[HN Gopher] Why does it take so long to get to Mercury? ___________________________________________________________________ Why does it take so long to get to Mercury? Author : 5faulker Score : 93 points Date : 2021-08-09 17:26 UTC (5 hours ago) (HTM) web link (www.esa.int) (TXT) w3m dump (www.esa.int) | mmaunder wrote: | This is super cool. I recommend reading the article and then | watching the animation. I'm no physicist, but it looks like the | fundamental idea is to use elliptical orbits to slip in front of | a planet along it's orbital track, and have the planet's gravity | slow the probe, and then get out of the way fast as the planet | passes. It's kind of the opposite of a cyclist drafting. | JoeAltmaier wrote: | Delta-v | JoeAltmaier wrote: | Only on HN can a correct comment be 'disagreed' with silently | to oblivion. :) | siavosh wrote: | I'm so curious what the navigation department for a place like | NASA/JPL consists of. Each probe/satellites mission is so custom, | do they have a software suite that they've refined over the last | 50 years, is it a couple physics professors etc, are there two | teams working independently to make sure no one messes up? | 7kay wrote: | They use GMAT, the General Mission Analysis Tool | https://opensource.gsfc.nasa.gov/projects/GMAT/index.php | amelius wrote: | Do Russians and the Chinese use a similar tool? | mishafb wrote: | I should have expected it to be matlab... | darkwater wrote: | Where do you see it being MATLAB? It just says that has a | MATLAB compatible syntax. This is the SF page, with some | screenshot: https://sourceforge.net/projects/gmat/ | MichaelZuo wrote: | It's a decent explanation for a phenomena that is not intuitively | obvious just by looking at a diagram of the solar system. I | wonder just how much fuel is needed to go directly without | gravitational slingshots. | azernik wrote: | The delta-V is around 16km/s. | | For high-efficiency chemical propulsion (e.g. hydrogen - which, | note, non-storable!) that's about 3.5-4x the exhaust velocity, | meaning around 98-99% of your spacecraft's mass needs to be | fuel. | | For electric propulsion (which has issues because of low thrust | and electrical power requirements), it's only about 0.5-1x the | exhaust velocity, so only maybe 50-70% of spacecraft mass needs | to be fuel. | | In any case, it's a LOT. | teslaberry wrote: | more importantly why is it so much easier to get to geostationary | orbit from the lunar surface, than it is from earth's surface ? | | and does this have major implications for the race to control the | moon, as whoever controls the moon, may have a much easier time | launching anything to geostationary orbit, including firing | directed energy weapons to that orbit, which, despite long | distance attenuation of the energy, still faces no interference | by an atmosphere as it would if directed from earth's surface. | | the military probably has that questions answered but you'd need | a security clearance to retrieve it. | ithkuil wrote: | "There is an art to flying, or rather a knack. The knack lies in | learning how to throw yourself at the ground and miss. ... | Clearly, it is this second part, the missing, that presents the | difficulties." | | -- The Guide | | Turns out that missing the Sun is much much easier. | radley wrote: | This reminded me of Kim Stanley Robinson's "2312" and the idea of | standing on Mercury in the narrow hospitable belt, looking at the | sun, and experiencing solar rapture. | | Apparent size of the sun from the planets | http://www.astronoo.com/en/children/sun-apparent-size.html | syncsynchalt wrote: | The short version: it takes much more fuel to fly directly from | the earth into the sun than it does to escape the solar system. | Because of this you need to use longer and slower methods. | bonzini wrote: | The problem is not flying into the Sun, it's slowing down to | get in an orbit around Mercury. | shadowgovt wrote: | It's both. The sun is actually pretty expensive to hit from | Earth... Earth's orbital velocity around the sun is about | 29.78 km/s. Even to fall into the sun, you need to kill most | of that (not quite zero is needed, of course, as the sun has | a wide radius... But closer to zero than simple Earth escape | velocity). | | The Parker Solar Probe, for example, pulled the trick off of | getting a high-eccentricity orbit around the sun by | slingshotting Venus seven times (the transfer orbit from | Earth to Venus is about 3 km/s delta-v). | | There's a pretty good overview of the math at the Space | stackexchange: | https://space.stackexchange.com/questions/38612/how-much- | les... | sxp wrote: | https://www.reddit.com/r/space/comments/1ktjfi/deltav_map_of... | is another way of looking at the problem. Once you're in Earth | orbit, you need 2.4+.68+.14+.68+1.73 = 5.63 km/s to get to the | moon. But you need 2.4+.68+.09+.28+2.06+6.31+1.22+3.06= 16.1 km/s | to get to Mercury. | | So if you don't use gravitational slingshots (which can be | modeled as ramming your spaceship into a planet's gravitational | field and bouncing off in the other direction), then you need ~3x | the delta-v to get to Mercury compared to the moon. And getting | to the Moon required one of the most powerful rockets in human | history. | | In theory, you could just use a rocket that's 3x as powerful as | the Saturn V to get enough fuel into orbit to get to Mercury via | a direct route. But this runs into engineering issues with | creating a rocket that big. Alternatively, you could develop a | way to refuel rockets in orbit and then launch 3x Saturn Vs with | one space probe and 2 fuel tanks. This is the plan for SpaceX | where they will launch a Starship with humans/robots and a | Starship with fuel to get enough fuel into orbit for a trip to | various parts of the Solar System. | | [edited to fix units] | firebaze wrote: | Please fix your units :) | | You don't need 16.1 m/sec to get to mercury, there's a 'k' | missing. To put it another way: that's off by about a factor of | 1000. | keanebean86 wrote: | How long until we can strap a massive laser to the moon and use | it to push ships around the solar system? | shadowgovt wrote: | Any old time. Powering and maintaining it would be a chore, | and since it's on a rotating body it wouldn't always be | pointing the direction we wanted it to, but solve those | problems and we're great. | | Also somewhat fun is the idea of "solar sailing." Both the | particulate solar wind and the raw solar radiation have | momentum, and by redirecting either one you can get a push. | Since increasing / decreasing the "height" of your orbit is | actually a function of causing acceleration in the direction | of orbit (or opposite that direction), we should be able to | change the shape of a vessel's orbit by deflecting outward- | streaming sunlight and particles so they're facing along the | orbital path (i.e. a 90-degree turn, with a 45-degree-angle | mirror). You'd need a lot of surface area for a reasonable | amount of delta-V in a human timeframe, but nothing we know | of makes this approach impossible. JAXA demonstrated it can | work (https://en.wikipedia.org/wiki/IKAROS); the Planetary | Society has been looking into it | (https://www.planetary.org/sci-tech/lightsail). | nitrogen wrote: | _since it 's on a rotating body it wouldn't always be | pointing the direction we wanted it to_ | | Could you put it on one of the poles? | 8note wrote: | It's also on a moving body, by reference to the sun | amanzi wrote: | I don't fully understand that chart, but it seems like it's | showing that it's harder to get to smaller planets than larger | ones - i.e. takes more effort (fuel) to get into a smaller | planet's gravitational pull, but you get a lot less benefit | since the gravitational pull is less. Is that a reasonable | assessment? | firebaze wrote: | https://en.wikipedia.org/wiki/Orbital_mechanics explains the | mathematics behind the delta-v calculations quite well. | bananabreakfast wrote: | It's actually the other way around. If takes more effort to | lower yourself into a large gravity well than a small one. | | Traveling anywhere outside of Earth's gravity means you are | always going to fall into a well, so the smaller the better. | The problem with Mercury is to get there you have to fall | into the Sun's gravity well. | | The sun has the biggest gravity well around, and Mercury is | deep down inside of it. If Mercury was bigger it would | actually be even harder to get there, since you'd then also | have to fall down it's large well too. | amanzi wrote: | Ah, got it - thanks. | b33j0r wrote: | Right on. Someone has mentioned aerobraking somewhere | above, but for those with fewer hours spent in Kerbal, this | may be of interest ;) | | A delta-v map will usually include atmosphere as a separate | route from the raw energy requirements of entering and | exiting gravity wells. Some of the counterintuitive aspects | of these flight plans involve "very nice, ok! But how do | you plan to slow down when you get there?" | | On arrival to the moon or mercury (or Minimus) you have to | be able to stop under your own power, which you have | necessarily brought in your tanks. And therefore mass. | | Too bad the resonant microwave propulsion thing was more | likely a poorly designed experiment (acceleration without | propellant). But... it violated our understanding of | physics, so we're stuck with bringing our gas everywhere. | Natsu wrote: | Now where's the version of the map that accounts for | lithobreaking[1] as well as aerobraking? :) | | [1] I feel that lithobreaking is slightly more correct than | lithobraking. | state_less wrote: | One might employ a solar sail/parachute. When you get near | Mercury, you'd have more than 4 times the force on the sail | too. | robotresearcher wrote: | [edit: typo was corrected, nevermind] | | 5.63 m/2 vs 16.1 m/s ? | | Neither 5.63 meters divided by 2, nor 16.1 meters per second | can be right, from context. | [deleted] | milansuk wrote: | The chart in the link says: Delta-V in km/s. | sxp wrote: | Yup. I fixed the units. | ithkuil wrote: | > then you need ~3x the delta-v to get to Mercury compared to | the moon. [..] > In theory, you could just use a rocket that's | 3x as powerful as the Saturn V to get enough fuel into orbit to | get to Mercury via a direct route | | I'm a bit confused. I have a vague memory that due to the | Tsiolkovsky rocket equation the amount of fuel needed to reach | a delta-v grow exponentially. How is it possible that you need | a 3x bigger rocket to reach 3x the delta-v? | sxp wrote: | I'm fuzzing the math a bit, but assume you have a Saturn V | style rocket that can get 100 tons into orbit. And that | consists of 50 tons of spaceship and 50 tons of fuel. Then | you get something like V_e * ln((50+50)/50) = .69 * V_e of | delta-v. But if you had two more rockets that launched an | additional 2 * 100 tons of fuel into orbit, you get V_e * | ln((250+50)/50) = 1.79 * V_e. This gives you 2.5x the | delta-v. Decrease the propellant mass fraction and this value | goes up. | | The numbers are completely made up and based on my time | playing KSP (https://xkcd.com/1356/) rather than real rockets | so change the numbers as you see fit. But my core point is | that orbital refueling greatly extends the usable delta-v of | a rocket once it's prepared in orbit since it's easier to | build 3x of a normal rocket than a rocket with 3x the | payload. | bee_rider wrote: | Huh, LEO really is just about halfway to almost anywhere if you | are willing to aerobrake, other than Mercury. | | I wonder why the chart doesn't consider the possibility to | aerobreak into the sun. | bagels wrote: | A lot more delta-v to get a transfer to the sun, then you | have to deal with the thermal problems which are pretty | difficult to overcome. If you're close enough to the sun to | be in its atmosphere, it's going to be pretty hot. | fred_is_fred wrote: | If you had a thousand (or million) satellites doing this, could | you actually measurably modify the orbits of planets? Mercury is | small enough that presumably 1000 satellites each doing 9 gravity | assists could change it in some way? | antognini wrote: | Yes, you can do this with Mercury. Mercury's orbit is chaotic, | so even modifications in its position on the order of | millimeters can build up to produce completely different long- | term dynamics. In particular, there's a few percent probability | that Mercury will get ejected from the Solar System before the | Sun dies. [1] It's also possible that Mercury will collide with | Venus. | | [1]: https://www.nature.com/articles/nature08096 | collaborative wrote: | Isn't space exploration then a bit like playing with fire? | Like, we could mess up some delicate balance by attempting | too many fly-bys | colechristensen wrote: | No. Everything gets hit by big rocks all the time and are | subject to all sorts of perturbations which dwarf anything | a little spacecraft can do to a planet. | Gibbon1 wrote: | Chaotic systems like mercury orbiting the sun have zone of | stability where all the potential orbits lay. The energy | from a space probe doing a flyby won't impart enough energy | to pop it out of that zone. I'm not going to do the math | but probably other planets have a larger effect on | Mercuries orbit than a space probe does. | ninju wrote: | Of course by slowing down the speed of the Mercury it would | affect is ability to be stable and it might end up be sucked | into the Sun :-( | baggy_trough wrote: | Because comets and asteroids have been zooming around for | billions of years, it's clear that the effect would be | negligible. | Andrew_nenakhov wrote: | Not negligible at all. Comets and asteroids do not target to | move a planet in some other direction, but a swarm of | satellites can do it in a coordinated way. Of course you will | need millions of satellites performing such dives over | million of years. | | But if your goal is to move the planet farther from the sun | because it becomes more luminous and threatens to scorch the | earth and you need to move away to cancel it, it is probably | doable. | baggy_trough wrote: | Let's take Voyager as an example. It weighs under 1000kg. 1 | million Voyagers then weighs 1x10*9 kg. | | A 1km radius rock asteroid, which is nowhere close to the | maximum, weighs an order of magnitude more than this. | ufmace wrote: | There's kind of an XKCD What If for this, though with Jupiter: | https://what-if.xkcd.com/146/ | | Summary is no for Jupiter, even the entire planet Earth | wouldn't affect it that much. Mercury is much smaller though, | so it might be possible to make a significant change, though | you're probably still talking about sending a significant | fraction of the mass of the whole planet Earth. | | You might be able to alter the orbit like that without | destroying the Earth if you figure out how to gather a ton of | mass already in space, asteroids or something maybe. But now | you've got to figure out how to redirect their orbit into the | exactly right one without throwing most of their mass around, | and do it with a reasonably-sized mission from Earth. | WJW wrote: | This is literally impossible to tell without more information. | | According to Wikipedia Mercury weighs about 3.285 x 10^23 kg, | about 10^20 times more than most satellites. You'd need _much_ | more than 1000 satellites (or have much heavier satellites) to | significantly impact the orbit. OTOH, if you 'd have sensitive | enough instruments you could measure the effect of even a | single gravity assist. | jwm1 wrote: | Cycling a large asteroid between Earth and Jupiter could be | used to slowly shift the orbit of Earth, most of the energy | coming from Jupiter. Astronomical engineering: a strategy for | modifying planetary orbits [https://arxiv.org/abs/astro- | ph/0102126] | dylan604 wrote: | There's a congressman that would like to have a converstation | with you about this to save Earth from climate change. ___________________________________________________________________ (page generated 2021-08-09 23:00 UTC)