[X] My Assistant

[Guest_BruceMoomaw_*]

Both Giotto and Stardust got clouted by several particles a sizable fraction of the mass of bullets, but travelling MUCH faster -- two such impacts knocked Giotto into a wobble that caused it to temporarily lose contact with Earth during its flyby, and Stardust got hit by four or five big enough to pierce the outer layer of its Whipple shields. (Its attitude-control system had been switched to an emergency high-thrust mode, so that even getting periodically shot -- there's no other word for it -- failed to shake its attitude stability.)

Presumably these are indeed fragile, easily crumbling clods, or they would have done a lot more damage. In fact, the local, high-density cloud of small particles that Stardust unexpectedly plowed through at one point when it was hundreds of km from Wild 2 seems to have been the result of a jet of such larger particles ejected from one of the comet nucleus' geysers -- which then evaporated their ice and exploded into puffs of their tinier component grains after they had traveled some distance from the comet, exactly like a fireworks display. (This model of explosively ejected fragile clods which then fragment further also seems to best explain the behavior of comets when they break up.) But, fragile or not, they are a real hazard for comet exploration spacecraft, and must be taken very seriously.

[edstrick]

The 70 km/sec bit was the very rough figure for Giotto relative to Halley.

[Marcel]

Something else: I wil try to take a look what happens with my C8. Could someone tell me what will be the exact time (corrected for one way light-time) of the impact ?

[odave]

Take a look at the Griffith Observatory page:

"The collision between the Deep Impact impactor and the nucleus of Tempel 1 is to be within a few minutes of 10:52 p.m. PDT on July 3 (allowing for the seven-minute light time delay between the actual collision and when its effects are seen here)."

Tempel 1 may be difficult in a C8. I had a gander in it a couple of weeks ago in my 12.5" dob and it took some some work to see. It's fairly diffuse and my skies are only so-so as far as light pollution goes. You'll probably need some really dark skies.

Good luck!

[Toma B]

Can someone tell me why all images taken so far are taken with Medium res camera?
They said that HRI is fixed...
HRI has 10x resolution of MRI...so why not HRI???

[abalone]

The 70 km/sec bit was the very rough figure for Giotto relative to Halley.

The accurate velocity was actually 68 km/s

[dvandorn]

Can someone tell me why all images taken so far are taken with Medium res camera?
They said that HRI is fixed...
HRI has 10x resolution of MRI...so why not HRI???

Because of the focus issue with the HRI, images taken by it have to be mathematically deconvoluted to give them proper focus. That deconvolution process is time-consuming. So, I imagine they are taking pictures with the HRI -- it is just taking some time for those pictures to be processed such that they are in focus.

-the other Doug

[4th rock from th...]

Hi,

Image contrast will suffer from the deconvolution.
Spatial resolution will be recovered, and I think that the images of the comet nucleous will be fine, but to image the coma, jets, and other large tenuous features around it you need contrast.

So my guess is that the MRI, even with lower resolution, might record this structures much better.
Also, there isn't much detail in the latest images, so in my opinion the HRI wouldn't give better results at the present time.

[lyford]

Hope this isn't a repost (I skimmed the topic but couldn't find it):

Dan Maas, of the awesome MER animations fame, has turned his rendering attentions towards Deep Impact.



Encounter Animation

(Requires QuickTime)

[Gsnorgathon]

Using optical measurements, the impactor's AutoNav software will steer itself to the brightest object in its line of sight, the Sun-facing side of Tempel 1's nucleus.

I sure hope there aren't any bright jets positioned so that the impactor ends up aiming off into empty space...

[Comga]

Image contrast will suffer from the deconvolution.
Spatial resolution will be recovered, and I think that the images of the comet nucleous will be fine, but to image the coma, jets, and other large tenuous features around it you need contrast.

So my guess is that the MRI, even with lower resolution, might record this structures much better.
Also, there isn't much detail in the latest images, so in my opinion the HRI wouldn't give better results at the present time.

If you look at the JPL website, you see that the MRI has a focal length of 2100cm and a diameter of 12. That makes it F/17.5. The HRI has a focal length of 10500cm and a diameter of 30. That makes it F/35 or twice as "slow" as the MRI. "Speed" helps in photographing dim things like the coma, which favors the MRI.

[Marcel]

Take a look at the Griffith Observatory page:

"The collision between the Deep Impact impactor and the nucleus of Tempel 1 is to be within a few minutes of 10:52 p.m. PDT on July 3 (allowing for the seven-minute light time delay between the actual collision and when its effects are seen here)."

Tempel 1 may be difficult in a C8.  I had a gander in it a couple of weeks ago in my 12.5" dob and it took some some work to see.  It's fairly diffuse and my skies are only so-so as far as light pollution goes.  You'll probably need some really dark skies.

Good luck!

Thanks ! But....impact will be 6:50 in the morning at my place. Sun rises 1 hour and 23 minutes before: it wil be daylight then !

[odave]

I'm shut out too, Tempel 1 will be below my horizon at impact time

From what I've read, the change in brightness from the impact is expected to be
too faint to be observed visually, but it could show up in CCD images.

Let's hope our more westerly friends have their cameras rolling!

[alan]

Gas jets shoot from Deep Impact’s target

Plumes of dust and gas shooting from Comet Tempel 1, captured in a Hubble Space Telescope image, have given a preview of what may be seen on 4 July when NASA's Deep Impact spacecraft smashes into the comet.

Hubble captured the brief spurt of activity on the icy body while performing a practice run for its observations on 4 July. The jet of gas and dust was seen in several images snapped over the course of eight hours on 14 June

http://www.newscientistspace.com/article/d...cts-target.html

[volcanopele]

I sure hope there aren't any bright jets positioned so that the impactor ends up aiming off into empty space...

I didn't know about that till last week. I just assumed that it would be like Huygens, Deep Impact would aim itself at the nucleus, then it would release the impactor, and finally DI would aim itself safely away. This ability to auto-navigate is a worry, not just for you and me, particularly in light of the new jet.