Student Ideas and Questions
Common Student Ideas
Asteroids never impact Earth (or) Asteroids no longer impact Earth.
Bridge to learning:
Explore the interactive Earth impact database and map to see the geographic and age distribution of known impact craters. Follow up with discussion: Are all impact craters old? What is the oldest and youngest crater you found?
An impact from a 100-meter (328-foot) asteroid, the smallest believed capable of causing regional devastation, is estimated to occur about once every 1000 years on average.
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 detect asteroids because of their motion. By taking measurements of an asteroid’s motion, we can calculate (for the short term) its estimated speed and direction in order to make a fairly reliable prediction of where to see it next, as long as there is a quick follow up observation. We can also confirm whether its magnitude is consistent with the first set of observations.
What’s the difference between an asteroid and a meteoroid?
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. Meteoroids can also be ejected from the surface of other planets during large asteroid impacts. Some Earth meteorites have been identified as originating from the Moon or Mars.
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.
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. Note that based on this analysis, the asteroid with the highest impact probability has only a 10% chance of Earth impact, beginning in 2095.
How do we know the shape of an asteroid? Are asteroids spherical?
Almost all asteroids have irregular shapes. Their masses are so low that gravitational forces have not been able to pull them into spherical shapes like the planets.
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. Some space probes have flown to asteroids and imaged them. Finally, a rough idea of an asteroid shape may be possible to determine from analyzing light curves of its rotation. But all of these techniques can be used to study only the largest asteroids. Since shapes for most asteroids are unknown, a sphere is used as a general model for determining the volume of an asteroid. The derived sizes are not actual measurements but estimates, since they are computed with some assumed 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 our 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:
- Altering the asteroid's orbit ( an idea proven feasible by the DART mission), or
- Fragmenting the asteroid into many small pieces that would not cause significant damage if they impacted Earth
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.
More information may be found at the NASA Planetary Defense Coordination Office.