I'm really looking forward to seeing more from New Horizons. The Pluto photos were amazing.
I just wish my PC would wake from sleep as reliably. I'm lucky if I can get it back up after one night, never mind five months.
This post by Phil Plait  has some details on how they determined that: The way the star blinked out from different locations makes sense if MU69 is not a single round object, but instead two roughly round objects, either orbiting one another as a close binary system or touching each other in what’s called a contact binary.
> an apparent magnitude of nearly 27
Wow. Just for reference, an 8.2 meter ground telescope (the Subaru in Hawaii) was able to capture objects at a magnitude of 27.7 after staring in the same spot for 10 hours. 
The target MU69 (fly-by on 1st Jan 2019) looks interesting. It's amazing that we can tell it might have a double-lobed shape from such a distance: https://en.wikipedia.org/wiki/(486958)_2014_MU69#Stellar_occ...
I'm wondering if radiation isn't one of the reasons. Hibernation means powered-down circuits, meaning there's no voltage on thin insulator barriers to degrade them faster. MOS gates come to mind. But I'm no expert on microelectronics. Power management does nothing for deep space probes because their RTGs work whether you want them or not.
Yes radiation would be the reason here. Power is not a concern - as another poster commented, the RTG is always on whether you like it or not!
Good call. I hadn't thought of that. I am not a rocket surgeon.
Most probably to save power. I found this link  from  that talks about it going into hibernation. But it also doesn't have much details.
 April 10, 2017: Nap Time for New Horizons: NASA Spacecraft Enters Hibernation [ http://pluto.jhuapl.edu/News-Center/News-Article.php?page=20... ]
 New Horizons News Archive [ http://pluto.jhuapl.edu/News-Center/News-Archives.php ]
Your welcome. But it appears that power saving isn't the reason. As another poster noted, the RTG powering New Horizons doesn't care if it the power provided by the RTG is used or not.
I did find this article  from when New Horizons was going to Pluto and if the reason is the same, then the hibernation is to prolong the life of the instruments: New Horizons spends much of deep cruise in hibernation - essentially long stretches of electronic slumber in which many of the spacecraft's subsystems (such as science instruments, navigational star trackers and most flight electronics) are turned off to extend their life, and the craft is spin-stabilized to minimize thruster usage.
 New Horizons Spacecraft to Collect More Data When "Hibernating" [ http://www.spaceref.com/news/viewsr.html?pid=41333 ]
>the craft is spin-stabilized to minimize thruster usage
So one good reason for it to hibernate is to save thruster fuel.
It's not obvious to me why the two (spin-stabilization and hibernation) would have to be coupled. Could it be due to antennas that are required for communication during active mission control?
That said, there's a whole lot of nothing out there. It makes sense to put the craft into a safer mode that has less risk of failure when there's nothing for it to do.
Most likely while spin stabilizing, many instrument packages that require some kind of orientation are useless, so it's better to turn them off - and also, to save their operational lifetimes (if something has a certain # hours of operating life it makes sense to turn them off to save life)
Simplicity of operations. Power management isn't an issue since the probe generates plenty of power from its RTG constantly. Instead of just constantly telling the probe to do nothing the probe has a built-in hibernation mode.
I rather look forward to a time when that's less necessary - the odds are slim, but non-zero, that the probe may have passed by something interesting while powered down, but we can never know.
I really have no idea what advances might enable that, though.
First, the amount of data this would generate is huge. The Pluto encounter was about a day worth of measurements that took over a year to downlink. The link is slow, about 1 kbit per second.
What instrument do you watch? The cameras are more like telescopes with very narrow fields of view. Slewing the spacecraft around aimlessly snapping pictures isn't doing science.
Next, space is big and empty. Many people have visions of dodging asteroids, planets, comets, and moons, but you only encounter these things when you aim for them. A typical journey across the solar system would encounter nothing larger than a grain of sand.
I think we'll always prefer to make three probes that need to sleep rather than one that doesn't. Until we begin to run out of targets we can know or strongly suspect beforehand will be interesting.
Stronger RTGs, maybe?
The plutonium decays at a fixed rate regardless of how much power the spacecraft is currently using.
More resilient electronics.
One thing that would enable that is cheaper cost to orbit, which would let us build heavier probes with more rugged hardware.
Anyone know why it was sleeping? I can't find that information. Power management while there was nothing interesting to do, maybe?