New Horizon completes Pluto FlyBy

Subject: New Horizon completes Pluto FlyBy
From: Alex Kemp
Date: Tuesday, 14 July 2015 17:00:34 +0100
To: Oliver Kemp, Micaela Kemp, Liisa Kemp, Davin Kemp

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The main photos + data will be received tomorrow, but at 11:50 UTC today New Horizon was due to make it’s closest approach (12,500km, 7,767 miles - about the same size as the diameter of the earth) to Pluto. Here is a taster:

It seems a little unbelievable to me, but it will take more than a year to radio all the data that the craft will have collected back to earth. What??? First, a little detail on getting there & then the reason why it takes forever for it to phone home.

It takes about 8 minutes for the light of the sun to reach the earth (it travels at 299,792,458 m / s) (300,000kps) (186,000 mps). It takes 5.3 hours for the same light to reach Pluto (and likewise for radio signals, since they are just another wavelength of light) (the precise time varies enormously, because Pluto’s orbit is so eccentric in both planes). That distance also means loss of power; sunlight at Mercury can melt lead (427℃/801℉), whilst Pluto suffers ‘snowstorms’ of nitrogen (it reaches -238℃/-396.4℉ (-273℃ is as cold as it is possible to get; even Helium becomes liquid at that temperature)).

New Horizon has travelled 2.97 billion miles (4.779 billion km) since January 19, 2006 to reach Pluto. With a delay of 4 or 5 hours in communication, it is impossible for real-time control of the probe. Therefore, NH Control aimed the probe at a ‘keyhole’ in space; it had to arrive at the centre of a 100km by 150km (60 miles x 90 miles) window of space within a 100 second span of time. After that point it was programmed to perform like some manic combination of robotic ballerino & photographer, twisting & turning this way & that, capturing all kinds of data from not just Pluto but also all 5 moons.

It is clear that the New Horizon’s probe is male, since it can only do one thing at a time. It can take photos & capture sensor data and store such data in it’s memory. It can also transmit such data back to Earth. However, it cannot do both at once (partly because of power concerns, but also because it needs to line-up it’s antenna on earth & stay lined up whilst it dribbles the info out of the aerial).

So, because it is so far away (32 times further away from us than the Sun) the radio signal from Pluto is extremely weak. That also means slow: ~1 kb / sec (that is kilobit--1,000 bits--not kilobyte, which would be kB & 1,024 bytes). 1kb translates to ~125 bytes (it depends on whether the signal has stop/check bits or not). Now there is more info from Emily Lakdawalla at the Planetary Society, but in brief, consider the highest resolution camera on the probe (the LORRI detector). This is 1,204 pixels square (tiny compared to most modern cameras). At 12 bits/pixel that is 12 million bits. Now, a lot of the background is black, so (lossless) compression reduces that to about 2.5 mb (megabit), which is about 42 minutes. For just one photo. Or 11 images per 8 hour session (which is all that they get from NASA). And there is far more than just a few photos to send.

The team prepared for this whole session by completely clearing the memory banks before approach. They now have to hope that a stray rock or bit of grit or gremlin doesn’t interfere. Hopefully, more tomorrow.