Overview
At The Asteroid News, our mission is to increase the public's understanding of asteroids. To that end, we choose the right tools for clear communication. We use nominal distance, expressed in Earth Radii. When reporting close approaches, we use the nominal distance (the most likely) in Earth Radii (Earth's mean radius ≈ 6,371 km or 3,959 miles, measured from the center to the surface).
Nominal Distance: The Actual Prediction, Not the Extremes
The nominal distance is the best-fit or most likely orbital solution calculated using the current data. In simple terms, “nominal distance” is the most probable distance the asteroid will pass by Earth, based on the latest observations; it represents the expected path that fits all known data.
Astronomers also compute minimum and maximum distances that reflect 3-sigma orbital uncertainty (“error bars”), meaning a 99.7% confidence range. These represent “worst-case” and “best-case” extremes; however, they do not reflect the most probable encounter distance. As more observations are reported, the uncertainty range typically shrinks, and the nominal distance may shift as the orbit becomes better understood.
The minimum and maximum distances are not predictions; they are the error. An analogy (imperfect) would be a hurricane cone of uncertainty. Using the minimum as if it were the prediction can trigger unnecessary alarms, and when the minimum is ruled out, it can lead to confusion about why the “prediction” changed. In simple terms: the minimum and maximum are the error, not the prediction.
Why Earth Radii Communicates “Closeness” More Intuitively Than Other Units
Earth Radii is not used by many space agencies or science writers for reporting close approaches; they use kilometers, miles, lunar distances, or astronomical units. When people read:
- “An asteroid will pass 1,000,000 kilometers from Earth” — many people ask: is that close?
- 37,399 km (~23,000 miles) sounds enormous.
- 0.00025 AU might seem tiny and alarming.
- 0.099 LD means “closer than the Moon,” but many people underestimate the Moon’s distance (~384,400 km, ~60.3 Earth Radii).
When answering “how close,” there is an implicit question behind the question: Are we safe? Kilometers, miles, AU, and LD answer “how far in absolute terms,” but they do not communicate “how close relative to Earth” as directly as Earth Radii does.
- 0 Earth Radii = Earth’s center
- 1 Earth Radii = Earth’s surface
- Geostationary satellites orbit at ~6.6 Earth Radii
- Lunar distance: 60.3 Earth Radii away
A major benefit of Earth Radii is that it lets readers create a scale model on the spot from anything round they may have on hand, from coins to balls. A distance of six Earth radii becomes three “coins” laid end to end.
A Practical Example: Apophis 2029
On April 13, 2029, asteroid 99942 Apophis (2004 MN4) will pass 5.97 Earth Radii from the center of Earth, which is more intuitive than saying 0.099 lunar distances, 0.00025 astronomical units, or 38,012 kilometers.
Our Position
We at The Asteroid News believe Earth Radii is the clearest, most intuitive, and most safety-relevant metric for communicating NEO close approaches, and that Earth Radii is the more intuitive tool for informing the public about NEO close approaches. We hope space agencies and science writers start using Earth Radii as well as one of the metrics for reporting NEO close approaches.