NASA’s DART (Double Asteroid Redirection Test) deliberately slammed into the Didymos’ tiny moon Dimorphos as a test to see if we can change the orbit of an asteroid by impact. This was a safe test because the impact was small enough to only nudge the tiny moon, no knock it out of orbit. Since the impact would slow down the moon, the worst case was the moon slows enough that it eventually lands on the parent asteroid.
DART impacted Dimorphos at about 15,000 mph (24,000 kph) and altered its 12-hour orbit by 32 ±2 minutes. While a 30-minute change would be easy to note, it’s far enough away that pinning down the amount of change will be difficult.
Scientists have a few methods of timing orbits. The first and least accurate is to simply measure the time it takes the moon to reach the same spot. Thanks to movement of the asteroid that changes the background stars, accuracy is limited in this method for near-Earth asteroids. It’s good enough to see that a change takes place, but distance and position are not accurate enough to get a precise reading. Imagine a clock tower with no hour or minute marks – you’re not going to be able to know precisely when it’s 6pm, but you’ll get reasonably close.
The second method is to catch the moon occulting (eclipsing) a star. Since a star is a point source (no size), it makes an excellent source to time the occultation. As the moon passes in front of the star it winks off. For a time, the point of light is blocked as the asteroid passes in front, then flashes back on. The light from the star is essentially parallel, so the eclipse shadow is the size of the asteroid. It’s invisible to the naked eye, but travels like a solar eclipse shadow.
If you know your precise location and time the event to a video frame, you get a very precise rendering of the time and position on the moon for your chord. If you have several people spaced out along the path at different distances from the center, then you get excellent information on the shape, size, and speed of the asteroid:
In this case, Didymos was much closer to Earth than the usual asteroid target, so it’s a much more difficult target. It’s the difference between a quarterback throwing passes to a stationary receiver to warm up and trying to throw the ball to a sprinting receiver. However, several teams of observers from the International Occultation Timing Association (IOTA) managed to successfully time occultations and increase the precision of the changed orbit of Didymos.
We record these occultations with video systems that overlay the GPS position and time on the frames. We have software that can record the brightness of the star and render it to a chart. Sometimes, the result is a clear, squared-off U that shows a drop in light levels. This time, the brightness varied more than usual (twinkling stars) and the drop was very short (it was moving fast), so only a few frames show the drop in light:
However, that brief downward spike is enough to tell scientists what the need to know!