Space history was made on February 12, 2001 when NASA’s NEAR spacecraft
became the first craft to land on an asteroid. What makes this landing even
more exceptional is that NEAR, managed by the Applied Physics Lab at Johns
Hopkins University, was not built to withstand a landing; its mission was to
orbit asteroid Eros and study the slow-moving rock from a distance. However,
with its main mission successfully completed, scientists thought they could
attempt an asteroid landing.

It fell to NEAR’s navigation team, based at JPL, to bring the craft in for a
semi-smooth landing. As the rendezvous drew closer, the JPL team had to
quickly crunch numbers to calculate the craft’s path as it plunged toward
Eros and relay commands back to NEAR to give it the best landing possible.
Bobby Williams, the head of the navigation team, talks about the pressure on
that eventful Monday morning.


Q: What was the asteroid landing like?

A: The days leading up to it were pretty chaotic. On Monday we got a really
early start at 2 am. Al Hewitt, the network operations person for the Deep
Space Network (the telecommunications system which talks to spacecraft),
called to say that the predictions for NEAR’s arrival at Eros we had sent
weren’t working. So we had an immediate panic attack

Pete Antreasian and Steve Chesley were waiting at their keyboards when the
numbers were made available, they then started processing. We were on a very
tight time schedule to get the number “how much earlier or later are we?” We
did that with a couple minutes to spare.

We found that it was 17 seconds late. We bumped the spacecraft’s clock back
to correct for the change and the spacecraft got to live 17 seconds over
again. We believe that made the difference – that if we hadn’t adjusted it,
it may have mapped a much bigger error on the ground. So that was the big
push-up for the morning.

We had to have pictures taken that were immediately downlinked which not
only required us to be on our toes, the spacecraft had to be officially set
up to do that. So it all worked — the pictures got down and a few moments
after they were taken, downlinked.

Q: How did the team feel?

A: The day before we touched down there was a lot of fatigue: we’d been
working pretty hard for the past month. Monday morning, all the fatigue
drained away. Everybody was pretty excited. It was the culmination of all
that hard work. It was like going in for your final exam, and you know
you’ll get an A and you feel really good when you come out.

Q: What makes a good team, especially in the face of doing an unprecedented
maneuver like the landing?

A: We didn’t over-train; we didn’t have a lot of blow-by blow simulation.
That makes everybody tired. My approach is always: lay it all out, simulate
little parts of it so that everybody knows what they have to do. They’re
smart people! We rely on their own innate abilities and their training, and
I think people respond to that.

Q: What will the first small body landing on an asteroid teach us about
future landings?

A: The fact that NEAR was able to land with no landing apparatus on the
spacecraft means that now they don’t have to over-design any kind of landing
apparatus on one that’s actually designed to land. We were extremely lucky
not to hit a rock or boulder and knock a solar array off. You wouldn’t want
that on a planned landing, where you have to take off again, or drop off a
rover. But now we know that we can survive an impact. In that sense we’ve
set the boundary; we know what the design constraints would be for a real
lander on an asteroid or comet.

Q: From the navigation point of view, what are the problems of landing on a
small body and how do you solve them?

A: For asteroids we know now that the key to landing is the models, like the
gravity fields and the solar pressure on the spacecraft. Because we had
those models fairly well-estimated, landing was a matter of planning and
using those models.

We found you can’t just arrive and land immediately, like we do at Mars. For
a small body that’s impractical, because you need to know the gravity, you
need to know the mass, and you can’t estimate those things until you get

Q: What is the NEAR navigation team’s future?

A: One important element is that we do navigation for many different
missions. Almost all of my group has only worked part-time on NEAR. We’re
used to working more than one mission. We have a couple people going to
other Discovery missions for the Applied Physics Lab, the CONTOUR mission,
which flies by at least two comets, and the Messenger mission, which goes to

You can email your questions to Bobby Williams at