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How to plot a satellite’s fall

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Update 20110925-pm(Asia): NASA has said that UARS fell back to Earth between 11:23 p.m. EDT Friday, Sept. 23 and 1:09 a.m. Sept. 24.

UARS reentered the atmosphere sometime between 0323 and 0509 GMT on 24 September. The mid-point of that groundtrack and a possible reentry location is 31 N latitude and 219 E longitude (green circle marker on the above map). Re-entry assessment by DoD's Joint Space Operations Center at Vandenberg AFB, CA. Credit: NASA

UARS fell back 20 years and nine days after its launch on a 14-year mission that produced some of the first long-term records of chemicals in the atmosphere. NASA has said the precise re-entry time and location of debris impacts have not been determined. During the re-entry period, the satellite passed from the east coast of Africa over the Indian Ocean, then the Pacific Ocean, then across northern Canada, then across the northern Atlantic Ocean, to a point over West Africa. The vast majority of the orbital transit was over water, with some flight over northern Canada and West Africa.

There have been a host of claims concerning the fall of UARS. SPACE.com has said it was flooded with emails from people claiming to have seen the spacecraft’s descent or to report potential debris sightings, but agency officials said these cases are unlikely. So far, NASA has not received any credible reports of debris, or of people on the ground who think they may have observed UARS burning up on its way through the atmosphere.

“Most of the time we do rely heavily on people on the ground,” Nick Johnson, chief orbital debris scientist at NASA’s Johnson Space Center in Houston, told reporters in a news briefing Saturday (Sept. 24). “UARS, whether it came in during the local day or local night, would have clearly been visible.” Johnson said there were some people along the western coast of North America that were hoping to capture views of the dying satellite. Since the agency did not receive any plausible reports, this would further suggest that UARS did, in fact, splash down in the ocean without ever reaching North America.

Reuters has reported that at 35 feet long and 15 feet in diameter, UARS was among the largest spacecraft to plummet uncontrollably through the atmosphere, although it is a slim cousin to NASA’s 75-tonne (68,000 kilogram) Skylab station, which crashed to Earth in 1979. Russia’s last space station, the 135-tonne (122,000 kilogram) Mir, crashed into the Pacific Ocean in 2001, but it was a guided descent. NASA now plans for the controlled re-entry of large spacecraft, but it did not when UARS was designed.

Six years after the end of its productive scientific life, UARS broke into pieces during re-entry, and most of it up burned in the atmosphere. Data indicates the satellite likely broke apart and landed in the Pacific Ocean far off the U.S. coast. Twenty-six satellite components, weighing a total of about 1,200 pounds, could have survived the fiery re-entry and reach the surface of Earth. However, NASA is not aware of any reports of injury or property damage. The Operations Center for JFCC-Space, the Joint Functional Component Command at Vandenberg Air Force Base, Calif., which works around the clock detecting, identifying and tracking all man-made objects in Earth orbit, tracked the movements of UARS through the satellite’s final orbits and provided confirmation of re-entry.

Update 20110924-pm(Asia): UARS has landed. NASA’s decommissioned Upper Atmosphere Research Satellite fell back to Earth between 11:23 p.m. EDT Friday, Sept. 23 and 1:09 a.m. EDT Sept. 24. The Joint Space Operations Center at Vandenberg Air Force Base in California said the satellite penetrated the atmosphere over the Pacific Ocean. The precise re-entry time and location are not yet known with certainty (from NASA Update No. 15).

Update 20110924: New updates from NASA on the re-entry path of UARS and possible times of re-entry. Background on predictive models of space debris. Description on kinds of debris that may be expected.

From mapping out the track of a hurricane to monitoring the melting of ice caps, satellites are heavily relied on in science. The Upper Atmosphere Research Satellite (UARS), which has been languishing in orbit for six years, is now on the verge of falling back to Earth, NASA announced in September 2011. In this photo, the UARS hangs in the grasp of the Remote Manipulator System during deployment from Space Shuttle Discovery in September 1991. Photo: NASA Marshall Space Flight Center

NASA Update No. 13 – the orbit of UARS was 85 miles by 90 miles (135 km by 140 km). Re-entry was expected between 11:45 p.m. Friday, Sept. 23, and 12:45 a.m., Sept. 24, Eastern Daylight Time (3:45 a.m. to 4:45 a.m. GMT). During that time period, the satellite was passing over Canada and Africa, as well as vast areas of the Pacific, Atlantic and Indian oceans. The risk to public safety was very remote. NASA is working to confirm the re-entry location and time and will provide an update shortly. [NASA Update No. 12 was not substantially different from No. 13]

NASA Update No. 11 – the orbit of UARS was 90 miles by 95 miles (145 km by 150 km). Re-entry is expected between 11 p.m. Friday, Sept. 23, and 3 a.m., Sept. 24, Eastern Daylight Time (3 a.m. to 7 a.m. GMT).  During that time period, the satellite will be passing over Canada, Africa and Australia, as well as vast areas of the Pacific, Atlantic and Indian oceans. The risk to public safety is very remote.

To foresee the paths of space junk so that collisions can be avoided, NASA developed one of the world’s most sophisticated predictive models, as Technology Review has explained. Called Legend (for “low-Earth to geosynchronous environment debris”), the three-dimensional model simulates the routes of all trackable space objects and even factors in new debris from future crashes. To take uncertainty and randomness into account, hundreds of scenarios are generated using the Monte Carlo method, a set of algorithms that can calculate risk factors in a complex environment. With Legend, NASA scientists use the average of multiple simulations to estimate the number, size, and type of objects that will collide—and approximately how often. Unlike models used by the U.S. Strategic Command Joint Space Operations Center, which detects and tracks large objects and screens active satellites daily for possible collisions within 72 hours, Legend includes smaller fragments and looks far into the future.

In place since 2004, the NASA model is constantly fed with data gathered from the results of ground tests and spacecraft that have broken up in orbit; from telescopes and radars viewing the sky; and from analysis of crater-marked spacecraft surfaces that have returned to Earth. That means new simulations must be run continually. Legend enables scientists to calculate the consequences of a particular breakup or collision and helps them alert managers at the space station that a piece of debris could be in its path. The model also advises soon-to-launch satellites of areas to avoid and will guide scientists as they attempt to develop and launch debris removal technology for the first time.

[Keep track of UARS via U.S. Strategic Command Joint Space Operations Center and NASA Orbital Debris Program Office.]

From NASA Orbital Debris Program Office: “After spacecraft (or parent body) breakup, individual components, or fragments, will continue to lose altitude and receive aeroheating until they either demise or survive to impact the Earth. Spacecraft components that are made of low melting-point materials (e.g., aluminum) will generally demise at higher altitudes than objects that are made of materials with higher melting points (e.g., titanium, stainless steel, beryllium, carbon-carbon). If an object is contained inside of a housing, the housing must demise before the internal object receives significant heating. Many objects have a very high melt temperature such that they do not demise, but some can be so light (e.g., tungsten shims) that they impact with a very low velocity. As a result, the kinetic energy at impact is sometimes under 15 J, a threshold below which the probability of human casualty is very low.”

This conceptual image shows the Upper Atmosphere Research Satellite, launched on Sept. 15, 1991, by the space shuttle Discovery. Originally designed for a three-year mission, UARS measured chemical compounds found in the ozone layer, wind and temperature in the stratosphere, as well as the energy input from the sun. Together, these measurements helped define the role of Earth's upper atmosphere in climate and climate variability. The 35-foot-long, 15-foot-diameter UARS was decommissioned on Dec. 14, 2005. Credit: NASA

Earlier: First, get the data from the owners, NASA. Second, read the onlnie news and watch TV, but keep track of what the orbital experts are saying – Space Track, Space Data Source, NASA’s Orbital Debris Program Office, the Joint Space Operations Center of U.S. Strategic Command. Third, keep handy some basics about the falling satellite (a few pointers follow).

Spread of surviving debris

NASA’s latest update: As of 10:30 a.m. EDT on Sept. 23, 2011, the orbit of UARS was 100 miles by 105 miles (160 km by 170 km). Re-entry is expected late Friday, Sept. 23, or early Saturday, Sept. 24, Eastern Daylight Time. Solar activity is no longer the major factor in the satellite’s rate of descent.

The satellite’s orientation or configuration apparently has changed, and that is now slowing its descent. There is a low probability any debris that survives re-entry will land in the United States, but the possibility cannot be discounted because of this changing rate of descent. It is still too early to predict the time and location of re-entry with any certainty, but predictions will become more refined in the next 12 to 18 hours.

Casualty risk assessment

NASA has been posting updates daily until about 24 hours before re-entry, and then at about 12 hours, six hours and two hours before re-entry. The updates will come from the Joint Space Operations Center of U.S. Strategic Command at Vandenberg Air Force Base, Calif., which works around the clock detecting, identifying and tracking all man-made objects in Earth orbit, including space junk.

As of Sept. 8, 2011, the orbit of UARS was 152 miles by 171 miles (245 km by 275 km) with an inclination of 57 degrees. Because the satellite’s orbit is inclined 57 degrees to the equator, any surviving components of UARS will land within a zone between 57 degrees north latitude and 57 degrees south latitude. It is impossible to pinpoint just where in that zone the debris will land, but NASA estimates the debris footprint will be about 500 miles long.

Advice: If you find something you think may be a piece of UARS, do not touch it. Contact a local law enforcement official for assistance.

UARS starboard view

UARS Reentry Predictions
The official source of reentry predictions for uncontrolled space objects is USSTRATCOM’s Joint Space Operations Center (JSpOC).
Normal procedure is for TIP (Tracking and Impact Prediction) messages to be prepared and released to the public (via the Space-Track.org website) at the following intervals: – T-4 days, T-3 days, T-2 days, T-1 day, T-12 hours, T-6 hours, and T-2 hours.
TIP messages provide the best estimates of reentry time and location but have large uncertainties. Even at T – 2 hours, the uncertainty of reentry time is on average +/- 25

UARS port view

minutes for nearly circular orbits. This equates to +/- 12,000 km on the Earth.
A final, post-reentry assessment message is normally issued within a few hours of reentry. This reentry prediction comes from the ‘Re-entry and Risk Assessment for the NASA Upper Atmosphere Research Satellite (UARS)’ document [get it here, pdf], NASA Orbital Debris Program Office, Lyndon B. Johnson Space Center.

Also see:
Re-Entry and Risk Assessment (498 KB PDF)
Frequently Asked Questions: Orbital Debris
NASA’s UARS satellite, launched in 1991 from the Space Shuttle, was the first multi-instrumented satellite to observe numerous chemical constituents of the atmosphere with a goal of better understanding atmospheric photochemistry and transport.

UARS recent orbital history

One Response

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  1. anonymous

    September 23, 2011 at 23:33


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