This has been a mile-a-minute type of day. I'll tell you, the excitement and drama of what's going on is up to a high pitch. Everyone's fingers are clammy!

I slept pretty well last night, though I did wake up at 4 a.m. to do some work. I sat in my bathrobe at my dining room table going through all these numbers, looking at the times and getting the idea that yes, this IS going to work!

When we run through the software in our testbed we actually can watch all the events that happen. We have support equipment that listens in to what the computer and the hardware are doing, including opening the airbags and rockets (even though there are no real rockets opening). We can make sure that all the events that are supposed to happen, happen and that they are happening at the right instant. We're actually trying to make sure that everything happens to within a fraction of a second of accuracy. When we do one of these tests, it produces a whole ream of paper that unfortunately we have to go through by hand to really understand what's going on. However, every time we run these tests the software causes the rockets to fire within a few thousandths of a second of when they're supposed to fire. This is better than we need, by a lot! It's so much fun to confirm that over and over again!

I spent the rest of today answering a lot of fast questions from the media. Our science team did a press conference and all of us were eagerly listening to see how they would handle the bombardment of questions. We're interested to know what kind of questions people have, so we can tune our answers to match the questions. One of the problems we have is that we're so familiar with this mission, we know all the details, that it's hard to really know how much other people know about it. So we basically try to listen to other people's questions and who's doing the asking and then try to judge how to explain all of this. There are a lot of complicated series of events. It's especially hard talking with television reporters because we have to explain things very succinctly and quickly, most importantly very quickly, without using any acronyms and trying to use words that people can see very visually. It takes practice and we're practicing.

Tonight, in my free time (which I have very little of!) I'm working on a script that I'm going to read during the landing. I'm trying to synchronize my talking with what I think should be going on at each given moment. I know there's going to be a lot of people listening, all over the world, and I have to make sure I say the right thing and that I don't misplace my tongue. Most importantly, I have to keep my concentration up because I have to get up at 1 a.m. to spend the early morning hours as mission manager for that phase of the descent and landing.

Just a few moments ago, we may have sent (we don't know yet for sure) the very last command to the spacecraft until after it lands. We do the best job we can aiming for Mars, but Mars moves! Mars' position is being influenced by Jupiter and the asteroids. Just today we started to see a tug of Mars. Right now Mars looks like the Moon from Earth; that's how big it is if you were on the spacecraft. We are less than a million miles from Mars. And at that distance Mars is starting to pull a little bit and the spacecraft is starting to very slowly accelerate toward Mars, as if it's falling into Mars. Hopefully it falls just the right way so it skims the atmosphere to slow down.

It looks like we are a little bit off target, but not too much. We're not going to do to much to change the direction of travel but we want to tell the software onboard exactly where it's going so the spacecraft will open the parachute at just the right height. Even though the software onboard Pathfinder is running and the clocks are ticking, anytime between now and 8:30 a.m., Pacific, Friday, we can send commands to the spacecraft to update these key parameters that control how the parachute gets opened. We have the ability to very quickly generate these commands based on the latest knowledge of where we're going. and that's based on information gathered by our navigators.

The Navigation team works together to take our best estimate of where we think we're going and figure out exactly how to take that information and propagate it to the ground. They can actually simulate the spacecraft flying through a computer-generated atmosphere and figure out how the lift and drag work. Pathfinder looks like a funny-shaped glider and we can actually go through the process of simulating that whole lift, drag and acceleration due to gravity (the tug of Mars) all the way down until that point where the parachute opens up and beyond. We do this over and over again with the new information we get.

The parachute will open up anywhere from about 5 km above the ground to as high as 11 km or more. In fact it looks like, from where we're coming right now, we're going to open the parachute a little on the high side, which means it'll take a longer time to soar to the ground. The spacecraft is soaring to the ground at a very, very high speed and once the parachute opens it has got a lot to do: get rid of the heat shields, repel down a 20-meter bridle, turn on the radar and start looking for the ground and begin the processing. On top of all of this, Pathfinder has to have enough time to figure out how high it is above the ground and how fast it's going in order to open the airbags. The airbags open about 1000 feet above the ground and once they open, the lander can no longer see the ground through the airbags. That's the reason that we do these updates-to get a better sense of where we're going and to get the parameters just right.

The command we sent about 15 minutes ago is still flying through space at this minute. It only takes about 1000 bits of information, which we send from the Deep Space Network in Goldstone, CA. These 1000 bits are shipped up one bit at a time, back-to-back. But even though it takes awhile to get all those bits out the antenna, by the time the last bit is out, the first bit is still flying through space. All those thousand bits are like little soldiers marching in series until they get to Mars, at which time they'll be received one at a time by the spacecraft.

My day isn't over yet. Now I must prepare for tomorrow's press conference...