Asteroid Passing Earth This Week: How Close, How Big
Is an asteroid passing Earth this week? Almost certainly. In most weeks, several known asteroids pass within a few dozen times the moon's distance, and now and then a small one comes closer than the moon. These flybys are routine. The figures that tell you whether one is worth a look are its size, its speed and how close it gets, and close-approach distances are given in "lunar distances," which makes them easy to picture. This article explains how to read each of those numbers. It also shows how to keep the closest flybys of the next two weeks on a spare screen, drawn to scale.
Is an asteroid passing Earth this week?
The headlines usually sound alarming ("stadium-sized asteroid to pass Earth"), but the event underneath is ordinary. Astronomers have cataloged tens of thousands of near-Earth asteroids, and their orbits cross the region around Earth's orbit all the time. Any given week has a list of close approaches. Most of them are far away by everyday standards and close only by the standards of the solar system.
This article doesn't name this week's flybys. The list changes from day to day, and new objects are found constantly. What follows is how to make sense of the list, wherever you read it.
How close is close? Using the moon as a ruler
Miles and kilometers stop meaning much at these distances, so close approaches are usually measured against the moon. One lunar distance (LD) is the average distance from Earth to the moon, about 238,900 miles or 384,400 kilometers.
| Lunar distances | Miles (approx.) | Kilometers (approx.) | What it means |
|---|---|---|---|
| 0.5 LD | 119,000 | 192,000 | Inside the moon's orbit, a genuinely close pass |
| 1 LD | 238,900 | 384,400 | As far away as the moon |
| 5 LD | 1.2 million | 1.9 million | Close for an asteroid, still far from Earth |
| 10 LD | 2.4 million | 3.8 million | A typical "close approach" in the news |
| 20 LD | 4.8 million | 7.7 million | About the outer edge of what gets reported |
Why "closest approach" is a single moment
An asteroid's distance changes all through its pass. The figure that gets reported is the closest point, together with the date and time it happens. Hours before and after that moment, the asteroid is already noticeably farther away.
How big is it?
Asteroid sizes are almost always estimates. Most near-Earth asteroids show up in a telescope as a single point of light, so their size is worked out from how bright they look. A small, reflective rock and a larger, darker one can look equally bright. That is why sizes are often given as a range, such as "between 30 and 70 meters," and why news stories compare them to buses, houses or stadiums.
A few things to keep in mind:
- Small ones are the most common. Most of the closest passes are rocks a few meters to a few tens of meters across.
- Big ones are tracked longest. Larger asteroids are brighter, so they tend to be found years ahead and their paths are known well.
- A range is honest. A single precise size for an asteroid that has never been imaged up close is a guess dressed up as a fact.
How fast is it going?
Speeds are given relative to Earth, usually in kilometers per second or miles per hour. They are typically several kilometers a second, and often more, which means tens of thousands of miles per hour. The number sounds dramatic, but for a viewer it mostly tells you how quickly the asteroid crosses the sky and how short its closest approach is.
Why some asteroids are found only days before
Many of the closest passes are by small asteroids that were discovered only days before they arrived. They are too faint to see until they get near. An orbit worked out from a few nights of observations still has some uncertainty in it. In that case, the honest way to describe the pass is as a range: how near it might come, how far it might stay, and a closest-approach time that could shift by hours or days. Each new observation narrows the range.
Can you see one?
Usually not without a telescope. Even a close pass is a faint point of light, and the small rocks that come nearest are the faintest of all. Amateur astronomers with good equipment sometimes photograph the brighter ones. For most people, the satisfying part is knowing that one is going by and where it sits compared with the moon.
Following asteroid flybys on GridTicker
GridTicker is a live ticker that runs in a browser on a spare screen. It has a feed for exactly this question. The asteroid card shows the closest asteroid passing in the next two weeks: how big it is, how fast it's going, and how far away it will be in the moon's distances. Next to those numbers is a picture of the pass. It shows the Earth, the moon, and the asteroid going by at its true distance, moving along its path. You don't have to convert lunar distances in your head because the moon is right there in the drawing for scale.
When an asteroid was found only days before and its orbit is still uncertain, the card says how near and how far it may pass, and how many hours or days either way. The card also says plainly that none of these asteroids is a danger to Earth.
Here's how to set it up:
- Open Settings → Space (/settings/space).
- Under Space feeds, turn on the asteroid feed for each screen where you want it.
- Add other sky feeds if you like. Meteor showers around their peaks and the northern lights are feeds of their own on the same page.
The guide's section on the sky cards explains each one: The moon, eclipses and the rest of the sky. GridTicker runs in any modern browser on Windows, macOS, Linux or a Raspberry Pi, with nothing to install. You can put it across the top of a monitor, on a TV in the corner or on a spare laptop and leave it running.
To have the next close pass on your screen, drawn at its true distance, set up a strip at https://app.gridticker.com/signup.