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Philae landing on the nucleus of Comet 67P C-G
jmknapp
post Nov 17 2014, 08:04 PM
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I'd think the way to go rather than assume a parabolic path or whatever would be to use a numerical method with small timesteps and an accurate model of the comet's gravitational field and crank through various scenarios until you get one that matches all the observations.

For changing direction--is it all that mysterious? Even if it came down flat as a three-legged cat, all it would take is an uneven surface under the three feet (like, say, a big chunk under one of them) to send it off in almost any direction, kind of like billiards.


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algorithm
post Nov 17 2014, 08:12 PM
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I saw a three legged dog jump out of a (stationary) lorry once!


Veered off to one side, but definetely no second landing site.
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fredk
post Nov 17 2014, 08:36 PM
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QUOTE (jmknapp @ Nov 17 2014, 08:04 PM) *
use a numerical method with small timesteps and an accurate model of the comet's gravitational field

Exactly what I had in mind. And with the expertise on this forum, not out of reach. We have good 3D models and total mass estimates. Assuming uniform density as a first approximation, it's straightforward in principle to solve Newton's equations taking the comet's rotation into account. You'd need the initial velocity vector of course, which you could estimate from the stated post-bounce velocity as well as the OSIRIS sighting.

In reality, many of these parameters would be poorly constrained, so you'd end up with a (probably large) uncertainty region around the second bounce location ("landing ellipse")...
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jmknapp
post Nov 17 2014, 09:58 PM
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QUOTE (elakdawalla @ Nov 17 2014, 12:53 PM) *
Given that we have SPICE information, I'm wondering if someone could produce a table giving altitude and sub-spacecraft lat and lon for both Philae and Rosetta in 5-minute time steps for the landing phase?


Here's a CSV file giving that data every minute:

Philae descent data

The latest SPICE files are from Nov. 15, but they still just have the initial touchdown.


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elakdawalla
post Nov 17 2014, 10:07 PM
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That is great. Here is the relevant info for the OSIRIS image times and the landing time. I guess altitudes are measured relative to some global mean, because the lander "sticks" at 158m upon landing?

15:14 Philae 1337 m, Rosetta 15.2 km
15:19 Philae 1068 m, Rosetta 15.2 km
15:23 Philae 848 m, Rosetta 15.2 km
15:34 Philae 158 m, Rosetta 15.2 km


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jmknapp
post Nov 17 2014, 10:13 PM
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Those are the numbers that come out--I'm thinking too that the altitudes are relative to the mean surface, so in this case the touchdown point on the actual surface is 158m different in altitude from the reference ellipsoid.


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sssalvi
post Nov 17 2014, 11:14 PM
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Re: Philae descent data in CSV format.

Does this data represent actual path taken data or is it a predicted trajectory?

Because as per this data the Philae was stationary for over 5 minutes ( 1535 to 1540 UT ) whereas it should have bounced immediately after touchdown.
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fredk
post Nov 17 2014, 11:18 PM
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QUOTE (elakdawalla @ Nov 17 2014, 11:07 PM) *
15:14 Philae 1337 m, Rosetta 15.2 km
15:19 Philae 1068 m, Rosetta 15.2 km
15:23 Philae 848 m, Rosetta 15.2 km
15:34 Philae 158 m, Rosetta 15.2 km

So that's a roughly 1200 m drop in the 20 minutes to first bounce. Looking at the OSIRIS image, the horizontal motion looks very roughly like 450-500 m over the same interval. So indeed the lander had considerably more vertical than horizontal velocity (at least as viewed from Rosetta).
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Keatah
post Nov 17 2014, 11:22 PM
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http://blogs.esa.int/rosetta/2014/11/15/our-landers-asleep/

..."With its batteries depleted and not enough sunlight available to recharge, Philae has fallen into 'idle mode' for a potentially long silence. In this mode, all instruments and most systems on board are shut down."

This implies the lander has active circuitry and some onboard systems have power. What would those be?
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djellison
post Nov 17 2014, 11:52 PM
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QUOTE (Keatah @ Nov 17 2014, 03:22 PM) *
This implies the lander has active circuitry and some onboard systems have power. What would those be?


Typically spacecraft have some low-level logic (on MER it's known as the battery control board - details here : http://trs-new.jpl.nasa.gov/dspace/bitstre...5/1/01-1682.pdf ) that arbitrates battery voltage, solar array input and deciding to powering on an actual flight computer.

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jmknapp
post Nov 18 2014, 01:01 AM
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QUOTE (sssalvi @ Nov 17 2014, 06:14 PM) *
Does this data represent actual path taken data or is it a predicted trajectory?


Don't know for sure--it might be the actual path taken to the first touchdown since it's dated Nov. 15 (LORB_DV_055_01_______00098.BSP on this page). It doesn't have any of the bounce data though--hopefully they'll add that when they figure it out.


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lunaitesrock
post Nov 18 2014, 01:49 AM
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QUOTE (djellison @ Nov 17 2014, 05:52 PM) *
Typically spacecraft have some low-level logic (on MER it's known as the battery control board - details here : http://trs-new.jpl.nasa.gov/dspace/bitstre...5/1/01-1682.pdf ) that arbitrates battery voltage, solar array input and deciding to powering on an actual flight computer.

Does Philae have small radioisotope heating units ~1W (RUH's) like the MER's do to keep the electronics box warm? Also, if the batteries completely drain and then the solar battery recharges some months from now, will the computer be able to reboot so Philae can resume operations?
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chemman
post Nov 18 2014, 02:19 AM
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QUOTE (lunaitesrock @ Nov 17 2014, 08:49 PM) *
Does Philae have small radioisotope heating units ~1W (RUH's) like the MER's do to keep the electronics box warm? Also, if the batteries completely drain and then the solar battery recharges some months from now, will the computer be able to reboot so Philae can resume operations?


No radioisotope units on board. See Emily's blog post here http://www.planetary.org/blogs/emily-lakda...-darmstadt.html It contains some information on the potential rebooting of the lander if solar conditions allow.


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jmknapp
post Nov 18 2014, 02:21 AM
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Turns out the CSV file I posted earlier has errors--I replaced the file with the corrected figures.


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climber
post Nov 18 2014, 03:39 AM
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Some news from Toulouse's SONC: http://www.ladepeche.fr/article/2014/11/18...c-veillent.html
In French. They say that they know Philae position within 100m but didn't find it yet.


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