APOD: Perihelion Approaches (2015 Aug 15)

[APOD Robot]

Perihelion Approaches

Explanation: This dramatic outburst from the nucleus of Comet 67P/Churyumov-Gerasimenko occured on August 12, just hours before perihelion, its closest approach to the Sun. Completing an orbit of the Sun once every 6.45 years, perihelion distance for this periodic comet is about 1.3 astronomical units (AU), still outside the orbit of planet Earth (at 1 AU). The stark image of the 4 kilometer wide, double-lobed nucleus in bright sunlight and dark shadows was taken by the Rosetta spacecraft's science camera about 325 kilometers away. Too close to see the comet's growing tail, Rosetta maintains its ringside seat to watch the nucleus warm and become more active in coming weeks, as primordial ices sublimating from the surface produce jets of gas and dust. Of course, dust from the nucleus of periodic comet Swift-Tuttle, whose last perihelion passage was in 1992 at a distance of 0.96 AU, fell to Earth just this week.

<< Previous APOD

This Day in APOD

Next APOD >>

[/b]

[Dad is watching]

Any ideas on the surface temperatures? Similar to what is seen on the moon?

[Boomer12k]

Dang...that is impressive...and soooooo cool to see it action!

:---[===] *

[Chris Peterson]

Any ideas on the surface temperatures? Similar to what is seen on the moon?

It is about -70°C - much colder than the sunlit Moon, but warmer than an icy body ought to be at 1.3 AU, probably because of rocks and dust on top of the surface ice.

[Dad is watching]

Any ideas on the surface temperatures? Similar to what is seen on the moon?

It is about -70°C - much colder than the sunlit Moon, but warmer than an icy body ought to be at 1.3 AU, probably because of rocks and dust on top of the surface ice.

Assuming that the dark side of the BBQ role is very much colder, and the possibility of reflected light and heat on the sun-ward side is minimal, there is little or no chance for the landers batteries to warm up enough (even at closest approach) to improve their electrical efficiency? It would have made more sense (in hind sight) to go with a nuclear power plant, wouldn't it?

[Chris Peterson]

Any ideas on the surface temperatures? Similar to what is seen on the moon?

It is about -70°C - much colder than the sunlit Moon, but warmer than an icy body ought to be at 1.3 AU, probably because of rocks and dust on top of the surface ice.

Assuming that the dark side of the BBQ role is very much colder, and the possibility of reflected light and heat on the sun-ward side is minimal, there is little or no chance for the landers batteries to warm up enough (even at closest approach) to improve their electrical efficiency? It would have made more sense (in hind sight) to go with a nuclear power plant, wouldn't it?

I don't think the problem is necessarily the temperature of the batteries, but the orientation of the solar panels.

Solar was the right choice. RTGs are expensive and heavy. Philae was a secondary component to the entire mission. Had it been specified to utilize a RTG, it would have completely changed the dynamics and economics of the mission- very possibly to something that wouldn't have been funded. And most of the risks would have still been there (the uncertainty in the whole landing profile).

[Aristaeus]

Did dust from the nucleus of periodic comet Swift-Tuttle actually fall to the earth or only into Earth's atmosphere?

[Chris Peterson]

:? Did dust from the nucleus of periodic comet Swift-Tuttle actually fall to the earth or only into Earth's atmosphere?

All meteor showers result in meteoritic dust that eventually falls to Earth. Tons per year. But the velocity, mass, and composition of the Perseid meteoroids is such that everything is burned up in the atmosphere (that is, converted to gas and dust). No meteorites survive to the ground.

[ta152h0]

that activity should dislodge Phylae ?

[Chris Peterson]

that activity should dislodge Phylae ?

Unlikely. Possibly outside of the occasional very active jet, most outgassing is a fairly gentle process. The density of the gas would still qualify as a hard vacuum, and the volume of high mass particles is small compared with the surface area.

[captainwiggins48]

The photo is overexposed so details are lost. Can they even reduce the aperture of Rosetta's camera lens? Remembering the extreme brilliance of Comet West (1975-76) whose nucleus was clearly visible in broad daylight, this should have been anticipated. Something other than mere reflected sunlight must be at play here.

[Chris Peterson]

The photo is overexposed so details are lost. Can they even reduce the aperture of Rosetta's camera lens? Remembering the extreme brilliance of Comet West (1975-76) whose nucleus was clearly visible in broad daylight, this should have been anticipated. Something other than mere reflected sunlight must be at play here.

You normally never want to reduce aperture, because that results in a loss of resolution. The Rosetta camera changes its exposure time to accommodate different conditions. This image was exposed to show the gas and dust ejection, which being much dimmer than the surface, resulted in an overexposed nucleus. Had the exposure been designed for the comet surface, we'd miss most of the detail in the jet.

The nucleus of Comet West was not visible. It was far too small to see as anything other than a point, and it was blocked from view by the coma, which was bright because it was tens of thousands of kilometers across, and was seen in forward scattered light. That same coma would have been nearly invisible from the surface of the comet itself.

[Ann]

In the Recent Submissions forum, there is an image of Comet 67P by Damian Peach. Interestingly, even though some background stars are clearly colored, the comet itself is pure white.

Does that mean that the comet has a dust tail but not a gas tail, and that it lacks a coma?

Ann

[Chris Peterson]

In the Recent Submissions forum, there is an image of Comet 67P by Damian Peach. Interestingly, even though some background stars are clearly colored, the comet itself is pure white.

Does that mean that the comet has a dust tail but not a gas tail, and that it lacks a coma?

All we see in this image is the coma and the dust tail. So obviously, there's a coma! We don't typically see the gaseous part of the coma, because gas is largely invisible. What we see is the dust. And both the coma and dust tail of any comet is basically white. Depending on the angle we see it with respect to the Sun, there may be a little wavelength dependent scattering that can shift that towards blue or yellow (which is often accentuated by processing).

67P certainly has an ion tail (which is what I assume you mean by "gas tail"). It's simply not bright enough to be seen in this exposure.

[Guest]

I would read the explanations for the photographs if the first sentence was a simple explanation without scientific terms.

I don't read any of the explanations.

[geckzilla]

I would read the explanations for the photographs if the first sentence was a simple explanation without scientific terms.

I don't read any of the explanations.

Which words are you having problems with?

[hoohaw]

Much as I love APOD, the caption writer is not a newpaperPerson: who, when, when, where, why, ALL in the first sentence or two. Instead, usually we have to wait until the end of the lengthy caption before we are told what we are seeing. Not good writing; sorry! (I do love APOD).

[geckzilla]

Much as I love APOD, the caption writer is not a newpaperPerson: who, when, when, where, why, ALL in the first sentence or two. Instead, usually we have to wait until the end of the lengthy caption before we are told what we are seeing. Not good writing; sorry! (I do love APOD).

I don't really like that method of writing. I especially despise the attention-grabbing headlines that are written by editors and often completely misrepresent the writer's views even expressed within the article itself. I'm surprised that you think a single paragraph is lengthy.

[saturno2]

Strange image

[MarkBour]

About the only thing that makes sense in the "writing criticism" comments to me, is that I find it hard to say the name "Comet 67P/Churyumov-Gerasimenko", let alone remember it correctly. Other than that, which cannot be avoided, I don't understand the objections.

Anyway, I have been hoping to see more of old 67PCG in this forum. This image is very satisfying. I'm wondering if there is any good source for a time-lapse of images over the last months? I looked at a couple of the mission links, and so far I only see about 1 image a week. Surely a lot more has been downloaded somewhere (?)

[John644]

I am curious to the question, if anyone has considered, using a comet, such as this one, as a vehicle to deliver deep space probes for exploration? I was reading the article, and that this comet has a 6 year turn around. In my mind, that would be a fantastic system booster to use. I think it would be extremely cost efficient for a transport. We've landed a probe on a comet, so the technology is there for the first part. We would need a means to maintain a line of communication, for system start ups, when a comet has reached it's outbound peak in order to send the system on it's continued journey for further exploration.
Wondering,
John

[Chris Peterson]

I am curious to the question, if anyone has considered, using a comet, such as this one, as a vehicle to deliver deep space probes for exploration?

It's an interesting idea. But keep in mind that the challenge of getting from the Earth to another place in the Solar System isn't the distance traveled, but the energy required to match an orbit. The more eccentric a comet's orbit, the greater differential velocity it will exhibit with respect to Earth while it is at Earth's distance from the Sun. So the sort of body that would be desirable to use as a vehicle is also the sort that will be hard to land on while it is near the Earth.

This isn't to say it can't be done, just that there are difficulties that aren't necessarily obvious on first examination.

[neufer]

I am curious to the question, if anyone has considered, using a comet, such as this one, as a vehicle to deliver deep space probes for exploration?

It's an interesting idea. But keep in mind that the challenge of getting from the Earth to another place in the Solar System isn't the distance traveled, but the energy required to match an orbit. The more eccentric a comet's orbit, the greater differential velocity it will exhibit with respect to Earth while it is at Earth's distance from the Sun. So the sort of body that would be desirable to use as a vehicle is also the sort that will be hard to land on while it is near the Earth.

Comets & asteroids might provide good spaceships for humans to travel into space & return while providing them water, shelter, radiation protection and (most importantly) something to do to pass the time (e.g., skating & sleigh sailing). This is especially important for human travel to inner space near the Sun.

<<2000 BD₁₉ is [a ~1km diameter] asteroid with the smallest perihelion of any numbered asteroid (0.092 AU—38% of Mercury's orbital radius). With its high eccentricity, not only does 2000 BD19 get very close to the Sun, but it also travels relatively far away from it. It is one of a small group of Aten asteroids that is also a Mars grazer. Its orbital elements indicate that may be an extinct comet.

It is estimated that 2000 BD₁₉'s surface temperature reaches ~920 K at perihelion, enough to melt lead and zinc, and nearly enough to melt aluminium. 2000 BD19 is considered a good candidate for measuring the effects of Albert Einstein's general theory of relativity because of how close it comes to the Sun.>>

• . https://en.wikipedia.org/wiki/Comet_Encke

---

Off on a Comet (French: Hector Servadac) is an 1877 science fiction novel by Jules Verne.

<<The story starts with a comet called Gallia, that touches the Earth in its flight and collects a few small chunks of it. The disaster occurred on January 1 of the year 188x in the area around Gibraltar. On the territory that was carried away by the comet there remained a total of thirty-six people of French, English, Spanish and Russian nationality. These people did not realize at first what had happened, and considered the collision an earthquake. They first noticed weight loss: Captain Servadac's adjutant Ben Zoof to his amazement, jumped twelve meters high. Zoof with Servadac also soon noticed that the alternation of day and night is shortened to six hours, that east and west changed sides, and that water begins to boil at 66 degrees Celsius, from which they rightly deduced that atmosphere became thinner and pressure dropped. At the beginning of their stay in Gallia they noticed the Earth with the Moon, but thought it was an unknown planet. Other important information was obtained through their research expedition with a ship, which the comet also took. During the voyage they discovered a mountain chain blocking the sea, which they initially considered to be the Mediterranean Sea and then they found the island of Formentera (before the catastrophe a part of the Balearic Islands), where they found a French astronomer Palmyrin Rosette, who helped them to solve all the mysterious phenomena. They were all on the comet which was discovered by Rosette a year ago and predicted a collision course with Earth, but no one believed the astronomer, because a layer of thick fog at the time prevented astronomical observations in other places.

As found by a new research expedition, the circumference of Gallia was 2320 km.

Involuntary travelers through the Solar system did not have any hope for long-term colonization of their new world, because they were lacking arable land. They ate mainly the animals that were left on the land carried away by Gallia. One strange phenomenon, they met was that the sea on the comet did not freeze, even though the temperature dropped below the freezing point (theory that the stationary water level resists freezing level for longer than when a rippled by wind). Once a stone was thrown into the sea, the sea froze in a few moments. The ice was completely smooth and allowed skating and sleigh sailing.

Gallia got to an extreme point of its orbit and then began its return to Earth. In early November Rossete's refined calculations showed that there will be a new collision with the Earth, exactly two years after the first, again on January 1. Therefore, the idea appeared to leave the comet collision in a balloon. The proposal was approved and the castaways made a balloon out of the sails of their ship. In mid-December there was an earthquake, in which Gallia partially fell apart and lost a fragment, which probably killed all Englishmen in Ceuta and Gibraltar. When on January 1 there was again a contact between the atmospheres of Gallia and Earth, the space castaways left in the balloon and landed safely two kilometers from Mostaganem in Algeria.>>

[bystander]

I am curious to the question, if anyone has considered, using a comet, such as this one, as a vehicle to deliver deep space probes for exploration? ...

http://asterisk.apod.com/viewtopic.php?f=31&t=35122