Smith nodded. "They're O.K." He looked at Hull. "A screamer is an emergency radio. There's one in every compartment. You've seen them." He pointed across the room, toward a red panel in the wall. "In there."
"But I thought it was impossible for a spaceship in flight to contact a planet by radio," Hull objected.
"Normally, it is," Smith admitted. "It takes too much power and too tight a beam to get much intelligence over a distance that great from a moving ship. But the screamers are set up for emergency purposes. They're like flares, except that they operate on microwave frequencies instead of visible light.
"The big radio telescopes on Luna and on the Jovian satellites can pick them up if we beam them sunward, and the Plutonian station can pick us up if we beam in that direction."
Hull looked much calmer. "But where do you get the power if the engines are gone? Surely the emergency batteries won't supply that kind of power."
"Of course not. Each screamer has its own power supply. It's a hydrogen-oxygen fuel cell that generates a hell of a burst of power for about thirty minutes before it burns out from the overload. It's meant to be used only once, but it does the job."
"How do they know where to find us from a burst like that?" Hull asked.
"Well, suppose we only had one screamer. We'd beam it toward Pluto, since it would be easier for an IP ship to get to us from there. Since all screamers have the same frequency—don't ask me what it is; I'm not a radio man—the velocity of our ship will be indicated by the Doppler Effect. That is, our motion toward or away from them can be calculated that way. Our angular velocity with respect to them can be checked while the screamer is going; they will know which direction we're moving, if we're moving at an angle.
"With that information, all they have to do is find out which ship is in that general area of the sky, which they can find out by checking the schedule, and they can estimate approximately where we'll be. The IP ship will come out, and when they get in the general vicinity, they can find us with their meteor detectors. Nothing to it."
"And," Captain Al-Amin added, "since we have eight screamers still left with us, we have plenty of reserves to call upon. There's nothing to worry about, Mr. Hull."