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I'd like to understand how does New Horizons space craft send its data back to Earth, billions miles away from it.

I read in a Time article:

"Also, at the distance of Pluto, we can only send data back at a rate that’s comparable with an old 1990s modem. Because of that, during the encounter, we’ll be taking many, many pictures, but those pictures will all be stored on the solid state memory and radioed back to the Earth months after the encounter."

If the required delay is really in months, then those images which we are all over Internet now must be about a month old?

A lot of details has been skipped from the news article, I'd like to understand how this is done, in simpler terms.

Thanks

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  • $\begingroup$ Welcome to Space.SE. There are different questions in your post, You should consider editing it to keep one, and post the other separately. Regarding the delay, this is not the time required for transit to Earth, but only that the bandwidth is limited (like an Internet connection that slows the transfer of a large file). $\endgroup$ – mins Jul 14 '15 at 21:11
  • $\begingroup$ I understand that bandwidth is limited but still the data is radioed from the space craft to Earth. Doesn't it takes time to reach Earth since the space craft is millions of miles away from the our planet.? $\endgroup$ – ahmad hamza Jul 14 '15 at 21:30
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Short answer

Yes, what you see in the news are recent images. It takes about 1 hour to the probe to send one image bit by bit, and about 4 hours for the bits to travel from the probe to Earth.

If New Horizons could do the scientific work, take images, and send them at the same time, this would be great, but that's not the case. While the probe had more time available for transmitting images in the previous days, now that the probe is close to Pluto and its moons the high-priority task is to collect scientific data (including images) and store them aboard the probe.

Data will be downloaded to Earth as other activities permit. Many more images are coming, some in the following days, but at only 1 image per hour, this will take months, so an image shot today might be transmitted to Earth only in several months.

All the details in the following full (and long) answer!


Labeled diagram of New Horizons
Source.


How slow is the communication between New Horizons and Earth?

From Wikipedia:

Communication with the spacecraft is via X band. The craft had a communication rate of 38 kbit/s at Jupiter; at Pluto's distance, a rate of approximately 1 kbit/s is expected. Besides the low bandwidth, Pluto's distance also causes a latency of about 4.5 hours (one-way). The 70 m (230 ft) NASA Deep Space Network (DSN) dishes are used to relay commands once it is beyond Jupiter.

It says that the traffic between the probe and the Earth is possible at about 1 kb/s. So to transmit an image it takes time: 1 kilobit image will take more or less 8 seconds, 1 megabit will take 1,000 seconds or 17 mn, etc.

(That's not completely accurate because additional control and correction bits are added to the pixel bits. But this is the principle.)


How long does it take to send an image file?

A LORRI image can be compressed aboard New Horizons, down to the size of 2.5 Mbits (depending on the content of the image). It takes about 42 mn to send this from the probe. But the signal has yet to travel to Earth. . . .


How long does it take to receive the image file?

Pluto and New Horizons are now at a distance of about 4.6 billion km. The radio waves travel at about 300,000 km/s. So it takes 4h20 to get to Earth.

To sum up:

  • T0: New Horizons sends the first bit of an image and continues sending bits during 42 mn
  • T0 + 42 mn: New Horizons sends the next image, and at T0+84 mn another one, etc.
  • T1 = T0 + 4h20: We receive the first bit of the first image on Earth.
  • T1 + 42 mn: We get the last bit of the first image, and then the first bit of the next image, etc.

How many images can be received in a month?

It has taken 5h02 for the first image. The next image can be received, and will be complete 42 mn later.

At this rate we are capable of receiving 34 images per day. In a month we can get 1045 images.

But actually this will be done in two phases: A browse catalog will be sent first to Earth in medium compressed format like explained so far, then the larger full resolution images (lossless compression). This is the current schedule:


Are images currently sent?

I you look at the downlink schedule provided by @TildalWave (Observation Playbook), you can see that images are currently sent:

  • Today and tomorrow, there are medium compressed image sets named NY Times to be downloaded.
  • High-resolution images will be sent planned 17-20th July.

Be ready to see them in the news!


This is very slow! Why?

New Horizons is very far, so we receive a very faint signal from it. To be able to decide if we are currently receiving a 0 or a 1 we need to accumulate received energy. To do that we need to listen the signal of a bit a longer time. So they must be sent at a slower rate.


Images cannot be sent continuously

New Horizons cannot send the images 24x7 to Earth:

  • Sometimes the Earth is not visible
  • Sometimes the antenna cannot be oriented in the direction of Earth (because the scientific instruments may need the probe to be oriented differently to work).
  • Sometimes data other than images must be transmitted.

The most important parts of the mission in this current Pluto fly-by phase is:

  • To collect data about the atmosphere. Pluto is moving away from the Sun, and so is cooling down, and its frozen particles atmosphere is going to change.
  • To take pictures of Pluto and its moons. New Horizons is now very close Pluto, and can shoot very detailed images, at a level never equaled in the past.

If you look at the spacecraft at the beginning of this post, you can see that the antenna cannot be oriented (to Earth) without changing other instruments and sensors orientation too, and that the LORRI imager and the dish are 90° apart, which limits the direction for imaging while transmitting to Earth.

So the high-priority data will be collected (images and other sensors output), and stored in memory. Later, at appropriate times, the probe will be oriented for a "call home" activity, so that stored data can be downloaded to Earth.


Where do images land?

The images are received by the Deep Space Network (NASA) with antennas at 3 locations around the world. At this time this is Madrid which is looking to New Horizons with its largest antenna (diameter 70 m). But as you see there is no signal, the communication is not active.

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  • $\begingroup$ I don't think they have to wait 4.5 hours between the pictures. They can just send the pictures back to back. Which is still very slow. Aside from that the Picture we see is 2.5 Mb, but the original and raw ich much bigger. In addition to pictures the probe collects other data. At this rate it would take 6 Month to send 15 Gb (which is not much for scientific data) $\endgroup$ – Stein Jul 14 '15 at 22:23
  • $\begingroup$ @Stein: You are right, I don't know what I was thinking. Adjusting... $\endgroup$ – mins Jul 14 '15 at 22:27
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    $\begingroup$ DSN Canberra is currently receiving i.stack.imgur.com/7l97A.png ;) (taken from DSN Now). See this Observation Playbook pluto.jhuapl.edu/Mission/The-Path-to-Pluto/… (PDF) for when to expect any data and of what. $\endgroup$ – TildalWave Jul 14 '15 at 22:31
  • $\begingroup$ Also, there is only one Deep Space Network (with three locations), and facilities might need to be used to communicate with other missions as well. $\endgroup$ – jamesqf Jul 15 '15 at 2:28
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To answer this part of the question:

then those images which we are all over internet now, are actually a month only or something like that?

In the last few days before the flyby, New Horizons sent us a few images. This is just a small part of the data that were collected (images and other instrument readings).
New Horizons can't take photos and send data to Earth simultaneously. So they planned the mission to have as much time as possible available for observations. To make sure we had at least some data in case of a catastrophic problem, some time was reserved for sending a few images to Earth just before the encounter. The rest of the data collected is stored aboard the spacecraft and will be sent when the observation phase of the mission has ended.

This image was taken on July 13th:

Pluto

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