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For first time, astronomers catch asteroid in the act of changing color

For first time, astronomers catch asteroid in the act of changing color

Last December, scientists discovered an “active” asteroid within the asteroid belt, sandwiched between the orbits of Mars and Jupiter. The space rock, designated by astronomers as 6478 Gault, appeared to be leaving two trails of dust in its wake — active behavior that is associated with comets but rarely seen in asteroids.

While astronomers are still puzzling over the cause of Gault’s comet-like activity, an MIT-led team now reports that it has caught the asteroid in the act of changing color, in the near-infrared spectrum, from red to blue. It is the first time scientists have observed a color-shifting asteroid, in real-time.

“That was a very big surprise,” says Michael Marsset, a postdoc in MIT’s Department of Earth, Atmospheric and Planetary Sciences (EAPS). “We think we have witnessed the asteroid losing its reddish dust to space, and we are seeing the asteroid’s underlying, fresh blue layers.”

Marsset and his colleagues have also confirmed that the asteroid is rocky — proof that the asteroid’s tail, though seemingly comet-like, is caused by an entirely different mechanism, as comets are not rocky but more like loose snowballs of ice and dust.

“It’s the first time to my knowledge that we see a rocky body emitting dust, a little bit like a comet,” Marsset says. “It means that probably some mechanism responsible for dust emission is different from comets, and different from most other active main-belt asteroids.”

Marsset and his colleagues, including EAPS Research Scientist Francesca DeMeo and Professor Richard Binzel, have published their results today in the journal Astrophysical Journal Letters.

A rock with tails

Astronomers first discovered 6478 Gault in 1988 and named the asteroid after planetary geologist Donald Gault. Until recently, the space rock was seen as relatively average, measuring about 2.5 miles wide and orbiting along with millions of other bits of rock and dust within the inner region of the asteroid belt, 214 million miles from the sun.

In January, images from various observatories, including NASA’s Hubble Space Telescope, captured two narrow, comet-like tails trailing the asteroid. Astronomers estimate that the longer tail stretches half a million miles out, while the shorter tail is about a quarter as long. The tails, they concluded, must consist of tens of millions of kilograms of dust, actively ejected by the asteroid, into space. But how? The question reignited interest in Gault, and studies since then have unearthed past instances of similar activity by the asteroid.

“We know of about a million bodies between Mars and Jupiter, and maybe about 20 that are active in the asteroid belt,” Marsset says. “So this is very rare.”

He and his colleagues joined the search for answers to Gault’s activity in March, when they secured observation time at NASA’s Infrared Telescope Facility (IRTF) on Mauna Kea, Hawaii. Over two nights, they observed the asteroid and used a high-precision spectrograph to divide the asteroid’s incoming light into various frequencies, or colors, the relative intensities of which can give scientists an idea of an object’s composition.

From their analysis, the team determined that the asteroid’s surface is composed mainly of silicate, a dry, rocky material, similar to most other asteroids, and, more importantly, not at all like most comets.

Comets typically come from the far colder edges of the solar system. When they approach the sun, any surface ice instantly sublimates, or vaporizes into gas, creating the comet’s characteristic tail. Since Marsset’s team has found 6478 Gault is a dry, rocky body, this means it likely is generating dust tails by some other active mechanism.

A fresh change

As the team observed the asteroid, they discovered, to their surprise, that the rock was changing color in the near-infrared, from red to blue.

“We’ve never seen such a dramatic change like this over such a short period of time,” says co-author DeMeo.

The scientists say they are likely seeing the asteroid’s surface dust, turned red over millions of years of exposure to the sun, being ejected into space, revealing a fresh, less irradiated surface beneath, that appears blue at near-infrared wavelengths.

“Interestingly, you only need a very thin layer to be removed to see a change in the spectrum,” DeMeo says. “It could be as thin as a single layer of grains just microns deep.”

So what could be causing the asteroid to turn color? The team and other groups studying 6478 Gault believe the reason for the color shift, and the asteroid’s comet-like activity, is likely due to the same mechanism: a fast spin. The asteroid may be spinning fast enough to whip off layers of dust from its surface, through sheer centrifugal force. The researchers estimate it would need to have about a two-hour rotation period, spinning around every couple of hours, versus Earth’s 24-hour period.

“About 10 percent of asteroids spin very fast, meaning with a two- to three-hour rotation period, and it’s most likely due to the sun spinning them up,” says Marsset.

This spinning phenomenon is known as the YORP effect (or, the Yarkovsky-O’Keefe-Radzievskii-Paddack effect, named after the scientists who discovered it), which refers to the effect of solar radiation, or photons, on small, nearby bodies such as asteroids. While asteroids reflect most of this radiation back into space, a fraction of these photons is absorbed, then reemitted as heat, and also momentum. This creates a small force that, over millions of years, can cause the asteroid to spin faster.

Astronomers have observed the YORP effect on a handful of asteroids in the past. To confirm a similar effect is acting on 6478 Gault, researchers will have to detect its spin through light curves — measurements of the asteroid’s brightness over time. The challenge will be to see through the asteroid’s considerable dust tail, which can obscure key portions of the asteroid’s light.

Marsset’s team, along with other groups, plan to study the asteroid for further clues to activity, when it next becomes visible in the sky.

“I think [the group’s study] reinforces the fact that the asteroid belt is a really dynamic place,” DeMeo says. “While the asteroid fields you see in the movies, all crashing into each other, is an exaggeration, there is definitely a lot happening out there every moment.”

Materials provided by Massachusetts Institute of Technology

asteroids

Asteroid tracking software updated to be 25 times faster

Modeling the shape and movement of near‑Earth asteroids is now up to 25 times faster thanks to new Washington State University research.

The WSU scientists improved the software used to track thousands of near‑Earth asteroids and comets, which are defined as being within 121 million miles or about 1.3 times the distance to the sun.

Their work provides a valuable new tool for studying asteroids and determining which of them might be on a collision course with Earth.

Matt Engels, a Ph.D. student who has been working with Professor Scott Hudson in the School of Engineering and Applied Sciences at WSU Tri‑Cities, is the lead author of a paper on the research in the July issue of Astronomy and Computing.

Researchers would like to have better information on asteroids, including which of them might crash into earth. The rocks also can provide valuable scientific information, answering fundamental questions about the creation of our solar system and providing a glimpse into our planetary past. Knowing more about individual asteroid composition also could open up new opportunities for possible asteroid mining.

NASA maintains a catalog that includes information on more than 20,000 near‑earth asteroids and comets. In the mid‑1990s scientists knew of less than 200 of such outer space rocks, but with better telescopes and more efforts at surveying, the numbers of known asteroids has grown dramatically.

But, there are only a trickle of papers that describe individual asteroids. Once a new asteroid is discovered, modeling it takes several months, if not longer, said Engels. The research is painstaking.

In the mid‑1990s, Hudson, who has an asteroid named after him, wrote the primary modeling software tool that researchers use to describe asteroids and their behavior. Using ground-based radar and optics data, the software helps researchers learn important information, such as an asteroid’s possible mineral make-up, current and future orbit, shape, and how it spins in space. In fact, Hudson co-authored a paper published in Science that determined that at least one asteroid, 1950 DA, has a very tiny chance of hitting earth during a precise 20‑minute period in March of 2880.

“The software was written for a super computer, so it’s really, really slow,” said Engels, who jumped at improving it for his PhD project. “It can take a long time to do the modeling to draw any conclusions from it, and it takes awhile to crunch the data to write a paper in the first place.”

Asteroids made from clay next to asteroids made using a computer model.
To check the accuracy of their computer model, the researchers compared their results to clay models of asteroids. The bottom right image comes from the computer model, and the bottom left is an image of the model asteroid. (Credit: WSU)

The new version of code works much faster. The researchers revised it to make operations work concurrently instead of performing one at a time. Because the work is very similar to the everyday graphics that modern computers use to crunch out nice displays, the researchers transferred the operations to the computer’s graphics processing units, or GPUs. GPUs are designed to perform complex mathematical and geometric calculations for graphics rendering and have a tremendous amount of power to do parallel calculations.

“It’s taking advantage of the horsepower that is used in computer graphics rendering,” Engels said. “It’s very cost effective and you don’t need a super computer. You can use a consumer level graphics card available for under $500.”

The improvements to the algorithms could also someday be used for a variety of other purposes, said Engels, who works as a research engineer at Pacific Northwest National Laboratory, such as for modeling systems in the electric power grid or gas and oil industry.

Engels is verifying the code with real asteroid data. He hopes to have it available to the astronomy community later this year.

Journal Reference: Astronomy and Computing

Materials provided by Washington State University

ZTF Spots Asteroid with Shortest Year 2019 LF6

ZTF Spots Asteroid with Shortest Year

Astronomers have spotted an unusual asteroid with the shortest “year” known for any asteroid. The rocky body, dubbed 2019 LF6, is about a kilometre in size and circles the sun roughly every 151 days. In its orbit, the asteroid swings out beyond Venus and, at times, come closer in than Mercury, which circles the sun every 88 days. 2019 LF6 is one of only 20 known “Atira” asteroids, whose orbits fall entirely within Earth’s.

“You don’t find kilometre-size asteroids very often these days,” says Quanzhi Ye, a postdoctoral scholar at Caltech who discovered 2019 LF6 and works with Tom Prince, the Ira S. Bowen Professor of Physics at Caltech and a senior research scientist at JPL, and George Helou, the executive director of IPAC, an astronomy center at Caltech.

“Thirty years ago, people started organizing methodical asteroid searches, finding larger objects first, but now that most of them have been found, the bigger ones are rare birds,” he says. “LF6 is very unusual both in orbit and in size—its unique orbit explains why such a large asteroid eluded several decades of careful searches.”

2019 LF6 was discovered via the Zwicky Transient Facility, or ZTF, a state-of-the-art camera at the Palomar Observatory that scans the skies every night for transient objects, such as exploding and flashing stars and moving asteroids. Because ZTF scans the sky so rapidly, it is well-suited for finding Atira asteroids, which have short observing windows.

“We only have about 20 to 30 minutes before sunrise or after sunset to find these asteroids,” says Ye.

To find the Atira asteroids, the ZTF team has been carrying out a dedicated observing campaign, named Twilight after the time of day best suited for discovering the objects. Twilight was developed by Ye and Wing-Huen Ip of the National Central University in Taiwan. So far, the program has discovered one other Atira asteroid, named 2019 AQ3. Before 2019 LF6 came along, 2019 AQ3 had the shortest known year of any asteroid, orbiting the sun roughly every 165 days.

“Both of the large Atira asteroids that were found by ZTF orbit well outside the plane of the solar system,” says Prince. “This suggests that sometime in the past they were flung out of the plane of the solar system because they came too close to Venus or Mercury,” says Prince.

In addition to the two Atira objects, ZTF has so far found around 100 near-Earth asteroids and about 2,000 asteroids orbiting in the Main Belt between Mars and Jupiter.

Ye says he hopes the Twilight program will lead to more Atira discoveries, and he looks forward to the possible selection by NASA of the Near-Earth Object Camera (NEOCam) mission, a proposed spacecraft designed to look for asteroids closer to the sun than previous surveys. NEOCam would pick up the infrared, or heat, signatures of asteroids. (Ye works at IPAC, which would process and archive data for the NEOCam mission, but is not part of that team.)

“Because Atira asteroids are closer to the sun and warmer than other asteroids, they are brighter in the infrared,” says Helou.”NEOCam has the double advantage of its location in space and its infrared capability to find these asteroids more easily than telescopes working at visible wavelengths from the ground.”

The International Astronomical Union Minor Planet Center listing for 2019 LF6 is at https://minorplanetcenter.net/mpec/K19/K19M45.html.

Materials provided by the California Institute of Technology