Friday, 21 April 2017

The ESA Rosetta Mission to a Comet – Where Did the Lander Philae End Up?

The ESA Rosetta Mission to a Comet – Where Did the Lander Philae End Up?

This is basically a follow-up to two earlier blogs, regarding the European Space Agency’s Rosetta mission to Comet 67-P.  They were based on a talk given by mission scientist Matt Taylor, at the University of Alberta, in Oct 2016.

The addendum concerns the plucky little lander, Philae, and is based on a paper in the journal Acta Astronautica entitled:

It’s a pre-print, so the title might change, and there might be some copy-editing, etc. that could affect the final product.  But, the main facts shouldn’t change.

Philae was the lander part of the Rosetta mission to comet 67/P, which touched down on November 12, 2014, after a 10 year mission to reach the comet.  The plan was for it to land, and anchor itself, with the help of some devices made for that purpose.  Then it would take scientific readings of various sorts.

The location of the landing site was determined while the spacecraft was in flight – it required close in examination of the comet to do that, since little was known of its detailed morphology from Earth-based observations. The landing sequence was then worked out, which involved a slow seven hour descent to the surface.

It did in fact, land, but then “bounced” (unplanned) into an unfortunate location.  The bounce lasted about 2 hours, and the lander was then a good kilometer from the target, though nobody knew exactly where it was, when it stopped bouncing.

It managed to do some scientific work, using on-board batteries, with all ten of the payload instruments.  But the ultimate location of the lander was poorly illuminated, so the solar panels couldn’t properly recharge, and Philae went into hibernation on November 15.  During that time, the temperatures were probably as low as -100 degrees Celsius, far below its operating range.

Then, signals from the lander were received in June and July 2015, on eight different occasions.  Basically, it re-awakened as the comet drew closer to the sun, and some of the instruments came back on with the rising temperatures.  But, though it awakened, it proved impossible to establish reliable ongoing communications.

That may have been due to damage to the electronics of the transmitter/receiver system, due to the cold.  Or, perhaps the distance to the orbiter was just too great to be able to maintain communications for long, at these times.  Perhaps the lander was just tilted in some weird way, obscuring the antennae or dust had been kicked up, covering up the solar panels to such a degree that they couldn’t generate significant power.   

At any rate, the signals from that first scientific sequence (FSS) and the later flashes of communication were extensively studied by the team back on Earth, to determine just what happened and where the lander likely ended up. Since the Rosetta spacecraft was still orbiting the comet, its camera could ultimately identify the exact location of the lander.
During the first few days after the FSS, the likely landing zone was triangulated to a location on the top edge of the duck’s head, so to speak.  Such matters as power, radio frequency and visibility constraints aided in the search.

Some further work, during the ensuing months, pinned that location down even more, to an ellipse measuring 22 by 106 meters, roughly the size of a football field (within the red lines in the photo).  No images of the region were possible, at the appropriate resolution, during this time as the orbiter was at a point in the orbit that was too far from the comet.

Two regions were finally fixed as the most likely spots, as a finer grained search took place.  Nonetheless, other areas in or near the ellipse were not entirely discarded.  By the summer, some areas of high reflectivity had been detected.  These fit with a general resemblance to the lander, but that was still not definitive.  Note that besides visual methods, high resolution 3D modelling was used to simulate the region, to help come up with the best viewing angles to pursue. As the orbiter got more data of the potential sites, these models would be further refined.

Eventually, one of the two regions was eliminated – the reflections that had been observed were actually from ice on a large boulder.

With one candidate out of the way, efforts were focussed on the other main region.  Now there were images captured of a lander-like structure, but they were still hidden in shadows.  The view of the presumptive Philae was an overhanging cliff from one direction and a nose-shaped rocky structure from the other.  Besides obscuring the possible lander, these also shadowed the general area.
Finally, in early September, everything lined up just right, so that a view over the obstructing rock was possible.  High resolution images (down to 5 cm per pixel) were now definitive.  And there it was, basically lying on its side, as can be seen in the detailed photo and enhanced photo.  It looks a little like it is still striving to climb up a mountain, a good symbol for a mission to deep space, in my opinion.

Since the mission ended on Sept 30, it was fortunate (though good fortune depends on good work) that a final resolution (in both senses of the word) was possible in time.  Now that we know that this comet had quite a solid surface to land on, later missions should be able to effect a smoother landing.  Or so we hope – with space missions, you never know.


·         Notes from Matt Taylor’s talk at the University of Alberta, Oct 4, 2016.


Science Fiction to Read

Now that you have read some real science (astronomy and astrophysics), you should read some science fiction.

Kati of Terra

How about trying Kati of Terra, the 3-novel story of a feisty young Earth woman, making her way in that big, bad, beautiful universe out there.

The Witches’ Stones

Or, you might prefer, the trilogy of the Witches’ Stones (they’re psychic aliens, not actual witches), which follows the interactions of a future Earth confederation, an opposing galactic power, and the Witches of Kordea.  It features Sarah Mackenzie, another feisty young Earth.

Or, try a hiking journal:

Sure, it’s not deep space, but it is still adventure and exploration.  You might want to give this trail a go, after reading up on it.

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