This is Emily Lakdawalla, blogger for the Planetary Society, and I'm going to talk to you about Stardust's flyby of comet Tempel 1 two weeks ago. When I scheduled myself to do a podcast about this flyby, I was a little worried. It's so easy for things to go badly wrong in space exploration. It was possible that for some reason or another I'd have nothing to talk about today. But in the words of the project scientist, Stardust's flyby of Tempel 1 wasn't 100% successful. It was 1000% successful!
NASA / JPL / Emily Lakdawalla
Animation of Stardust's flyby of Wild 2
This animation is composed of 24 frames captured by Stardust as it flew by Wild 2 on January 2, 2004.
Two years later, Stardust flew past Earth and released the sample capsule. The capsule landed successfully on January 19, 2006, bringing with it the first ever particles collected from a comet. Since then, the tiny dust samples have been analyzed in laboratories around the world. We've found out that the early solar system was a dynamic and turbulent place, with particles that formed close to the Sun somehow mixed up inside a comet, which must have formed far from the Sun.
Meanwhile, another small mission, Deep Impact, launched on January 12, 2005 for a totally different kind of mission to a comet. The particles that Stardust collected came from the surface of a comet. Deep Impact's goal was to see inside a comet, by smashing a large copper projectile into it at several kilometers per second. Analyzing the dust thrown up from the impact would tell us what materials formed the inside of the comet, and analyzing the size and shape of the impact crater would tell us how those materials were put together.
Deep Impact encountered comet Tempel 1 on July 4, 2005, and was, if you'll pardon the pun, a smashing success. It was billed as hitting a bullet with another bullet while watching from a third bullet. In fact the impactor that Deep Impact used to smash Tempel 1 was a robotic spacecraft in itself, with a navigational camera and a computer that autonomously steered the impactor to the flattest possible crash site, which was between two distinctive circular features on Tempel 1's surface. Our images from the Deep Impact mission include these navigational camera views taken by the impactor craft as it dove to its doom, as well as more distant shots from the flyby craft.
There was one small problem with the Deep Impact mission. The dust from the impact didn't clear in time for the flyby craft to be able to see the impact site. So we learned about the composition of the comet, and we were able to learn something about how it was put together from the shape of the splash of impact ejecta. But we never saw that crater, which would have told us even more.
Both of those missions ended with perfectly capable spacecraft already out in solar orbit. Deep Impact was sent on to a successful flyby of a second comet, Hartley 2, last year. When mission planners realized it'd be possible to send Stardust on to Tempel 1, it was a no-brainer. Stardust would be the first spacecraft ever to visit a comet for a second time. In the six years separating the Deep Impact and Stardust encounters, comet Tempel 1 completed one orbit around the Sun, passing through perihelion, when solar heating made it most actively spit out jets. Scientists looked forward to seeing how the comet had changed after perihelion passage. And they also wanted a second chance to see that crater.
As Stardust approached the comet, the mission tried to manage expectations. Stardust was an old spacecraft. It had barely any fuel left, so little, in fact, that there was some concern about it lasting through the flyby. The mission was forced to keep the science plans simple to make sure that the spacecraft wouldn't run out and lose pointing control before it had a chance to send the data back to Earth. Another problem was that the mission couldn't be positive that they had timed the flyby right to have the right side of the comet in sunlight for Stardust to see the Deep Impact crater.
As they approached, Tempel 1 played coy. Its position was well known from Earth, but this wasn't accurate enough to steer the spacecraft to a 200-kilometer flyby. For that, Stardust would have to do its own navigation, picking up the comet in its camera. Stardust started looking for Tempel 1 in December, but didn't spot it for weeks. This was kind of a tense time, because the longer it took Stardust to spot the comet, the more potential there was that it would need to make a larger rocket burn in order to achieve the flyby, and fuel was short. Finally, on January 18, they spotted the comet, and were able to plan the rest of the burns.
NASA / JPL / Cornell / animation by Emily Lakdawalla
Stardust's flight past Tempel 1 (version 3)
This animation consists of 61 images of Tempel 1 taken by Stardust during its flyby on February 15, 2011. The images have been rotated 180 degrees (so that illumination appears to be coming from above) and aligned.
I stayed up late on Valentine's Day to see the first images come in. The spacecraft was supposed to fly past the comet, turn to Earth, and send five pictures taken near closest approach before relaying all the rest of the data. I was thrilled when the mission confirmed the success of the flyby, but what I really wanted to see was those pictures. It turned out though that the spacecraft didn't get the command to send the best pics first. Instead, the 72 pictures came down in order, one every fifteen minutes. The first images clearly showed the lumpy shape of the comet. I decided to go to sleep and see the new pictures the next morning.
It took only an instant's look at those high-res pictures to confirm that the flyby had been a success. The high-res images clearly revealed the two circular features that'd been so prominent in Deep Impact's images of the comet. The old spacecraft had done it -- it managed to track the comet and return the photos that Deep Impact couldn't, showing us the impact site.
Which is not to say that the impact crater was obvious. I was hoping for a splashy feature around an obvious circular hole, but there was no visible splash. It was more subdued than the science team expected it to be. It took some really detailed comparisons of the old and the new pictures from Deep Impact and Stardust to confirm that there really were changes to the surface. After some judicious image processing you can make out an irregularly shaped rim about 150 meters across, and a mound in the middle. What that crater tells us, co-investigator Peter Schultz said, is that the surface of the comet where we hit it is weak and fragile. It's fluffy.
There were other changes to the comet. A prominent feature in the Deep Impact photos was a smooth tongue of material that looked like something flowed across the surface. Scientists wanted to look at that thing again and see if it flowed farther, or retreated, or didn't change in six years. Lo and behold, it had changed, and it retreated. They don't know what made it in the first place, but whatever it is, it's receding.
Stardust didn't only see old territory; it saw parts of Tempel 1 that Deep Impact didn't see. The back-side of the comet turned out to be as interesting, maybe even more interesting-looking than the front side. And the particular path of Stardust past Tempel 1 produced 3D information, from which the science team is now constructing a detailed shape model for the comet. Co-investigator Jessica Sunshine said that this 3D information has already produced surprises, making it clear that some things they thought were hills turned out to be depressions.
Stardust didn't just send back camera data; it also has dust detectors, and data from one of them was turned into this cool sound clip. Each burst of sound represents a spatter of dust slamming into the spacecraft's shields. Scientists are decoding that data to learn about how many impacts there were, and how big the particles were.
The Stardust spacecraft has performed loyally much beyond its primary mission, but its fuel is now almost gone, which means its functional life is almost over. It's still taking photos of Tempel 1 as it travels farther and farther away, but once that work is done, it's going to be decommissioned, its transmitter turned off forever.
The science team is busy analyzing the data that Stardust returned; I'll be looking out for scientific publications about the comet, and will post results at planetary dot org slash blog. Thanks for listening, and tune in to my next podcast on March 18, when I'll preview MESSENGER's orbital mission at Mercury.