The population of natural Earth satellites

Our planet may frequently capture small asteroids into orbit (Image: Detlev van Ravenswaay/Science Photo Library)

Mikael Granvik, Jeremie Vaubaillonc, Robert Jedickea
Abstract
We have for the first time calculated the population characteristics of the Earth’s irregular natural satellites (NES) that are temporarily captured from the near-Earth-object (NEO) population.
The steady-state NES size-frequency and residence-time distributions were determined under the dynamical influence of all the massive bodies in the solar system (but mainly the Sun, Earth, and Moon) for NEOs of negligible mass.
To this end, we compute the NES capture probability from the NEO population as a function of the latter’s heliocentric orbital elements and combine those results with the current best estimates for the NEO size-frequency and orbital distribution.
At any given time there should be at least one NES of 1-meter diameter orbiting the Earth. The average temporarily-captured orbiter (TCO; an object that makes at least one revolution around the Earth in a co-rotating coordinate system) completes (2.88±0.82) rev around the Earth during a capture event that lasts (286±18) days. We find a small preference for capture events starting in either January or July.
Our results are consistent with the single known natural TCO, 2006 RH120, a few-meter diameter object that was captured for about a year starting in June 2006.
We estimate that about 0.1% of all meteors impacting the Earth were TCOs. (sciencedirect.com)

Read also: Hundreds of tiny moons may be orbiting Earth

ROSAT satellite- latest news

(update)
From bbc/news:

“…Just as for Nasa’s UARS satellite, which plunged into the atmosphere in September, there was high uncertainty about the final moments of Rosat.
But if the timings are correct, any wreckage would probably have dived into the Indian Ocean – although no eyewitness reports have yet come in.
If anything did manage to make landfall, the likely areas to be affected would have been Myanmar and China….”

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ROSAT – latest news
Last update: 23 October 2011, 02:45 UTC (04:45 CEST)
On Sunday, 23 October 2011, between 1:45 UTC (3:45 CEST) and 2:15 UTC (4:15 CEST) the german ROentgen SATellite ROSAT has re-entered Earth’s atmosphere. There is currently no confirmation if pieces of debris have reached Earth’s surface.
www.dlr.de

Read also:
1. Europe Safe From Falling German Satellite Debris
2. If Germany’s Satellite Falls on Your House, Who Pays for Repairs?
3. What Are the Odds You’ll Get Struck by the Falling ROSAT Satellite?

What Are the Odds You’ll Get Struck by the Falling ROSAT Satellite?

This exclusive image was made by Ralf Vandebergh, who said: “”It is false-color to increase certain visible contrasts. A very special detail visible is the shadow of the body (the telescope) on the solar panels! You can see the angle with the sun and the observer (me) as ROSAT passed not overhead but [at] 51.4 degrees northern latitude. This is a very difficult observation as the object is very small.

Not long after re-emerging en masse from our underground bunkers and panic rooms, having successfully avoided being squashed by a falling NASA satellite on Sept. 24, humanity has learned that the sky is falling yet again. Another huge piece of space debris, a 2.6-ton, defunct German telescope called the Roentgen Satellite (ROSAT), will crash back to Earth Saturday or Sunday (Oct. 22 or 23), and the chances it will hit someone are even greater this time around.

The odds are 1-in-2,000 that a chunk of ROSAT will strike a person. For the UARS satellite that fell into the southern Pacific Ocean in September, the odds were 1-in-3,200. According to Heiner Klinkrad, head of the European Space Agency’s Orbital Debris Office, ROSAT poses a higher risk than UARS because more of its mass is expected to survive atmospheric re-entry and reach Earth’s surface. [Photos: Germany’s ROSAT Satellite Falling to Earth]

“The fact that the ROSAT re-entry risk estimate is higher than for UARS lies in the surviving mass, which, percentage-wise, is considerably higher for ROSAT than for UARS, and hence, the net mass reaching ground is higher for ROSAT than for UARS,” Klinkrad told Life’s Little Mysteries, a sister site to SPACE.com. “This is due to the ROSAT internal mirror assembly that is very resistant to [heat] during re-entry.”

Typically, when a satellite crashes to Earth, only 20 to 40 percent of its mass survives; the rest burns up from heat generated by friction between the satellite and particles in the atmosphere, Klinkrad said. Because ROSAT’s mirrors ? which collected X-rays and extreme ultraviolet light emitted by celestial objects ? resist heat, they reduce the percentage of the spacecraft that will burn up, and over half of the spacecraft’s mass, about 1.7 tons of it, is expected to reach the surface. [If a Satellite Falls On Your Home, Who Pays for Repairs?]

According to scientists in NASA’s orbital debris office at Johnson Space Center in Houston, calculating the risk of space debris hitting someone requires first working out how much debris makes landfall. Analysts then make a grid of how the human population is distributed around the globe. Oceans, deserts and the North and South poles are largely devoid of people, for example, whereas coastlines are brimming with them. In short, the analysts must figure out which patches of Earth have people standing on them.

Throwing in a few more minor details, such as the latitudes over which satellites spend most of their time orbiting ? ROSAT will most likely fall between 53 degrees north and 53 degrees south latitudes ? the scientists calculate how likely it is that a piece of space junk will strike the ground where a person happens to be. This time around, the odds are 1-in-2,000, and there’s a one-in-several-trillion chance that not only will a person get hit, but that person will be you.

Two dead satellites have crashed to Earth in as many months, after years of gradually getting dragged down to lower and lower orbits. More will re-enter the atmosphere in the future. With this in mind, you may be interested to know the overall risk of getting struck in a given year, or in your lifetime.

“The annual risk of a single person to be severely injured by a re-entering piece of space debris is about 1 in 100 billion,” Klinkrad said. In the course of a 75-year lifetime, then, the odds of getting injured by space junk would be a little less than 1 in 1 billion. If this sounds scary, it probably shouldn’t. By comparison, “the annual risk that a single person gets struck by a lightning is about a factor 60,000 higher, and the risk of a serious injury from a motor vehicle accident is about 27 million times higher than the risk associated with re-entry events.”
http://www.space.com

App tracks ROSAT satellite’s crash to Earth


ROSAT, a defunct X-ray telescope, is crashing to Earth sooner than expected owing to enhanced solar activity, says Johann-Dietrich Wörner, executive director of DLR, the German lab in charge of the mission. It was thought that the 2.4 tonne spacecraft would deorbit in late October or early November, but Wörner says the re-entry date is now going to be between 20 October and 25 October.

“Increased solar radiation activity has enlarged the atmosphere, increasing the friction on the satellite. So it will come down earlier than expected,” he says. But he can’t say where in that date range the satellite is most likely to deorbit – it depends on fast-fluctuating atmospheric conditions. Heatproof optics mean much more of ROSAT’s mass is expected to survive re-entry compared to the NASA UARS satellite that fell into the Pacific Ocean last month….. Continue reading App tracks ROSAT satellite’s crash to Earth

Remains of satellite may never be found, NASA says

A six-ton NASA science satellite crashed to Earth on Saturday, leaving a mystery about where a ton of space debris may have landed.

The U.S. space agency said it believes the debris ended up in the Pacific Ocean, but the precise time of the bus-sized satellite’s re-entry and the location of its debris field have not been determined.

The Upper Atmosphere Research Satellite, or UARS, ended 20 years in orbit with a suicidal plunge into the atmosphere sometime between 11:23 p.m. on Friday and 1:09 a.m. EDT on Saturday (0323 to 0509 GMT Saturday), NASA said.

The satellite would have been torn apart during the fiery re-entry, but about 26 pieces, the largest of which was estimated to have weighed 330 pounds (150 kg), likely survived the fall, officials said………… Continue reading Remains of satellite may never be found, NASA says

UARS Will Hit the South Pacific Today

Orbital scientists say that the falling Upper Atmosphere Research Satellite (UARS) will not impact the ground over US territory. According to the latest predictions, it will splash in the South Pacific Ocean, a little to the north of New Guinea.

Over the past few days, experts have been hard at work in analyzing the trajectory the UARS took when it began its descent, as well as all the other factors that may be involved in altering this course.

This is very complex task, especially when considering that even solar activity can influence the rate at which a spacecraft is influenced and pulled by the atmosphere. However, researchers from NASA and the US Air Force (USAF) managed to understand UARS’ path in more detail.
In addition, amateur astronomers and skywatchers have also been keeping an eye on the satellite, working with NASA to centralize the data. In the end, this proved to be useful for narrowing down the possible time windows when the satellite was expected.

A post published on the NASA website on Thursday, September 22, indicates that the UARS has entered a 115-by-120 mile (185-by-195 kilometer) orbit around the planet, and that reentry is therefore expected to occur sometimes during Friday afternoon (EDT).

Experts also said that there will be night at the location where the impact is expected to occur, giving people an early warning about the incoming spacecraft. “The satellite will not be passing over North America during that time period,” the NASA post said yesterday.

“It is still too early to predict the time and location of re-entry with any more certainty, but predictions will become more refined in the next 24 to 36 hours,” the agency announced. The Joint Space Operations Center of US Strategic Command contributed to tracking the satellite as well.

Expert Ted Molczan, who is using United States Strategic Command (USSTRATCOM) orbital elements to determine where the 6.5-ton spacecraft will impact the ground, says that scientists will not be able to determine exactly where UARS will impact until it does.

“I am making these estimates to maintain awareness of the approximate decay time, to maximize my chances of seeing the event,” Molczan explains, as quoted by Space. He says that the satellite’s debris trail will cover almost 500 kilometers (310 miles) in length.

“If, within a few hours of the decay, it appears that it will occur on a revolution that spends some time above my horizon, then, weather permitting, I will go out and watch for it during the several minutes in which it might pass,” he concludes.

http://news.softpedia.com/news/UARS-Will-Hit-the-South-Pacific-Today-223214.shtml

Second big satellite set to resist re-entry burn-up

Incoming! (Image: Max Planck Institute for Extraterrestrial Physics)

Even if NASA’s 6-tonne UARS satellite does not cause any injury or damage when it re-enters the Earth’s atmosphere today, there is more space junk headed our way next month. A defunct German space telescope called ROSAT is set to hit the planet at the end of October – and it even is more likely than UARS to cause injury or damage in populated areas.

No one yet knows where UARS (Upper Atmosphere Research Satellite) will fall to earth. Although most of the craft’s mass will be reduced to an incandescent plasma, some 532 kilograms of it in 26 pieces are forecast to survive – including a 150-kilogram instrument mounting.

NASA calculates a 1-in-3200 chance of UARS causing injury or damage. But at the end of October or beginning of November, ROSAT – a 2.4-tonne X-ray telescope built by the German aerospace lab DLR and launched by NASA in 1990 – will re-enter the atmosphere, presenting a 1 in 2000 chance of injury.

The higher risk stems from the requirements of imaging X-rays in space, says DLR spokesperson Andreas Schütz. The spacecraft’s mirrors had to be heavily shielded from heat that could have wrecked its X-ray sensing operations during its eight-year working life. But this means those mirrors will be far more likely to survive a fiery re-entry.

Broken mirror, bad luck

On its ROSAT website, DLR estimates that “up to 30 individual debris items with a total mass of up to 1.6 tonnes might reach the surface of the Earth. The X-ray optical system, with its mirrors and a mechanical support structure made of carbon-fibre reinforced composite – or at least a part of it – could be the heaviest single component to reach the ground.”

At the European Space Agency in Darmstadt, Germany, the head of the space debris office, Heiner Klinkrad, agrees that ROSAT’s design means more of it will hit the surface. “This is indeed because ROSAT has a large mirror structure that survives high re-entry temperatures,” he says.

ROSAT was deactivated in 1999 and its orbit has been decaying since then. “ROSAT does not have a propulsion system on board which can be used to manoeuvre the satellite to allow a controlled re-entry,” says space industry lawyer Joanne Wheeler of London-based legal practice CMS Cameron McKenna. “And the time and position of ROSAT’s re-entry cannot be predicted with any precision due to fluctuations in solar activity, which affect atmospheric drag.”

Solar swelling

US Strategic Command tracks all space objects and the US-government-run Aerospace Corporation lists both upcoming and recent re-entries on its website. But ROSAT is not yet on the upcoming list because its re-entry time is far from certain.

The moment a craft will re-enter is difficult to predict because it is determined by two main factors. First, the geometry of the tumbling satellite as it enters the upper atmosphere, which acts as a brake. Second, the behaviour of the upper atmosphere itself, which grows and shrinks with the amount of solar activity, says Hugh Lewis, a space debris specialist at the University of Southampton, UK.

“Solar activity causes the atmosphere to expand upwards, causing more braking on space objects. The reason UARS is coming back sooner than expected is a sudden increase in solar activity. Indeed, we expect to see a higher rate of re-entries as we approach the solar maximum in 2013,” he says.

But don’t expect it to be raining spaceships – what’s coming down is partly a legacy of 1990s space-flight activity. “Some of the re-entries we see today [with UARS and ROSAT] are a heritage of years with high launch rates, which were a factor of two higher than they are today,” says Klinkrad.

“The trend is towards smaller satellites, with more dedicated payloads,” he says, rather than “all-in-one” satellite missions on giant craft like UARS. That means debris from future missions should be smaller.

http://www.newscientist.com/article/dn20955-second-big-satellite-set-to-resist-reentry-burnup.html