Why We Track Asteroids Like the One That Flew by This Week


Earlier this week, on March 8, asteroid 2013 TX68 came within about 3.1 million miles of Earth, astronomers estimate. Original predictions suggested its closest approach might be within the orbits of geostationary satellites around the Earth, or it could be as far away as 9.5 million miles. Additional data changed the distance range to between 3 million and 15,000 miles. At about 100 feet in diameter, the object was too small to be seen at the 3.1 million-mile distance, but it clearly did not hit Earth.

We keep an eye on such space debris. NASA’s Center for Near-Earth Object Studies at the Jet Propulsion Laboratory in California currently tracks 13,947 near-Earth objects, defined as those coming within 130 million miles of our planet’s orbit. The center considers about 12 percent of those potentially hazardous, according to Paul Chodas, manager of the center. That means they come within 5 million miles and pose about a one in 1 billion chance of hitting Earth in the next 100 years. (TX68 isn’t one of them.)

Asteroids travel on elliptical orbits around the Sun, explains Judit Györgyey Ries, an asteroid observer and researcher at the University of Texas at Austin’s McDonald Observatory. An asteroid’s path changes slightly from the effect of gravity when it passes close to a planet or from the energy of it absorbing and emitting sunlight.

The orbit of asteroid TX68. Image credit: NASA/JPL-Caltech

The more data scientists collect on a specific asteroid, the more accurate their predictions of its path and probability of colliding with Earth. TX68 is a perfect example. It was first observed by the Catalina Sky Survey in October 2013, while approaching Earth at night. Three days later, the asteroid passed into the daytime sky and could no longer be observed. Based on those three days of data, TX68 appeared to have a four in 1 billion chance of hitting Earth.

That may sound like a long shot, but the odds were nevertheless four times higher than the threshold NASA has set for potentially hazardous objects. “That caught our attention,” Chodas says. Then Italian astronomer Marco Micheli, with the European Space Agency, noticed faint traces of the asteroid in archived telescope images, which directed a search for more archived images. Based on that additional data, TX68’s potential for impact dropped back to the more acceptable one-in-a-billion chance.

All calculations come with uncertainty, of course, and with asteroids, that uncertainty grows the farther into the future the orbit projection. At the scale of the Earth, this uncertainty equals large distances, on the order of millions of miles. (For perspective, the average distance from Earth to the Moon is about 239,000 miles.) That makes it important for scientists to continue to monitor known objects.

Now scientists know where to look for TX68 when it returns to our part of the solar system. If it turns up where expected, that will decrease uncertainty about its future orbit. If not, says Györgyey Ries, the uncertainty will grow.

Three years ago, a meteor about 60 feet wide broke up in the atmosphere over Chelyabinsk, Russia. Observers didn’t see it coming because of its small size and approach from the direction of the Sun, but the dashcam and smartphone recordings of its fiery descent and glass-shattering sound wave were subsequently seen worldwide.

Any object between about 100 and 165 feet should burn up and disintegrate in the atmosphere, Chodas says, with some small meteorites reaching the ground, as they did in Chelyabinsk. NASA mostly worries about roughly 1000 known objects measuring at least one kilometer, or about six-tenths of a mile.

NASA-funded surveys began scanning the night sky in 1998 for near-Earth objects, and about 1500 NEOs are now detected each year. The strategy, according to Chodas, is to find as many of these objects measuring 330 feet and larger as possible, to provide as much time as possible for attempts to deflect a potential impact. For example, preparations for diverting a large asteroid of 650 to 1000 feet might involve building and launching a rocket, which would take years.

“You would just have to nudge it,” Chodas says. “Presumably, we could launch as heavy a rocket as we possibly could to run into the asteroid and change its velocity slightly. A change of one meter per second would likely be enough to divert it from impact.” NASA has plans for two missions to test deflection methods.

In January, NASA announced that its NEO detection and tracking project, now called the Planetary Defense Coordination Office, will supervise all NASA-funded projects working to find and characterize asteroids and comets passing near Earth's orbit and also coordinate response to potential impact threats.

For Chodas, TX68’s fly by presented an opportunity. “We know this particular asteroid can’t impact Earth in the next 100 years,” he says. “It is more of an opportunity to remind people we are working on the problem so that, if an asteroid should be headed for Earth, we would have enough warning time, possibly decades, to do something about it.”

But as Györgyey Ries notes, “I only worry about the ones we don’t know of.” 

More Details Emerge About 'Oumuamua, Earth's First-Recorded Interstellar Visitor

In October, scientists using the University of Hawaii's Pan-STARRS 1 telescope sighted something extraordinary: Earth's first confirmed interstellar visitor. Originally called A/2017 U1, the once-mysterious object has a new name—'Oumuamua, according to Scientific American—and researchers continue to learn more about its physical properties. Now, a team from the University of Hawaii's Institute of Astronomy has published a detailed report of what they know so far in Nature.

Fittingly, "'Oumuamua" is Hawaiian for "a messenger from afar arriving first." 'Oumuamua's astronomical designation is 1I/2017 U1. The "I" in 1I/2017 stands for "interstellar." Until now, objects similar to 'Oumuamua were always given "C" and "A" names, which stand for either comet or asteroid. New observations have researchers concluding that 'Oumuamua is unusual for more than its far-flung origins.

It's a cigar-shaped object 10 times longer than it is wide, stretching to a half-mile long. It's also reddish in color, and is similar in some ways to some asteroids in our solar system, the BBC reports. But it's much faster, zipping through our system, and has a totally different orbit from any of those objects.

After initial indecision about whether the object was a comet or an asteroid, the researchers now believe it's an asteroid. Long ago, it might have hurtled from an unknown star system into our own.

'Oumuamua may provide astronomers with new insights into how stars and planets form. The 750,000 asteroids we know of are leftovers from the formation of our solar system, trapped by the Sun's gravity. But what if, billions of years ago, other objects escaped? 'Oumuamua shows us that it's possible; perhaps there are bits and pieces from the early years of our solar system currently visiting other stars.

The researchers say it's surprising that 'Oumuamua is an asteroid instead of a comet, given that in the Oort Cloud—an icy bubble of debris thought to surround our solar system—comets are predicted to outnumber asteroids 200 to 1 and perhaps even as high as 10,000 to 1. If our own solar system is any indication, it's more likely that a comet would take off before an asteroid would.

So where did 'Oumuamua come from? That's still unknown. It's possible it could've been bumped into our realm by a close encounter with a planet—either a smaller, nearby one, or a larger, farther one. If that's the case, the planet remains to be discovered. They believe it's more likely that 'Oumuamua was ejected from a young stellar system, location unknown. And yet, they write, "the possibility that 'Oumuamua has been orbiting the galaxy for billions of years cannot be ruled out."

As for where it's headed, The Atlantic's Marina Koren notes, "It will pass the orbit of Jupiter next May, then Neptune in 2022, and Pluto in 2024. By 2025, it will coast beyond the outer edge of the Kuiper Belt, a field of icy and rocky objects."

Last month, University of Wisconsin–Madison astronomer Ralf Kotulla and scientists from UCLA and the National Optical Astronomy Observatory (NOAO) used the WIYN Telescope on Kitt Peak, Arizona, to take some of the first pictures of 'Oumuamua. You can check them out below.

Images of an interloper from beyond the solar system — an asteroid or a comet — were captured on Oct. 27 by the 3.5-meter WIYN Telescope on Kitt Peak, Ariz.
Images of 'Oumuamua—an asteroid or a comet—were captured on October 27.

U1 spotted whizzing through the Solar System in images taken with the WIYN telescope. The faint streaks are background stars. The green circles highlight the position of U1 in each image. In these images U1 is about 10 million times fainter than the faint
The green circles highlight the position of U1 in each image against faint streaks of background stars. In these images, U1 is about 10 million times fainter than the faintest visible stars.
R. Kotulla (University of Wisconsin) & WIYN/NOAO/AURA/NSF

Color image of U1, compiled from observations taken through filters centered at 4750A, 6250A, and 7500A.
Color image of U1.
R. Kotulla (University of Wisconsin) & WIYN/NOAO/AURA/NSF

Editor's note: This story has been updated.

8 Useful Facts About Uranus
Uranus as seen by the human eye (left) and with colored filters (right).
Uranus as seen by the human eye (left) and with colored filters (right).

The first planet to be discovered by telescope, Uranus is the nearest of the two "ice giants" in the solar system. Because we've not visited in over 30 years, much of the planet and its inner workings remain unknown. What scientists do know, however, suggests a mind-blowing world of diamond rain and mysterious moons. Here is what you need to know about Uranus.


Uranus is the seventh planet from the Sun, the fourth largest by size, and ranks seventh by density. (Saturn wins as least-dense.) It has 27 known moons, each named for characters from the works of William Shakespeare and Alexander Pope. It is about 1784 million miles from the Sun (we're 93 million miles away from the Sun, or 1 astronomical unit), and is four times wider than Earth. Planning a trip? Bring a jacket, as the effective temperature of its upper atmosphere is -357°F. One Uranian year last 84 Earth years, which seems pretty long, until you consider one Uranian day, which lasts 42 Earth years. Why?


Most planets, as they orbit the Sun, rotate upright, spinning like tops—some faster, some slower, but top-spinning all the same. Not Uranus! As it circles the Sun, its motion is more like a ball rolling along its orbit. This means that for each hemisphere of the planet to go from day to night, you need to complete half an orbit: 42 Earth years. (Note that this is not the length of a complete rotation, which takes about 17.25 hours.) While nobody knows for sure what caused this 98-degree tilt, the prevailing hypothesis involves a major planetary collision early in its history. And unlike Earth (but like Venus!), it rotates east to west.


You might have noticed that every non-Earth planet in the solar system is named for a Roman deity. (Earth didn't make the cut because when it was named, nobody knew it was a planet. It was just … everything.) There is an exception to the Roman-god rule: Uranus. Moving outward from Earth, Mars is (sometimes) the son of Jupiter, and Jupiter is the son of Saturn. So who is Saturn's father? Good question! In Greek mythology, it is Ouranos, who has no precise equivalent in Roman mythology (Caelus is close), though his name was on occasion Latinized by poets as—you guessed it!—Uranus. So to keep things nice and tidy, Uranus it was when finally naming this newly discovered world. Little did astronomers realize how greatly they would disrupt science classrooms evermore.

Incidentally, it is not pronounced "your anus," but rather, "urine us" … which is hardly an improvement.


Uranus and Neptune comprise the solar system's ice giants. (Other classes of planets include the terrestrial planets, the gas giants, and the dwarf planets.) Ice giants are not giant chunks of ice in space. Rather, the name refers to their formation in the interstellar medium. Hydrogen and helium, which only exist as gases in interstellar space, formed planets like Jupiter and Saturn. Silicates and irons, meanwhile, formed places like Earth. In the interstellar medium, molecules like water, methane, and ammonia comprise an in-between state, able to exist as gases or ices depending on the local conditions. When those molecules were found by Voyager to have an extensive presence in Uranus and Neptune, scientists called them "ice giants."


Planets form hot. A small planet can cool off and radiate away heat over the age of the solar system. A large planet cannot. It hasn't cooled enough entirely on the inside after formation, and thus radiates heat. Jupiter, Saturn, and Neptune all give off significantly more heat than they receive from the Sun. Puzzlingly, Uranus is different.

"Uranus is the only giant planet that is not giving off significantly more heat than it is receiving from the Sun, and we don't know why that is," says Mark Hofstadter, a planetary scientist at NASA's Jet Propulsion Laboratory. He tells Mental Floss that Uranus and Neptune are thought to be similar in terms of where and how they formed.

So why is Uranus the only planet not giving off heat? "The big question is whether that heat is trapped on the inside, and so the interior is much hotter than we expect, right now," Hofstadter says. "Or did something happen in its history that let all the internal heat get released much more quickly than expected?"

The planet's extreme tilt might be related. If it were caused by an impact event, it is possible that the collision overturned the innards of the planet and helped it cool more rapidly. "The bottom line," says Hofstadter, "is that we don't know."


Although it's really cold in the Uranian upper atmosphere, it gets really hot, really fast as you reach deeper. Couple that with the tremendous pressure in the Uranian interior, and you get the conditions for literal diamond rain. And not just little rain diamondlets, either, but diamonds that are millions of carats each—bigger than your average grizzly bear. Note also that this heat means the ice giants contain relatively little ice. Surrounding a rocky core is what is thought to be a massive ocean—though one unlike you might find on Earth. Down there, the heat and pressure keep the ocean in an "in between" state that is highly reactive and ionic.


Unlike Saturn's preening hoops, the 13 rings of Uranus are dark and foreboding, likely comprised of ice and radiation-processed organic material. The rings are made more of chunks than of dust, and are probably very young indeed: something on the order of 600 million years old. (For comparison, the oldest known dinosaurs roamed the Earth 240 million years ago.)


The only spacecraft to ever visit Uranus was NASA's Voyager 2 in 1986, which discovered 10 new moons and two new rings during its single pass from 50,000 miles up. Because of the sheer weirdness and wonder of the planet, scientists have been itching to return ever since. Some questions can only be answered with a new spacecraft mission. Key among them: What is the composition of the planet? What are the interactions of the solar wind with the magnetic field? (That's important for understanding various processes such as the heating of the upper atmosphere and the planet's energy deposition.) What are the geological details of its satellites, and the structure of the rings?

The Voyager spacecraft gave scientists a peek at the two ice giants, and now it's time to study them up close and in depth. Hofstadter compares the need for an ice-giants mission to what happened after the Voyagers visited Jupiter and Saturn. NASA launched Galileo to Jupiter in 1989 and Cassini to Saturn in 1997. (Cassini was recently sent on a suicide mission into Saturn.) Those missions arrived at their respective systems and proved transformative to the field of planetary science.

"Just as we had to get a closer look at Europa and Enceladus to realize that there are potentially habitable oceans there, the Uranus and Neptune systems can have similar things," says Hofstadter. "We'd like to go there and see them up close. We need to go into the system." 


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