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Tuesday, 9 November 2010

Hartley 2 roundup: A dry ice comet?

All links are numbered, and listed at the end.

Last week, as most space nuts can tell you, [1]the cunningly re purposed Deep Impact spacecraft and its veteran control team managed to blast past the core of comet [2]Hartley 2. The probe took images by the thousand, and [3]infrared spectral data by the bucket, which will be keeping researchers busy for decades.

Right: The Deep Impact spacecraft, undergoing preparation at Ball Aerospace. Image courtesy of Ball Aerospace.

The probe, and the team, have performed amazingly well. The team has made this look easy- but its not! The Deep Impact craft shot by the comet at around 27,000 miles an hour and the pointing and programming of [4]the imaging sensors had to be bang on. The probe has, over the whole course of the flyby delivered tens of thousands of images, 23,000 from the medium resolution imager alone, and all told they plan to bring down 120,000 images.

The probe was launched in 2005 to explore comet Tempel 1 in a unique way: by bombing it with a huge copper projectile, blasting parts of its subsurface into space where the flyby probe ( the Deep Impact craft proper ) could scan it.

Image above: Deep Impact watches as a copper slug slams into the crust of comet Tempel 1. Image courtesy of Cal tech.

[5]The initial mission was a resounding success, and it transpired that the robot explorer had enough fuel in its tanks for another mission. Originally the craft was to fly to [6]comet Boethin, but this comet has basically vanished, so Deep impact was re targeted to Hartley 2.
The extended mission has been dual purpose; prior to the approach to Hartley 2 the spacecraft was using its telescopes to [7]search nearby stars for exoplanets

Why comets? Why Hartley 2?

Image above: Comet Hartley 2, as seen from Earth in this long exposure. Image courtesy of JPL. 

Comets are complex and constantly changing places, despite their small size. Among the most often quoted reasons for their study are: To study the carbon ([8]possibly pre-biological) chemistry that goes on in them and on their surfaces, to learn about their origins and how they fit into the history of our solar system, to decipher their structure and composition, to find out [9]what they contributed to the chemistry of early earth, and to (hopefully) understand how one on a collision course with us could be deflected or destroyed.

Image above: In 1908 [10]a comet core came down over Tunguska, Siberia. Only a small one, around 50 meters across, and it detonated before it hit the ground; but the blast did this to a thousand acres of forest. Such events are thankfully very rare. Image courtesy of Time Magazine.

Comets are the ancient solar system in capsule form, and part of the history of Earth, and perhaps the chemistry that gave rise to life in the bargain. And they keep throwing us [11]curve balls, which makes scientist all the more eager.

Which comet was question of what Deep Impact could reach with its remaining fuel reserves. Originally comet Boethin was to be targeted, but it disappeared, perhaps because it disintegrated naturally - comets are unstable things made of material that can be [12]less strong than fresh snowfall.

Hartley 2 was picked specifically because a small, cheekily active, comet like this had never been seen up close before. The science team are hoping that the comparison will help them determine which parts of the comets are due to ongoing processes, and which are relics from ancient times that can inform us on how the solar system formed, and what role comets played in that.

The price is also something worth boasting about: By re-using an existing spacecraft NASA has been able to pull off a [13]discovery class space mission for a tenth of the normal cost (around $40 million US compared to $350 million US for a full mission)- here's to many more examples of lowering the cost of visiting new worlds by using our heads!

So what has [14]Deep Impact/Epoxi found?

To kick off a review of the very very preliminary findings, we go straight to the horses mouth:

Video above : The press briefings on the preliminary results of the EPOXI mission. Courtesy of NASA/JPL/UMD.

The first thing that strikes me is the feel of this press conference: They've done something amazing and they know it! The elation in the clip shown of the control room  just as the first image came down is palpable, and flows out of the recording into the press conference. The human experience is important- that elation at those 'exploration moments' and seeing something never seen before- it is the reason why so many people find space exploration so compelling! The comment by Ed Weiler: "This is not virtual, this is Earth seeing the nucleus of Hartley 2 for the first time" sums it up well.

The science return will take a long time to work through, but even on approach the surprises began to mount up. Even from a distance the the could tell that the nucleus wasn't a nice spherical shape, by watching the jets coming from the comet. Data from scans run by the mighty [15]Arecibo radio dish seemed to [16]confirm this.

But a much bigger surprise followed: The emission of dust grains seems to be tied to the emission of dry ice- frozen CO2, the gas humans exhale when we breath. This suggests that the most active region of the comet is not powered by sublimating water, but [17]by primordial CO2, something that is literally unheard of for a comet. [18]On most comets the main gas being released is water vapour - and [19]water is present in this comet. But the water does not seem to go up and down with the dust emission anything like as much as the CO2 does.

As Deep impact tore towards the chunk of ices and organic chemistry the [20]crafts infrared spectrometer measured a huge spike in the abundance of the gas cyanogen, but there was no accompanying release of dust particles, making it a mystery how this gas was released (jets would have released dust particles to). Exactly what this means isn't yet clear, but if the origin of the cyanogen can be pinned to a specific area, or specific jets,  then the combined data could give a clearer pictures of how the comet is spinning.

The close up images begin to unravel the story:

Video above: The first five Deep Impact images of the close encounter released have been nicely made into this 5 X real speed video, courtesy of Machi on [21]

Video above: NASA animation of the flyby. Courtesy of NASA/JPL/UMD

The animations show some firsts for space exploration right away: The mission has imaged the jets and the surface features that they are  related to, all at once. Looking at the dumbbell structure of the comet, with a smooth neck connecting two rough surface lobes, it is clear that the jets are coming from the rough, active areas- in fact they appear to be coming from specific features. The team also spots weird 'clumps' of unknown material, which they believe may be the remnants of past activity.

The clumps were a surprise: The smooth area is thought to be fine grains that have collected in the middle, and resembles smooth areas on asteroids, such as [22]Itokawa which was visited by the epic [23]Hayabusa mission. However Hartley 2 is a very different beast than Itokawa, so while the boulders on Itokawa are likely to be rock, the 'clumps' on Hartley 2 are a complete mystery as a frozen comet is mainly ices! Clumps of ice should be seared down to almost nothing by solar radiation sublimating them, leaving a relatively smooth surface.

Image left: An enhanced version of  one of the close flyby images of Hartley 2. The brilliant white columns of jets are clearly visible in daylight, on the terminator, and over the night side of the nucleus. Image courtesy of NASA/JPL/UMD.

The images also show that the jets are running on the day side, on the terminator, and on the night side- as Dr Sunshine says this means the science team has a lot of work to do, as the comet jets are presumed to be driven by heat from the sun! There have been indications of this before, but this is the first clear evidence. Exactly how long the jets can last isn't known, it depends on how the jets end - do they clog up, or to they run dry? But the comet looses around a meter of material across its whole surface  every time it swings through the warm inner solar system, so it may have  relatively short lifespan left. The shape of the comet is itself a clue, but so far the team is unsure what direction it is pointing them in.

On the subject of the comets spin- at the moment, given what we know about comets its unlikely that the spin  will provide enough [24]centripetal force to snap the comet at its neck and leave it smeared across the sky as rubble. If the jets were pushing the comet to spin much faster then it might- but at the moment it seems that if anything they are slowing the spin. Right now they can only say its probably spinning about its shortest axis, and unlikely to break up due to centripetal force anytime soon.

The close flyby data from the infra red spectrometer hasn't been  looked at yet, but should give us a detailed idea of what gasses this odd comet is releasing, and clues to its internal makeup.

The future of Deep Impact?
The probe doesn't have enough fuel do fly to another comet, but it is in good health, and has enough fuel and power to run its instruments. The team are waiting on instructions from NASA and input on how  the probe could still be used for a future mission, for example [25]asteroid tracking. Currently they are looking to finish operations at the end of this year.

Hartley 2 itself had one more spectacle up its sleeve: As the Earth has passed through its wake the planet may have been been bombarded by [26]a storm of small debris from the comet. This would show up in our skies as a meteorite shower informally nicknamed the '[28]Hartleyids'

There will undoubtedly be much more follow up to these preliminary results over the months and years to come, but for now I'd just like to say; My heartfelt congratulations to the Deep Impact team!

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