"Pure what?" said Anti, but received no reply. She looked over the circle huddled around the scanner. Nona was curled near Cameron, sleeping peacefully. Docchi leaned forward with uncomfortable intensity. Jeriann was beside him but he didn't seem to notice her. "How long does this go on?" said Anti. "I'm getting tired of freezing in the dark." Actually she didn't mind it; cold that would kill others still bothered her hardly at all.
"Until we know," said Docchi. "All the way to Centauri if it takes that long."
"How can we know?"
"We'll find out as soon as we measure relative speeds," answered Docchi. "The scanner is similar to radar but it uses gravity, which makes things rather difficult. We can't send out an impulse and see how long it takes to get back because it travels instantaneously as far as we're concerned."
"Then there isn't any way? They seem to know how fast we're going."
"Better astronomical equipment," said Docchi. "We're a bigger object and they were able to measure our light shift, until we stopped illuminating the whole dome."
"And now they can't tell because they can seldom see us?"
"The contrary, if they're on their toes. They should guess that we're putting most of the power into the drive."
"Then how can we find out?" said Anti.
"Triangulation," said Docchi. "When we first saw them it was from the front. In past weeks they've crept up until they're nearly broadside. Now I hope they'll drop back. It may take weeks to tell, especially if our speeds are almost evenly matched."