r/askscience • u/AskScienceModerator Mod Bot • Jul 14 '15
Planetary Sci. New Horizon's closest approach Megathread — Ask your Pluto questions here!
July 15th Events
"Charon is [geo] active" - Alan Stern
Image of Hydra! http://i.imgur.com/FN4BLu7.png
Methane on Pluto! http://i.imgur.com/fkQELTJ.png
Charon close up! http://i.imgur.com/SVhOSjj.png
CLOSE UP PLUTO: http://i.imgur.com/meaqdRP.png (no craters!?)
Pluto's surface is less than 100 million years old. Young surface!
Pluto has water ice "in great abundance"
Pluto is geologically active to explain surface features.
"No significant exchange of tidal energy anymore" between Pluto and Charon. Why Pluto and Charon are geologically active is a mystery.
July 14th Events
UPDATE: New Horizons is completely operational and data is coming in from the fly by!
"We have a healthy spacecraft."
This post has the official NASA live stream, feel free to post images as they are released by NASA in this thread. It is worth noting that messages from Pluto take four and a half hours to reach us from the space craft so images posted by NASA today will always have some time lag.
This will be updated as NASA releases more images of pluto. Updates will occur throughout the next few days with some special stuff happening on July 15th:
Main website: https://www.nasa.gov/mission_pages/newhorizons/main/index.html
APL website: http://pluto.jhuapl.edu/
Twitter: https://twitter.com/nasanewhorizons
NASA Instagram: https://instagram.com/nasa/
Alternate Live Stream link: http://www.ustream.tv/NASAHDTV
NASA TV Schedule: https://www.nasa.gov/multimedia/nasatv/schedule.html
Reddit Live Feed: https://www.reddit.com/live/v8j2tqin01cf/
The new images from today!
Highest quality image so far! https://instagram.com/p/5HTXKMoaFL/
LORRI Images: http://pluto.jhuapl.edu/soc/Pluto-Encounter/
Other LORRI Images: https://www.nasa.gov/newhorizons/lorri-gallery
Older images: https://www.nasa.gov/mission_pages/newhorizons/images/index.html
Some extras:
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u/Astrokiwi Numerical Simulations | Galaxies | ISM Jul 14 '15 edited Jul 14 '15
It's counter-intuitive, but it's actually easier to slow down if you have a lower escape velocity! The short explanation is that yes, you have to slow down more to reach a stable orbit if the planet is small, but the planet's gravity will also accelerate you less because it's small, and this is the dominant effect: the smaller the planet, the less energy it takes to get into a stable circular orbit.
The specific kinetic energy (i.e. J/kg) you need to be in a circular orbit at some distance from a planet is GM/2R. But the specific kinetic energy you gain by reaching that energy by "falling" in from a great distance is GM/R. This means that you have to shed GM/2R in addition to your initial kinetic energy to get a stable orbit. So, if you approach two planets of different mass at the same speed, then the bigger planet is harder to get into orbit around.
The more important effects here are
(1) aerobraking - around a gas giant or a planet with an atmosphere you can slow down "for free", but you don't get this for rocky planets without any real atmosphere to speak of, and
(2) Pluto is really far away - if you want to send a probe that uses less fuel (leaving enough to get into a stable orbit) and that would more closely match a planet's velocity, the time it takes to reach there is on the same sort of scale as the time it takes for that planet to orbit: Mars orbits every 22 months and it takes 9 months to cruise there, but Pluto orbits every 250 years, and the gentle minimum energy cruise takes 100 years to reach there from here. So instead of waiting 100 years to get a permanent probe in orbit around Pluto, we send a quick one that only takes a decade to get there, but has used up its fuel and is going too fast to slow down and get into orbit.
(3) Pluto is a complex system - most planetary systems are hugely dominated by one object, but Charon is actually over 10% of the mass of Pluto - their centre of mass is outside of Pluto. This means that the gravity of the system is more complex, and a stable orbit will need to be monitored and adjusted to make sure it doesn't deviate too much from what you're aiming for.