Catching up to a 43-foot space telescope that is zipping around the planet at 17,000 miles an hour takes more than just putting the shuttle's pedal to the metal, NASA officials say.
By the time the Endeavour's crew finally attempts to capture the Hubble Space Telescope -- sometime before dawn Eastern time tomorrow -- the astronauts will have solved a complex problem in orbital mechanics and completed a critical act in the 11-day repair drama.
If the crew misses the rendezvous on the first try, the shuttle will have little fuel for a second attempt.
And even if a second try were successful, the extra fuel consumption would likely shorten the mission, curtail the repairs and cancel plans to boost Hubble to a higher and longer-lasting orbit.
Here's how the orbital race is run:
When Endeavour was launched at 4:27 a.m. yesterday, Hubble had just passed overhead at a maximum altitude of 368 miles.
By the time the shuttle reached orbit minutes later, it was below the space telescope and trailing it by almost 6,000 miles.
To catch up, Endeavour had to go faster. But the laws of physics require actions that seem to contradict earthbound intuition.
Because of the way speed and altitude are linked in orbital physics, accelerating in the direction of flight makes the shuttle go higher, and slower, said NASA spokeswoman Barbara Schwartz.
To catch Hubble, Endeavour began in a lower, and therefore faster, orbit about 357 miles above the Earth at the high point,and 342 miles at the low point.
In that elliptical orbit, the crew was closing the gap between Endeavour and Hubble by 375 miles during each 90-minute trip around the world.
The crew's task is to gradually raise the shuttle's orbit and slow down until Endeavour is flying in formation with Hubble, at an identical altitude and speed.
That's accomplished with a series of 10 carefully calculated firings of the shuttle's Orbital Maneuvering System (OMS) thrusters.
Using data from star trackers and inertial measuring devices in both spacecraft, NASA computers precisely calculate the position, orbit and speed of both spacecraft, and issue instructions to Endeavour's crew on the precise timing and duration of the thruster firings.
"They burn [to accelerate] in the direction of the orbit, and that raises the altitude," said Ms. Schwartz.
After its first adjustment yesterday morning, she said, Endeavour was still closing the gap, but more gradually -- gaining just 242 miles per orbit. It was also five miles higher than it had been, and only 4,478 miles behind Hubble.
By early tomorrow, if all goes well, Endeavour will have matched Hubble's orbit, which ranges from 361 miles to 368 miles high.
When Endeavour closes to within eight miles of the telescope, shuttle pilot Ken Bowersox, 37, will take the shuttle's joystick and fly the spaceship manually, using the smaller Reaction Control System (RCS) thrusters.
At first, he will approach with the shuttle's nose pointed toward the telescope and its payload bay toward space. But as the distance closes, Ms. Schwartz said, he will dip the shuttle's nose toward Earth and approach Hubble from below, payload-bay-first.
The seemingly awkward approach is designed to keep hydrazine fuel exhaust from the shuttle's thrusters pointed as much as possible away from the telescope. That's to answer astronomers' fears that the fuel might contaminate Hubble's mirrors and other sensitive parts.
When the telescope is within reach, Swiss mission specialist Claude Nicollier will reach out with Endeavour's mechanical arm and grab it, then lower it onto a receptacle in the payload bay, where it will remain throughout five days of repairs.