Sizing Up Near-Earth Asteroids

by Amy Mainzer
Scientist and Engineer

Asteroids. The word conjures images of pitted rocks zooming through space, the cratered surfaces of planets and moons, and for some, memories of a primitive video game. Just how hazardous are these nearest neighbors of ours? We think that one contributed to the extinction of the dinosaurs, giving rise to the age of mammals. How likely is this to happen again?

The Wide-field Infrared Explorer (WISE) mission, an infrared telescope launching in about a year, will observe hundreds of near-Earth asteroids, offering unique insights into this question. The risk posed by hazardous asteroids is critically dependent on how many there are of different sizes. We know that there are more small asteroids than large ones, but how many more, and what are they made of?

Asteroids reflect sunlight (about half of which is the visible light that humans see), but the sun also warms them up, making them glow brightly in infrared light. The problem with observing asteroids in visible light alone is that it is difficult to distinguish between asteroids that are small and highly reflective, or large and dark. Both types of objects, when seen as distant points of light, can appear equally bright in visible light. However, by using infrared light to observe asteroids, we obtain a much more accurate measurement of their size. This is because the infrared light given off by most asteroids doesn’t depend strongly on reflectivity.

This image of near-Earth asteroid 433 Eros reveals that its ancient surface has been scarred by numerous collisions with other small objects. Image credit: NASA/JPL/JHUAPL

WISE will give us a much more accurate understanding of how many near-Earth asteroids there are of different sizes, allowing astronomers to better assess the hazard posed by asteroids. The danger posed by a near-Earth asteroid depends not only on its size, but also on its composition. An asteroid made of dense metals is more dangerous than one of the same size made mostly of less dense silicates. By combining infrared and visible measurements, we can determine how reflective the asteroids are, which gives us some indication of their composition.




    27 Responses to “Sizing Up Near-Earth Asteroids”

  1. Peter Says:
    November 13th, 2008 at 9:58 pm

    Seeing as how Jupiter was struck by a comet a few years ago, are there any plans to track objects which might be on collision courses with other bodies in the solar system besides earth?

    Mainzer says

    There is a program in place at JPL to look for impacts on other planets in our solar system as well as Earth. The JPL near-Earth object observation program ( predicted a possible impact on Mars on January 30, 2008, although subsequent observations ruled out the possibility a couple of weeks before that date. The JPL near-Earth Object office tracks possible impacts on all the planets in our solar system, although somewhat less precisely than it tracks potential Earth impactors.

  2. mary anne smit Says:
    November 15th, 2008 at 11:58 pm

    when will info be available from april/may 1999 of night sky of large rock object /asteroidal ? is there info on rocky objects that do not fill the critera of a “typical asteroid or small bodies object. attention to Amy Mainzer thank you .

  3. Bill Wright Says:
    November 25th, 2008 at 7:31 pm

    Hi Amy,

    As I watch the History’s specials on different matters of the cosmos - questions for you “scientists” at JPL come to mind. Where is the scientific method employed in your organizations work? If I am remembering my scientific method properly, it is a method to eliminate subjectivity as much as possible. Matters such as replication and testability of hypothesis are important parts? Are there alternate universes that you are using as a second and third rep? Have things that do not have the ability to be ran through the scientific process become objective of late? In my focus of science it is necessary for publication that the work be able to be reproduced by anyone utilizing the same methods.

    I dont mean to sound off - but it seems to me if we are going to solve some of our own earthly problems - it might be useful to utilize some of your talents/knowledge here on earth - things such as energy, resource use and sustainability. Then all of us could collectively explore the various unknows of the tomorrow and universe.

    It is sad to me that our time is spent on things we cannot control rather than on things we can - where is the history’s channel public outreach toward our current problems? And where is the distinction between science and an adventure.

    I welcome your thoughts and becoming educated on how astronomy is a scientific discipline.


  4. Derek Says:
    November 30th, 2008 at 8:26 pm

    How do scientist see the asteroid that is supposed to come very near to the Earth in or around the year 2022. It must be so very very far away at this point, the mathmatic’s and the ability to even see this asteroid are astounding. How is this done?

    Thanks, Derek

    Mainzer says

    Good question, Derek. It turns out that most asteroids aren’t found when they’re that far away. The Earth and the asteroid are both always going around the sun like they’re on a racetrack, with the Earth lapping the asteroid since it’s usually on the inside track. So you can imagine that we usually see them at one close approach, allowing us to predict when the next close approach will be. Astronomers generally find new asteroids by taking a series of snapshots of a particular part of the sky, then looking for things that move between the frames. Stars move very slowly if at all, and more distant objects like galaxies do not appear to move at all, whereas the motion of asteroids stands out. Once we identify all the asteroid candidates in each snapshot, we have to make sure it’s the same asteroid that we’re seeing from frame to frame - sometimes there are more than one in a frame. We can figure this out by using what we know about the laws of gravity and orbital dynamics to calculate possible orbits for the detections. Once this is done, we have a tentative knowledge of the orbit, which allows us to make predictions about where the object should be in the future. We can improve our knowledge of an asteroid’s orbit by making more observations of it. This is why JPL’s Near-Earth Object tracking system is constantly updating impact predictions as new observations are fed in.

  5. Jon Hutto Says:
    December 1st, 2008 at 7:17 am

    Well I guess I don’t need the meteor portion of my car insurance policy!

    Being more able to determine danger from no danger will really be of a big help to the people tracking near earth objects. Sounds like a fun project. Need some help? :)

    Mainzer says

    Hi, Jon -
    No, you definitely don’t need asteroid insurance! Just to be clear, WISE will not find all the potentially hazardous objects out there - far from it. However, it will observe a statistically significant fraction of the population down to small sizes. This will give us a better understanding of how many asteroids we should expect expect to find.

  6. Magnus in Sweden Says:
    December 5th, 2008 at 8:14 am

    The universe is so amazing! I’m deeply impressed with the efforts done to explore the unknown. Maybe one day, asteroids will be mined for minerals, water and other valueable material (in space, water IS expensive) when man starts to go beyond the solar systems limits. Now is really a fascinating period of time to live in.

    It’s interesting how the science and literature (science fiction) seems to have a close relationship and inspire each other. Ones vision (e.g. Carl Sagan) can turn into reality by inspiring (NASA) scientists.

    Amy, keep up the good work! I’ve seen you in a couple of episodes of “The Universe” on the History Channel and really hope to see more of you in the future!


  7. John K Says:
    December 10th, 2008 at 3:54 pm

    amy, I saw your demonstration with the yellow and blue paints. I thought it was a good analogy but its still hard to imaginge that the 2 would be close enough to mix together to form green at the second bang. John K in Phoenix

  8. Patricio Says:
    December 10th, 2008 at 7:41 pm

    Cheers from CHILE Amy. You know we got here one of the most clearest skies … so I really like your presentations in The Universe (by Discovery Channel in Chile). BEST REGARDS

    Mainzer says

    Chile is indeed one of the most amazing places on Earth for astronomy! And it is a beautiful country.

  9. Andre Garcia Says:
    December 11th, 2008 at 9:05 am

    Dear Dr.Amy Mainzer,

    It was a great surprise to see new faces on scientific world (I saw you on History Channel serie “The Universe”) , mainly women, which are getting more and more good positions in the scientifc world.

    Two questions: 1-how do the effect of friction and tide effect (earth against the asteroid) could deteriorate the impact of a possible asteroid ? 2- I believe that several nuclear devices (it would need millions of Giga Joules for small 1 km diameter granite asteroid) won’t be enough to evaporate these introdures rather to deviate would be smarter. Am I correct?

    Best regards.

    Andre Garcia

    Mainzer says

    Hi, Andre -
    How close an asteroid has to get to Earth before its shape is affected by Earth’s gravity is an interesting question. Some speculate that when the asteroid Apophis makes its close approach to Earth in 2029, the asteroid could undergo some amount of plastic deformation as a result of Earth’s gravitational pull.

  10. Luis Delgado Says:
    December 11th, 2008 at 2:27 pm

    Hi Amy:

    My concern is about how behaviors affect perceptions of life, in that sense I would like to ask: How would you think that the reactions of ordinary people if you look at an apocalyptic asteroid? And what think is the best way of alerting the world population?



  11. Carl Hoffman Says:
    December 22nd, 2008 at 1:36 pm

    Hi Amy,
    I was glad to find your blog! I really enjoy you on The Universe along with the chinese physicist fellow.Both of you amaze and enthrall me with your dialog and pure openess.I hope to see you guys again,soon…Carl

  12. johnny Says:
    January 5th, 2009 at 2:05 pm

    “By seeing what’s behind the dust, Spitzer has shown us star and planet formation is a very active process in our galaxy,” Churchwell said.

    from an 04 jpl press release, in the const. centaurus, a planet rich environment in the star forming region, is this a possible target for the wise mission and what kind of surprises await us?

  13. Ely Says:
    January 12th, 2009 at 11:10 pm

    Hi Amy,

    Asteroids is very amazing object in the outer space of the universe, floating around the outer space. but, it is very dangerous object. it can destroy planets and moons even human being life and animal life. so i suggest to continue study hard this object. Godspeed! Ely From Mindanao,

  14. Scott S Says:
    January 16th, 2009 at 7:51 pm

    Hi Amy,
    Great job on The Universe episode about meteors and asteroids that recently aired. My question is: Are you still refining the path of Apophis, and what is the likelihood of it impacting the earth in either 2029 or 2036? I recently heard that there’s a zero chance, but prior, I read that the chances were about 1 in 45,000 for 2036, which is a bit scary. Can you shed some more light on this one? And will WISE track Apophis, and if so, is there a website that we can keep up to date on WISE, including other near-Earth threat-tracking? Thanks for your time.


    Mainzer says

    Hi, Scott-

    Astronomers are still refining Apophis’ orbit. The next close approach is in early 2013, at which time we’ll probably be able to get radar observations of it which will really nail down the orbit. Right now, the odds of impact are zero in 2029 and about 1/45,000 in 2036, but they will almost certainly be reduced to zero once we get a better look at it in 2013. The major source of uncertainty in the 2036 encounter is something called the Yarkovsky force, which is a thermal effect that depends on the asteroid’s physical properties. The Near-Earth Object Office is the clearinghouse for all this information:

    WISE is not going to see Apophis during its six month lifetime, unfortunately. But it will see a lot of other near-Earth asteroids with similar sizes, and this should help us learn about how many there are and how strongly they are affected by the Yarkovsky force.

  15. johnny Says:
    January 22nd, 2009 at 10:17 pm

    are “shooting stars” a potential hazard as well, what can wise do to determine if these earth sized meteors flying through space have the potential to cause catastophic occasions to “worlds” in their path?

    Mainzer says

    Hi, Johnny-

    No need to worry about shooting stars - most everything you see is the size of a grain of sand. The very brightest shooting stars are about the size of a pea! So they are not a hazard. Fortunately, the very largest asteroid in the main asteroid belt, Ceres, is only a quarter the diameter of the Moon, so there aren’t any Earth-sized asteroids anywhere nearby.

  16. George Says:
    January 28th, 2009 at 2:53 am

    Amy, your job sounds fun!

    Good luck with the question has to do with evaluating the composition of these asteroids. Can we detect water? How about some of the more rare metals? Seems to me that you guys have a good oportunity to educate the public and politicians on what these things are made of and propose some future science missions as well as the eventual mining of these asteroids.

    It’d be nice to see these things as a good source of rare metals and H2o instead of being afraid of them.

    Mainzer says

    That’s an interesting question. The DAWN mission is going to visit the asteroids Vesta and Ceres, and it will look at their compositions using a variety of instruments. We think that Ceres probably has a thick layer of water ice underneath its surface crust. Certainly asteroids contain lots of minerals, and we already know how to send missions to land on them or run into them. However, towing an asteroid back to Earth is another matter - they’re very massive! I don’t expect that that will happen anytime soon.

    I don’t think anyone should be afraid of asteroids. Impacts just don’t happen often enough to warrant fear. That said, even though the probability of an impact is small, the damage it would cause could be large, which means we should pay some attention to it. However, this must be compared to the hazards posed by, say, climate change and habitat destruction, where the probability of it happening is large, and the damage is also large. Some biologists (like E. O. Wilson at Harvard) calculate that we’re in the middle of a mass extinction event that will be as large as the K/T extinction - caused by us chopping down forests and paving everything. Such a problem calls for immediate and largescale attention. Certainly that’s not as glamorous a way to go as an asteroid impact, but it’s a much more urgent problem.

  17. Lars Hansson Says:
    January 28th, 2009 at 4:40 am

    Hi Amy,

    Regarding the movement of a newly found asteroid, how do you determine its movement in the third dimension from a series of two dimensional snapshots?

    Best regards,

    Mainzer says

    Hi, Lars-
    What you need are multiple observations of the asteroid over time. If you have the right cadence of observations, the three dimensional properties of its orbit fall out naturally. Think of it this way: If you’re watching a car go around a racetrack off in the distance, sometimes it appears to move sideways fast, but when it’s moving toward you, it hardly appears to move at all. If you have enough observations of it as it moves around the track, you can infer its path in three dimensions. This is because the laws of gravity provide a powerful constraint on what it has to be doing – for example, you know that the car can’t suddenly levitate! Similarly, if we have enough data points spaced out over time, we know the range of possible orbits the asteroid could be in, revealing its three dimensional motion.

  18. Ed Says:
    February 7th, 2009 at 2:27 pm

    Hi Amy,

    You said that Apophisis is predicted to undergo some plastic deformation. My question is - how was this hypothesis made? Can WISE also determine the asteroid’s structure and composition at this distance?

    Also please correct me if I’m wrong but doesn’t it take less gravitational pull to alter an object’s orbital trajectory than to deform it? Ergo an asteroid undergoing deformation is also being pulled closer to Earth (and moon)?

    Should I sell the house and and move to the opposite side of the planet by 2027? :)

    Thank you.


  19. Bob Turcott Says:
    February 12th, 2009 at 12:46 pm


    great job on your presentations on the universe programs!!! I have enjoyed them, your presentaion with the paint of cosimic background radiation was cool!!! very notable comment which was pretty cool and correct is all humans are pretty wierd!!! Thats so true! I can only imagine what other life would be out there in the cosmos!!!

    I am a principal infrared systems opto-mech design Engineer and always enjoy learning more about other appliactions for infrared!!!

    Bob T

  20. Glenn K Says:
    February 14th, 2009 at 8:34 pm

    Dr. Mainzer:

    Respectful “questions three” from a kibbitzing curmudgeon:

    1. What will provide the source of visible light data for the WISE-observed asteroids {especially, terrestrial vs. orbital instrument(s)}, or is WISE multi-spectral?

    2. Does the comparison method intended to be used between the two data sets require contemporaneous observations [i.e., does the relative geometry of the asteroid with respect to Earth at the time of observation matter in spectral comparison]?

    3. Do you have to take into account the chromatic (and/or atmospheric, if applicable) aberrations of WISE and the visible light observer, if any?

    In the mission summary, it sounds like a straightforward set of observations. However, when you start thinking about all the controls that you might have to introduce in the whole experiment, it gets complicated pretty fast. I’m sure I am missing a lot, and may even be considering non-essential variables, depending on the mission goal. This sounds like a “fun” project, and I wish you and your team the best of success.

    Very Respectfully,
    Glenn K.

  21. Tim C Says:
    March 19th, 2009 at 6:53 pm

    Hi Amy,
    Just a quick question. Does the current NEA research also include the search for potential moon impacts?
    I haven’t seen any documentaries about NEA’s that deal with the potential devastation that NMA’s might create.

  22. Greg Mellon Says:
    November 12th, 2009 at 1:41 am

    Hi Amy,

    Most of what you talk about is WAY over my head but I watch the Universe shows several times to try to get the most out of it!

    I failed high school physics but somehow you make me want to understand.


  23. Mithridates (from Page F30) Says:
    November 17th, 2009 at 11:09 pm

    You might be a good person to ask about a question I’ve always had: Dawn will be arriving at Ceres in 2015 but will we have the opportunity to obtain better ground-based observations in the years before its arrival? I’m curious whether we will remain at our current level of knowledge until Dawn’s arrival or whether there is anything to look forward to before then.

    24 Themis has also turned out to be quite the interesting object with ice on its surface and it seems like it might even be an easier target to reach than Ceres. Will the Hubble or anywhere else be turning its eye on 24 Themis now that we’ve found out what an interesting object it is?

    Oh, and one more question: are there any gravity simulators online where one is able to input the size and mass of an object and then come up with a reasonable simulation of what it would be like to be on the surface? A demonstration of what it would be like to walk/jump/pour water etc. on the surface of Ceres for example would be extremely interesting and I had assumed that something like that would be available online but as of yet have never been able to find one.

    Thanks in advance.

    JPL Media Relations responds:

    I will pass your questions along to the folks at the Dawn mission and Hubble Space Telescope.

  24. Scott S. Says:
    December 11th, 2009 at 4:34 pm

    Why was the launch of WISE delayed? we need that thing up there scouting the skies!!!! What happened??

    JPL Media Relations responds:

    The WISE launch was delayed for a few reasons as is common with rocket launches. One delay came about because bad weather caused another launch on the east coast to be pushed back — some of the same United Launch Alliance crew working the launch of the Wideband Global SATCOM-3 (WGS-3) mission were needed for the WISE launch on the west coast. WISE was delayed again due to a problem with the motion of one of its booster steering engines. This issue was resolved by swapping out an anomalous component, and WISE soared into space on the morning of Dec. 14. Read more at: .

  25. CsendesMark Says:
    December 25th, 2009 at 5:23 pm

    Hi Amy,

    First, I wish successful hunt for those unexpected discoveries!

    What will happen to WISE after it’s 6 month mission?
    It will be too warm to make more science in the infrared?
    Why WISE orbiting the Earth’s day-night border? It is for constant temperature?

    Best regards.

  26. Francisco Fajre Says:
    July 23rd, 2010 at 9:12 pm

    Hi Amy, i live in Chile ( here Tololo ), i see History Channel, i like pulsar you now what ist the near pulsar to the eart. Thak and good tv show in History Channel.

  27. shade Says:
    November 1st, 2011 at 9:15 am

    Do you think the mining of asteroids in outer space will become a reality in your lifetime?
    Why or Why not?

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