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61 votes
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Why do scientists use specialized units for distance when metric units are perfectly adequate?

From Kepler to the 1960's, the AU was the best measurement basis available for astronomy. Planetary motions could be expressed precisely in AU, but the relation of the AU to terrestrial units was only ...
John Doty's user avatar
  • 3,364
25 votes
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Why did NASA collect so much data about electrical phenomena at the Apollo 13 launch site?

According to an article from the Lunar and Planetary Institute (archive.org link): As a result of the electrical disturbances experienced during the Apollo 12 launch, several experiments were ...
Russell Borogove's user avatar
23 votes
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If I wanted to reconstruct an entire Apollo mission's crewed spacecraft trajectories, what are the key sources of historical data I'd look for?

To answer the question literally: you'd be looking for NASA Apollo Trajectory (NAT) data files. The report Apollo Mission 11, Trajectory Reconstruction and Postflight Analysis Volume 1 (PDF) provides ...
Ludo's user avatar
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19 votes
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How do we know what percentage of NEOs we've discovered?

NEOs are mostly found as dots in images taken by various telescopes, often those of amateurs (as in not paid, nothing about skill or equipment). By taken repeated images days apart moving dots can be ...
GremlinWranger's user avatar
12 votes

Why do scientists use specialized units for distance when metric units are perfectly adequate?

In general, people find really enormous numbers hard to fathom, and this is true even for scientists. When using the metric system, it's not so bad hard to understand kilo-, mega-, and occasionally ...
Barmar's user avatar
  • 229
11 votes
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Fuel vs food on the trip to Mars?

Fuel requirements will probably dominate food and other consumables by an order of magnitude or more, so you can't save mass by shortening the trip. The exact tradeoff depends on the assumptions you ...
Russell Borogove's user avatar
10 votes
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How precise are spacecraft trajectory measurements?

The injection accuracy of the launch vehicle is typically measured in m/s, as the velocity change required to get the spacecraft exactly on the desired trajectory. This wraps up all the dimensions of ...
Mark Adler's user avatar
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9 votes
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How is foot-pounds of energy defined?

Is it... the gravitational potential energy of one pound hoisted one foot in a constant gravitational field...? Yes indeed it is! To be energy, the pound has to be parallel to the foot. $$E = \int \...
uhoh's user avatar
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8 votes
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What are these structures on the Lunar Ranging Retro Reflector (LRRR) arrays for?

The Apollo 11 EASEP handbook gives basic information on the first version. Of the LRRR, it has just two pages of text... There are some (poorly reproduced) images labelling the main parts. It's ...
Andy's user avatar
  • 5,178
8 votes

What forms of water ice have been observed and verified in the solar system?

Actually, Ice VII has been discovered in diamonds on Earth. The water is first trapped in the diamond as the latter is formed deep in the mantle. Then when the diamond cools at the surface its rigid ...
Oscar Lanzi's user avatar
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8 votes
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What is the order of magnitude forces due to Earth's magnetic field, sunlight, drag, oblateness and tidal forces compare?

Here is a nice graph of part of what you are asking for. It's from the book Satellite Orbits; Models, Methods, Applications by Oliver Montenbruck and Eberhard Gill, Springer, 2000. The figure and ...
Alexander Vandenberghe's user avatar
7 votes
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How is atmospheric temperature measured from a satellite?

I'm not sure if I understand the basics behind this well enough to explain it but I'll give it a try anyway. I've looked mainly at this report and on the wiki on satellite temperature measurements. ...
Alexander Vandenberghe's user avatar
7 votes
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What forms of water ice have been observed and verified in the solar system?

The only form of ice that we see naturally in bulk on Earth is Ice I, all within the sub-h variety. There's no place on Earth that gets cold enough for any other form--but that's not necessarily true ...
Justin Eiler's user avatar
7 votes
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How are precision trajectory measurements made of trans-Neptunian spacecraft?

Doppler and ranging are used routinely. They are both two-way, with the Doppler turning the frequency around and the ranging turning around a pseudo-noise signal. This is complicated, only for these ...
Mark Adler's user avatar
  • 58.2k
7 votes
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How can rockets measure their own speed relative to the ground?

You can put measuring devices anywhere you like, but the best answers are always computed by measuring many different ways simultaneously. If you have enough independent methods, you can detect and ...
Ryan C's user avatar
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6 votes
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How will OSIRIS-REx scan and characterise the near-earth asteroid Bennu?

OSIRIS-REx is packed all full of good stuff. I'll throw together a quick list of the scanning ones you're interested in. Also of note is that the entire spacecraft will be making that scanning ...
M.A.H.'s user avatar
  • 1,266
6 votes

How does an atmospheric probe measure temperature during descent?

As described in extensive detail by Al Seiff and T.C.D. Knight in their May 1992 paper in Space Science Reviews, Vol 60; The Galileo Probe Atmosphere Structure instrument (which can be read without ...
Tom Spilker's user avatar
  • 18.3k
6 votes
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How would 2 spacecraft be launched to be in sync in opposite directions?

STEREO A and B, 2006-047A and B, (29510 and 29511) were launched together from "Cape Canaveral Air Force Station in Florida on a Delta II 7925-10L launcher into highly elliptical geocentric ...
uhoh's user avatar
  • 149k
6 votes
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Can a spacecraft use an accelerometer to determine its orientation?

If multiple accelerometers are spread around the vehicle, their readings can be combined to determine angular speed (from centripetal acceleration) and angular acceleration somewhat easily. There ...
CourageousPotato's user avatar
5 votes
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Why do the LAGEOS' satellites have four germanium corner cube reflectors out of over 400?

D.A. Arnold "Optical and Infrared Transfer Function of the Lageos Retroreflector Array", 1978, NGR 09-015-002 (P179) confirms that they're for infrared measurements, and adds some interesting info: ...
Bob Jacobsen's user avatar
  • 12.7k
5 votes

Could a Kuiper Belt object fly by probe measure star distances by parallax better than Gaia?

To use the longer base line of 100 AU instead of 2 AU, we need measurements from both the probe in the Kuiper Belt and another probe near the earth. But the data transfer over 100 AU distance is very ...
Uwe's user avatar
  • 1,714
5 votes

What instruments and techniques measured Mars' atmospheric D/H ratio which suggests (all of) it's water didn't evaporate after all?

I don't have access to the Science paper in question (would have to register, boo, that ain't open) so I don't see what Scheller et al cited for their D/H sources (the abstract makes it look like the ...
Erin Anne's user avatar
  • 11.5k
5 votes
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Local atmospheric pressure and local surface gravity in Jezero crater?

Perseverance has been making pressure measurements daily it seems for a while: https://mars.nasa.gov/mars2020/weather/ 750 Pa / 101325 Pa = ~0.007 atm I believe most surface probes include an 'weather'...
BrendanLuke15's user avatar
5 votes
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Significance of measure of dispersion in Orbit determination output

It's all about choosing how much tolerance you have for uncertainty. The more sigmas you use, the higher your confidence becomes that the object is within that many standard deviations of the ...
Ryan C's user avatar
  • 7,972
5 votes

Measuring Oxygen Quantity

Assuming you are asking about the oxygen in the Service Module tanks - the stuff used as a reactant in the fuel cells - quantity was measured by means of capacitance probes in the tanks. (Source: ...
Organic Marble's user avatar
4 votes
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Could an Igloo work on Mars?

On most of Mars, the air pressure is ~600 Pa, which means any exposed ice will sublimate. You'd have to seal the ice in with a layer of another material. Also, if you were to evaporate ice to a ...
Hobbes's user avatar
  • 128k
4 votes

Could daily variations of weight on Earth really be 0.003%?

From a book in french about tides: If a man with a mass of 102 kg is standing on earth, the constant gravitational force of Earth to the man is 1000 N, of the Sun 0.61 N or 610 mN and of the Moon 0....
Uwe's user avatar
  • 49k
4 votes

When were the presence of specific elements on asteroids first identified?

The first asteroid (Ceres) was discovered on 1801. Initial studies on asteroids were primarily on positional parameters and nature of orbit. The first ever attempt on predicting the composition of ...
Nilay Ghosh's user avatar
  • 1,022
4 votes
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Could daily variations of weight on Earth really be 0.003%?

The total gravitational pull of the Moon is $g_M = 3.3\cdot10^{-5} \frac{m}{s^2}$ or $0.0003\%$ of Earth's gravity. This is one order of magnitude smaller than the stated value. At all times, most of ...
asdfex's user avatar
  • 15.1k
4 votes

Measuring the pressure and temperature of Pluto's atmosphere using stellar occultations

Stellar occultations of distant atmospheres produce light curves that are almost entirely due to refraction, not opacity (absorption), in those atmospheres. Ray paths of light rays encountering the ...
Tom Spilker's user avatar
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