Steve Chesley, Co-Investigator, JPL
What is your role on the SD-NExT team? What is your team’s role in the SD-NExT mission?
I’m on Stardust-NExT’s science team. The engineering team is in charge of getting the spacecraft to the comet and the science team’s job is to investigate the comet itself. I actually have two roles on the science team. Originally, I joined the team because I’m an expert in asteroid and comet orbit determination. I also study the comet’s rotation.
How is your job important to the mission’s success?
The navigators need to know where the comet is in its orbit in order to get the spacecraft to the right area. Though they can use the spacecraft’s camera to steer it to the right position, the camera cannot tell them exactly how far away the spacecraft is from the comet. They use my orbit information to let them know what time the flyby will occur and when to start taking the imagery.
The science team also needs to know where Tempel 1 is in its rotation at rendezvous – when the spacecraft flies by the comet. That’s complicated because comet Tempel 1 has a big rotation problem. Both the comet’s rotation and orbit are affected by thrusting caused by gasses leaving the surface of the comet. Outgassing, we call it. It occurs when ices on the comet’s surface sublime – go straight from a solid to a gas and shoot off the surface. Outgassing impacts the comet’s rotation rate and direction. We have to make predictions on how the orbit and spin state change over time in reaction to those factors.
Since this is our second visit to the comet, the science team’s hope is to be able to see a lot of the terrain that was previously imaged by the Deep Impact mission – to observe how it may have changed. Then, though it’s not a primary mission goal, we hope to see the crater. If we are able to see it, it’s because we were able to figure out what the orientation of the comet would be at the time of the flyby and control the spacecraft arrival so that the crater created by Deep Impact is illuminated by the Sun and therefore visible. Not so easy when both the spacecraft and the comet are speeding along at tens of kilometers per second!
What are some of the challenges that accompany your job with the SD-NExT mission?
Well, this outgassing I mentioned is, at a very fundamental level, random. Regions on the comet will become active, then later, inactive. We believe the comet’s landscape is changing and that the sources of the jets are moving around and becoming more and less active from revolution to revolution around the Sun. So it’s very difficult to predict in advance what they will be doing in February of 2011.
So, the rotation rate of the comet is changing in a way that is hard to predict. From past observation we believe that around perihelion (when the comet is closest to the Sun), it goes through some kind of a deceleration, then a big acceleration, and finally another deceleration. From about June of 2010 until the encounter, however, the comet will be in the daylight sky and we don’t have a way to observe 2011’s changes in acceleration.
So we have to make predictions based on things that are random at some level, and that adds significant challenge. We’ve watched the comet for decades now, and we’ve seen the changes from revolution to revolution. They’re modest, so we’re not expecting some dramatic change. But comets do dramatic things every now and then, so there’s no ruling that out. We’re extrapolating from what happened in the past, and we’ll find out in February whether or not the comet did the same thing or something completely different.