now--Gunderson against the pirate. Preston
dropped back again to take advantage of the Patrol ship's protection.

"I'm going to try a diversionary tactic," Gunderson said on untappable
tight-beam. "Get ready to cut under and streak for Ganymede with all you
got."

"Check."

Preston watched as the tactic got under way. Gunderson's ship traveled
in a long, looping spiral that drew the pirate into the upper quadrant
of space. His path free, Preston guided his ship under the other two and
toward unobstructed freedom. As he looked back, he saw Gunderson
steaming for the pirate on a sure collision orbit.

He turned away. The score was two Patrolmen dead, two ships wrecked--but
the mails would get through.

Shaking his head, Preston leaned forward over his control board and
headed on toward Ganymede.

* * * * *

The blue-white, frozen moon hung beneath him. Preston snapped on the
radio.

"Ganymede Colony? Come in, please. This is your Postal Ship." The words
tasted sour in his mouth.

There was silence for a second. "Come in, Ganymede," Preston repeated
impatiently--and then the sound of a distress signal cut across his
audio pickup.

It was coming on wide beam from the satellite below--and they had cut
out all receiving facilities in an attempt to step up their transmitter.
Preston reached for the wide-beam stud, pressed it.

"Okay, I pick up your signal, Ganymede. Come in, now!"

"This is Ganymede," a tense voice said. "We've got trouble down here.
Who are you?"

"Mail ship," Preston said. "From Earth. What's going on?"

There was the sound of voices whispering somewhere near the microphone.
Finally: "Hello, Mail Ship?"

"Yeah?"

"You're going to have to turn back to Earth, fellow. You can't land
here. It's rough on us, missing a mail trip, but--"

Preston said impatiently, "Why can't I land? What the devil's going on
down there?"

"We've been invaded," the tired voice said. "The colony's been
completely surrounded by iceworms."

"Iceworms?"

"The local native life," the colonist explained. "They're about thirty
feet long, a foot wide, and mostly mouth. There's a ring of them about a
hundred yards wide surrounding the Dome. They can't get in and we can't
get out--and we can't figure out any possible approach for you."

"Pretty," Preston said. "But why didn't the things bother you while you
were building your Dome?"

"Apparently they have a very long hibernation-cy