# Tag Info

106

Because the earth goes very fast around the sun. If you want to get to the sun, you need to slow down almost completely so that your speed relative to the sun becomes almost zero. If you don't slow down (almost) completely, your probe will miss the sun when you 'drop' it, so it will eventually come back and you'll end up in an elliptical orbit. Kind of like ...

99

Given a pair of objects that are gravitationally bound to each other, they will orbit around their common barycenter (center of mass of the system). The object to be most logically deemed the moon will be the one of lesser mass because it will be further from the barycenter than its companion. For example, Pluto has a gravitationally bound companion named ...

74

Gravity isn't just about mass, but about distance, too. Our moon has a surface gravity of about 1/6th of Earth, because it is small and less dense than the Earth is. Surface gravity of a body is inversely proportional to the square of its radius, holding mass constant. That means that if you compressed the moon such that it was $\frac{1}{\sqrt{6}}$th of its ...

71

Maybe some visual intuition for what actually happens in the Hohmann transfer helps? It's already very close to what you are describing. In the top arc, the spacecraft (yellow), is going a bit slower than Mars (red), so it's indeed "waiting" for the planet to catch up to it. It only touches the orbit of Mars in a point, but that's all we need if we time ...

60

Imagine you have a very heavy book and a bookcase, and your goal is to put the book on the top shelf of the bookcase. How much time would you spend doing that? Maybe five seconds, maybe fifteen. Would going much slower help you? No, it would not, because simply carrying the book is exhausting to you. You would never be able to hold the book up for an entire ...

51

What immediately springs to mind is the Martian moon Phobos, orbiting the planet in 7 hours 39 minutes. That's a fair bit quicker than the 24 hour 37 minute sidereal period of Mars. From the surface of the planet, Phobos and Deimos will therefore appear to cross the sky in opposite directions. Other solar system examples include the small inner Jupiter moons ...

46

Shot answer: Was Sputnik-1 "only for beep" - no, it wasn't :) It was technical test of R-7 as space launcher and test of spacecraft in orbit (athough very simple spacecraft). Also scientists at least tried to make atmosphere research with Sputnik-1. (From my current search results I'm not sure they got much.) Long answer: It's current state of my ...

44

Wouldn't i inevitably spiral to sun surface even if i was faster than 0km/s ? No. On reasonable timescales, an orbit will have a fixed distance of closest approach, called "periapsis." (These timescales shorten if you're close enough to what you're orbiting that an atmosphere can drag you down). You don't really need to "drop in straight line" (which ...

43

A very good question! The reason is essentially to do with tides. And a slightly over-simplified summary is: If the moon orbits more slowly than the rotation of the parent body (as our Moon does, 12 degrees per day while the Earth rotates about 360 degrees per day) then the moon will gradually orbit further and further away. If the moon orbits faster than ...

40

As far as I know, there has not been a space mission that would have been impossible without a theory of relativistic physics. It is true that the relativistic effects are clearly visible in GPS clocks. However, if the theory didn't exist, they'd just classify it under "weird observation" and trim the clocks to match ground station clocks. The weird ...

40

Primarily, locations of spaceports would change. California, not Florida would host the NASA's main launch site. Russia would be in slightly better position, able to send rockets over the Black Sea, nicer inclinations than currently available from Baikonur - although Vostochny wouldn't happen or would be closer to Chita. ESA could forget about French Guiana, ...

39

Why is it the most energy efficient to change orbit inclination while crossing the equator? Specifically, it's most efficient to do a plane change at one of the two "nodes" where the origin orbital plane intersects the destination plane. ANASIS-II is destined for geostationary orbit, so its destination plane is the plane of the equator. Any orbit ...

38

It depends whether or not you want to orbit or land softly upon Mars, or just hit it. For the former, you have to match orbits with it, that probably means burning more fuel. For the latter, you can skip the Mars orbit injection and just crash. This is quite fuel efficient, especially as the reduced delta-V requirements mean that you need less fuel for the ...

37

You've hit on a really interesting question. To answer this, I'm going to look at JPL Horizons, using the center of the Earth and the center of the Moon as the distances provided. I'm going to look at each of the Apollo missions, with the time that they were orbiting the Moon, showing the max distance, with 10 minute increments included. All distances in kms,...

35

Yes, it is. Given two spherical, uniform, bodies one with mass $m_1$ and radius $r_1$ and the other with mass $m_2$ and radius $r_2$, then the surface acceleration due to gravity will be equal when $$r_2 = \sqrt{\frac{m_2}{m_1}} r_1$$ For the Moon to have the same surface gravity as the Earth, we can plug in suitable numbers, and you end up with a radius ...

33

Changing orbits requires delta-v. To reach the Sun, you need to subtract delta-v such that your velocity relative to the Sun is near zero, which allows you to "fall straight down" into the Sun - your required delta-v is nearly equal to your orbital speed. To escape the solar system, you need to add sufficient delta-v in order to reach escape ...

31

Not a soft landing. A soft landing requires the spacecraft having a thrust-to-weight ratio greater than one (otherwise it just falls faster and faster). Ion engines have a very low thrust to weight ratio, much smaller than one. On the moon, the surface acceleration is 1.625m/s², so the thruster must provide at least 1.625N of force for every kg of spacecraft....

30

Is such an orbit even possible? TL;DR: If the Sun wasn't around, yes, such an orbit is possible. But since the Sun is around, such an orbit is impossible. About the name of the orbit Quoting from Emily Lakdawalla, who has a bit more gravitas than some random file blogger, What is a geostationary orbit like at Mars? I have to pause here for a brief ...

29

So let's break down the answer into bite sized chunks... How do you transfer a spacecraft from one solar system body to another? There are two main things you need to do. Set up an orbit that intercepts the orbit of your target planet/moon. Time it so that your spacecraft intercepts the target orbit at the same time as the planet/moon you're trying to ...

29

The diagram you show is the digital version of a drawing by someone with an Etch-a-Sketch: completely inaccurate. The diagram below is accurate, showing Pioneer 10 & 11 and Voyager 1 & 2 trajectories in a heliocentric, inertial reference frame, of course with the ecliptic N-S dimension collapsed. No retrograde, no dog-legs between planets. Every now ...

28

Yes, it is possible. As James K observed in a comment, the surface gravity of Uranus is slightly less than that of Earth, but its mass is 14 times larger. If Earth were orbiting Uranus, it would be a very large moon, but it would still be considered a moon, and thus a moon with a higher surface gravity than its planet. The reason this is possible is that ...

28

I don't know what the USSR was trying to do with it, but I know what the US Navy did with it. Researchers at the Johns Hopkins University's Applied Physics Lab used the Doppler shift on the 20 MHz tone to determine Sputnik-1's orbit, plus ionospheric electron density and a couple of other things (like a transmitter frequency offset of ~1 kHz from the ...

27

A great aid to intuition is to remember one principle about orbit changes: if the engine is off, the orbiter always returns to same point one orbit later. So for any orbit change, if you want to do only a short burn, it has to be at a point that is common for both the current orbit and the destination orbit. This applies to inclination changes, altitude ...

25

Sputnik 1 was pressurized with nitrogen at 1.3atm. The period of the beeping was tied to a pressure sensor. The logic was being that if anything (such as a micrometeoroid) penetrated the satellite, the change in pressure would detect this and inform the scientists on the ground. This simple test had scientific value for the later programs with living samples ...

22

I believe the answer is yes, but just barely. The distance from the Earth to the moon varies significantly over time, from 356,400 to 406,700 km. I plugged the dates of orbital entry and departure for each of the lunar Apollo missions (8, 10-17) into pyephem to find the ranges of lunar distance. At Apollo 13's flyby, the moon was one day past apogee and ...

20

The center of the Earth is, for any reasonable approximation, in one of the focus points of an elliptical orbit. For a circular orbit, there is only one focus point, so the center of the Earth is in the center of the orbit. The plane of the orbit thus would intersect both the center of the Earth as well as the launching site. If the launch site was on the ...

20

Does pork-chop plots exist for Earth-Moon System? No, because the concept doesn't make much sense. The Earth-Mars configuration and Mars' eccentricity makes the cost to send a vehicle from Earth to Mars vary by a huge amount. Pork chop plots are a useful way to visualize these huge variations. The cost of sending a vehicle to the Moon on the other hand ...

20

Escaping the solar system requires adding orbital velocity to the spacecraft. Similarly, getting closer in the solar system requires removing orbital velocity. It turns out Earth is more out of the Sun's gravity well than it's in it. In other words, the simple answer is that Mercury is "farther away" in terms of the change of velocity that's ...

19

In a two body system the total energy, the sum of kinetic energy and potential energy is constant for each body. If the total energy is constant for each body, the total energy for the whole system is also constant. So energy conservation is valid for each body alone as well as the system. In a three body system energy may be exchanged between the bodys. ...

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