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honouring the planet, by making use of its wreckage.’ She faced Roberta defiantly. ‘I suppose you think either position is illogical.’
    â€˜Not at all. One would have to have a very stunted emotional imagination not to have some response to this, the ruin of a world, of, presumably, a planetary biosphere every bit as mature and rich as that of Datum Earth itself. But what you’re doing here is neither right nor wrong. It simply is.’ She glanced around the sky. ‘Where is Cyclops?’
    Stella swam over next to her and pointed. ‘Up there, at four o’clock.’
    Looking that way, Dev could see only a disc of blackness, occluding the stars. He said, ‘Actually what you see is just the baffle, shielding the radio telescope from leakage from the habitats, the shuttles.’ He tapped a console, and a big display tablet brought up an image of a vast, lacy dish: the antenna of the spaceborne radio telescope itself.
    Roberta glanced up at the baffle, itself an immense structure. ‘A shame I can’t see it with the naked eye, but I sense the scale.’
    Stella said, ‘You know, astronomy, and particularly radio astronomy, was one of the first great science programmes for the Next, once we had organized our society sufficiently. An area where huge advances in knowledge were available based on a simple expansion of technological scale. We began with a trio of super-Arecibos. On the Datum this was a major radio telescope with its dish built into a volcanic caldera in Puerto Rico. We constructed much larger dishes in calderas on one particular Earth, near Olduvai Gorge, at Pinatubo, one at Yellowstone in North America – a long-dead copy of the parent volcano on the Datum. If you visualize these positions, spread around the globe of the world, you will see we had coverage of the equatorial sky for twenty-four hours of the day.
    â€˜But these efforts will be surpassed as we move into space. Our first design was Cyclops, out there. A single parabolic dish antenna five kilometres wide. We named it after a pre-Step Day proposal, from a century ago, to build such a telescope from a conglomeration of a thousand smaller antennas, constructed on the ground. It might be unfinished, but it’s good enough to have acquired the clearest version yet of the Invitation.’ She fished her own tablet out of her bag, and tapped it to bring up data on the signal. ‘In some ways it’s a classic SETI discovery. An extremely strong signal. Polarized, as if it has been broadcast by a radio telescope of the kind we can build ourselves. The frequency is around the minimum of the background noise of the Galaxy. We’re aware of a lot of detail below the signal’s top-level structure, but much of it is lost in the noise. And what we have is complex. Not decipherable, so far anyhow.’
    â€˜Which is,’ Roberta said calmly, ‘why we’re all here.’
    Dev said, ‘We still don’t know where it’s coming from. The source is stationary against the background of the stars. The source appears to be in Sagittarius—’
    â€˜It’s logical that it should be.’ Roberta glanced over her shoulder, and Dev just knew she was looking straight towards the position of the Sagittarius constellation in the sky. ‘The overwhelmingly most likely location of high intelligence is towards the centre of the Galaxy. The spiral arms, where we live, are waves of star birth washing around the galactic disc. But at the core, where the stars are crowded close, where the energy fluxes are enormous – a dangerous place, but where the first worlds rich in rock and metal formed billions of years before Earth – that is where the peak of galactic civilization must reside. And all of that lies in the direction of Sagittarius.’
    Lee said, ‘And you think it’s imperative that we pick up this Invitation and figure it out.’
    Roberta looked back