New OSIRIS-REx photos of Bennu sample site after sample grab

Nightingale before and after sample grab
Click here and here for original images.

The OSIRIS-REx science team today released the photos the spacecraft obtained in its last fly-by of the asteroid Bennu on April 7th of the Nightngale sample site, showing how it changed after the sample grab in October 2020.

The two photos to the right compare that site, with the top image taken before the sample grab and the bottom image taken on April 7th.

Comparing the two images reveals obvious signs of surface disturbance. At the sample collection point, there appears to be a depression, with several large boulders evident at the bottom, suggesting that they were exposed by sampling. There is a noticeable increase in the amount of highly reflective material near the TAG [touch-and-go] point against the generally dark background of the surface, and many rocks were moved around.

Where thrusters fired against the surface, substantial mass movement is apparent. Multiple sub-meter boulders were mobilized by the plumes into a campfire ring–like shape — similar to rings of boulders seen around small craters pocking the surface.

Jason Dworkin, the mission’s project scientist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, noticed that one boulder measuring 4 feet (1.25 meters) across on the edge of the sampling site seemed to appear only in the post-TAG image. “The rock probably weighs around a ton, with a mass somewhere between a cow and a car.”

Dante Lauretta, of the University of Arizona and the mission’s principal investigator, later pointed out that this boulder is likely one of those present in the pre-TAG image, but much nearer the sampling location, and estimates it was thrown a distance of 40 feet (about 12 meters) by the sample collection event.

The sample grab arm penetrated the surface by more than a foot, and that event is marked by that dark depression at the center of the site.

OSIRIS-REX will now prepare for its May 10th engine firing that will send it on its route back to Earth to return that sample in September 24, 2023. After that the spacecraft could be sent on another mission, this time to the potentially dangerous asteroid Apophis.

South Korea’s leader announces his nation’s goals in space

The new colonial movement: Moon Jae-in, South Korea’s president since 2017, on March 25th gave his first speech focused on his nation’s goals in space, outlining plans to encourage private enterprise as well as achieving an unmanned mission to the Moon by 2030.

His speech listed three main programs. First, they are developing their own home-built rocket, dubbed the KSLV-2, which they hope to launch on its first orbital test flight by October of this year.

Second, he touted a project to send a probe to the asteroid Apophis in 2029. I described this probe in my November 2020 report on a science conference focused entirely on Apophis. If all goes well, they hope to have the probe fly in formation with the asteroid as it makes its close approach that year.

Third, he committed his nation to landing an unmanned lander on the Moon by the end of this decade. (Sound familiar?)

While much of this was the typical photo-op stuff that politicians love, designed mostly to enhance their public image, Moon did make it clear their goals are also to foster a new private aerospace industry that would compete in the emerging new space market.

Moon underscored the role of the private sector in enhancing Korea’s space development capabilities. To that end, he said, the government will step up efforts to build an “innovative industrial ecosystem that nurtures global space companies such as SpaceX.”

Another issue he put forth was strengthening international competitiveness of made-in-Korea satellite systems, in the lead-up to the introduction of 6G wireless networks, self-driving vehicles, and other products and services enabled or enhanced by satellites.

All-in-all, it is actually surprising that up to now South Korea has not made its presence felt in space. This announcement suggests they now intend to change that.

Radar images of Apophis during its March close approach of Earth

Apophis as seen by radar March 9, 2021
Click for full image.

Using two radar dishes, Green Bank in West Virginia and Goldstone in California, astronomers were able to produce radar images of the asteroid Apophis during its most recent close fly-by of Earth on March 10th.

The image to the right, cropped and reduced to post here, shows Apophis on March 9th. If you go to the full image you can also see the March 10th and 11th images, which appear to show the asteroid in different orientations as it rotated.

These images represent radar observations of asteroid 99942 Apophis on March 8, 9, and 10, 2021, as it made its last close approach before its 2029 Earth encounter that will see the object pass our planet by less than 20,000 miles (32,000 kilometers). The 70-meter radio antenna at the Deep Space Network’s Goldstone Deep Space Communications Complex near Barstow, California, and the 100-meter Green Bank Telescope in West Virginia used radar to precisely track Apophis’ motion. At the time of these observations, Apophis was about 10.6 million miles (17 million kilometers) from Earth, and each pixel has a resolution of 127 feet (38.75 meters).

The data obtained has firmly removed any chance Apophis will impact the Earth in the next 100 years. However, it still could hit us late in the 22nd century.

These observations were originally planned to also include data from the Arecibo Observatory, but that telescope was destroyed in December when its instrument platform collapsed. If it had been operational, these radar images would have had much better resolution.

Apophis: the asteroid that may someday hit the Earth

Apophis' path past the Earth in 2029
From the conference’s logo.

Last week planetary scientists held a three-day virtual conference focused on the asteroid Apophis, whose orbit regularly places it close to the Earth with a real possibility that it might someday hit the Earth.

When Apophis was originally discovered early in 2004, it wasn’t at first seen as an out-of-the-ordinary asteroid. Not much was known about it other than its albedo and that it was not trivial in size, probably around 1,000 feet in diameter. All this changed in December 2004, when further observations suggested that it had a 2.7% chance of impacting the Earth in 2029, making it the most dangerous asteroid ever discovered.

Fortunately, all subsequent observations of the asteroid have since eliminated any possibility of an impact, at least through the year 2068. Depending on what happens during the 2029 and 2036 fly-bys of Earth, there is a very small chance that it could hit the Earth in 2068, though the data says the chances of that happening are quite small (1 in 150,000).

On April 13, 2029 however Apophis will still zip past the Earth less than 20,000 miles above its surface, well within the orbit of all geosynchronous satellites. For viewers in Europe and Africa it will be a naked eye object, with a magnitude of -3.

Because the date of that close fly-by is so well known, and because this asteroid still poses a threat to the Earth, the planetary science community is very much interested in taking advantage of that close approach. From the conference’s webpage:

Knowledge is the first line of planetary defense, and the 2029 Apophis encounter is a once-per-thousand-year opportunity for investigating an asteroid as large as 350 meters passing within 6 Earth-radii. Time is of the essence as we have less than a decade to plan Earth-based and possible in-situ missions whose measurements can deliver unprecedented detailed knowledge on the physical nature of Apophis as the prototype example (poster child) of potentially hazardous asteroids.

I attended that conference, and am here now to give a summary.
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Avalanches on an asteroid

A new analysis predicts that when the asteroid Apophis flies past the Earth in 2029, the close fly-by will cause avalanches on the asteroid.

If asteroids pass close to Earth, they begin to experience the effects of our planet’s gravity. Just like the moon pushes and pulls the oceans, creating the tides, asteroids are susceptible to tidal forces from our planet. To judge what effect this will have on Apophis, scientists need to know what it’s made of. Their best guess is based on photos taken by a Japanese spacecraft named Hayabusa, which took detailed pictures of an Apophis-sized asteroid named Itokawa. Those images revealed that the asteroid wasn’t a solid mass of rock spinning through space, but rather a giant clump of debris held together loosely by gravity. “You look at the [Hayabusa] pictures and you’re like, ‘Uh, that’s a pile of rocks, dude.’ It’s very likely that Apophis is similar,” says astrophysicist Derek Richardson of the University of Maryland, College Park.

To show that Earth’s gravity could cause some of these rocks to tumble, Richardson and his colleagues developed a computer model that allowed them to place virtual sand piles across the surface of a model asteroid with roughly the same dimensions as Apophis. By factoring in the gravity from the asteroid, the tidal force from Earth, centrifugal force caused by the asteroid’s rotation, inertial forces, and other effects, the team was able to predict how the particles on the surface of the asteroid would behave on approach. The results confirm that Earth’s tidal forces would be strong enough to cause tiny avalanches on the asteroid, the team reported online ahead of print in Icarus.

Need I note that there are a lot of uncertainties here? Because they are using what is known about a different asteroid, all of their assumptions about Apophis’s properties in their computer model could be very wrong.

Still, this is interesting, because it does demonstrate that an asteroid could be significantly disturbed simply by flying past a planet.

An detailed analysis of the tumbling of the asteroid Apophis, detected by radar observations in January, suggests it will be easier to predict the asteroid’s orbit in the future.

The sky isn’t falling: A detailed analysis of the tumbling of the asteroid Apophis, detected by radar observations in January, suggests it will be easier to predict the asteroid’s orbit in the future.

The gentle but persistent nudging [of the Yarkovsky effect] arises when sunlight is absorbed by a rotating object and then reradiated as heat in some other direction. In particular, if Apophis were spinning retrograde (opposite the way Earth does), then over time its orbit would change in a way that increases the chance of impact in 2036. But now we can rest easy, because Apophis appears to be tumbling as it orbits the Sun. That’s the conclusion reached by a team of telescopic observers who monitored the asteroid’s light curve as it passed near Earth in January. Apophis is spinning around two axes at the same time, implying that any Sun-warmed surfaces are radiating heat in all directions, not just one in particular.

It is very difficult to measure the Yarkovsky effect, thus making it very difficult to precisely calculate the orbits of many near Earth asteroids. In the case of Apophis, however, it appears the astronomers have gotten a good handle on the problem.

New data has allowed scientists to lower the chance that the asteroid Apophis will hit the Earth in a future orbit.

New data has allowed scientists to lower the chance that the asteroid Apophis will hit the Earth in a future orbit.

Recent observations from Pan-STARRS PS1 telescope at Haleakala, Hawaii have reduced the current orbital uncertainty by a factor of 5, and radar observations in early 2013 from Goldstone and Arecibo will further improve the knowledge of Apophis’ current position. However, the current knowledge is now precise enough that the uncertainty in predicting the position in 2029 is completely dominated by the so-called Yarkovsky effect, a subtle nongravitational perturbation due to thermal re-radiation of solar energy absorbed by the asteroid. The Yarkovsky effect depends on the asteroid’s size, mass, thermal properties, and critically on the orientation of the asteroid’s spin axis, which is currently unknown. This means that predictions for the 2029 Earth encounter will not improve significantly until these physical and spin characteristics are better determined.

The new report, which does not make use of the 2013 radar measurements, identifies over a dozen keyholes that fall within the range of possible 2029 encounter distances. Notably, the potential impact in 2036 that had previously held the highest probability has been effectively ruled out since its probability has fallen to well below one chance in one million. Indeed only one of the potential impacts has a probability of impact greater than 1-in-a-million; there is a 2-meter wide keyhole that leads to an impact in 2068, with impact odds of about 2.3 in a million.

The second paragraph basically says that the keyholes that might bring Apophis back to Earth are very small, making it unlikely that the asteroid will fly through any one of them in 2029. The first paragraph however notes that it will be impossible to chart the asteroid’s course accurately enough to rule out this possibility until we have more data on the asteroid itself.

New data from the Herschel Space Telescope suggests that the asteroid Apophis is bigger and less reflective than previous believed.

More Apophis news: New data from the Herschel Space Telescope suggests that the asteroid is bigger and less reflective than previous believed.

Instead of 900 feet across, they now estimate it has a diameter of about 1070 feet. And knowing that the asteroid has a lower albedo means that astronomers will be better able to gauge the effect of solar radiation on Apophis’s orbit.

On Wednesday Apophis will pass the Earth at a distance of 9 million miles, allowing astronomers to gather more data about this asteroid’s orbit and composition.

On Wednesday Apophis will pass the Earth at a distance of 9 million miles, allowing astronomers to gather more data about this asteroid’s orbit and composition.

Having crossed outside Earth’s orbit, Apophis will appear briefly in the night-time sky. Wednesday 9 January will afford astronomers the rare opportunity to bring a battery of telescopes to bear: from optical telescopes to radio telescopes to the European Space Agency’s Infrared Space Observatory Herschel. Two of the biggest unknowns that remain to be established are the asteroid’s mass and the way it is spinning. Both of these affect the asteroid’s orbit and without them, precise calculations cannot be made.

Using a solar sail to deflect an earth-destroying asteroid

solar sail mission to Apophis

In a paper published today on the Los Alamos astro-ph preprint website, two Chinese scientists have proposed using a solar sail for deflecting any asteroid that happens to be aimed at the earth. The diagram to the right is their simulated mission to impact the asteroid Apophis, which will pass close to the earth in 2029 and — depending on whether that flyby puts it through a very small 600 meter-wide mathematical “keyhole” — could then return in 2036 on a collision course.

The idea is to use the sail to slow the spacecraft down enough so that it starts to fall towards the sun. The sail is then used to maneuver it into a retrograde orbit. When it impacts the asteroid the impact will therefore be similar to a head-on collision, thereby imputing the most energy in the least amount of time with the least amount of rocket fuel. In their Apophis simulation, a mission, weighing only 10 kilograms (about 22 pounds), launched around 2025, and hitting the asteroid in this manner in 2026, would deflect its flyby in 2029 enough to guarantee it will not fly through the “keyhole” and therefore eliminate any chance of it hitting the earth in 2036.

Obviously many questions must be answered before such a mission should fly.
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