This week marks the official end of the hope for any signals from the Philae lander on 67P/CG. Out of power, chilled to near absolute zero, probably covered in dust, and zooming away from the Sun and Earth, there was little hope. But ESA had been listening and sending instructions in case the tough little lander might accumulate enough power to send a signal. With no signal since July, efforts have been abandoned. (ESA is still searching its imagery acquired last year to see if we can spot the precise location of Philae.)
In the official announcement, ESA points out that had Philae landed in its planned , sunny location, it would have overheated and ceased operation by March. They also note that the fact that Philae survived the very rough landing and the unfavorable environment in a crevasse “greatly exceeding the specifications of its various electronic components,” and “was quite remarkable.”
Hear, hear!
Meanwhile, science teams continue to crunch away at the data acquired by Rosetta in the last year. One study published in February considers what we can infer about the internal structure of the comet, based on gravitational perturbation and radio propagation [1]. Several measures indicate that the comet is much less dense than water, which is consistent with information about other comets. But is this because of hollow caverns inside, or a homogenous “fluffy” consistency. Answering that question required getting close up to the comet, orbiting for a time at 10KM above the comet.
The investigation combined Rosetta’s measurements of the proportions of ice and dust, gravitational perturbation of the orbit of Rosetta around 67P/CG, and radio propagation between Philae and Rosetta.
As the ESA blog noted, analysis of the (tiny) gravitational perturbation was done by measuring the Doppler shifts on signals from Rosetta to Earth (!), which requires removing the effects of the Sun, planets, asteroids—the whole solar system. PI Martin Pätzold remarks, with aplumb, “Thankfully, these effects are well understood and this is a standard procedure nowadays for spacecraft operations.” In addition, the calculation had to consider the effects of solar wind and the comets out gassing.
This is measurement is significant, very nice work, all.
The blog notes that the famous double lobed shape of 67P/CG made this measurement a bit easier, because there are substantial differences between the thin and fat cross sections.
In the end, the team concludes that there is no evidence of large empty spaces inside the comet, and it is likely that the comet is about 70-75% porosity. I.e., it is a fluffy mixture of ice and dust, fairly uniform throughout.
The blog notes that in the final crash dive into the comet (in September), there will be one more chance to obtain even more precise radio Doppler measurements, which could reveal small caverns, if there are any.
- Pätzold, M., T. Andert, M. Hahn, S. W. Asmar, J. P. Barriot, M. K. Bird, B. Häusler, K. Peter, S. Tellmann, E. Grün, P. R. Weissman, H. Sierks, L. Jorda, R. Gaskell, F. Preusker, and F. Scholten, A homogeneous nucleus for comet 67P/Churyumov–Gerasimenko from its gravity field. Nature, 530 (7588):63-65, 02/04/print 2016. http://dx.doi.org/10.1038/nature16535
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