New Horizons late cruise, 500 Millions kms - ~200 million kms Options

[Alan Stern]

Is there a PPoD archive? I got here a bit late and due to my busy job might not make it every day.

(It seems like just yesterday when I had time to spend all day at my desk F5ing Where Is NH? until it crossed Lunar orbit.)

We're implementing an archive, also a subscribe (i.e., push) service. Should be just a couple of weeks now...

[Alan Stern]

We're implementing an archive, also a subscribe (i.e., push) service. Should be just a couple of weeks now...

We got first light on Hydra in July, data only revealed it this month; story here: http://pluto.jhuapl.edu/news_center/news/20140912.php

[Aldebaran]

A question from a layman:

New Horizons is currently in Sagittarius as viewed from the Earth, and given its relative proximity to Pluto and the relatively low gravity of the Pluto system, it seems likely from the geometry that it will continue in that direction (roughly towards the centre of the galaxy). I realise that it may be diverted to a Kuiper body object, which may change that bearing)

Given that we have a solar orbital velocity of 200km/s around the galaxy, if we superimpose New Horizon's solar-centric velocity of 14.6km/s (towards Sagittarius), does this mean that NH will very gradually spiral towards the galactic centre? - or is that a naive idea?

[Gerald]

It will very roughly move along a Kepler ellipse around the galactic center.
But because the field of gravity of the Milky Way is poorly approximated by a mass point, instead better by a disk, the trajectory will additionally oscillate about the galactic plane.

[Aldebaran]

It will very roughly move along a Kepler ellipse around the galactic center.
But because the field of gravity of the Milky Way is poorly approximated by a mass point, instead better by a disk, the trajectory will additionally oscillate about the galactic plane.

Thank you.

[John Sargent]

LOONNNGGG time watcher, first time poster.....

I was telling my 7 year old son about New Horizons (he was born on the one-year aniversary of the launch). He was particulary interested in the remoteness of Pluto and was astonished when I told him about how the sun is much less bright way out there on the edge of the solar system. Does anyone have a good practical way of simulating the lighting conditions out there? For example, is the amount of available light similar to the moonlight on a full moon night? Thanks

[john_s]

Sure- sunlight is about 1000x fainter on Pluto than on Earth (~32x further from the sun, and the sun's brightness goes as 1/R^2). That sounds like a lot, but the human eye is amazingly adaptable- we can see fairly well in full moonlight, which is about a million times fainter than sunlight.

Illumination on Pluto is something like ordinary subdued indoor domestic lighting like you might have at home in the evening. If you have a camera which reports exposures settings you can check this- for a pair of similarly-exposed photos, illumination of the scene is proportional to (f-stop)^2/(exposure time). ["^2" means "squared"]

[edit] As an example, if you expose an outdoor scene at 1/250th second at f11, and an indoor scene (with the same ISO) at 1/8th second at f2, the brightness ratio of the two scenes is about 950x.

John

[Explorer1]

The Sun would be a point, not a disc, but still be over 200 times the brightness of the full moon (an average, given the elliptical orbit).
Phil Plait has a good article on this:
http://blogs.discovermagazine.com/badastro...sun-from-pluto/

[djellison]

I believe it's about 250x brighter than a full moon. About 1/1500th as bright as the Sun at Earth.

Someone may want to check my maths on this....but....

One way to express it is with a camera. If you go outside with a DSLR and open the aperture fairly wide - you might get an exposure of, say, 1/4000th of a second ( the quickest exposure on many off the shelf consumer DSLR's )

Find a place where the same aperture results in an exposure of about 1/3rd of a second. That's how much darker it is on Pluto compared to the Earth.

Alternatively - it's like going from an aperture of F2 to F38....but without the added benefit of great depth of field

Doug

[Gerald]

I guess, you intended to write "F1 to F38".
For the other values I got about the same.

[djellison]

Yeah - I as thinking of my 50mm F1.4 - but rounded up

[Roby72]

A good way to simulate the brightness of the sun at pluto imho is when a solar eclipse at earth comes close to the totality phase.
I think about 15 to 10 minutes before totality arrives is just about the same brightness level at pluto..but this is only rough guess and depends on the eclipse geometry.
Could one do the math for this please ?

Shadows during this eclipse phase on earth are strange because of the thin sun sickle..shadows at pluto must be still more sharper with almost no penumbra visible.


Robert

[ngunn]

shadows at pluto must be sharper with almost no penumbra visible.

I have on just a couple of occasions managed to observe shadows cast by Venus. They are very faint because of course the surface brightness of Venus is thousands of times less than the Sun. But they are also very sharp, making the shadows of near and far tree branches impossible to distinguish. In terms of angular size our Venus is about the same as Pluto's Sun.

[Gerald]

A good way to simulate the brightness of the sun at pluto imho is when a solar eclipse at earth comes close to the totality phase.
I think about 15 to 10 minutes before totality arrives is just about the same brightness level at pluto..but this is only rough guess and depends on the eclipse geometry.
Could one do the math for this please ?
...

With a month of 29.5 days and an angular size of 32' for Sun and Moon, I get 3.2 seconds before/after totality to reduce brightness by a factor of 1500:
29.5d * 32' * 4 / 360° / 1500 / pi = 3.2 s.
(The dependence of the area of the visible part of the Sun from time should converge towards linear near totality in this idealized case of zero totality time, 2nd order equivalent to a horizontally shrinking rectangle with constant height 32'. A circle (area r²pi) is a little smaller than a circumscribed square (area 4r²), therefore the factor 4/pi.)
It's a little longer before/after totality for a larger angular size of the Moon, but further detail would be off topic in this thread.

[hendric]

A fun experiment would to be get a room and cover the windows so that only 1/1000 of the area is window is uncovered. That would give an idea of how bright it would be inside a house anyways.