Satellite Re-entry

A controlled re-entry: the space shuttle leaves a trail of ionized gas across the Texas sky, en route to a landing at the Kennedy Space Center.

Introduction

Sooner or later all good things must come to an end. Objects in high orbits merely die with their power supplies; solar cells degrade with time due to radiation exposure and damage due to debris, radioisotopic thermo-electric generators eventually are unable to supply sufficient power. Satellites in low Earth orbit have one last chance to perform - when they sink back into the atmosphere below about 100 km and are consumed in an orgy of fire. A re-entry has the added bonus of being visible to daylight observers too, as a chaotic series of intertwined contrails across the sky.

Satellite re-entries can be quite spectacular and viewed by large numbers of people as the craft will often be seen over a few hundred to a few thousand kilometers before it finally succumbs. For example Sputnik 2 was witnessed by many people during its descent on 14th April, 1958. In the ten minutes it took to travel from over New York to the Amazon, it descended from 130 km to around 60 km, leaving a trail of `sparks' some 100 km long, the satellite itself a multitude of brilliant colors.

Re-entry Prediction

Space-Track.org

The U.S. Department of Defense (DoD)'s Space-Track.org website, provides a 60 Day Decay Prediction of expected re-entries of objects in unclassified orbits. For objects near decay, the site provides Trajectory Impact Prediction (TIP) messages, which estimate the time and location of decay.

Unfortunately the lifetime of a satellite is notoriously difficult to predict. As the orbit (specifically the perigee) lowers, atmospheric drag increases. This varies according to atmospheric conditions, the solar cycle, the mass and surface area of the satellite, to name a few factors. It retains all the chaos of weather forecasting with little of the certainty; only in the final hour or two of the satellite's life can useful predictions be made. Even then the craft may skip along the upper atmosphere, like a stone skimming across a pond, due to aerodynamic interactions. Thus re-entry predictions made a couple of months in advance will pinpoint a day of demise with an error of a week or so. The geographical location can not be predicted with certainty (though potential areas can be).

A synopsis of decayed/decaying satellites is found here.

Aerospace Corporation

Aerospace provides short term predictions for satellites and substantial hardware within 5 days of predicted decay.

Private Individual Predictions

Harro Zimmer

Harro, also a SeeSat-L subscriber, posts his decay reports on SeeSat-L. He provided decay reports on the Starshine satellite's demise.

Of course, another way to get the latest updates is to subscribe to SeeSat-L.

Planned re-entries

Though most re-entries are a hit and miss affair, a few can be viewed with some certainty, where mission controllers have deemed it necessary to `pilot' a craft back into the atmosphere in order to dispose of it as was the reentry of Mir on March 23, 2001. Then there are the periodic re-entries of the Progress M supply ferry for the International Space Station, or one of the ascent stages of a launch vehicle falling to Earth due to having (intentionally) insufficient orbital velocity.

Space Shuttle re-entries normally are joyous events for observers, but on February 1, 2003, many bore witness to the final minutes of Columbia and the crew of STS 107. SeeSat-L relayed the sad observations by Mike McCants, Robert Fenske Jr., Rob Matson, Harald Edens. and Rick Baldridge. Rick recorded one of the earliest videotapes of pieces falling away from Columbia, which was assigned reference number EOC2-4-0056 by the Columbia Accident Investigation Board.

In happier times, NASA provided TV coverage of the STS-93 re-entry over Texas on July 27, 1999.

Additional photo and link to the STS-103 re-entry over Texas on December 27, 1999.

Pictured below is an example of the re-entry of an external tank (ET), discarded during the ascent of each space shuttle:

The STS-55 ET on its way back to Earth.

The ET provides the fuel and oxidizer for the shuttle's main engines during the first 8.5 minutes or so of the ascent. After main engine cut-off (MECO) the tank is separated from the shuttle. At this point in time both shuttle and ET are in an elliptical orbit whose apogee is close to the operating orbital altitude of the shuttle, but with a perilously low perigee. The shuttle performs an OMS (orbital maneuvering system) burn some 45 minutes into the mission to circularize the orbit and thus prevent a premature return to Earth. The ET is destined to re-enter over the Pacific/Indian Ocean (ascent trajectory dependent) some 80 minutes or so after the launch. The earlier launches (involving two orbital maneuvering system, OMS, burns) resulted in the ET re-entering around 185,000 feet above the Indian Ocean (on one occasion a South African pilot reported seeing the re-entry). Current direct ascent trajectories result in ET re-entry over the Pacific Ocean, south of Hawaii.

After separation the ET cruises half-way round the world to its re-entry point, visible to observers from Newfoundland, across Europe to Africa, as a mag. +1 object of reddish hue, accompanying the white orbiter of similar brightness. During earlier missions a tumble was induced (by venting excess gas) to negate aerodynamic lift and helped guide it toward a predetermined area; a large re-entry footprint is planned for in order that no pieces of the tank should come within 370 km of any land mass, nor enter foreign air-space, to avoid areas of human activity and the polar regions

The re-entry of the STS-41C ET (6th April, 1984, from a direct ascent) was observed by Paul Maley of the Johnson Space Center Astronomical Society, from Mauna Kea, Hawaii, in the pre-dawn sky:

At 0519 (local time) a bright glow pierced the horizon, varying slowly in brilliance. Three complete brightness variations were visible before the glow was unexpectedly obscured as the tank was hidden by a small cinder cone, but the wake remained luminous as three bright lines in the sky broken by two dark areas. This was later found to be due to the tank tumbling; the broad cylindrical side and the small end alternately faced the observer as it tumbled at 45 degrees per second. As the glow emerged from its short eclipse it had brightened to at least mag. -6, continually pulsating as it tumbled. Disintegration was beginning. The brilliant mass slowly began to separate into a sparkling cluster of individual pieces, some climbing upward, gaining lift for a short span of new life. The swarm gradually formed an elliptical group of perhaps as many as 30 to 50 visible pieces as bright as mag. 0 and others as dim as mag. +4. The steady stream of glittering fragments covered a span of about 10 degrees long and 5 degrees high as it sped through the southern constellations of Ara, Pavo, Indus and Grus. The orange-yellow `glob' had given birth to a host of brilliant blue-white fragments, some unvarying in brilliance with others cascading end over end.

Above left image: Close up of the ET fragments during re-entry.
Above right image: Contrails arising from the ET break-up paint the sky.

137 seconds after it had begun it was all over, but a glance upward revealed the steady light of the mag. +2 250 km high orbiter climbing into its parking orbit. At 0549 there was still an odd bluish cloud in the same spot where the tank had disintegrated. This contrail-like cloud was photographed until 0553.

Paul also witnessed a re-entry of the shuttle itself during STS-69 on the 18th September, 1995, as the vehicle streaked across the southern US states on its way to the Kennedy Space Center:

I am happy to report we observed the spectacular reentry of STS-69 at 1122GMT this morning. In its typical fashion (I've seen 6 or 7 reentries of the orbiter before) we observed a red incandescent trail in the wake of Endeavour which itself was -3 magnitude and yellow (oval shape). Skies were mostly cloudy (low clouds) but we had quite excellent views anyway. I was able to get the parking lights in front of the Mission Control Center turned off so that we could better observe the low elevation pass (maximum elevation 17 degrees) as it flew directly over Waco, Texas to our north. The trail, which repeats at certain missions, also took it over McDonald Observatory. This sighting was different from STS-70 which occurred with the sun 3 degrees above the horizon. This time sunrise was 40 minutes after the predawn pass. Orbiter altitude was 37 nautical miles, velocity Mach-16.

The above ET re-entry account and accompanying images were taken from an article in Spaceflight (Vol. 26, December 1984) with the author's permission and are © Paul D. Maley.

Paul Maley, who is an experienced satellite observer, has finally produced an excellent web site on satellite observing including several pictures of shuttle re-entries.

The shuttle web site normally provides re-entry maps and listings of major cities (normally only 24-48 hours before the planned landing) in the US (and sometimes Canada) where re-entry can be observed.

NASA's Orbital Debris Quarterly has additional information on satellite re-entries.

Links: to the VSO Home Page, observing guide, and satellite predictions.

Contact: webmaster@satobs.org