The AU-to-miles conversion is the last surviving imperial bridge in American astronomy. NASA computes interplanetary trajectories in metric — always metric, since the Mars Climate Orbiter burned up in 1999 because Lockheed Martin used pound-force-seconds and JPL expected newton-seconds. The $327 million mistake eliminated imperial units from NASA's flight software permanently. But when the press office writes a headline — "New Horizons Reaches Pluto After 3 Billion Miles" — they convert from km to miles. Every American space mission runs on two sets of numbers: the metric ones the engineers use, and the imperial ones the public reads.
Multiply AU by 92,955,807.3. That's the formula. It's the kilometer figure — 149,597,870.7 — divided by 1.609344, the exact number of kilometers in a statute mile. (The US survey mile of 1,609.347 meters was retired on January 1, 2023 — a date that passed with roughly zero public attention. The international mile is now the only legal mile in the United States.)
The conversion traces back through two layers of metrology: first the IAU 2012 definition of the AU in meters, then the 1959 International Yard and Pound Agreement that fixed the inch at 25.4 mm, the foot at 0.3048 m, and the mile at 1,609.344 m. The AU-to-miles conversion is a chain of four international agreements, none of which were written with astronomy in mind, all of which astronomers now depend on.
Jupiter orbits at 5.2 AU — 483 million miles. Neptune at 30 AU — 2.8 billion miles. Voyager 1 at 163 AU — 15.2 billion miles. The numbers get big fast. The AU-to-miles conversion is mostly used for the outer planets, because those are the distances that sound most impressive to a public audience. Nobody in a press release says "Pluto is 39.5 AU away." They say "3.7 billion miles." The AU sounds clinical. The miles sound like a road trip.
Solar System in miles
| Body | Distance (AU) | Distance (miles) | |
|---|---|---|---|
| Mercury | 0.39 | 36,000,000 | 88 Earth days per orbit |
| Venus | 0.72 | 67,200,000 | Our closest planetary neighbor |
| Earth | 1.00 | 92,955,807 | ±1.7% over the year (elliptical) |
| Mars (opposition) | 0.52 | 48,300,000 | Closest approach, every 26 months |
| Mars (conjunction) | 2.52 | 234,000,000 | Far side of the Sun |
| Jupiter | 5.2 | 483,000,000 | 12 Earth years per orbit |
| Saturn | 9.6 | 892,000,000 | Cassini took 7 years to get here |
| Uranus | 19.2 | 1,780,000,000 | Discovered by Herschel in 1781 |
| Neptune | 30.0 | 2,790,000,000 | Predicted by Le Verrier, found by Galle |
| Pluto | 39.5 | 3,670,000,000 | New Horizons: 9.5 years, 1 flyby |
Frequently Asked Questions
Why does the Sun-Earth distance change if the AU is fixed?
The AU is now a fixed definition, not a live measurement. The actual Earth-Sun distance varies because Earth's orbit is an ellipse with an eccentricity of about 0.017. At perihelion (around January 3), Earth is 0.983 AU from the Sun — about 91.4 million miles. At aphelion (around July 4), it's 1.017 AU — about 94.5 million miles. The 1.0 AU value is the semi-major axis: the average of the closest and farthest points. The seasons come from axial tilt, not distance variation. The Northern Hemisphere is actually closer to the Sun in January than in July.
How many miles does a spacecraft travel to get to Mars?
A Hohmann transfer — the most fuel-efficient path — covers about 300 million miles over 8-9 months. That's a curved arc, not a straight line. At the moment of launch, Mars might be 48 million miles away in a straight line. The spacecraft doesn't aim at where Mars is. It aims at where Mars will be when it gets there. The trajectory is a transfer ellipse that touches Earth's orbit at one end and Mars's at the other, covering roughly half an orbit — about 3 AU of actual travel distance. Straight-line distance: misleading. Trajectory length: the number that matters.
Should I use miles or km for astronomical distances?
If you're doing science: kilometers. Every journal, every mission, every international collaboration uses metric. If you're explaining space to an American lay audience: miles, because 93 million is a number people have heard before and 150 million kilometers is not. The engineering work happens in metric. The translation to miles is a courtesy — signal, not science. This page exists because sometimes you need the number in the unit your audience understands.
Engineering Context
NASA's Eyes on the Solar System — the public visualization tool — converts all internal metric coordinates to miles for its US-facing display. The conversion runs thousands of times per frame as it renders the real-time positions of spacecraft and planets. Each conversion is a multiply by 0.621371 — the km-to-miles factor — derived from the exact 1 mile = 1.609344 km relationship. The rendering runs at 60 frames per second. At 60 fps, over a 10-minute session, the AU-to-miles conversion runs 36,000 times per object. It's a multiply. It costs nothing. But it's the bridge between a metric Solar System model and an imperial-reading public, and that bridge is crossed millions of times a day without the public ever knowing it's there. Length Conversion Guide.
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