What does New Horizons use to keep itself traveling (so far), while keeping all of its monitoring and communication systems active?
I did read that it leveraged force from a gravity sling shot from Jupiter, but what else contributes? Solar?
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3$\begingroup$ Are you asking what's propelling it or what it's using to power its onboard systems? $\endgroup$– JoeJul 14, 2015 at 19:26
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3$\begingroup$ @Joe I was originally trying to ask about both, but my question and its title keep getting edited by other people so who knows! $\endgroup$– RickyJul 14, 2015 at 22:51
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$\begingroup$ Feel free to edit your question to make your intent clearer. $\endgroup$– JoeJul 15, 2015 at 4:30
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1$\begingroup$ Regarding propulsion, think of spacecraft more like billiard balls rolling downhill than like cars. They get a burst of initial speed from a rocket, then spend the rest of their time just coasting. Where they end up depends on the shape of the terrain. (There are other complications, but start with this mental model.) $\endgroup$– JoeJul 15, 2015 at 4:42
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2 Answers
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Space is basically a vacuum, so there's no air resistance. A probe that's been launched will travel at the same speed indefinitely. Because New Horizons is moving away from the Sun, it loses some speed to overcome the Sun's gravity.
New Horizons was launched on the fastest rocket they could get. Then it used a gravity assist from Jupiter to gain some more speed. Since then, it's been coasting. New Horizons does carry small thrusters which are used for small course and attitude changes.
To power its systems, New Horizons uses a Radioisotope thermoelectric generator. Plutonium pellets generate heat from atomic decay, this heat is converted to electricity. At the start of the mission the RTG produced 4500 W of heat, which was converted to 290 W of electricity. During the Pluto encounter, ca. 200 W was available (due to plutonium decay and deterioration of the thermocouples).
Most of the heat from the RTG seems to be radiated out through the radiator panels on the outside of the RTG. Some of the heat is conducted through the spacecraft frame to the propellant tank to keep it from freezing. The spacecraft is well-insulated. When the electronics aboard draw more than 150W the heat they produce is enough to keep the spacecraft at its operating temperature (+10 to +30 °C). When the spacecraft is in hibernation, electric heaters are used.
NH contains a series of louvres that can be opened to radiate away excess heat, these were designed to be used mostly during the initial phases of the flight (nearer the Sun).
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$\begingroup$ Thanks. I had assumed it had something to with space's lack of air/atmosphere. Does this theory scale? As in, does it mean we could launch much bigger (maybe manned) crafts that travel similar distances/durations in the same way? $\endgroup$– RickyJul 14, 2015 at 7:22
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2$\begingroup$ Yes. The exception is when you're in orbit around Earth (or another body with an atmosphere). Low Earth orbits (below 1000 km or so) experience drag from the upper layers of the atmosphere, so e.g. the ISS constantly slows down and has to be reboosted a few times a year. $\endgroup$– HobbesJul 14, 2015 at 7:32
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$\begingroup$ I'm still pretty impressed that they get a full 200W out of that. $\endgroup$ Jul 14, 2015 at 14:13
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1$\begingroup$ Less impressive when you consider they throw away 4 kW of heat to produce 200 W of electricity. RTGs aren't really efficient, but they're the best we have available for this application (i.e. no moving parts, for instance). $\endgroup$– HobbesJul 14, 2015 at 19:17
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$\begingroup$ Well, at least part of those 4kW is probably used to keep the systems from freezing? That seems more effective than using electrical heating. Does it need to use radiators at all now, being so far from sun? $\endgroup$– jkavalikJul 15, 2015 at 6:11
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Propulsion
Until someone solves the N-body problem every spacecraft needs some kind of propulsion to correct its course during the mission. New Horizons uses a Hydrazine based propulsion system including four 4.4 N main thrusters and twelve 0.9 N attitude control thrusters. Its 77 kg fuel tank allows a total post launch delta-v of somewhere over 290 m/s (650 mph).
New Horizons has carried out several course corrections since its launch. The last at the time of writing was only a few weeks ago.
Electrical Power
Because of the vast distances from the sun New Horizons will operate at Solar power is impractical. At the elliptical orbit of Pluto the strength of the sun varies from 0.0012 to 0.0004 times what we get on Earth.
Instead of trying to collect the tiny amount of sunlight available New Horizons uses a Radioisotope Thermal Generator (RTG). Specifically, a General Purpose Heat Source — Radioisotope Thermoelectric Generator (GPHS-RTG) manufactured by General Electric Space Division (now part of Lockheed Martin).
The GPHS-RTG that New Horizons carries is actually a spare from the Cassini mission. It provided about 250W at launch, and the decay of the Plutonium causes the power output to drop by approximately 5% every four years, meaning that by now the RTG should be providing about 200W of power.
This graphic shows the position of the RTG on New Horizons.
For some more reading on New Horizons power systems see The Pluto-New Horizons RTG and Power System Early Mission Performance, Geffrey K. Ottman and Christopher B. Hersman. (source of the above graphic)
answered Jul 14, 2015 at 7:53
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1$\begingroup$ This is the correct answer for 'What powers Pluto?'. Unfortunately, OP was asking about propulsion :P $\endgroup$ Jul 14, 2015 at 10:23
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1$\begingroup$ @VedantChandra I was trying to also cover the "while keeping all of its monitoring and communication systems active?" part of the question. $\endgroup$ Jul 14, 2015 at 12:03
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1$\begingroup$ @VedantChandra I see what you wanted to do and narrowed the scope of this question and asked another one as a placeholder to migrate part of the ForgeMonkey's answer there, but one thing that we want to preserve with edits is also validity of already existing answers. ForgeMonkey answered both parts of the question, as it was originally asked. I'm afraid I have to roll the question back to its previous revision, so the edit doesn't invalidate any answers. Thanks anyway, I know you wanted to help out! $\endgroup$ Jul 14, 2015 at 13:22
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3$\begingroup$ The need for propulsion has nothing to do with solving the N-body problem. The N-body problem is perfectly solved with numerical integration. The propulsion is needed because we cannot launch something to the incredibly exact velocity vector and exact time needed to, for example, fly by Jupiter to encounter Pluto and then flyby Pluto 10,000 km away on the front side. We can't even determine that exact vector exactly enough because we don't know exactly enough where Jupiter will be when we get there and where Pluto will be when we get there. $\endgroup$ Jul 14, 2015 at 14:19
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8$\begingroup$ Numerical integration doesn't perfectly solve the N-body problem. It resolves it to a certain threshold of accuracy. Granted, as long as the solution is more precise than the accuracy of our engines and navigation, it's good enough. $\endgroup$ Jul 14, 2015 at 18:11