Rosetta's comet mission discussion thread

[dlgoff]

I haven't read all the post in this thread so maybe it's been mentioned before. Anyway, I just heard that the craft bounced twice before coming to rest. What a cool visualization this brings.

 

[Borg]

I haven't read all the post in this thread so maybe it's been mentioned before. Anyway, I just heard that the craft bounced twice before coming to rest. What a cool visualization this brings.

For some reason, I keep picturing a piece of litter blowing along the street when I read about that.

 

[enorbet]

I just read the cells are only receiving sunlight some 1.5 hours/day and of course at that distance it is less energy per unit area, so quite insufficient. I don't know if this means that when it is closer things could improve (it was amazing enough that it "slept for 10 years and then awakened") but currently they are holding drilling off till near nominal charge when there's little to lose.

This turn of events is disappointing but really just a minor setback considering the number of bold firsts it has already achieved, and the orbiter is in perfect shape and will continue to do good Science for a long time. The most impressive thing to me is how "self-contained" the project is. They had so little information as to what to expect and instead of having a reconnaissance flight before final design and fabrication, they engineered an adaptable system that did both "by the seat of the pants" in one mission. That is some phenomenal engineering. That it even made orbit with such a wacky shaped object is laudable. That they had confidence in the orbiter to align itself to where the lander could drop like a brick with any modicum of accuracy demonstrates fantastic expertise and serious chutzpah (I almost said cajones) :) What a team!

 

[marcus]

I haven't read all the post in this thread so maybe it's been mentioned before. Anyway, I just heard that the craft bounced twice before coming to rest. What a cool visualization this brings.

Yes! the touchdown times were quoted as 15:33, 17:26, and 17:33, and the takeoff speed from the first bounce was estimated at 0.38 m/s, as I recall from the press conference.
One can make a sloppy estimate of the surface gravity from that (assuming uniform field over the range of interest).
The first bounce lasted twice 56 minutes, or twice 3360 seconds. So if the acceleration were uniform it would be g = 0.38/3380 m/s² = 0.113 mm/s²

This agrees with the order of magnitude estimate I've been hearing of 10^-5 Earth gee,
a hundred thousandth of Earth surface gravity.

 

[dlgoff]

Yes! the touchdown times were quoted as 15:33, 17:26, and 17:33, and the takeoff speed from the first bounce was estimated at 0.38 m/s, as I recall from the press conference.
One can make a sloppy estimate of the surface gravity from that (assuming uniform field over the range of interest).
The first bounce lasted twice 56 minutes, or twice 3360 seconds. So if the acceleration were uniform it would be g = 0.38/3380 m/s²

You're a true science freak. I love you man.

Edit: BTW No disrespect intended.

 

[dlgoff]

For some reason, I keep picturing a piece of litter blowing along the street when I read about that.

Ever play Asteroids?

 

[Imager]

Yes! the touchdown times were quoted as 15:33, 17:26, and 17:33, and the takeoff speed from the first bounce was estimated at 0.38 m/s, as I recall from the press conference.
One can make a sloppy estimate of the surface gravity from that (assuming uniform field over the range of interest).
The first bounce lasted twice 56 minutes, or twice 3360 seconds. So if the acceleration were uniform it would be g = 0.38/3380 m/s2 = 0.113 mm/s2

This agrees with the order of magnitude estimate I've been hearing of 10-5 Earth gee,
a hundred thousandth of Earth surface gravity.

If it helps, the video mentioned they estimated 38cm/sec bounce went about about 1Km. The second bounce was 3cm/sec for 7min. I didn't hear the ESA mention the height for that bounce.

 

[marcus]

"true science freak"
That is a serious honor coming from you, DL. It takes one to know one. : ^)

 

[dlgoff]

"true science freak"
That is a serious honor coming from you, DL. It takes one to know one. : ^)

Note my edit on Post # 95. I feel better now.

 

[marcus]

If it helps, the video mentioned they estimated 38cm/sec bounce went about about 1Km. The second bounce was 3cm/sec for 7min. I didn't hear the ESA mention the height for that bounce.

Thanks for the confirmation, Imager! 7 minutes is 420 seconds which is twice 210 seconds so let's see if we get the same takeoff speed they do.
Multiply 210 s by 0.113 mm/s². Well we don't get their 3 cm/s, more like 2.4 cm/s, but it is close enough.

 

[enorbet]

Thank you, Vagn, for that enlightening paper you linked. It supports what I suspected, that not only has the mission as a whole raised the bar but it extends all the way back to component level. This mission is very likely something of a "game changer", at least one could hope.

 

[Borg]

Ever play Asteroids?

Of course. :)

Now that I've seen the calculations by Marcus, It sounds like it bounced like a piece of litter in extreme slow motion.

 

[marcus]

I underestimated the surface gravity at the landing site by assuming acceleration g was uniform during the first jump, whereas it starts greater at the surface and falls off with altitude. So 0.113 mm/s², a kind of average, underestimates the real surface gravity, which I guess would be more like 0.13 mm/s² or 0.14.
That would agree with the 3 m/s takeoff speed on the second hop. Let's say 0.13.
That fits with the estimate they gave for the escape velocity from the surface, at the landing site. As I recall they said it was about 0.5 m/s.

DL, I treasure the compliment without reservation! Regardless of whether deserved : ^)

BTW anyone who can correct or improve the estimates here, or supply better data, is cordially invited to do so.

 

[OmCheeto]

OUR LANDER’S 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.

"Prior to falling silent, the lander was able to transmit all science data gathered during the First Science Sequence," says DLR's Stephan Ulamec, Lander Manager, who was in the Main Control Room at ESOC tonight.

That sure was an exciting 3 days. :)

But I wonder though, will the comet becomes more active, and would the glow of the coma act as a 24 hour light source? Do we have any comet experts on staff?

wiki on cometary coma
The coma is generally made of ice and dust. Water dominates up to 90% of the volatiles that outflow from the nucleus when the comet is within 3-4 AU of the Sun.

And where is 67P anyways?

Ah ha!
[/quote]
Comet 67P (Churyumov-Gerasimenko)
2.97 AU from the sun

Not a New Comet
Discovered in 1969, Comet 67P (Churyumov-Gerasimenko) is a Jupiter family comet measuring 4.5km across. It has an orbital period of 6.5 years and its closest point to the Sun at perihelion is 1.2458 AU (186m kilometres). Although Comet 67P won’t be visible with the naked eye from Earth, the Rosetta Spacecraft will make sure it is seen around the world in unprecedented detail.
[/quote]

Sweet! That site has little buttons you can push to change the date. The comet will pass through the orbit of Mars around May 30, 2015.
Kind of reminds me of a program I typed into my computer from a book one day, only it doesn't take 10 minutes to render the image.
Anyone else remember this book?

Published in 1982

 

[harrylin]

[...] "Prior to falling silent, the lander was able to transmit all science data gathered during the First Science Sequence," says DLR's Stephan Ulamec, "[..]

That sure was an exciting 3 days. :)

But I wonder though, will the comet becomes more active, and would the glow of the coma act as a 24 hour light source? Do we have any comet experts on staff?[..]

Apparently they did dare to do some drilling:
http://www.theguardian.com/science/2014/nov/14/philae-comet-lander-drills-hammers-rosetta

Combining those different news reports, it looks like they actually pulled it off - to get data from all their instruments including from drilled samples just before the lander went in stand-by. :)

If so, [update, I just heard that this is indeed the case] then it's a huge success despite the problems.

I don't know about the glow, but surely when the lander gets closer to the sun, much more energy will be collected in 1.5 hours (if that is still valid after the drilling and hammering) than currently. It would be great if the lander manages to send back more data when things are getting hot - that would be the cream on the cake. :D

 

[Jonathan Scott]

Although the drill successfully executed its pre-planned procedure, I didn't hear any confirmation that the drill actually made contact with the surface or successfully picked anything up. If it didn't actually pick anything up, the other experiments to analyse the results would probably not be very useful.

It's also unfortunate that the lens cap on the APXS spectrometer apparently failed to open, and again this could only be determined after the experiment was thought to have worked normally according to plan.

I can understand that they want to be as optimistic as possible, reporting "80% of planned science data" was obtained. However, it's still not clear whether that contained anything very useful.

 

[Jonathan Scott]

But I wonder though, will the comet becomes more active, and would the glow of the coma act as a 24 hour light source? Do we have any comet experts on staff?

My guess is that reflected light from any outgassing or dust would be extremely tenuous (the coma occupies a vast volume of space). Even if the outgassing were very strong (which would probably threaten the stability of the surface) the reflected light would be far weaker than direct sunlight.

 

[Doug Huffman]

The concern with increased insolation is too much heat, too hot PV's band gap shrinking due to thermal charge carriers.

 

[OmCheeto]

The concern with increased insolation is too much heat, too hot PV's band gap shrinking due to thermal charge carriers.

I wonder how warm it will get.

Rosetta takes comet's temperature
between 13 and 21 July, 2014

...scientists determined that its average surface temperature is about –70ºC.
The comet was roughly [3 AU] from the Sun at the time..., meaning that sunlight is only about a tenth as bright [as on earth].
Although –70ºC may seem rather cold, importantly, it is some 20–30ºC warmer than predicted for a comet at that distance covered exclusively in ice.​

And the batteries have to be warmed up to 0°C to just to start charging.

http://www.spaceflight101.com/play-by-play-philae-landing.html

During hibernation, Philae will dedicate all generated power to keeping its secondary batteries warm as they need to be at a temperature of 0°C to start charging. To achieve that, the lander would need about 50 to 60 Watt-hours of power per day plus the additional power to boot up and initiate communications. This is only possible if the illumination situation improves significantly as the comet moves closer to the sun and the overall geometry enters a more favorable setting – at least that’s the hope, although odds are looking slim.
​

hmmm...

____date___AU from sun
07/17/14_________3.815
11/15/14_________2.967

I think I'll send a tweet to Rosetta to get a new temperature reading. :)

 

[Dotini]

This September 5 "color close-up" from 62km may be worth a quick glance.
https://plus.google.com/11350700917...6057110080496657778&oid=113507009175485747967

 

[enorbet]

Yes the Lander is asleep but the boys back home certainly are not. If you'd like too see some truly diligent sleuthing results take a peek at this http://blogs.esa.int/rosetta/2014/11/16/philae_spotted_after_first_landing/

 

[Dotini]

Yes the Lander is asleep but the boys back home certainly are not. If you'd like too see some truly diligent sleuthing results take a peek at this http://blogs.esa.int/rosetta/2014/11/16/philae_spotted_after_first_landing/

I found the following interesting comment within the comments section of the ESA blog:

"The MUPUS instrument was deployed but the surface was so hard that the shaft of its hammer broke as reported in the BBC's "Sky AT Night" special tonight.. The question of, is the surface rock or ice remains an open one, for us at least. The harpoons may have fired as planned, but bounced off the very hard subsurface, the ice screws were deployed but could not dig into the subsurface for the same reason. The plot thickens as they say."

At about the 34:20 mark in this Rosetta mission results video, we hear the OSIRIS Principle Investigator, Holger Sierks, remark upon the color of the comet. He makes the enigmatic statement that although the color is gray, the bright areas look less reddish, and the comet would look red if it weren't so dark.

 

[Greg Bernhardt]

Mysterious 'Song' Coming From Comet 67P Recorded By Rosetta
http://www.space.com/27734-rosettas-comet-sounds-like-this-magnetic-oscillations-video.html

 

[OmCheeto]

Interesting pictures of Philae after her first landing:

OSIRIS SPOTS PHILAE DRIFTING ACROSS THE COMET
Released 17/11/2014 3:00 pm

They still haven't found her final landing spot.

The following comment has me puzzled:

"The image taken after touchdown, at 15:43 GMT, confirms that the lander was moving east..."
​

How does one decide, which way is north, on a rotating dumbbell?

 

[Dotini]

BBC reports, http://www.bbc.com/news/science-environment-30083969#

"...data was pulled off the robot just before its sagging energy reserves dropped it into sleep mode.
Little of the results have so far been released by the various instrument teams. The one major exception is MUPUS.

This sensor package from the German space agency's Institute for Planetary Research deployed a thermometer on the end of a hammer.

It retrieved a number of temperature profiles but broke as it tried to burrow its way into the comet's subsurface.

Scientists say this shows the icy material underlying 67P's dust covering to be far harder than anyone anticipated - having the tensile strength of some rocks.

It also helps explain why Philae bounced so high on that first touchdown.

The 4km-wide comet has little gravity, so when key landing systems designed to hold the robot down failed at the crucial moment - the probe would have been relying on thick, soft, compressive layers to absorb its impact.

However much dust it did encounter at that moment, it clearly was not enough to prevent Philae making its giant rebound."

ROMAP (Rosetta Lander Magnetometer and Plasma Monitor) reports,
http://www.igep.tu-bs.de/forschung/weltraumphysik/projekte/rosetta/comet_en.html
http://www.igep.tu-bs.de/forschung/welt ... _ROMAP.pdf

 

[Jonathan Scott]

How does one decide, which way is north, on a rotating dumbbell?

By the right hand rule, as usual?

 

[OmCheeto]

By the right hand rule, as usual?

Yes. I know that. But is it magnetic north, or geoorsolarrotationalyaxially north?

Sorry to be too succinct. I'm always assuming. Very bad habit of mine.

 

[Jonathan Scott]

But is it magnetic north, or geoorsolarrotationalyaxially north?

For things like that, north is defined purely by the main rotation axis.

 

[OmCheeto]

For things like that, north is defined purely by the main rotation axis.

Any idea what the xyz rotation axis is, in relation to our solar system's main "planetary" plane of orbits?

Sorry to speak in such simpleton terms, but it's been 1/3 of a lifetime since I've spoken the language.

hmmm... Never mind. I spend 20 seconds thinking about it, and the math, which I also no longer know, hurt my brain.

I spent at least 30 minutes the other day, relearning geometry, trying to figure out how to physically make an angle of 67.5°.
I eventually did it, but it hurt my brain.

Ugh. Time for a nap.

Don't ever get old, JS. You are our only hope.

 

[lpetrich]

I checked 67P/Churyumov–Gerasimenko - Wikipedia, and it referred me to Measuring comet 67P/C-G | Rosetta - ESA's comet chaser

North Pole: Right ascension: 69 degrees; Declination: 64 degrees