Because we don't know what they're like and can't take a chance! I know!' he added bitterly. 'We're going to make contacts and try to find out all we can about them—especially where they come from. I suppose we'll try to make friends—but we haven't much chance. We can't trust them the fraction of an inch. We daren't! They've
