By Alan Duffy Thursday 13 November 2014
Photo: Comets represent a perfectly preserved frozen fossil from the messy business of planet formation. (Supplied: ESA/Rosetta/Navcam)
After a 6 billion kilometre journey, the Rosetta spacecraft has finally landed on a comet to send its discoveries back to Earth and send the mark of humanity through space, writes Alan Duffy.
The European Space Agency (ESA) has made history today with the first successful landing of a probe onto the surface of a comet.
It has taken incredible ambition and effort to achieve such a hazardous and unique landing, yet the scientific goals far outweighed the risks. We can now drill into a comet and directly sample material dating back billions of years, from a time when the planets of our Solar System first formed.
Comets represent a perfectly preserved frozen fossil from the messy business of planet formation. We will also discover if comets are responsible for bringing that all important ingredient of life itself to Earth - water.
After a 6 billion kilometre journey lasting over a decade, the Rosetta spacecraft chased down the 4km-long comet Churyumov-Gerasimenko/67P, coming to within 100km of the target.
This is equivalent to hitting the bullseye of a dartboard in Perth from Sydney. With a billion euro ($1.4bn) dart. While blindfolded (as the mission was powered down for almost the entire journey).
This was the precision with which ESA's Space Operations Centre in Darmstadt had calculated the gravity from all the objects in the Solar System to ensure that everything went to plan.
Yet it was still with relief that the automatic timer awoke Rosetta from hibernation on January 20, 2014, and sent a global wake-up tweet: "Hello, world!"
At such close range, Rosetta was able to take the most detailed pictures of Comet 67P ever, revealing the first of many surprises. The comet appeared to be made of two objects stuck together in what's called a contact-binary resembling a cosmic rubber duck.
The comet appeared to have dunes, rocky slopes, sharp jagged edges and enormous boulders strewn across the surface. It will take astronomers years to fully explain features that would be more familiar to a planet than a comet with low gravity and no weather to shape its features. This picture was a far cry from the benign 'dirty-snowball' picture of a comet and compounded the already difficult task of landing onto the surface.
Over the next few months, Rosetta orbited ever closer to the comet, taking ever more detailed images looking for a safe place to land the Philae probe. On September 15, 2014, landing site 'J' was chosen, roughly corresponding to the crown of the duck's head.
Even here the ground is rocky with potentially lander-destroying shards of ice or probe-tipping boulders within the expected landing site area. The entire mission hinged on perfect timing of when to detach the lander from Rosetta as it flew around the comet, as without thrusters the entire seven hours walking-speed fall couldn't be adjusted. Instead, after separation was confirmed at 20.03pm AEDT on November 12, 2014, ESA joined a worldwide online audience in watching a live stream of the fall, unable to do anything more.
In what will now be remembered as a historic first in space exploration the 100kg, the fridge-sized Philae lander confirmed at 3.05am AEDT on November 13, 2014, that it had landed safely - 500 million km from Earth.
It had planned to fire harpoons to attach itself to the comet to prevent it bouncing, but unfortunately the ground was so soft that it didn't trigger (much like an airbag won't if the car is only bumped gently) and the probe appears to have bounced at least once, meaning there were two landings this morning!
More worryingly, it's not clear if the screws on each leg have drilled into the surface to fix Philae in place. One of the major mission goals is to drill into the surface, and as any DIY-enthusiast knows, you have to apply pressure to your drill or else it skits across the wall. The same thing may happen to Philae if it's not securely fastened. In the next day a decision will be made as to whether to manually fire the harpoon, but regardless, the lander has made history by safely landing onto a comet.
Philae has batteries that will last for 2.5 days and after that will rely on solar panels to provide power for the next few months. As the comet approaches the Sun, its surface will heat up and sublime into gas, forming the famous comets tail, and potentially covering the panels in the process.
As a result of the limited time available, ESA has planned to undertake key science goals over the coming days. One of the main tasks will be to drill into the comet to take pristine samples of material dating back to a time when the planets themselves were first forming. Most excitingly, these samples can test if the ratio of water isotopes (heavier versions of the chemical) on the comet is similar to the oceans of Earth, revealing comets as the origin of at least some of the water on our planet, raining down as frozen ice to fill the seas over billions of years.
The future for Philae is not going to be a very long one - as the comet heats up, the craft may be destroyed by the gas rushing off the surface, starved of solar power by dirt covering its panels, or simply heated too much by the Sun to function any longer.
Yet Rosetta will continue to orbit this frozen world, and will follow its trajectory back into deep space towards Jupiter, slowly running out of power as the intensity of the Sun's light diminishes. The comet will have a silent companion for thousands if not millions of years into the future.
Although Rosetta has earned its place in space exploration history, it has one last mission to perform. Attached to the craft is a 7.5cm nickel disk with 1,000 different languages micro-etched onto the surface, meaning they can be read with nothing more advanced than a microscope, preserving this cultural archive for the future.
Dr Alan Duffy is a Research Fellow at Swinburne University of Technology in the Centre for Astrophysics and Supercomputing. View his full profile here.
