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Why does it take so long to get to Mercury?
- JoeAltmaier 5y agoDelta-v
- JoeAltmaier 5y agoOnly on HN can a correct comment be 'disagreed' with silently to oblivion. :)
- tempestn 5y agoWhen you just say "delta-v", it means nothing to anyone who doesn't already know what that means, and it provides no additional value to those who do, so the comment has no purpose. At least a complete sentence like, "This illustrates why it's difficult to overcome the large delta-v between Earth and Mercury," would say the same thing in a way that could actually be useful to someone. Even better if it defined delta-v as well.
- JoeAltmaier 5y agoRight because HN is a tutorial? Nobody here can understand much without detailed explanations. The purpose is, so folks can look up delta-v and start to understand. It's not necessary to recapitulate everything on the web in a post. For instance I google 'orbital mechanics delta v' and the whole story is right there for the taking.
- MichaelZuo 5y agoIt’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 5y agoThe 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.
- MichaelZuo 5y agoAh thanks, Project Orion makes a lot more sense in that context!
- azernik 5y agoYeah, people tend to pretty easily understand how the rocket equation is exponential in the required ∆V, but the way it's inversely exponential in exhaust velocity is less well understood. Good specific impulse plus good thrust is a combo that requires nuclear-explosion-scale energy, ie Project Orion or Nuclear Salt Water Rockets (an option that is, shockingly, even more insane than Orion).
- syncsynchalt 5y agoThe 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 5y agoThe problem is not flying into the Sun, it's slowing down to get in an orbit around Mercury.
- shadowgovt 5y agoIt'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-less-delta-v-would-it-take-to-reach-the-sun-using-venus-and-earth-flyby https://space.stackexchange.com/questions/38612/how-much-les...
- sxp 5y agohttps://www.reddit.com/r/space/comments/1ktjfi/deltav_map_of_the_solar_system/ 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 5y agoPlease 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 5y agoHow long until we can strap a massive laser to the moon and use it to push ships around the solar system?
- shadowgovt 5y agoAny 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); https://en.wikipedia.org/wiki/IKAROS); the Planetary Society has been looking into it (https://www.planetary.org/sci-tech/lightsail https://www.planetary.org/sci-tech/lightsail).
- fred_is_fred 5y agoIf 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?
- ninju 5y agoOf 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 5y agoBecause comets and asteroids have been zooming around for billions of years, it's clear that the effect would be negligible.
- Andrew_nenakhov 5y agoNot 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 5y agoLet'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.
- Andrew_nenakhov 5y agoSolar sails give you essentially free power to fly around the solar system. So it is just a matter of time. With millions of years (and sun burning out is not exactly a fast process), this is quite doable.
- siavosh 5y agoI'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 5y agoThey use GMAT, the General Mission Analysis Tool https://opensource.gsfc.nasa.gov/projects/GMAT/index.php https://opensource.gsfc.nasa.gov/projects/GMAT/index.php
- amelius 5y agoDo Russians and the Chinese use a similar tool?
- DataGata 5y agoI would be willing to bet money they use the same tool.
- mishafb 5y agoI should have expected it to be matlab...
- darkwater 5y agoWhere 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/ https://sourceforge.net/projects/gmat/
- radley 5y agoThis 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 http://www.astronoo.com/en/children/sun-apparent-size.html
- jefurii 5y agoThe journey to Mercury is even more impressive in his earlier book "The Memory Of Whiteness" where you also get fascinating meditations on the intersections of physics, metatheater, and music theory.
- 4wsn 5y agoWhat's solar rapture? Sounds like a Scientology ritual.
- Sharlin 5y agoIn "Blue Mars" they build a city on tracks on Mercury, always staying at the terminator as the planet slowly rotates under it.
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- addingnumbers 5y agoHopefully on the dusk terminator and not the dawn terminator? If a mechanical problem slows you down would we rather see it start getting colder, or hotter as we work on correcting it?
- radley 5y ago
- ithkuil 5y ago"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.
- teslaberry 5y agomore 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.
- mmaunder 5y agoThis 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.
- fogof 5y agoThis reminds me of a fun brain teaser: Of all the planets, which is Earth closest to on average?
- rrdharan 5y agohttps://physicstoday.scitation.org/do/10.1063/PT.6.3.20190312a/full/ https://physicstoday.scitation.org/do/10.1063/PT.6.3.2019031...
- ballmerspeak 5y agoIt's Mercury. https://www.youtube.com/watch?v=SumDHcnCRuU https://www.youtube.com/watch?v=SumDHcnCRuU
- ars 5y agoThat's easy - it's Earth. :)
- bagels 5y agoNot wrong.
- bobsmooth 5y agoCGP Grey did a video on this. Mercury is the mostest-closest for all the planets.
- SilasX 5y agoYes! This was a surprise to me to learn as well. I mean, it was in a different context (how to get to the sun), but the same issue applies, of having to shed a ton of angular momentum/orbital energy. (If you're curious about the opposite direction, of objects with the least angular momentum, that would be either bosons, or Texas's four-day-long "rotating" power outages.)
- gorgoiler 5y agoAhem. Unclear if this is intentional or an artefact of their blog software, but I couldn’t help but notice that the phrase “vacuum of space” in the article has an extraneous . It is quite visible to the naked eye. Shurely shome mishtake?
- dcow 5y agoHow much energy do we have to shed? Are we making the earth fall towards the sun every time we launch a space craft?
- bagels 5y agoRockets aren't very massive compared to the earth... yet?
- phendrenad2 5y agoI wonder how hard it would be to build a big shock absorber and fire it directly at Mercury. Maybe delicate electronics and sensors could survive the impact if the force is spread out over a long enough time.