[meteorite-list] The Quest To Find Philae

[Ron Baalke]

http://blogs.esa.int/rosetta/2015/06/11/the-quest-to-find-philae-2/

The quest to find Philae
European Space Agency
June 11, 2015
 			
Rosetta and Philae teams continue to search for the current location of 
the lander, piecing together clues from its unexpected flight over the 
surface of Comet 67P/Churyumov-Gerasimenko after its initial landing on 
12 November.

While Rosetta continues to study the ever-changing comet from a distance, 
the mission teams have been trying to narrow down the location of Philae 
on the comet nucleus using a variety of data, including imaging, magnetic 
field, and radio wave measurements.

Philae first touched down at the Agilkia landing site on the head of Comet 
67P/C-G at 15:34 GMT on 12 November 2014, confirmation arriving back at 
Earth via Rosetta 28 minutes later. Unfortunately Philae's harpoons did 
not deploy and the ice screws alone proved insufficient to secure the 
lander at Agilkia. As a result, the lander rebounded for an additional 
two-hour flight before finally coming to rest at a site now known as Abydos.

[Image]
The last moments of Philae's descent, the imprint of its touchdown, and 
subsequent drift away from Agilkia was captured by Rosetta's OSIRIS camera. 
All times in UT onboard spacecraft time. Credit: ESA/Rosetta/MPS for OSIRIS 
Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA

Both Rosetta's navigation camera and the high-resolution OSIRIS camera 
successfully identified the first touchdown point, with Philae's down-looking 
ROLIS camera providing high-resolution views of the location  from as 
close as 9 metres altitude. The bouncing lander was then identified in 
OSIRIS  and NAVCAM images shortly after it had left Agilkia. Somewhat 
later, another OSIRIS image was thought to show Philae above the horizon 
of the large depression known as Hatmehit  on the comet's head.

[Image]
Rosetta's OSIRIS wide-angle camera captured this view of Comet 67P/Churyumov-Gerasimenko 
on 12 November 2014 at 17:18 GMT (onboard spacecraft time), with what 
is thought to be Philae above the rim of the large depression (marked). 
The image has been used to guide subsequent lander search efforts, and 
provides the basis for trajectory reconstructions.
Credits: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA

Magnetic field measurements from Philae's ROMAP provided further details 
on subsequent events as the lander flew above the comet, including precise 
timing of the various contact points. Initially, the lander flew in a 
stable orientation, but is then thought to have struck a surface feature 
at 16:20 GMT, after which it tumbled. A third touchdown occurred at 17:25 
GMT, followed by a much shorter bounce of just a few minutes, before Philae 
finally arrived at Abydos at 17:32 GMT. Overall, Philae is thought to 
have travelled more than a kilometre from its initial contact point.

Images taken by its ROLIS and CIVA cameras, along with telemetry and data 
returned by its instruments during its nearly 60 hours of surface operations, 
have built up a picture of the final landing site. The lander is thought 
to be in a rough piece of terrain, tilted up against a cliff, and mostly 
in shadow.

In the days and weeks after landing, the OSIRIS team continued the search 
for Philae by making a detailed examination of new images of the comet 
as they came in. However, this proved to be a daunting task, given the 
rough terrain, the small size of the lander, and the distance of Rosetta 
from the comet.

The highest resolution images taken of the region of interest after Philae's 
landing were part of a dedicated search that took place in mid-December, 
at distance of roughly 18 kilometres from the surface of the comet. At 
this distance, the OSIRIS narrow-angle camera has a resolution of 34 centimetres 
per pixel. The body of Philae is just 1 metre across, while its three 
thin legs extend out by up to 1.4 metres from its centre.

Taking into account the size, reflectivity, and orientation of Philae, 
along with the "point spread function" or intrinsic resolution of the 
camera optics, the OSIRIS team expect Philae to be no more than a few 
pixels across in their images. Their examination of the head of Comet 
67P/C-G revealed many initial candidates for Philae in the form of bright 
spots just a few pixels wide, as illustrated in the montage below:

[Image]
Approximate locations of five lander candidates initially identified in 
high-resolution OSIRIS Narrow Angle Camera images taken in December 2014, 
from a distance of about 20 km from the centre of Comet 67P/C-G. The candidates 
are circled in the close-ups, identifying Philae-sized features approximately 
1–2 m across. The contrast has been stretched in some of the images to 
better reveal the candidates. All but one of these candidates (top left) 
have subsequently been ruled out of consideration due to constraints including 
the reconstructed lander trajectory and topography at the landing site. 
The candidate at top left lies near to the current CONSERT ellipse.
Credits: Centre image: ESA/Rosetta/NAVCAM – CC BY-SA IGO 3.0; insets: 
ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA

The question then is which, if any, of these candidates is the real Philae?

Fortunately, more information is available. The initial trajectory taken 
by Philae as it departed from Agilkia could be worked out from the OSIRIS 
images. This constrained the problem a bit, but only as far as the second 
bounce.

Fortunately, it was possible to narrow down the lander's final location 
by using the radio signals sent between Philae and Rosetta as part of 
the CONSERT experiment after the final touchdown. Combining data on the 
signal travel time between the two spacecraft with the known trajectory 
of Rosetta and the current best shape model for the comet, the CONSERT 
team have been able to establish the location of Philae to within an ellipse 
roughly 16 x 160 metres in size, just outside the rim of the Hatmehit 
depression.

[Image]
By combining radio-wave data exchanged between Rosetta and Philae as part 
of the CONSERT experiment with trajectory data and the current model of 
the shape of 67P/C-G, the location of Philae has been determined to lie 
within an ellipse measuring approximately 16 x 160 m. The CONSERT ellipse 
corresponds to the outer limits of the various solutions identified for 
the likely landing site of Philae, although this depends to some extent 
on the shape model assumed. More detailed work, including a full account 
of all the errors involved, is currently in progress by the CONSERT team.
Credits: ESA/Rosetta/Philae/CONSERT

The ellipse represents the team's current best estimate of the outer bounds 
of the likely location of Philae based on a number of simulations, but 
further work is being carried out to quantify things more rigorously in 
terms of the statistical likelihood of Philae being inside this region. 
The location of the ellipse also depends on the assumed shape model of 
the comet: as this is constantly being refined, some slight revisions 
in the positioning of the ellipse remain possible.

Nevertheless, the CONSERT ellipse rules out the majority of the candidates 
shown in the montage above. But there is at least one candidate near to 
the ellipse, as well as a number of other bright spots in the vicinity.

[Image]
The current 16 x 160 m CONSERT ellipse overlaid on an OSIRIS narrow-angle 
camera image of the same region. The location and size of the ellipse 
is not accurate at the pixel level, and may also change as further CONSERT 
data analysis proceeds and more detailed comet shape models become available. 
The OSIRIS image is a slightly cropped 2 x 2 mosaic of NAC images taken 
from a distance of approximately 18 km from the surface of the comet on 
13 December 2014. At this distance, the resolution of NAC is approximately 
34 cm per pixel, and the full mosaic covers roughly 1.3 km.
Credits: Ellipse: ESA/Rosetta/Philae/CONSERT; Image: ESA/Rosetta/MPS for 
OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA

"We have identified several possible lander candidates in OSIRIS images, 
both inside the CONSERT region of interest and nearby," says Holger Sierks, 
OSIRIS principal investigator at the Max Planck Institute for Solar System 
Research (MPS) in Germany.

Holger adds: "That said, it is important to note that the viewing geometry 
during our December search was such that Rosetta was 90 degrees to the 
Sun-comet direction and in a terminator orbit. Philae's solar panels could 
have been well illuminated, but still hidden in the rugged terrain from 
Rosetta's perspective, making it difficult or impossible to spot."

[Image]
Zooming in towards the current CONSERT ellipse, a number of bright dots 
are seen in the region. As only one (at most) of these could be the lander, 
the majority must be associated with surface features on the comet nucleus.
Credits: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA

Furthermore, and as the image above shows, bright features are common 
on the surface of the nucleus. The challenge of identifying Philae is 
made even more difficult by the fact that many of them are transient. 
For example, small-scale regions of the nucleus may "glint' under favourable 
illumination conditions, thus appearing in some images but not others.

To address this issue, scientists working with OSIRIS team member Philippe 
Lamy at the Laboratoire d'Astrophysique de Marseille (LAM) and the Institut 
de Recherche en Astrophysique et Planétologie (IRAP) in France began searching 
for special sets of OSIRIS images.

In particular, they looked for images taken before and after Philae's 
landing under nearly similar illumination conditions, to reduce the chances 
of being fooled by transient surface features glinting. In that way, if 
something new appeared after landing, it could be Philae.

Scanning a broad area encompassing the expecting landing zone, the team 
identified a promising candidate that is seen on two images taken on 12 
and 13 December, a month after the 12 November landing, but not on an 
image taken earlier, on 22 October. The candidate is also featured in 
the montage earlier in the post, at top-left.

The movie below zooms in to the region of interest on the image taken 
on 13 December. (Click here for more movie download options). 

[Movie]
This movie shows a zoom into the 13 December 2015 OSIRIS narrow-angle 
camera image taken from a distance of about 20 km from the centre of Comet 
67P/C-G. A large number of bright spots are seen: as only one (at most) 
of them could be Philae, the majority must be associated with surface 
features on the comet nucleus. The movie ends on a promising candidate 
located just outside the CONSERT error ellipse (marked): this candidate 
was not seen in 22 October images, but appears in images taken on both 
12 and 13 December. Credits: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA

Additional images below show the region containing the candidate as seen 
on 22 October from a distance of 10 km from the centre of the comet (roughly 
8 km from the surface), and then on both 12 and 13 December from 20 km 
from the centre (18 km from the surface).

[Image]
"Before" and "after" comparison images of a promising candidate located 
near the CONSERT ellipse as seen in images from the OSIRIS Narrow-Angle 
Camera. Each box covers roughly 20 x 20 m on Comet 67P/C-G.

The left-hand image shows the region as seen on 22 October (before the 
landing of Philae) from a distance of about 10 km from the centre of Comet 
67P/C-G, while the centre and right-hand images shows the same region 
on 12 and 13 December from 20 km (after landing). The candidate is only 
seen in the two later images.

The illumination conditions are broadly similar in the three images and 
the same topography can be recognised in each case. The difference in 
distance at which the images were taken yields a difference in resolution 
and thus the December images have been resampled and interpolated to match 
the scale of the October image. As a result, the candidate covers more 
pixels calculated for a Philae-sized object seen by the OSIRIS narrow-angle 
camera from a distance of 18 km to the surface.
Credits: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA

The "after" images have been resampled and interpolated to be on the same 
scale as the "before" image: this results in the bright spot covering 
more pixels than in the original data.

"Although the pre- and post-landing images were taken at different spatial 
resolutions, local topographic details match well, except for one bright 
spot present on post-landing images, which we suggest is a good candidate 
for the lander," says Philippe Lamy, member of the OSIRIS team at the 
Laboratoire d'Astrophysique de Marseille, adding: "This bright spot is 
visible on two different images taken in December 2014, clearly indicating 
that it is a real feature on the surface of the comet, not a detector 
artefact or moving foreground dust speck."

[Graphic]
This graphic shows the solar panels on Philae's body. Simulations of the 
day-night cycle and landing site topography show considerable variations 
in the illumination as local features cast shadows at different times. 
The light blue coloured sections indicate the portion of solar panels 
thought to be illuminated at the exact moment the 13 December image was 
taken. While Philae's legs and feet appear bright in images acquired during 
its descent to the surface, the simulations suggest that the Sun would 
not reach the lower portions of the lander at the time the 13 December 
image was taken. Under these illumination conditions, Philae would only 
cover a few pixels in the OSIRIS images taken from 20 km.
Credits: DLR

But is this really Philae? Unfortunately, it is impossible to be sure.

On one hand, analyses carried out at the Philae Science Operations and 
Navigation Center (SONC) at CNES suggest that this candidate satisfies 
a number of constraints regarding, for example, illumination and radio 
visibility in this region.

On the other, the candidate is located just outside the ellipse currently 
identified by CONSERT, although as mentioned earlier, improved shape models 
and continued CONSERT data analysis may alter its position.

Also, given the relatively long seven week interval between the "before"
and 'after" images, it is possible that this object is due to a physical 
change at that location on the nucleus, perhaps as fresher material was 
newly exposed. The relative lack of significant illumination in this region 
at the time argues that such changes are unlikely, but they cannot be 
completely ruled out.

Ultimately, a definitive identification of this or any other candidate 
as being Philae will require higher-resolution imaging, in turn meaning 
closer flybys. This may not be possible in the near-term, as issues encountered 
in navigating close to the comet mean that the opportunity to make flybys 
at significantly less than 20 km from the surface may be on hold until 
later in the mission. But after the comet's activity has subsided, Rosetta 
should be able to safely operate in close proximity to the comet nucleus 
again.

The other possibility of further refining Philae's location would come 
if the lander were to receive enough power to wake-up from its hibernation 
and resume its scientific study of 67P/C-G. Then, CONSERT could be used 
to perform additional ranging measurements and significantly reduce the 
uncertainties on the lander's location.

At the moment, Philae is still in hibernation, but the mission team remain 
hopeful that, as the comet moves closer to the Sun along its orbit, the 
lander will receive enough power in the coming weeks or months to wake 
up and transmit a signal to Rosetta.

"The conditions for Philae's wake-up are becoming more and more favourable 
as the comet approaches the Sun," says Lander Project Manager Stephan 
Ulamec. "The team at DLR's Lander Control Center has continued to prepare 
long term operations for Philae and its instruments in the hope that it 
does wake up soon."

In the meantime, the team continue to search through all available data. 
Patrick Martin, ESA's Rosetta mission manager says: "Follow-up work, such 
as the identification of candidates in other images taken from 20 km or 
less, along with improved reconstruction of the local topography, may 
help further narrow down the location of Philae."

Matt Taylor, ESA's Rosetta project scientist, adds: "Accurately locating 
the lander is of great scientific value, in particular for the joint orbiter 
and lander CONSERT experiment to get the best assessment of the interior 
structure of the comet nucleus. Knowing where Philae is would provide 
important context for the lander measurements and valuable information 
for its possible future operation. In the meantime, Rosetta is continuing 
to observe the comet from a range of distances as the comet's activity 
increases."

 

Notes and credits

This blog post was prepared in collaboration with CNES, DLR, LAM, and 
MPS.

The candidate discovered by the team of Philippe Lamy at the Laboratoire 
d'Astrophysique de Marseille (LAM) was identified by G. Faury of AKKA 
Technologies (a contractor of LAM and IRAP). E. Jurado and R. Garmier 
of CS-SI (a contractor of CNES) at the Philae Science Operations and Navigation 
Center at CNES (SONC), A. Herique and Y. Rogez of the CONSERT team at 
Institut de Planétologie et d'Astrophysique de Grenoble, and P. Heinisch 
of the ROMAP team at IGEP, the Technical University of Braunschweig, were 
also involved.

CONSERT: CONSERT is the Comet Nucleus Sounding Experiment by Radiowave 
Transmission. The principal investigator is Wlodek Kofman of the Institut 
de Planétologie et d'Astrophysique de Grenoble (IPAG), France. The CONSERT 
instrument was designed built and operated by IPAG, LATMOS (Laboratoire 
Atmosphères, Milieux, Observations Spatiales, France) and MPS (Max Planck 
Institute for Solar System Research, Germany) and was financially supported 
by CNES, CNRS, UJF, DLR, and MPS.

OSIRIS: The scientific imaging system OSIRIS was built by a consortium 
led by the Max Planck Institute for Solar System Research (Germany) in 
collaboration with CISAS, University of Padova (Italy), the Laboratoire 
d'Astrophysique de Marseille (France), the Instituto de Astrofísica de 
Andalucia, CSIC (Spain), the Scientific Support Office of the European 
Space Agency (The Netherlands), the Instituto Nacional de Técnica Aeroespacial 
(Spain), the Universidad Politéchnica de Madrid (Spain), the Department 
of Physics and Astronomy of Uppsala University (Sweden), and the Institute 
of Computer and Network Engineering of the TU Braunschweig (Germany). 
OSIRIS was financially supported by the national funding agencies of Germany 
(DLR), France (CNES), Italy (ASI), Spain (MEC), and Sweden (SNSB), and 
the ESA Technical Directorate.

ROLIS (ROsetta Lander Imaging System) is a descent and close-up camera 
on the Philae Lander. It has been developed by the DLR Institute of Planetary 
Research, Berlin.

ROMAP: ROMAP is the Rosetta Lander Magnetometer and Plasma Monitor. The 
contributing institutions to ROMAP are: Institut für Geophysik und Extraterrestrische 
Physik, Technische Universität Braunschweig, Germany; Max-Planck Institut 
für Sonnensystemforschung, Göttingen, Germany; Hungarian Academy of Sciences 
Centre for Energy Research, Hungary; and Space Research Institute Graz, 
Austria. The co-principal investigators are Hans-Ulrich Auster (Technische 
Universität, Braunschweig) and István Apáthy, KFKI, Budapest, Hungary.

Rosetta is an ESA mission with contributions from its Member States and 
NASA. Rosetta's Philae lander was provided by a consortium led by DLR, 
MPS, CNES, and ASI.