APOD: Sky Full of Arcs (2024 Aug 17)

[APOD Robot]

Sky Full of Arcs

Explanation: On August 11 a Rocket Lab Electron rocket launched from a rotating planet. With a small satellite on board its mission was dubbed A Sky Full of SARs</a> (Synthetic Aperture Radar satellites), departing for low Earth orbit from Mahia Peninsula on New Zealand's north island. The fiery trace of the Electron's graceful launch arc is toward the east in this southern sea and skyscape, a composite of 50 consecutive frames taken over 2.5 hours. Fixed to a tripod, the camera was pointing directly at the South Celestial Pole, the extension of planet Earth's axis of rotation in to space. But no bright star marks that location in the southern hemisphere's night sky. Still, the South Celestial Pole is easy to spot. It lies at the center of the concentric star trail arcs that fill the skyward field of view.

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[Ann]

Turn, turn, turn! Yes, the Earth turns, turns, turns on its axis! Which is why the sky seems to turn, turn, turn all around us. So if you point a camera at the night sky and don't compensate for the Earth's rotation, you will get a picture of star trails.

Sky Full of Arcs
Image Credit & Copyright: Rory Gannaway

You will have to excuse me for not paying attention to the satellite launch trail in the APOD. As for star trails images, I think they are beautiful, but also very frustrating. That's because I can almost never identify any of the stars in them. With one exception, of course - Polaris! Well, if we are looking at the north celestial pole, of course:

Star trails near the north celestial pole. The bright little Polaris arc right next to the north celestial pole stands out like a sore thumb. Credit: Fabrizio Melandri

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Polaris is clearly not in the APOD, so we are dealing with the south celestial pole. But if so, we should have a good chance of seeing the Magellanic Clouds, yet they are not in the picture.

This ESO image makes it easier to make sense of the star trails near the southern pole:

Star trails over Atacama Desert Cacti. Credit: ESO/B. Tafreshi

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Larger sizes of the image are here.

You can see the Large Magellanic Cloud at upper right and the Small Magellanic Cloud at right in the ESO image, near 3 o'clock. They are not in the APOD. But to the lower left of the Small Magellanic Cloud in the ESO image is a blue-looking star which is Beta Hydrus. This star, which is spectral class G2IV and only a tiny bit bluer than the Sun, is located at latitude -77 degrees, so it is a southerly star indeed. Could this be the star whose trail is seen in the APOD near the edge of the image at upper left (seemingly being crossed by the trail of the satellite)?

Ann

[florid_snow]

I always appreciate your posts Ann, and I also love playing "what's that star"! We can figure it out with some free planetarium software like Stellarium and the launch location and time. Looking at Rocketlab's press release, we also get an idea why maybe this photographer was taking a two hour star trail photo: "The Electron lifted off from Pad B at Rocket Lab’s Launch Complex 1 in New Zealand at 9:18 a.m. Eastern after a two-hour delay caused by ground winds." Silly that they gave US time, but that is about 1 a.m local time, so the photo time is likely centered around local midnight. The launch location is 39 S 178 E.

Plugging that in, we see that the bright blue star on the left of the image that the rocket trail bisects is likely to be Achenar. That means Canopus is the brightest star at the 7 o'clock position, and alpha and beta Centauri are near the 3 to 4 o'clock position, just barely not touching the rocket trail.

All that said, beta Hydrus is just so very close to the celestial pole. I think it's visible here at about the 10 o'clock position, but very close to the pole, not touching the rocket trail. So what is the one you're talking about that is intersected by the rocket trail at about the 11 o'clock position? The other stars in this region are the lovely blue and gold contrasting alpha and beta Toucan. I think the rocket trail goes through beta Toucan and then Achenar. What do you think?

Always fun to play "what's that star"! Have a great weekend everyone

[johnnydeep]

So, can I assume that the trail of the Electron rocket was made by a separate set of exposures? It can't just be there over the 2.5 hours of the 50 exposures to make the star trails, can it?

[johnnydeep]

I was going to joke about how this "Electron" rocket is powered solely by batteries creating an electron stream jet, and while that's not strictly true, electrons DO figure prominently it its propulsion:

Design
Electron uses two stages with the same diameter (1.2 m (3 ft 11 in)) filled with RP-1/LOX propellant. The main body of the rocket is constructed using a lightweight carbon composite material.[24]

Both stages use the Rutherford rocket engine, the first electric-pump-fed engine to power an orbital rocket.[17] The electric pumps are powered by lithium-polymer batteries. The second stage uses three batteries which are "hot swapped", two of the batteries are jettisoned once depleted to shed mass.[25] There are nine Rutherford engines on the first stage and one vacuum-optimized version on the second stage.[26][27][28] The first stage engines deliver 162 kN (36,000 lbf) of thrust and the second stage delivers 22 kN (4,900 lbf) of thrust. Almost all of the engines' parts are 3D printed to save time and money in the manufacturing process.[17][24]

Electric-feed rocket cycle. The oxidizer and fuel are fed to the pump which increases the pressure before injecting it into the combustion chamber. The pumps are actuated by an electric motor powered by batteries. An inverter converts the batteries' DC electricity to the AC needed by the motor. The fuel is also circulated around the outside of the combustion chamber and nozzle to prevent it from overheating. - The electric-pump-fed engine is a bipropellant rocket engine in which the fuel pumps are electrically powered, and so all of the input propellant is directly burned in the main combustion chamber, and none is diverted to drive the pumps. This differs from traditional rocket engine designs, in which the pumps are driven by a portion of the input propellants.

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[florid_snow]

So, can I assume that the trail of the Electron rocket was made by a separate set of exposures? It can't just be there over the 2.5 hours of the 50 exposures to make the star trails, can it?

Yes, it probably is. I mean, it could be done either way. But practically, with wide launch windows, the exact time of a rocket launch is so uncertain, easy for them to delay 10 minutes, or 2 hours in this case. So imagine you are the photographer, hiked out to a semi-rural location, get the shot set up at the previously published time, you start taking 3-minute long exposures. But the rocket doesn't launch. You can't get service on your phone. What to do? Just keep taking 3-minute exposures so that you don't miss it. And voila, 2.5 hours later, you've got 49 nice frames for a star trail photo, and 1 frame that includes the launch.

Trying to photograph lightning strikes is a similar experience. But when you get a good one, it's amazing.

[johnnydeep]

So, can I assume that the trail of the Electron rocket was made by a separate set of exposures? It can't just be there over the 2.5 hours of the 50 exposures to make the star trails, can it?

Yes, it probably is. I mean, it could be done either way. But practically, with wide launch windows, the exact time of a rocket launch is so uncertain, easy for them to delay 10 minutes, or 2 hours in this case. So imagine you are the photographer, hiked out to a semi-rural location, get the shot set up at the previously published time, you start taking 3-minute long exposures. But the rocket doesn't launch. You can't get service on your phone. What to do? Just keep taking 3-minute exposures so that you don't miss it. And voila, 2.5 hours later, you've got 49 nice frames for a star trail photo, and 1 frame that includes the launch.

Trying to photograph lightning strikes is a similar experience. But when you get a good one, it's amazing.

Ok, thanks. That makes some sense.