launch decision were particularly tough. Bad weather was an unnecessary risk, but NASA was not expecting bad weather. Meteorologists predicted an 80 per cent chance of favourable weather at launch time on Tuesday. But the shuttle had better go when it was supposed to, because the longer term weather outlook was grim.

By Tuesday morning, Galileo's keepers were holding their breath. The countdown for the shuttle launch was ticking toward 12.57 p.m. The anti-nuclear protesters seemed to have gone quiet. Things looked hopeful. Galileo might finally go.

Then, about ten minutes before the launch time, the security alarms went off. Someone had broken into the compound. The security teams swung into action, quickly locating the guilty intruder … a feral pig.

With the pig safely removed, the countdown rolled on. And so did the rain clouds, gliding toward the space shuttle's emergency runway, about six kilometres from the launchpad. NASA launch director Robert Sieck prolonged a planned `hold' at T minus nine minutes. Atlantis had a 26-minute window of opportunity. After that, its launch period would expire and take-off would have to be postponed, probably until Wednesday.

The weather wasn't going to budge.

At 1.18 p.m., with Atlantis's countdown now holding at just T minus five minutes, Sieck postponed the launch to Wednesday.

Back at the SPAN centre, things were becoming hectic. The worm was spreading through more and more systems and the phones were beginning to ring every few minutes. NASA computers were getting hit all over the place.

The SPAN project staff needed more arms. They were simultaneously trying to calm callers and concentrate on developing an analysis of the alien program. Was the thing a practical joke or a time bomb just waiting to go off? Who was behind this?

NASA was working in an information void when it came to WANK. Some staff knew of the protesters' action down at the Space Center, but nothing could have prepared them for this. NASA officials were confident enough about a link between the protests against Galileo and the attack on NASA's computers to speculate publicly that the two were related. It seemed a reasonable likelihood, but there were still plenty of unanswered questions.

Callers coming into the SPAN office were worried. People at the other end of the phone were scared. Many of the calls came from network managers who took care of a piece of SPAN at a specific NASA site, such as the Marshall Space Flight Center. Some were panicking; others spoke in a sort of monotone, flattened by a morning of calls from 25 different hysterical system administrators. A manager could lose his job over something like this.

Most of the callers to the SPAN head office were starved for information. How did this rogue worm get into their computers? Was it malicious? Would it destroy all the scientific data it came into contact with? What could be done to kill it?

NASA stored a great deal of valuable information on its SPAN computers. None of it was supposed to be classified, but the data on those computers is extremely valuable. Millions of man-hours go into gathering and analysing it. So the crisis team which had formed in the NASA SPAN project office, was alarmed when reports of massive data destruction starting coming in. People were phoning to say that the worm was erasing files.

It was every computer manager's worst nightmare, and it looked as though the crisis team's darkest fears were about to be confirmed.

Yet the worm was behaving inconsistently. On some computers it would only send anonymous messages, some of them funny, some bizarre and a few quite rude or obscene. No sooner would a user login than a message would flash across his or her screen:

Remember, even if you win the rat race—you're still a rat.

Or perhaps they were graced with some bad humour:

Nothing is faster than the speed of light…

To prove this to yourself, try opening the refrigerator door before
                   the light comes on.

Other users were treated to anti-authoritarian observations of the paranoid:

The FBI is watching YOU.

or

Vote anarchist.

But the worm did not appear to be erasing files on these systems. Perhaps the seemingly random file-erasing trick was a portent of things to come—just a small taste of what might happen at a particular time, such as midnight. Perhaps an unusual keystroke by an unwitting computer user on those systems which seemed only mildly affected could trigger something in the worm. One keystroke might begin an irreversible chain of commands to erase everything on that system.

The NASA SPAN computer team were in a race with the worm. Each minute they spent trying to figure out what it did, the worm was pushing forward, ever deeper into NASA's computer network. Every hour NASA spent developing a cure, the worm spent searching, probing, breaking and entering. A day's delay in getting the cure out to all the systems could mean dozens of new worm invasions doing God knows what in vulnerable computers. The SPAN team had to dissect this thing completely, and they had to do it fast.

Some computer network managers were badly shaken. The SPAN office received a call from NASA's Jet Propulsion Laboratories in California, an important NASA centre with 6500 employees and close ties to California Institute of Technology (Caltech).

JPL was pulling itself off the network.

This worm was too much of a risk. The only safe option was to isolate their computers. There would be no SPAN DEC-based communications with the rest of NASA until the crisis was under control. This made things harder for the SPAN team; getting a worm exterminating program out to JPL, like other sites which had cut their connection to SPAN, was going to be that much tougher. Everything had to be done over the phone.

Worse, JPL was one of five routing centres for NASA's SPAN computer network. It was like the centre of a wheel, with a dozen spokes branching off—each leading to another SPAN site. All these places, known as tailsites, depended on the lab site for their connections into SPAN. When JPL pulled itself off the network, the tailsites went down too.

It was a serious problem for the people in the SPAN office back in Virginia. To Ron Tencati, head of security for NASA SPAN, taking a routing centre off-line was a major issue. But his hands were tied. The SPAN office exercised central authority over the wide area network, but it couldn't dictate how individual field centres dealt with the worm. That was each centre's own decision. The SPAN team could only give them advice and rush to develop a way to poison the worm.

The SPAN office called John McMahon again, this time with a more urgent request. Would he come over to help handle the crisis?

The SPAN centre was only 800 metres away from McMahon's office. His boss, Jerome Bennett, the DECNET protocol manager, gave the nod. McMahon would be on loan until the crisis was under control.

When he got to Building 26, home of the NASA SPAN project office, McMahon became part of a core NASA crisis team including Todd Butler, Ron Tencati and Pat Sisson. Other key NASA people jumped in when needed, such as Dave Peters and Dave Stern. Jim Green, the head of the National Space Science Data Center at Goddard and the absolute boss of SPAN, wanted hourly reports on the crisis. At first the core team seemed only to include