[HN Gopher] Why does it take so long to get to Mercury?
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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.
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