APOD: So Long & Thanks for All the Fish (2009 Nov 23)

[neufer]

• Starbuck: It is an evil voyage, I tell thee. If Ahab has his way,
  neither thee nor me, nor any member of this ship's company will ever see home again.

http://apod.nasa.gov/apod/ap091123.html

Rosetta is a European Space Agency-led robotic spacecraft mission launched in 2004, intended to study the comet 67P/Churyumov-Gerasimenko. Rosetta consists of two main elements: the Rosetta space probe and the Philae lander. The spacecraft will also flyby and examine two asteroids on its way to the comet.

http://antwrp.gsfc.nasa.gov/apod/ap080908.html

In May 2014, the Rosetta craft will enter a slow orbit around the comet and gradually slow down in preparation for releasing a lander that will make contact with the comet itself. The lander, named "Philae", will approach Churyumov-Gerasimenko at relative speed around 1 m/s and on contact with the surface, two harpoons will be fired into the comet to prevent the lander from bouncing off. Additional drills are used to further secure the lander on the comet.

Once attached to the comet, expected to take place in November 2014, the lander will begin its science mission:

• * Characterisation of the nucleus
  * Determination of the chemical compounds present, including enantiomers
  * Study of comet activities and developments over time

The exact surface layout of the comet is currently unknown and the orbiter has been built to map this before detaching the lander. It is anticipated that a suitable landing site can be found, although few specific details exist regarding the surface.>>

<<Philae (previously known as RoLand) is the lander that accompanies the Rosetta spacecraft. It is designed to land on Comet 67P/Churyumov-Gerasimenko shortly after arrival. The lander is named after Philae island in the Nile, where an obelisk was found that was used to translate the Rosetta Stone. The lander is designed to touch down on the comet’s surface after detaching itself from the main spacecraft body and “falling” towards the comet along a ballistic trajectory. It also will deploy harpoons to anchor itself to the surface, and the legs are designed to dampen the initial impact to avoid bouncing. Communications with Earth will use the orbiter spacecraft as a relay station to reduce the electrical power needed. The mission duration on the surface is planned to be at least one week, but an extended mission lasting months is possible. The main structure of the lander is made from carbon fiber, shaped into a plate maintaining mechanical stability, a platform for the science instruments, and a hexagonal “sandwich” to connect all the parts. The total weight is about 100 kg. Its “hood” is covered with solar cells for power generation.>>

[geckzilla]

The APOD description states that the comet's diameter is 15 km but the Wikipedia article says it's 4 km. I wonder which is correct?

[bystander]

The JPL Small Body Database and the ESA Rosetta site both have it at 4 km. Hubble measured it at 5x3 km.

[neufer]

The JPL Small Body Database and the ESA Rosetta site both have it at 4 km. Hubble measured it at 5x3 km.

Churyumov-Gerasimenko has a rather interesting orbital history. Comets are regularly nudged from one orbit to another when they encounter Jupiter or Saturn in close proximity. For this comet it was calculated, that before the year 1840 it was completely unobservable due to its perihelion distance of about 4.0 AU. At this time Jupiter shifted that distance to about 3.0 AU. Later on, in the year 1959, another encounter with Jupiter pushed it to about 1.28 AU, where it is now.

A seal from the Indus Valley Civilization recovered at Dholavira in India.

<<[In his book Comet, Carl Sagan] reproduces an ancient Chinese manuscript (the Book of Silk) that shows comet tail varieties: most are variations on simple comet tails, but the last shows the comet nucleus with four bent arms extending from it, recalling a swastika. Sagan suggests that in antiquity a comet could have approached so close to Earth that the jets of gas streaming from it, bent by the comet's rotation, became visible, leading to the adoption of the swastika as a symbol across the world.>>

[gigi]

What a beautiful crescent photo of Planet Earth. However, I am a little confused by the location of the crescent. Doesn't the sun generally move east to west? If Antarctica is lit and the rest is in darkness, it appears as thought the earth is rotating south to north or north to south and that can be right! What am I missing?

http://apod.nasa.gov/apod/ap091123.html

[Chris Peterson]

What a beautiful crescent photo of Planet Earth. However, I am a little confused by the location of the crescent. Doesn't the sun generally move east to west? If Antarctica is lit and the rest is in darkness, it appears as thought the earth is rotating south to north or north to south and that can be right! What am I missing?

The Earth spins west to east, but its spin axis is tilted with respect to its orbital axis. When this image was taken, it was approaching summer in the southern hemisphere- that is, the Sun during the day was well south of the equator. So southern polar latitudes were lit for the better part of a full 24-hour day. The image was taken from the night side of Earth, which is why we only see a small part that is in daylight. But as always, half the planet is lit, and half is not, and the sun is generally seen to rise in the east and set in the west (although not exactly so by a fair margin).

[DavidLeodis]

I agree. It is a great and fascinating image.

I soon though gave up trying to find out what the UPD/LAM/IAA/RSSD/INTA/UPM/DASP/IDA stand for in the credit. but still :)ing.

[gigi]

What a beautiful crescent photo of Planet Earth. However, I am a little confused by the location of the crescent. Doesn't the sun generally move east to west? If Antarctica is lit and the rest is in darkness, it appears as thought the earth is rotating south to north or north to south and that can be right! What am I missing?

The Earth spins west to east, but its spin axis is tilted with respect to its orbital axis. When this image was taken, it was approaching summer in the southern hemisphere- that is, the Sun during the day was well south of the equator. So southern polar latitudes were lit for the better part of a full 24-hour day. The image was taken from the night side of Earth, which is why we only see a small part that is in daylight. But as always, half the planet is lit, and half is not, and the sun is generally seen to rise in the east and set in the west (although not exactly so by a fair margin).

To Chris Peterson, thank you so much for a clear and perfectly sensible explanation. You helped me visualize it. I am new to this bulletin board but have enjoyed APOD for many years as an amateur aficionado of the night sky-- ever since my dad used to drag out his self made reflector telescope (ground the glass and all) to let me look at the moon and planets when I was 9 years old. "gigi"

[Syniurge]

According to http://www.esa.int/esaCP/SEMXJY3VU1G_index_0.html the photo is untouched.
So I'm confused.. why don't we see stars and city lights not covered by clouds on this APOD?

[rstevenson]

I'd guess that an exposure sufficient to show us stars and city lights would greatly over-expose the crescent Earth.

Rob

[owlice]

: sighs :

: the radio in her head has switched songs :

According to http://www.esa.int/esaCP/SEMXJY3VU1G_index_0.html the photo is untouched.
So I'm confused.. why don't we see stars and city lights not covered by clouds on this APOD?

Perhaps because of the distance from which it was taken?

[Chris Peterson]

I'd guess that an exposure sufficient to show us stars and city lights would greatly over-expose the crescent Earth.

Exactly. The properly exposed crescent tells us that a typical "daylight" exposure was used. That's not going to show stars or illuminated areas on the night side. For a typical camera sensitivity, daylight exposures are on the order of 1/100 second or faster, while exposures that can capture stars are on the order of a second or longer.