A native of Houston, Theilig studied geology at the University of Texas, Austin, partly because of the appeal of outdoor fieldwork. Two of her professors were involved in NASA projects studying the Moon and Mars, so before graduating in 1976, she was already participating in research about channels on Mars. That summer, through a NASA-sponsored internship, she was at JPL assisting the Viking imaging team when the Viking 1 lander set down on Mars. She directed herself toward a career in solar system exploration by earning a Ph.D. in geology with a concentration in planetary studies from Arizona State University, Tempe, in 1986. She returned to JPL in 1987 as a National Research Council associate, investigating lava flows on Earth to aid interpretation of the NASA Magellan spacecraft's radar imaging of lava flows on Venus. Seeking more active participation in mission operations, Theilig joined the Galileo project seven months before the spacecraft was launched in 1989.
Q: What is the Galileo mission?
A: The mission was designed to study the Jupiter system. It has consisted of both a probe that studied the upper atmosphere and an orbiting spacecraft that has 12 instruments on it to study Jupiter, its moons, its rings and its magnetosphere [a vast bubble of charged particles that surrounds the planet].
Q: What results from Galileo have been most exciting to you?
A: For me, it has been the discovery of the potential for liquid water on Europa and the discovery of a type of lava flows on Io that haven't occurred on Earth for a billion years or so.
Q: What is still ahead for Galileo?
A: We still have three Io encounters ahead of us. The first two were designed for polar passes, and one of the reasons for that is to get magnetic-field information about Io. After that, we go by Io a third time, and that encounter will also focus on fields and particles. Then we go into a long orbit around Jupiter and pass by Amalthea, one of the smaller inner moons. Then we go within one Jupiter radius of the cloud tops of Jupiter, then finally on a controlled impact into Jupiter in September 2003.
Q: What do you like best about your job?
A: What I like best is working with the dedicated Galileo team members who work together to get the best science we can. Their creativity to overcome problems has been a hallmark of Galileo throughout the mission.
Q: How did you first become interested in space exploration?
A: Growing up, I watched all the early manned exploration. Then, when I got into college in the 1970s and we started seeing new and different worlds, that was the time of early planetary exploration, and that's what really hooked me.
Q: What advice would you give to young people interested in careers linked to space exploration?
A: Get the best education you can. Get a good basic knowledge with a concentration in science and math that will serve you well. Don't box yourself in. Stay flexible with your options, even if it means you don't stay with the field you started in. Take opportunities to get involved in projects and clubs that go beyond what you learn in classroom activities.
Q: What are one or two of your main interests apart from your Galileo work?
A: Travel is one. I enjoy seeing new places. And another is my involvement in a faith community.
Q: What are the biggest remaining mysteries about Jupiter?
A: Europa -- the water layer there and the potential for life.