Starship Asterisk*

An Active Sun During a Total Eclipse

Explanation: Sometimes, a total eclipse of the Sun is an opportunity. Taking advantage of such, the above image shows the solar eclipse earlier this month as covered and uncovered by several different solar observatories. The innermost image shows the Sun in ultraviolet light as recorded over a few hours by the SWAP instrument aboard the PROBA2 mission in a sun-synchronous low Earth orbit. This image is surrounded by a ground-based eclipse image, reproduced in blue, taken from Gabon. Further out is a circularly blocked region used to artificially dim the central sun by the LASCO instrument aboard the Sun-orbiting SOHO spacecraft. The outermost image -- showing the outflowing solar corona -- was taken by LASCO ten minutes after the eclipse and shows an outflowing solar corona. Over the past few weeks, our Sun has been showing an unusually high amount of sunspots, CMEs, and flares -- activity that was generally expected as the Sun is currently going through Solar Maximum -- the busiest part of its 11 year solar cycle. The above resultant image is a picturesque montage of many solar layers at once that allows solar astronomers to better match up active areas on or near the Sun's surface with outflowing jets in the Sun's corona.

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That's rather neat It's almost like the sun has burned a hole in the moon and is showing through.

A most interesting and well timed composite. But, hmmm, you've got me thinking (and nitpicking) again ...

APOD Robot wrote:Sun-orbiting SOHO spacecraft

The Moon moves around the Sun, but is not Sun-orbiting. The Moon orbits the Earth.

Similarly, SOHO moves around the Sun, but is not Sun-orbiting. SOHO orbits the First Sun–Earth Lagrangian Point.

Here is a schematic of the orbit from the SOHO site:

Nitpicker wrote:The Moon moves around the Sun, but is not Sun-orbiting. The Moon orbits the Earth.

The Moon also orbits the Sun. If the Earth disappeared, the Moon would continue on an almost identical path with respect to the Sun as it is currently on.

Similarly, SOHO moves around the Sun, but is not Sun-orbiting. SOHO orbits the First Sun–Earth Lagrangian Point.

SOHO follows a complex path around the L1 point that has a 6-month period. It doesn't actually orbit that point, because there is nothing there to orbit. It would be better to say that SOHO is in a solar orbit that is perturbed by the Earth... not unlike the Moon's orbit around the Sun.

Chris Peterson wrote:The Moon also orbits the Sun. If the Earth disappeared, the Moon would continue on an almost identical path with respect to the Sun as it is currently on.

Touche. If the Earth disappears, so does the argument.

Chris Peterson wrote:SOHO follows a complex path around the L1 point that has a 6-month period. It doesn't actually orbit that point, because there is nothing there to orbit. It would be better to say that SOHO is in a solar orbit that is perturbed by the Earth... not unlike the Moon's orbit around the Sun.

Hmmm, I like your description, too, but the SOHO site says this:
http://sohowww.nascom.nasa.gov/about/orbit.html

SOHO moves around the Sun in step with the Earth, by slowly orbiting around the First Lagrangian Point (L1), where the combined gravity of the Earth and Sun keep SOHO in an orbit locked to the Earth-Sun line. The L1 point is approximately 1.5 million kilometers away from Earth (about four times the distance of the Moon), in the direction of the Sun.

Nitpicker wrote:Hmmm, I like your description, too, but the SOHO site says this:
http://sohowww.nascom.nasa.gov/about/orbit.html

SOHO moves around the Sun in step with the Earth, by slowly orbiting around the First Lagrangian Point (L1), where the combined gravity of the Earth and Sun keep SOHO in an orbit locked to the Earth-Sun line. The L1 point is approximately 1.5 million kilometers away from Earth (about four times the distance of the Moon), in the direction of the Sun.

Indeed. But they are using the term "orbit" a bit casually. Halo orbits aren't described by ordinary orbital dynamics, because they are essentially around a virtual mass, not a real one.

Well, a barycentre is often just a point in space, too. Perhaps we can agree that this is just another semantic argument over the word "orbit"? I'll concede if you will.

Nitpicker wrote:Well, a barycentre is often just a point in space, too. Perhaps we can agree that this is just another semantic argument over the word "orbit"? :blah: I'll concede if you will.

I'm not saying they are using orbit wrong, because it has different meanings. But since we generally think of orbits in terms of a small mass around a large one, it's easy to get confused by what a halo orbit really is. The standard equations for describing orbits don't describe anything around a barycenter, either, since the equations again assume a very large difference in mass between the two bodies, so the smaller one orbits around the center of the large one.

Anyway, whatever you want to call it, don't think it's remotely Keplerian. The path is non-planar, non-conic, and isn't around a central mass.

Nitpicker wrote:

Chris Peterson wrote:The Moon also orbits the Sun. If the Earth disappeared, the Moon would continue on an almost identical path with respect to the Sun as it is currently on.

Touche. If the Earth disappears, so does the argument.

Chris Peterson wrote:SOHO follows a complex path around the L1 point that has a 6-month period. It doesn't actually orbit that point, because there is nothing there to orbit. It would be better to say that SOHO is in a solar orbit that is perturbed by the Earth... not unlike the Moon's orbit around the Sun.

But if the Sun disappeared, both the Earth and the Moon (and all the other planets in the no longer solar system) would just go off on a tangent, right?

Is there any chance that the Earth-Moon system would get captured by Jupiter and end up in orbit around Jupiter?

Ann

Chris Peterson wrote:

Nitpicker wrote:Well, a barycentre is often just a point in space, too. Perhaps we can agree that this is just another semantic argument over the word "orbit"? I'll concede if you will.

I'm not saying they are using orbit wrong, because it has different meanings. But since we generally think of orbits in terms of a small mass around a large one, it's easy to get confused by what a halo orbit really is. The standard equations for describing orbits don't describe anything around a barycenter, either, since the equations again assume a very large difference in mass between the two bodies, so the smaller one orbits around the center of the large one.

Anyway, whatever you want to call it, don't think it's remotely Keplerian. The path is non-planar, non-conic, and isn't around a central mass.

I wasn't thinking about whether simple or more complex kinetic models are required to sufficiently describe any of these orbits. I was thinking about orbits in terms of their kinematics only. If C goes around B and B goes around A, it is more straightforward to say C orbits B, than to say C orbits A, though I concede the latter is still true.

Ann wrote:But if the Sun disappeared, both the Earth and the Moon (and all the other planets in the no longer solar system) would just go off on a tangent, right?

Is there any chance that the Earth-Moon system would get captured by Jupiter and end up in orbit around Jupiter?

It seems very, very unlikely. Earth would be flying outwards at about 30 km/s, Jupiter at about 13 km/s. They'd have to be in just the right positions when the Sun disappeared so that their trajectories intersected in exactly the right way... too close and Earth would slingshot, too far and it would exceed Jupiter's escape velocity. And of course, the two orbits aren't coplanar, so being in the right position is difficult, maybe even impossible.

Nitpicker wrote:I wasn't thinking about whether simple or more complex kinetic models are required to sufficiently describe any of these orbits. I was thinking about orbits in terms of their kinematics only. If C goes around B and B goes around A, it is more straightforward to say C orbits B, than to say C orbits A, though I concede the latter is still true.

Keep in mind that the halo orbit's maximum deviation from the L1 point is only about 0.1% of the solar orbit circumference. In other words, it would require a very accurate measurement to even detect that SOHO isn't in a solar orbit (although still not a Keplerian orbit, because at the Earth-Sun L1 point the orbital period is less than a year; that's why the L1 point can't support a stable orbit).

Chris Peterson wrote:

Ann wrote:
But if the Sun disappeared, both the Earth and the Moon (and all the other planets in the no longer solar system) would just go off on a tangent, right?

Is there any chance that the Earth-Moon system would get captured by Jupiter and end up in orbit around Jupiter?

It seems very, very unlikely. Earth would be flying outwards at about 30 km/s, Jupiter at about 13 km/s. They'd have to be in just the right positions when the Sun disappeared so that their trajectories intersected in exactly the right way... too close and Earth would slingshot, too far and it would exceed Jupiter's escape velocity. And of course, the two orbits aren't coplanar, so being in the right position is difficult, maybe even impossible.

The minimum relative velocity starts at ~17 km/s at a point where Jupiter's escape velocity (at ~4.2 AU) is just ~0.65 km/s. The Earth would have to hit Jupiter's atmosphere just right to lose all of this tremendous excess kinetic energy but without burning up entirely as a giant meteorite in the process. Unfortunately the first aerobraking event would be so severe that the Earth would shortly skip reentry into Jupiter permanently. (So the answer is no.)

However, as the Earth approaches the limb of Jupiter (at ~60 km/s) frictional forces on the near side of the Earth will spin the Earth so rapidly that large "blobs of liquid Earth" will fly off tangentially from the far side of the Earth and go into orbits with perijovian heights up to the diameter of the Earth above the surface.

APOD Robot wrote:Sun-orbiting SOHO spacecraft

That's a reasonable enough description for me. Sorry for doubting you, APOD Robot. <backflip emoticon>

Nitpicker wrote:

APOD Robot wrote:
Sun-orbiting SOHO spacecraft

That's a reasonable enough description for me. Sorry for doubting you, APOD Robot. <backflip emoticon>

It can only be attributable to human error. The 9000 series is the most reliable computer ever made. No 9000 computer has ever made a mistake or distorted information. They are all, by any practical definition of the words, foolproof and incapable of error.

Nomad must have been series 8999.

Nit, your picks are most entertaining, thank you.

neufer wrote:The minimum relative velocity starts at ~17 km/s at a point where Jupiter's escape velocity (at ~4.2 AU) is just ~0.65 km/s. The Earth would have to hit Jupiter's atmosphere just right to lose all of this tremendous excess kinetic energy but without burning up entirely as a giant meteorite in the process. Unfortunately the first aerobraking event would be so severe that the Earth would shortly skip reentry into Jupiter permanently. (So the answer is no.)

Thanks. I was carrying on this discussion in the middle of the night while running an imaging session. I kind of thought I could count on you to run the actual numbers.

I expect, however, that there are theoretically possible scenarios for an Earth capture, if we include additional interactions with other gas giants.

Chris Peterson wrote:
I expect, however, that there are theoretically possible scenarios for an Earth capture, if we include additional interactions with other gas giants.

That would be a challenge

Captured asteroids that became moons probably almost all relied on the Sun (or at least Jupiter) being their third body and that's not an option in this case.

Neufer wrote:

Nitpicker wrote:

APOD Robot wrote:
Sun-orbiting SOHO spacecraft

That's a reasonable enough description for me. Sorry for doubting you, APOD Robot. <backflip emoticon>

It can only be attributable to human error. The 9000 series is the most reliable computer ever made. No 9000 computer has ever made a mistake or distorted information. They are all, by any practical definition of the words, foolproof and incapable of error.

Beyond wrote:
Nomad must have been series 8999.

One of the twin(prime)s: 8999 & 9001.

neufer wrote:

Chris Peterson wrote:
I expect, however, that there are theoretically possible scenarios for an Earth capture, if we include additional interactions with other gas giants.

That would be a challenge :!:

I think that's an understatement.

Great, now there are runaway Earths everywhere in my Universe Sandbox simulation thanks to you two. And a lot of crash sites in Jupiter's cloud tops.

Is that a comet just to the right of the solar mask?

Billsey wrote:Is that a comet just to the right of the solar mask?

Looks to me like a cosmic ray hit on the detector.