Student Ideas and Questions
Common Student Ideas
Asteroids never impact Earth (or) Asteroids no longer impact Earth.
Bridge to learning: Go to Guiness World Records to read about the most recent impact crater (note: current as of May 2026).
An impact from a body the size of the Chelyabinsk meteorite of 2013 (17 meter diameter) is expected to occur once per century.
Common Student Questions
How do we know if we are imaging the same asteroid on two different nights, since it has moved?
We can detect asteroids because of their motion compared to the stationary background stars. By making repeated measurements of an asteroid’s position, we can calculate its estimated speed and direction in order to make a fairly reliable prediction of where to see it next. It’s important to make follow up observations as soon as possible, preferably within the same night, in order to increase the likelihood of locating the same asteroid. As more detections are added over multiple nights, the asteroid’s orbit becomes more accurately defined.
Typically, Rubin Observatory observes the same patch of sky twice in each night and returns every 3-4 days, making it very effective at identifying new Solar System objects and determining their orbits.
What’s the difference between an asteroid and a meteoroid?
The general distinction is size: Objects less than one meter in diameter are considered meteoroids. The smallest asteroid ever studied was 2015 TC25, which was observed when it made a close flyby of Earth in October 2015. Its diameter was two meters.
Both asteroids and meteoroids are space rocks that orbit the Sun. They could be primordial material left over from the formation of the Solar System, or remnants from comets or asteroids. Meteoroids can also be ejected from the surface of other planets, due to large asteroid impacts. Some meteorites on Earth have been identified as coming from the Moon or Mars.
What are the most dangerous asteroids known at this time?
The Sentry Earth Impact Monitoring program maintains an impact risk table of the most interesting potential future impact threats based on current observations.
The European Space Agency Near-Earth Objects Coordination Centre maintains a similar but more extensive Risk List and plot.
How do we know the shape of an asteroid? Are asteroids spherical?
Only a very small fraction of known asteroids have been studied in enough detail to determine their shapes. One way to determine an asteroid’s shape is by mapping its surface with radar. Another way is to use two or more telescopes to observe the asteroid simultaneously, a technique called interferometry. A rough idea of an asteroid shape may be derived from analyzing how its brightness changes as it rotates (its “light curve”). And a few asteroids have been imaged directly by space probes. But most of these techniques can be used to study only the largest asteroids.
The laws of physics suggest that almost all asteroids have irregular shapes, because their masses are so low that gravitational forces cannot form them into spherical shapes like the planets. Since shapes for most asteroids are unknown, they are typically modeled as spheres. Most asteroid sizes are not actual measurements but estimates, since they are computed with some assumed (model) values.
Why does the probability of impact for a newly-discovered asteroid often increase with additional observations before decreasing?
The initial set of possible orbits have a large range of positions due to uncertainties. As additional observations are made, some of the possible orbits are eliminated (many of which do not pose a risk of Earth impact). But if the small set of possible orbits that are potential Earth impactors are not ruled out, the probability of impact will increase.
For more information about this idea, check out Phil Plait’s blog post on this topic.
How do scientists make an estimate of the total number of asteroids when all of them have not yet been discovered?
The estimate is based on a fragmentation model. Think about this: If you struck a rock with a hammer, it would break into pieces. Some of the pieces would be larger, but the number of smaller pieces would be greater by a certain predictable amount. We have found with a high degree of certainty all of the larger asteroids. Knowing this number, we can apply our model to predict the total number of smaller sizes of asteroids that are likely to exist.
If we found an asteroid on a collision course with Earth, what could we do about it?
There isn’t a single answer to this question, because many variables must be taken into account: the time until impact, the orbit and speed of the asteroid, and the size and composition of the asteroid. Most techniques fall into two categories:
Deflecting the asteroid’s orbit so that it misses the Earth, or
Fragmenting the asteroid into small pieces that would not cause significant damage if they impacted Earth
More information may be found in this Planetary Society article.
The NASA DART (Double Asteroid Redirection Test) Mission was the first-ever mission dedicated to demonstrating one method of asteroid deflection by changing an asteroid’s motion in space through kinetic impact. In September 2022, the DART spacecraft deliberately crashed into the small asteroid moonlet Dimorphos. It shortened Dimorphos’s orbit around its parent asteroid Didymos by about 33 minutes and changed the overall orbit of the binary asteroid around the Sun by 0.15 seconds.
It is critically important that asteroids are detected as early as possible, because there is no way to deflect or destroy an asteroid that’s detected hours before impact. No known weapon system could intercept the asteroid because of the velocity at which it travels–an average of 12 miles per second.