Høydepunkter fra Reddit-AMA med Alan Stern

Nettstedet Reddit (hva, er du ikke der forlengst?) holder jevnlig “Ask Me Anything”-sesjoner, som har fått så høy status etterhvert at også president Obama har stilt opp. Igår var det hovedforsker i New Horizons-prosjektet Alan Sterns tur, og han fikk mange og gode spørsmål. Vi har samlet de beste nedenfor.

What has surprised you the most in the data that you’ve gathered so far?

For me, Charon’s dark pole. We all think this is hysterical, because Charon had a dark pole in the simulation we did of what the encounter would be like. It was also a surprise because Charon’s rotational light curve (i.e. its pattern of light and dark) is very subtle, so while we knew Pluto was going to have dark spots, Charon very well could have been uniform.

Do you have any other object in the Kuiper Belt chosen for New Horizons to attempt to fly past after Pluto?

Last summer, we discovered five KBOs, three of which were possibly in reach of the spacecraft. We’re deciding between 2014MU69 (aka 11-blahblahblah aka PT1) and 2014PN70 (aka PT3). We can only go to one, and we’ll announce that in the fall. Meanwhile, we’re kinda busy with this whole Pluto encounter. This is all pending NASA approving an extended mission for us (We’ll make a proposal next summer after all this cools off, fingers crossed).

What are your biggest worries for possible things going wrong with the flyby?

There are these thing called IMUs on the spacecraft (we have two). The ones that were on the Stereo spacecraft were from the same batch as ours and they broke. 🙁 We can’t control that. The odds of both of them breaking would be bad.

Am I correct that Pluto has rings, and are you studying them part of this mission?

We don’t know if Pluto does or does not have rings. They’re possible, especially because of Pluto’s crazy outer moon system, and if there’s a source in the system that can actively supply them, like Enceladus does for the Saturn system. Also centaur (10199) Chariklo has rings, and no one expected that. As part of the hazard search we looked for them, but the best time to see rings is in something called forward scattering. We pass behind Pluto and see if we see any dust lit up by the sun, kind of like how it’s hard to see when you drive into bright light because all the crap on your windshield gets lit up. This is how Jupiter’s rings were discovered by Voyager, and we have some observations that we will take as we leave the Pluto system that will look for them.

I’ve heard that Pluto’s atmosphere was discovered after occultations of the dwarf planet across a star. Has anything similar been found on Charon? How likely is it that Charon has an atmosphere too?

Using that same technique from Earth, we haven’t seen an atmosphere on Charon. But those measurements weren’t as sensitive as the spacecraft’s will be. If there is an atmosphere, there would be an upper limit of pressure in nanobars (one billionth of a bar). For reference, Pluto’s atmosphere has microbar level pressure (one millionth of a bar) and Earth’s atmosphere has pressure of a bar. Anything is possible, but another reason that the probability of Charon having an atmosphere is low is because it can’t really hold onto one, gravitationally-speaking. Pluto can– it’s eight times more massive.

If you could use today’s technology for instruments, computing and memory, how much of a difference would it have made in the way Pluto is observed by NH? (I am asking because the more I learn about flyby planning, the more I learn about technical constraints)

We’re limited in other ways, weirdly. For example, LORRI, our high resolution imager, has an 8-inch (20cm) aperture. The diffraction limit (how much an 8” telescope can magnify) is 3.05 mircorad. which is just over half the size of single pixel 4.95 microrad. So if we swapped out the current sensor with a higher res one, we couldn’t do much better because of the laws of physics. A bigger telescope would solve that problem, but then it would make the spacecraft heavier, which require more fuel to send to Pluto AND a longer time to get there, because the spacecraft is more massive. We launched Pluto on the largest, most powerful rocket available at the time (the Atlas V, with extra boosters), so again we’re limited by physics: “At the time” doesn’t mean best ever. The Saturn V rocket, which sent astronauts to the moon, was actually more powerful.1

More megapixels also means more memory. For example, LORRI images are made up of a header and then the 1024×1024 array of numbers that make up our image and go from 0 to 65535 (216). There’s not really a way to make that info smaller if we went to 2048×2048. We could downlink a compressed version, but we want the full info eventually.

We could have a bigger hard drive. At some point either a very short time ago or a in the next few days, we’re wiping the entire hard drive in prep for the encounter. So having a larger hard drive would have been nice, and yeah we could use that, and today’s tech would probably get us a bigger one.. We are filling up said HD during the encounter. On the other hand, we will be downlinking stuff until the end of 2016, so bigger hard drive means we need the spacecraft to survive even longer to finish it (we are not planning on it breaking, but it’s a risk, so we are downloading a compressed version of everything, which will all be down in November, but think really lossy JPEGS).

Our downlink rate is actually limited by the spacecraft power. We have all the plutonium we could get our hands on, but there was actually a shortage at the time. As a result, even Voyager has a higher downlink rate then we do. 🙁 It’s still really cool we can run all our instruments with less power than an incandescent light would use.

So yeah, the things that would make our mission better, super smartphone tech can’t really fix. It’s all physics. And lack of Plutonium (We wants moar!!! Tell your congressfolk, we can’t go to the outer solar system on solar. Us planetary folk would love missions to Uranus and Neptune.)