Deep Impact: a retrospective
Back in January 2005, NASA launched the Deep Impact spacecraft on a trajectory to study comets. Four months later, on July 4, as it approached comet Tempel 1, it fired a 370kg impactor from a distance of 864,000 km – more than that double the distance from the Earth to the Moon. 24 hours later, the impactor collided with the comet at a speed of over 10 kilometers per second, carving out a crater with the force of 4.8 tons of TNT. As cometary material blasted into vapour, the instruments on board Deep Impact recorded and analysed the resulting flash of light to determine exactly what comet Tempel 1 is made from. This done, Deep Impact’s mission was complete. But since the spacecraft was still in good working order and had plenty remaining fuel, NASA decided to extend the mission and see what else they could learn about the Solar System.
The mission was renamed EPOXI, from the two separate tasks assigned to the spacecraft: the extrasolar planet observations, called Extrasolar Planet Observations and Characterization (EPOCh), and the flyby of comet Hartley 2, called the Deep Impact Extended Investigation (DIXI). In the course of completing this new extended mission, Deep Impact travelled further than any other deep space comet explorer, and was responsible for a number of scientific breakthroughs before finally failing nine years after launch. The last successful communication with the craft was on August 8 2013, and a month later the science team reluctantly agreed that, without the ability to instruct it to position its solar panels correctly, it was unlikely ever to power up again. Deep Impact was dead, and the mission was over.
“Deep Impact has been a fantastic, long-lasting spacecraft that has produced far more data than we had planned,” said Mike A’Hearn, the Deep Impact principal investigator at the University of Maryland in College Park. “It has revolutionized our understanding of comets and their activity.”
“Six months after launch, this spacecraft had already completed its planned mission to study comet Tempel 1,” said Tim Larson, project manager of Deep Impact at JPL. “But the science team kept finding interesting things to do, and through the ingenuity of our mission team and navigators and support of NASA’s Discovery Program, this spacecraft kept it up for more than eight years, producing amazing results all along the way.”
The extended EPOXI mission was started by turning Deep Impact around and sending back to Earth from Tempel 1, so as to use the planet’s gravity to slingshot it out towards comet Hartley 2. It passed by Earth in December 2007, and arrived at Hartley 2 in 2010 where it collected new imagery and data for transmission back to Earth.
- The flash of light from the initial impact experiment was a great deal fainter than expected. This revealed that comets (or at least, this particular comet) had a soft, snowy surface. This disproved the theory of the time, which stated that comets must have hard dense crusts from the repeated exposure to the Sun’s heat, and which would help reduce the amount of outgassing, and therefore the size of the cometary coma.
- Observations of Tempel 1 and Hartley 2 showed that the gas emitted from the smooth equatorial region of a comet (visible in the image) is composed of water vapour, while the gas from the polar regions is carbon dioxide. This may be because of seasonal effects (the equator gets hot enough to melt water ice but the poles do not), or there could have been a factor in the formation of comets billions of years ago that caused these different compounds to concentrate in different parts of the comet.
- A later mission to re-examine the surface of Tempel 1 and compare how the impactor’s crater had changed over time revealed that parts of the crater had changed dramatically while others had not changed much at all. This revealed that the erosion of surface features on comets is not a gradual uniform process, but is rather more cataclysmic: A series of events like landslides occur in individual areas.
- It has been known for over a century that comets contain various ices, including Carbon Monoxide and Carbon Dioxide, which can be identified by studying the gas that comes off as the ice warms. However, EPOXI found that the proportions of these ices in a comet depend on whether the comet is a short-term comet orbiting near the Sun, or a long-term comet which drifts very far out. The difference in composition suggests that short term comets aren’t just close to the Sun at the moment, but that they formed there as well, which flies in the face of long-held belief that short-term comets formed way out past Neptune in the Kuiper belt, and that long term comets originate from even further in the distant, remote Oort cloud.
- The emissions from Hartley 2 contained a great deal of large water-ice crystals. Presumably these melt and sublimate as they fly out into the cometary coma, which explains why some comets have been measured as holding more water in their tail and coma than could possibly have melted directly off the core.
- While travelling between the two comets, Deep Impact collected imagery of the Moon and Mars, helping to confirm the presence of water on the Moon, and Methane in the Martian atmosphere.
- It was also used to carefully study and confirm earlier discoveries of exoplanets around six seperate stars
All in all, Deep Impact was an incredibly successful mission, which is happily something we’ve come to expect from these robotic explorers of the Solar System. Rovers on Mars, Infrared telescopes, and the Deep Impact cometary explorer are all examples of machines designed to last long enough to complete one task, but that then continue working on a steady stream of cleverly-designed missions for years past their designed lifespan.