# Are the planets where we expect them to be

• Bjarne
In summary, the planets are there when probes arrive, but there is error in the expected position. The film is discussing these mundane errors with regards to spacecraft.
Bjarne
I saw a film some time ago at NGC.
It was told that the planets not are exactly there where expect them to be, when space probes arrives, but I did not got the point..
What is wrong ?

Can you give the title of the film? I haven't heard of that problem, but I can tell you that the planets are there when probes arrive, otherwise the probes would miss, and we wouldn't have all the cool pictures we get from the probes on and around those planets.

But perhaps the film was talking about planets being just a tiny bit out of their expected position; maybe enough difference to be measured, but not enough to make the probe miss? That seems pretty unlikely, since the margine for error on interplanetary probe missions is very small. I'd like to search for the qoute from the film itself, if you can remember the title (and if it's in English).

LURCH said:
Can you give the title of the film? I haven't heard of that problem, but I can tell you that the planets are there when probes arrive, otherwise the probes would miss, and we wouldn't have all the cool pictures we get from the probes on and around those planets.

But perhaps the film was talking about planets being just a tiny bit out of their expected position; maybe enough difference to be measured, but not enough to make the probe miss? That seems pretty unlikely, since the margine for error on interplanetary probe missions is very small. I'd like to search for the qoute from the film itself, if you can remember the title (and if it's in English).

It's long ago
Yes it's only a tiny bit

More likely the course corrections the probes make are due to error in the probe's trajectory, not error in the expected position of the planet.

Bjarne said:
I saw a film some time ago at NGC.
It was told that the planets not are exactly there where expect them to be, when space probes arrives, but I did not got the point..
What is wrong ?

Since you cannot supply a source to determine what is truly being discussed, I suspect the problem is that the target planet is not where it is expected to be relative to the the incoming the space probe. Our imperfect modeling of the behavior solar system is a part of the total error. For example, there is about a 1 km uncertainty in Mars position over the last/next 10-20 years; errors increase moving forward or backward in time.

Additional error arises from not quite knowing the state of the probe. The probes navigate by dead reckoning, with occasional Kalman updates based on Deep Space Network measurements. DSN range rate measurements are incredibly precise (~ 1 mm/sec). Measurements of range, are also quite good (~ 1 m). Things get worse normal to line of sight. Cross range position error (observable by using multiple antenna) is on the order of a kilometer. Transverse velocity is not directly observable.

These are errors with respect to Earth -- and that brings us back to the uncertainties in the ephemerides.

twofish-quant said:
I suspect its more along the mundane lines alluded to above. The Pioneer anomaly is observable only in the tiny handful of vehicles that have gone well beyond the planets; the flyby anomaly is a tiny possibly unexplained change in velocity during a planetary flyby. Neither of these pertains to the "planet not being where we expect them to be."

Also just to give you an idea of the types of effects that do impact spacecraft , look at this http://en.wikipedia.org/wiki/Yarkovsky_effect
Force due to solar radiation pressure is much smaller than thrust uncertainty. It is an effect, but not much of one, during transit. (Note: This is not quite the same as the Yarkovsky effect. The Yarkovsky effect is a second order effect that results from tumbling. We do not like our spacecraft to be tumbling.) Torque due to solar radiation pressure does make for small but persistent perturbative effects during long term operations. For this reason, the Mars Reconnaissance Observer underwent am eight day solar radiation pressure calibration at the end of the cruise phase to Mars.

## 1. Where do we expect the planets to be?

Based on our current understanding of the solar system, we expect the planets to be in their respective orbits around the sun.

## 2. How do we determine the expected position of planets?

Scientists use mathematical equations and observations from telescopes and satellites to determine the expected positions of planets in their orbits.

## 3. Have the planets always been where we expect them to be?

No, our understanding of the solar system and the positions of planets has evolved over time as we continue to gather more data and make new discoveries.

## 4. Are there any factors that could cause the planets to deviate from their expected positions?

Yes, gravitational interactions between planets, asteroids, and other objects in the solar system can cause slight variations in the expected positions of planets.

## 5. Do we ever need to adjust our predictions for the positions of planets?

Yes, as our technology and understanding of the solar system improves, we may need to make adjustments to our predictions for the positions of planets. This is especially true for distant or newly discovered planets.

Replies
12
Views
2K
Replies
15
Views
2K
Replies
45
Views
4K
Replies
8
Views
446
Replies
2
Views
1K
Replies
19
Views
2K
Replies
12
Views
2K
Replies
14
Views
3K
Replies
6
Views
2K
Replies
6
Views
1K