Fixing a Far-out Flaw
The article in Science magazine had an arresting headline: "Galileo Hits a Snag." The subject wasn't the great but long-gone scientist's problems with the Inquisition; it was a peculiar failure in the spacecraft bearing his name.
Galileo the spacecraft is on its way to Jupiter. Launched from the space shuttle in October 1989, Galileo is taking the scenic route to its giant target. To conserve fuel (and thus weight), its fabricators at the Jet Propulsion Laboratory in California sent it on a complex looping path that uses the gravitational fields of other planets as if they were slingshots. Earth's field was one of the slings; Galileo will swing by the home world again during December 1992 for a last gravitational acceleration toward its goal.
There was some concern about this tricky routing, because it entailed such a long flight time to Jupiter. Nothing serious went wrong, though, until trouble arose in some fairly simple machinery: the unfolding system for an umbrella-like high-gain communications antenna.
The antenna does look like an umbrella, but a gigantic one made of gold-plated mesh stretched over graphite ribs---or at least that's how it would look if it had unfurled properly. According to Science, the same type of antenna has worked perfectly on six different earth-orbiting communications relay satellites. But this April, when controllers radioed the signal for Galileo's main antenna to unfurl and get to work, something stuck.
This could be bad news for experimenters waiting for word from Jupiter. (Their number includes one Alaskan: Juan Roederer, of the UAF Geophysical Institute, is chief theoretician on an international team that put together the energetic particle detector system aboard the spacecraft.) The big antenna can return 134,000 bits of data every second from space near Jupiter, enough to generate a complete image of the planet every minute; the two little antennas already properly deployed can send back only 10 bits a second from that far out.
Bad news it may be, but not deadly, for a couple of reasons. First, Galileo has had a successful mission so far. (Roederer reports "terrific results" from the spacecraft's instruments as it looped around Venus and passed through Earth's comet-shaped magneto- sphere.) Even if the main antenna doesn't open soon, the Jet Propulsion Laboratory team expects that more good data will come in from Galileo flyby past the asteroid Gaspra this October, when the smaller antennas can each send 1200 bits of information home every second. Even if they are the only ones functioning by December 1995, all would not be lost. They would do, if just barely, to send back data gathered by the Jupiter probe itself, an instrument package to be sent on a 75-minute suicide mission into the planet's atmosphere.
Second, the engineers have a good chance of fixing the recalcitrant antenna. They have lots of time; it will be four years before Galileo will reach its destination. They have the equivalent of a bigger hammer---the temperature increase as the spacecraft goes from the frigid chill of the asteroid belt to the heat of sunstruck space near Earth. Roederer says the engineers refer to an "intense thermal exercising" of the mechanism as warmed metals expand, which might just loosen whichever jammed widget caused the difficulty.
They also have an identical antenna within reach, right inside the Spacecraft Assembly Facility in Pasadena. They're putting that earthbound antenna through its paces, cycle after cycle of opening and closing, hoping to catch it in the act of jamming to provide clues about what went wrong far out in space.
It's a great image---engineers doing their best to break an oversized umbrella so they can fix its faraway twin. I wonder how they answer when their children ask, "What did you do at work today?"
