# 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 ...

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 ...

52

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 ...

45

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 ...

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 ...

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 ...

34

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

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 ...

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

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 ...

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 ...

27

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 ...

27

Yes, but only because the Earth rotates. If you throw the ball, you will end up in a slightly different spot. If you were at a high point at the equator and threw the ball due East at higher than orbital velocity, you would rotate by the time the ball came back and it would be slightly over your head, because you have moved to a different spot in the orbit. ...

25

A first-order approximation to see whether an orbit could be stable is to calculate the radius of the Hill sphere of the parent: $$R_H \approx a\sqrt[3]{\frac{m}{3M}}$$ With the mass or the ISS ($m$), the mass of the Earth ($M$), and the orbital radius of the ISS ($a$), we see that the region the ISS dominates gravitationally has a radius of less than 2 ...

23

The lowest orbit achieved would probably be PFS-2, a small satellite deployed from Apollo 16's service module. It was intended to go into a 55x76-mile orbit (88.5x122 km), but due to variations in the Moon's gravity field, it made passes of six miles (9.6 km) or less before crashing into the Moon's surface. There are very few stable low orbits around the ...

22

What's the root cause of the disparity between what the article says and what's shown in the video? There is no disparity. The article says you need a minimum velocity of two inches per second [bold emphasis mine]: The engineer added, “Simple trigonometry led to the conclusion that pushing an object away at two inches per second within a 30-degree cone ...

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 ...

18

"Lowest possible lunar orbit..." As pointed out in comments and in answers to the linked questions Are low, polar lunar orbits in general relatively stable? Moon orbit station-keeping delta-V budget What's the floor for stable retrograde lunar orbits? Besides Luna, what celestial bodies exhibit lumpy gravity? very close orbits around any body ...

15

Circular orbits at different altitudes require different speeds, so if you start with a radial separation, the spacecraft and station will tend to drift further apart unless they accelerate themselves radially to close the distance. The effect is small at small distances, larger at long distances. To a first approximation, the separation is the difference ...

13

Based on the calculations presented by @uhoh I generated a plot showing the necessary delta-V for a fly-by mission, i.e. entering into a Hohmann transfer with a far point intersecting the orbit of a planet to get into a circular orbit at the same radius as a planet Note that this does not include any methods to save fuel (aero-braking, swing-by) and ...

13

It’s a long journey, but it’s all “downhill” — once the spacecraft leaves the moon’s gravitational sphere of influence, Earth’s gravity brings it home. The process of leaving the moon is called “trans-Earth injection” or TEI; the rocket engine on the CSM fires for about two and a half minutes, adding about 1000 m/s to the spacecraft's speed in lunar orbit, ...

12

The orbital mechanics of satellites are independent from the mass of the satellite. As long as the sats mass is tiny compared to the mass of Earth. The total mass of the ISS is much larger than the mass of the dragon capsule itself, the same is true for the volume and surface of both. So the atmospheric drag of both changes only very little after docking.

12

There are multiple methods by which a satellite may determine where it is. The traditional approach is ground tracking. As David Hammen has mentioned, in this case ground stations detect where the satellite is and how it is moving. This information is then used to calculate the orbit. If necessary, the satellite may be told about this orbit, but more ...

12

The Moon is not a perfect sphere with homogenous density, there are mass concentrations (called the Mascons). So the lunar orbit is not a perfect circle or ellipse. Low orbits change their shape without losing energy under the influence of the Mascons. If the orbit loses too much height at one point the object may crash into the lunar surface. The orbit is ...

12

Are there any known examples of this situation? Yes! In addition to Phobos mentioned in this answer and from Astronomy SE: How did “oddball” Valetudo, Jupiter's new prograde moon, end up in a wider orbit? Why are most of Jupiter's moons retrograde?

12

I've got a set of Keplerian orbital elements $e_0$, $a_0$, $i_0$, $\omega_0$, $\Omega_0$, and $\theta_0$, and I'd like to get to a different orbit with orbital elements $e$, $a$, $i$, $\omega$, $\Omega$, and $\theta$. How do I calculate (a) the amount of delta-v I'll need for this maneuver or set of maneuvers, and (b) which maneuver or set of maneuvers I ...

11

You're correct. On a perfectly spherical, atmosphere-free Earth, with no obstacles as tall as you, with a uniformly spherical gravitational field, it would be possible; the low point of the orbit would be at the altitude you threw the ball from, a couple of meters above the surface.

10

Orbits beneath synchronous orbits have a higher angular velocity than their planets rotation, orbits above have a slower angular velocity. Drag (atmospheric or tidal) would try to match the angular velocity to the planets rotation. So below a synchronous orbit objects get slower, above it they would speed up (and slow down the rotation of the body they are ...

10

Your comment on this answer has, I think, led me to understand what you are really asking about. What I am saying is that, presuming the inclination to be ZERO (Plane of orbit parallel to the equatorial plane), the entire plane keeps shifting from pole to pole - parallel to the equatorial plane. You want to move the orbit like this: If that is the right ...

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