Starship Asterisk*

Chris Peterson wrote:At a dark site, the gegenschein isn't all that rare. I see it several times a year, and could see it more often if I made the effort.

Nitpicker wrote:
The brighter portion near the antisolar point (a possible concentration of particles at the L2 Lagrangian point, so I've just read) is still dimmer than the light pollution in my suburban skies.

• The L2 Lagrangian point is not dynamically stable so it is not clear why dust would necessarily collect there.

http://history.nasa.gov/SP-349/ch6.htm wrote:

[b][color=#0000FF]The Gegenschein cannot be associated with Earth because seen from Pioneer and seen from Earth it is in different directions in space. [/color][/b]

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<<Pioneer 10 produced new information about the Zodiacal Light, the faint band of light along the Zodiac, believed to be an effect of sunlight reflected from particles in interplanetary space. The slight enhancement of the glow exactly opposite to the Sun in the sky - the Gegenschein - could be caused by distant particles illuminated like miniature full moons opposite the Sun, or by a stream of particles extending as a comet's tail from Earth.

The imaging photopolarimeter was turned on March 10, 1972, seven days after Pioneer 10 was launched. During the first few weeks of the mission, when the Sun angle was about 26 degrees from the spacecraft's spin axis, only that part of the sky more than 60 degrees from the Sun line could be inspected by the imaging photopolarimeter. So observations concentrated on the Gegenschein. It quickly became apparent that the Gegenschein could not be associated with Earth because, although the spacecraft had not moved much farther from the Sun, it had moved ahead along the orbit of the Earth. While the direction of the Gegenschein was directly away from the Sun as seen from the spacecraft, this direction was by this time different from the direction of the Gegenschein from Earth. So the anti-solar glow was confirmed as being associated with the light reflected from particles spread around the Solar System, not from particles associated with the Earth itself. Experimenters later measured the faint glow of the Gegenschein to near the orbit of Mars, again confirming its interplanetary nature.

As Pioneer 10 moved away from the Sun, the brightness of the Gegenschein also decreased, thereby showing that it results from particles in the inner Solar System. There was, however, a decrease in the rate at which the brightness faded within the asteroid belt, which indicates that the particles responsible for the counterglow increase somewhat within the belt. But beyond the belt there is virtually no Gegenschein.

As Pioneer moved out from Earth, it became possible to start mapping the whole of the sky to look at the Zodiacal Light. Scientists found that the Zodiacal Light also decreases in brightness as the square of the distance from the Sun. The rate of decrease slowed within the asteroid belt, indicating that particles responsible for the Zodiacal Light, although concentrated in the inner Solar System, also increase somewhat within the asteroid belt itself. But beyond three and a half times Earth's distance from the Sun, the Zodiacal Light is negligible, and experimenters were able to record the integrated starlight from the Galaxy tree of the Zodiacal Light for the first time. Since the Zodiacal Light's brightness, at 2.41 times Earth's distance from the Sun, is less than one tenth that at Earth's orbit, experimenters conclude that the asteroid belt beyond this distance does not contribute significantly to the Zodiacal Light as seen from Earth. Zodiacal Light brightness fades almost completely at a distance of 3.3 times Earth's distance from the Sun. This is where particles would have to circle the Sun in orbits having a period of half that of Jupiter. Jupiter's gravity appears to sweep the Solar System clear of such particles beyond this resonance orbit. Thus, there is virtually no Zodiacal Light nor Gegenschein beyond the asteroid belt.>>

neufer wrote:

Nitpicker wrote:
The brighter portion near the antisolar point (a possible concentration of particles at the L2 Lagrangian point, so I've just read) is still dimmer than the light pollution in my suburban skies.

• The L2 Lagrangian point is not dynamically stable so it is not clear why dust would necessarily collect there.

Nitpicker wrote:

Chris Peterson wrote:At a dark site, the gegenschein isn't all that rare. I see it several times a year, and could see it more often if I made the effort.

How rare is rare? A dark site is rare to most people. And several times a year at a dark site doesn't exactly sound common. The brighter portion near the antisolar point (a possible concentration of particles at the L2 Lagrangian point, so I've just read) is still dimmer than the light pollution in my suburban skies.

Fernangarc54 wrote:I pose an open question: Can we assume that Gegenschein is a visual reflection of what would have been our modest planetary ring?

tdurow3 wrote:
I took a picture of the glory mentioned in today's APOD back in 2011.

http://en.wikipedia.org/wiki/Opposition_surge wrote:

[b][color=#0000FF]The opposition effect brightens the area around Buzz Aldrin's shadow due to retroreflective properties of lunar regolith. Buratti et al. have suggested that its brightness increases by some 40% between a phase angle of 4° and one of 0°, and that this increase is greater for the rougher-surfaced The principal cause of the lunar opposition surge is shadow-hiding rather than coherent backscatter.[/color][/b]

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<<The opposition surge (sometimes known as the opposition effect, opposition spike or Seeliger effect) is the brightening of a rough surface, or an object with many particles, when illuminated from directly behind the observer. The term is most widely used in astronomy, where generally it refers to the sudden noticeable increase in the brightness of a celestial body such as a planet, moon, or comet as its phase angle of observation approaches zero. It is so named because the reflected light from the Moon and Mars appear significantly brighter than predicted when at astronomical opposition.

Two physical mechanisms have been proposed for this observational phenomenon: shadow hiding and coherent backscatter.

The phase angle is defined as the angle between the observer, the observed object and the source of light. In the case of the solar system, the light source is the Sun, and the observer is situated on Earth. As the phase angle of an object lit by the sun decreases, the object's brightness rapidly increases. This is mainly due to the increased area lit, but is also partly due to the intrinsic brightness of the part that is sunlit. This is affected by such factors as the angle at which light reflected from the object is observed. For this reason, a full moon is more than twice as bright as the moon at first or third quarter, even though the visible area illuminated appears to be exactly twice as large.

When the angle of reflection is close to the angle at which the light's rays hit the surface (that is, when the sun and the object are close to opposition from the viewpoint of the observer), this intrinsic brightness is usually close to its maximum. At a phase angle of zero degrees, these shadow areas become negligible. The celestial body in effect becomes an imperfect mirror. When phase angles approach zero, there is a sudden increase in apparent brightness, and this sudden increase is referred to as the opposition surge.

The effect is particularly pronounced on regolith surfaces of airless bodies in the solar system. The usual major cause of the effect is that a surface's small pores and pits that would otherwise be in shadow at other incidence angles become lit up when the observer is almost in the same line as the source of illumination. The effect is usually only visible for a very small range of phase angles near zero. For bodies whose reflectance properties have been quantitatively studied, details of the opposition effect — its strength and angular extent — are described by two of the Hapke parameters. In the case of planetary rings (such as Saturn's), an opposition surge is due to the covering of shadows on the ring particles. This explanation was first proposed by Hugo von Seeliger in 1887.

An alternate theory for the increase in brightness during opposition is that of coherent backscatter. In the case of coherent backscatter, the reflected light is enhanced at narrow angles if the size of the scatterers in the surface of the body is comparable to the wavelength of light and the distance between scattering particles is greater than a wavelength. The increase in brightness is due to the reflected light combining coherently with the emitted light. Coherent backscatter phenomena have also been observed with radar. In particular, recent observations of Titan at 2.2 cm with Cassini have shown that a strong coherent backscatter effect is required to explain the high albedos at radar wavelengths.

The existence of the opposition surge was first recorded in 1956 by Tom Gehrels during his study of the reflected light from an asteroid. Gehrels' later studies showed that the same effect could be shown in the moon's brightness. He coined the term "opposition effect" for the phenomenon, but the more intuitive "opposition surge" is now more widely used. Since Gehrels' early studies, an opposition surge has been noted for most airless solar system bodies. No such surge has been reported for gas giant, nor for bodies with pronounced atmospheres.>>

Nitpicker wrote:Beautiful. I've just learnt a new word, too: gegenschein. But I think the notable bright stars in the background are Rigel and Betelgeuse.

astromandan wrote:If we're sticking with German, I believe the word for "the glory" is "heiligenschein"

Ann wrote:What a lovely picture!

Note the big round emission nebula between Betelgeuse and Bellatrix at about three o'clock. That's the Lambda Orionis nebula.


Ann

Chris Peterson wrote:

Nitpicker wrote:

Chris Peterson wrote:At a dark site, the gegenschein isn't all that rare. I see it several times a year, and could see it more often if I made the effort.

How rare is rare? A dark site is rare to most people. And several times a year at a dark site doesn't exactly sound common. The brighter portion near the antisolar point (a possible concentration of particles at the L2 Lagrangian point, so I've just read) is still dimmer than the light pollution in my suburban skies.

Well, rare in this case is not in the slightest. The gegenschein is always there. So it's just a question of being in a dark site with good weather, not having a Moon, and looking. The current wording is much better- it is seldom discerned because most people don't have good observing conditions.

An observation made several times a year, for most astronomical phenomena, is not rare. I only see a full Moon a few times a year. Is a full Moon rare? I only see Jupiter at opposition once a year. Is that a rare event?

In fact, a rarely discernable faint glow known as the gegenschein ... can be seen 180 degrees around from the Sun in an extremely dark sky.