Starship Asterisk*

Flying Past Neptune's Moon Triton

Explanation: What would it look like to fly past Triton, the largest moon of planet Neptune? Only one spacecraft has ever done this -- and now, for the first time, images of this dramatic encounter have been gathered into a movie. On 1989 August 25, the Voyager 2 spacecraft shot through the Neptune system with cameras blazing. Triton is slightly smaller than Earth's Moon but has ice volcanoes and a surface rich in frozen nitrogen. The first sequence in the video shows Voyager's approach to Triton, which, despite its unusual green tint, appears in approximately true color. The mysterious terrain seen under the spacecraft soon changed from light to dark, with the terminator of night soon crossing underneath. After closest approach, Voyager pivoted to see the departing moon, now visible as a diminishing crescent. Next July, assuming all goes well, the robotic New Horizons spacecraft will make a similar flight past Pluto, an orb of similar size to Triton.

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AMAZING!!!

I was surprised a the greenish color, I guess I never took real notice.

It looks like a HOLLYWOOD ANIMATION.....Can't wait until some rover lands on it and start sending back images.

:---[===] *

I'd never heard the term "cantaloupe terrain" -- unique to Triton -- before reading the Wikipedia entry on Triton, today. I have since discovered that cantaloupe melons are named after one of several towns in Italy named Cantalupo, meaning literally: "singing wolf". It then occurred to me that "terrain" is perhaps not an ideal word to describe the surface of any heavenly body (Earth not being heavenly), but I can't think of a better word. I then wondered whether "terrain" is a correct word to describe a surface composed of ices. I then wondered whether there are countries other than Australia where cantaloupe melons are known as rockmelons. I don't know if there is any rock on Triton, but the area of "cantaloupe terrain" sure looks like a rockmelon to me.

I very much enjoyed the movie. Thank you.

Are the background stars in the movie real or were they added for visual effect?

I remember the day very well. I was on holiday in Italy and heard the news on a US Forces radio network saying that the spacecraft had just passed Neptune but that the signals would not arrive at Earth for a further 4 hours. It was enticing to think there were these packets of radio waves en route somewhere between there and here strung out over many millions of km. An unusual visualisation. Those that missed the Earth will now be 25 light years away in the direction of Gemini.

Nosepicker wrote:Are the background stars in the movie real or were they added for visual effect?

Looks to me like they represent the correct star fields, but were added in post processing, not captured by the camera.

How much time (days, hours, minutes??) does the video represent? I saw the date of the event.

Fantastic. I am a bit surprised that nobody had put this together in a movie before. And I suppose it was a lot of work. It looks great. There are obviously people at NASA (and some related companies) who do a lot of work animating, either with direct data such as this, or with a lot more artistic input, such as for example, the famous "7 minutes of terror" video. I'm wondering how those people come into the industry, where they are employed, etc. I guess I'm just wondering out loud, does NASA (or JPL, etc.) post job openings for animators? I wonder how many such people they employ in this way. There must be no end of ideas of what to render, but resources would limit them.

Nitpicker wrote:I then wondered whether there are countries other than Australia where cantaloupe melons are known as rockmelons.

We've got a hami melon in the fridge. It's a variant of the cantaloupe even though it doesn't look very similar. The same word in Cantonese is used for hami melon as it is for cantaloupe. As far as I can tell, Hami is just the name of a city, much like the Italian etymology of cantaloupe. That's about all the rockmelon trivia I have for you.

I want to wish the New Horizons team the best and some luck sprinkled in

Chris Peterson wrote:

Nosepicker wrote:Are the background stars in the movie real or were they added for visual effect?

Looks to me like they represent the correct star fields, but were added in post processing, not captured by the camera.

I would have expected the star field to drift a bit, relative to Triton, unless the spacecraft was approaching at a very particular angle

Nitpicker wrote:I would have expected the star field to drift a bit, relative to Triton, unless the spacecraft was approaching at a very particular angle

The approach was almost linear, as was the departure, and I doubt the position of Triton changed much over such a short interval. The star field changed completely between the two, but the camera was on the surface when that happened, so it isn't seen.

Chris Peterson wrote:

Nitpicker wrote:I would have expected the star field to drift a bit, relative to Triton, unless the spacecraft was approaching at a very particular angle

The approach was almost linear, as was the departure, and I doubt the position of Triton changed much over such a short interval.

I don't know what the time interval of the approach or departure in the movie was, exactly, but I played around in Stellarium for a while, looking at the line of sight from Neptune to Triton (which I understand is pretty close to the path taken by the spacecraft), and even over a period of a few minutes or hours, the background star field drifted noticeably, relative to Triton. I would be interested to learn more of the details, re precise time and position of the craft.

Nitpicker wrote:I don't know what the time interval of the approach or departure in the movie was, exactly, but I played around in Stellarium for a while, looking at the line of sight from Neptune to Triton (which I understand is pretty close to the path taken by the spacecraft), and even over a period of a few minutes or hours, the background star field drifted noticeably, relative to Triton. I would be interested to learn more of the details, re precise time and position of the craft.

The video covers about ten days, three days before the encounter to seven days after. The star fields are not static; the field before the encounter is drifting at a different rate than the one seen after the encounter.

I'd be very surprised if it isn't an accurate composition.

Chris Peterson wrote:

Nitpicker wrote:I don't know what the time interval of the approach or departure in the movie was, exactly, but I played around in Stellarium for a while, looking at the line of sight from Neptune to Triton (which I understand is pretty close to the path taken by the spacecraft), and even over a period of a few minutes or hours, the background star field drifted noticeably, relative to Triton. I would be interested to learn more of the details, re precise time and position of the craft.

The video covers about ten days, three days before the encounter to seven days after. The star fields are not static; the field before the encounter is drifting at a different rate than the one seen after the encounter.

I'd be very surprised if it isn't an accurate composition.

Well, that information contradicts the Wikipedia article on Voyager 2, which describes its curved path from very low over the North Pole of Neptune, towards Triton, over a period of less than 24 hours. Triton orbits Neptune with a period of about five days, I think. So, it is unclear to me how the background star field, on approach to Triton in the movie, could appear so static over a period of three days.

Here are a couple of animated gifs I made using Solar System Simulator with a coarse interval (10 minutes) clearly showing the background stars. The 15° fov was disorienting so I made a 90° too.
http://www.geckzilla.com/astro/voy2_tri ... _15fov.gif
http://www.geckzilla.com/astro/voy2_tri ... _90fov.gif

Nitpicker wrote:

Chris Peterson wrote:The video covers about ten days, three days before the encounter to seven days after. The star fields are not static; the field before the encounter is drifting at a different rate than the one seen after the encounter.

Well, that information contradicts the Wikipedia article on Voyager 2, which describes its curved path from very low over the North Pole of Neptune, towards Triton, over a period of less than 24 hours.

I don't think there's any contradiction. The rapid flyby was less than 24 hours. The video is made up of frames from well before to well after that.

Triton orbits Neptune with a period of about five days, I think. So, it is unclear to me how the background star field, on approach to Triton in the movie, could appear so static over a period of three days.

I agree. But the fact that it isn't rendered as static is certainly suggestive of the fact that somebody actually considered the background.

Perhaps the video is synthesized, with the actual imagery mapped onto a sphere (which is very much in the area of specialization of the author), in which case the view may reflect a different path, or a time distorted path.

Chris Peterson wrote:

Nitpicker wrote:

Chris Peterson wrote:The video covers about ten days, three days before the encounter to seven days after. The star fields are not static; the field before the encounter is drifting at a different rate than the one seen after the encounter.

Well, that information contradicts the Wikipedia article on Voyager 2, which describes its curved path from very low over the North Pole of Neptune, towards Triton, over a period of less than 24 hours.

I don't think there's any contradiction. The rapid flyby was less than 24 hours. The video is made up of frames from well before to well after that.

Triton orbits Neptune with a period of about five days, I think. So, it is unclear to me how the background star field, on approach to Triton in the movie, could appear so static over a period of three days.

I agree. But the fact that it isn't rendered as static is certainly suggestive of the fact that somebody actually considered the background.

Perhaps the video is synthesized, with the actual imagery mapped onto a sphere (which is very much in the area of specialization of the author), in which case the view may reflect a different path, or a time distorted path.

First, although generally realistic in appearance, I'm convinced the video is not meant to be precise in stellar background content or dynamics during the flyby. In content I've been unable to make heads or tails out of the star background - I can't find any matching stars (searching large and small fields of view) even knowing the accurate coordinates of the receding moon. With that said, please read on.
Second, using Horizons, I plotted the absolute value, or magnitude, of the view drift over 10 days (240 hours) in 30-min increments. Edit:The gray bar represents star background partial invisibility because Triton' in the way (Note: the width of the bar is defined as when Triton just fills is ½ the video FoV in the vertical direction). I also chose 0 hours and 0° to be at closest approach.














The right-side plot shows that on approach, the star background (or FoV center) drifts 32° in ~70 hours, but after the flyby, the maximum drift amplitude is only 4° over the next 140 hours. This is exactly the general trend that is displayed in the video. It's the detailed background / FoV drift that's not shown. Now, as I see it, comes the creative part of the video. The image sequence appears based on the Narrow Angle camera on Voyager 2. The FoV of the camera is ~0.4° x 0.4°! Horizons data for Triton's angular size crudely, but definitively scales, the video FoV as being < 1°! The best resolution at closest approach is also consistent with the narrow camera field. So here's where the dynamics break down. I believe Paul has created a good illusion here. He as created the feel of a wide angle camera by "slowing" down the FoV view drift. Consider the post-flyby view drift of 4° on a 0.4° x 8° FoV. During the last 30 seconds or so of the video, the drift would be a dizzying 5 to 10 FoVs! Geck found 15° disorienting. An FoV <1° would be really distracting and the video would most likely not be appreciated.

I think the purpose of the star field is to enhance the visual experience, but Paul may have purposely scaled back the background drift rate to make the experience more comfortable / palatable. It's clear he followed the general dynamical trend. Also, the primary focus of the video is intended to be Triton - to enjoy Triton's carefully recreated realism. The video could have been done without stars. Maybe that would have been better, but my opinion is the stars are an added extra. Paul addressed them in a reasonable manner I think. It's ironic though that the stars ended up being a big focus. I think he did a good job at the pseudo star-background realism; it's not trivial to show or understand clearly what's going on.

Lastly, I'm normally pretty good at star field recognition, but maybe I missed it here, maybe due to the clever scaling of the background drift rate.
If anyone has identified specific members in the star field(s), I'd appreciate hearing about it.

geckzilla wrote:Here are a couple of animated gifs I made using Solar System Simulator with a coarse interval (10 minutes) clearly showing the background stars. The 15° fov was disorienting so I made a 90° too.
http://www.geckzilla.com/astro/voy2_tri ... _15fov.gif
http://www.geckzilla.com/astro/voy2_tri ... _90fov.gif

So Geck, What the Heck did you do to get flyby date into the Simulator? When the APOD was posted, I immediately went to the Simulator with great hope, but low and behold, I couldn't enter a date before 1990! Instead I entered Jan 1, 1990 to get a sense of the star field(s) but it was a no-go. Today, as a last resort, I clicked your link hoping I had an old link or something, but NOoooo, the same 1990 limit!!

Do you have any idea what's going on??

Wow, I don't quite have time at the moment, to absorb all the new info since my last post, but a quick thanks to geck, alter (because ego is like a last name) and Chris for their investigations so far.

And Chris, I don't know why I assumed the video must have started after Neptune was passed by, but I did. I now agree that there is no contradiction.

I really did focus mainly on Triton in the movie, but confess to being distracted by the limited, yet specific movement of the (re)synthesised background stars. Everyone's a critic!

Thanks Nosepicker for asking such an interesting question and getting everyone thinking!

alter-ego wrote:

geckzilla wrote:Here are a couple of animated gifs I made using Solar System Simulator with a coarse interval (10 minutes) clearly showing the background stars. The 15° fov was disorienting so I made a 90° too.
http://www.geckzilla.com/astro/voy2_tri ... _15fov.gif
http://www.geckzilla.com/astro/voy2_tri ... _90fov.gif

So Geck, What the Heck did you do to get flyby date into the Simulator? When the APOD was posted, I immediately went to the Simulator with great hope, but low and behold, I couldn't enter a date before 1990! Instead I entered Jan 1, 1990 to get a sense of the star field(s) but it was a no-go. Today, as a last resort, I clicked your link hoping I had an old link or something, but NOoooo, the same 1990 limit!!

Do you have any idea what's going on??

Set it to 1990, submit the form, and look at the URL. It's a long string of variables and values. Find 1990 and change it to 1989. I think there is a reason they don't let you go that far back... like it's less accurate, or something. But for this purpose it is accurate enough.

GhostScript wrote:Thanks Nosepicker for asking such an interesting question and getting everyone thinking!

Indeed. And say hi to Aunty Fruit and Uncle Bum, from me.

and all this time funny and scientist went together, like ice cold and beer !

alter-ego wrote:First, although generally realistic in appearance, I'm convinced the video is not meant to be precise in stellar background content or dynamics during the flyby. In content I've been unable to make heads or tails out of the star background - I can't find any matching stars (searching large and small fields of view) even knowing the accurate coordinates of the receding moon.

I've identified the star fields, and they are the same constellation - the Big Dipper

During the approach, you see four bright stars roughly horizontal, they are, left to right: Alcor, Mizar, Alioth and Megrez. The FoV ~17.3°
Post-flyby, is tricky, but the 3 bright stars are: Phad (upper left), Megrez (middle) and Alioth (right). The FoV ~17.3°

So how can that be?
The post-flyby FoV is created by translating the approach FoV by ~3.5°, the inverting it! Interestingly, Triton's receding position is within 3.5°of its approach position, but inverting the receding-Triton FoV, makes it virtually unrecognizable.

It is true the star fields have identifiable members but it's clear that the background is only for effect and more enjoyable experience.

Pretty nifty!

FYI, the actual approach constellation is Sagittarius, and the post-flyby constellation is Camelopardalis.