Cold Peril: The Ulysses Spacecraft

"When Ulysses was launched in 1990 the RTG produced 285 watts. Now it's down to 207 watts - barely enough power to run the science instruments and the heaters at the same time," notes Angold.

Inside Ulysses the temperature varies from place to place. "Many of the science instruments are already below freezing (0 C)," says Ulysses thermal engineer Fernando Castro. "That's OK, because they can operate at low temperature." But the fuel lines are another matter. They're hovering about 3 degrees above zero, "and if they freeze we're in trouble."

Fuel lines are critical to the mission. They deliver hydrazine propellant to the ship's eight thrusters. Every week or so, ground controllers fire the thrusters to keep Ulysses' radio antenna pointing toward Earth. The thrusters won't work if the hydrazine freezes. No thrusters means no communication. The mission would be lost.

About eight meters of fuel line snake through the spaceship. Every twist and turn is a possible cold spot, a place where the hydrazine can begin to solidify. "If the hydrazine freezes anywhere, I don't know if we can safely thaw it again," worries Castro. When hydrazine thaws, it expands, possibly enough to rupture the fuel lines. Ulysses' propellant would fizzle uselessly into space.

The temperature at any given point along the fuel lines is bewilderingly sensitive to what's going on elsewhere in the spacecraft. Turning on a scientific instrument "here" might cause a chill "over there," because it takes power away from one of the heaters. Firing a thruster, playing back or recording data: almost anything could upset Ulysses' delicate thermal balance.

The complicated interior of Ulysses. Dark blocks are science instruments and other devices. Fuel lines, denoted by red, blue and green, lead from a central hydrazine tank to the thrusters.

Even the simple act of sending the spacecraft a message can cause problems. Systems engineer Andy McGarry recalls, "last month we were sending some new commands to Ulysses when the temperature began to drop, as much as 0.8 degrees C near the fuel lines. We were less than a degree from the freezing point of hydrazine - too close for comfort."