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Space Shuttle Challenger disaster

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An iconic image of the accident.

The Space Shuttle Challenger accident occurred on the morning of January 28 1986, at 11:39 EST, when Space Shuttle Challenger was destroyed 73 seconds into its flight (at the start of mission STS-51-L, the 25th of the STS program and Challenger's 10th) due to the failure of an O-ring seal in the aft (lower) segments of the right solid rocket booster (SRB).

The seal failure caused a flame leak from the SRB that impinged upon the adjacent external propellant tank and aft SRB connecting strut. Within seconds the flame caused structural failure of the external tank, and the orbiter broke up abruptly due to aerodynamic forces. The crew compartment and many other vehicle fragments were eventually recovered from the ocean floor.

The launch was televised live, although most viewers saw it tape-delayed later that day. Christa McAuliffe had been expected to be the first teacher in space, and students worldwide had expected to watch a television broadcast of her delivering a science lesson from space.

U.S. manned space flights did not resume until over two years later, with the launch of the space shuttle Discovery on September 29 1988 with the "Return to Flight" mission STS-26.

Crew

The crew of STS-51-L.

Pre-Launch conditions and delays

Launch delays

Challenger's launch was originally set for 2:43pm EST on January 22. Delays with STS-61-C caused the launch date to be pushed back to the 23rd and then to the 24th. Launch was re-scheduled for the 25th due to bad weather at the Transoceanic Abort Landing (TAL) site in Dakar, Senegal. NASA decided to use Casablanca as the TAL site, but because it was not equipped for night landings the launch had to be moved to the morning (Florida time). Predictions of unacceptable weather at KSC (Kennedy Space Center) caused the launch to be re-scheduled for 9:37am EST on the 27th. Launch was then delayed 24 hours when the pad technicians could not remove a 'closing fixture' from the orbiter's hatch. When the fixture was finally sawed off, cross winds at the SLF (Shuttle Landing Facility) exceeded the limits for a Return to Launch Site (RTLS) abort. Launch on the 28th was delayed two hours when a fire detection system failed during liquid hydrogen tanking procedures. The launch finally began at 11:38am EST on Tuesday, January 28.

Ice on launch pad

File:Sts-33 closeout 01.jpg
A closeout photo showing ice on the launch pad

The night before launch, local temperatures dropped to the low 20s Fahrenheit (about -5 degrees Celsius) causing ice formation in the region. This was unusually cold; some nearby Floridians had the rare problem of removing a layer of ice from their car windshields if they left for work before dawn.

Several launch pad water systems were opened slightly and allowed to flow into drains to keep the standing water from freezing and cracking the pipes. However the drains themselves froze and caused overflows. High wind gusts spread the water around the pad, causing additional ice formation. The ice team quickly went into action and began clearing away ice that posed a threat to Challenger's TPS (Thermal Protection System). That team had experience with a similar (although, not as bad) incident on pad 39-A before the launch of STS-51-C. That launch had been postponed.

The temperature at liftoff was 36 °F (2 °C), 15 °F (8 °C) colder than any previous launch. The liquid oxygen and hydrogen stored in the large central foam-insulated External Tank were maintained at cryogenic temperatures, while the solid rocket boosters and their rubber O-rings remained at ambient temperature, only slightly above the freezing point of water.

Engineers at Morton Thiokol (manufacturer of the solid rocket boosters) knew that the temperatures were outside of the design range of the O-rings. They strongly objected to the launch, but were overruled by senior Thiokol management.

NASA and Thiokol management knew about serious O-ring erosion on earlier Shuttle flights. They planned steps to correct the problem, but decided to keep flying the shuttle in the interim, treating it as an acceptable flight risk.

January 28 launch and failure

Liftoff

File:Sts-33 d67 01.jpg
Camera D-67 captures grey smoke emitting from the right-hand SRB

At 6.6 seconds before liftoff, the three space shuttle main engines (SSME) ignited. Until liftoff actually occurs, the SSMEs can be safely shut down and the launch aborted if necessary. At liftoff time (T=0), the three SSMEs were at 100% of rated performance and began throttling up to 104% of rated performance under computer control. At this moment, the two SRBs were ignited and hold-down bolts were released with explosives, freeing the vehicle from the pad. With the first vertical motion of the vehicle, the gaseous hydrogen vent arm retracted from the ET but failed to latch back. Review of film shot by pad cameras showed that the arm did not re-contact the vehicle. The post-launch inspection of the pad also revealed that kick springs on four of the hold-down bolts were missing. They were also ruled out as a contributing factor in the accident.

Later review of launch film showed that at T+0.678, strong puffs of dark grey smoke emitted from the right-hand SRB near the aft strut that attaches the booster to the ET. The last smoke puff occurred at about T+2.733. The last view of smoke around the strut was at T+3.375.

Initial ascent

The Challenger lifts off. Video

As the vehicle clears the tower, the SSMEs are operating at 104% of rated performance, and control switches from the LCC (Launch Control Center) at KSC to the MCC (Mission Control Center) in Houston, Texas. Times are given in seconds after launch. Flight controller call signs or crew name identify the speaker. "Intercom" means crew-to-crew communication. Speakers: 

T+5.000 seconds:
DPS: Liftoff confirmed.
Flight: Liftoff...

To prevent aerodynamic forces from tearing the shuttle apart, the SSMEs must throttle down to limit velocity in the dense lower atmosphere. The throttle down operation begins at about T+28 seconds. At T+35.379, the SSMEs throttle back further to the pre-planned 65%. 

T+19.859 seconds:
Booster: Throttle down to 94.
Flight: Ninety four...

T+40.000 seconds:
Smith, intercom: There's Mach 1.
Scobee: Going through 19,000.

T+48.900 seconds:
Booster: Three at 65.
Flight: Sixty-five, FiDO...
FIDO: T-del confirms throttles.
Flight: Thank-you.

At T+51.860, the SSMEs begin throttling back up to 104% as the vehicle approaches Max Q (the area of maximum aerodynamic pressure on the vehicle, approximately 720 pounds per square foot or 34 kilopascals). 

T+57.000 seconds:
Scobee, intercom: Throttling up.

Plume

File:Sts-33 e207 01.jpg
Camera captures plume on right SRB

At about T+58.788, a tracking film camera captures the beginnings of a plume near the aft attach strut on the right SRB. Unknown to those on Challenger or in Houston, ignited gas had begun to leak through a growing hole in one of the right-hand SRB's joints. Within a second, the plume becomes well defined and intense. Even had the crew or mission control known, nothing could have been done. Everything else was apparently normal and the crew called the expected "go" as the three SSMEs throttled up.

At an altitude of 35,000 feet, Challenger passes through Mach 1.5 (1.5 times the speed of sound). 

 T+62.000 seconds:
 Smith, intercom: "Thirty-five thousand, 
    going through one point five."

T+68.000 seconds:
CAPCOM: Challenger, go at throttle up.
Scobee: Roger, go at throttle up.

Failure develops

At T+72.525, later analysis of telemetry data showed a sudden lateral acceleration to the right, which may have been felt by the crew. At T+72.564 - liquid hydrogen pressure in external tank begins decreasing due to rupture caused by SRB flame. 

T+73.000 seconds:
Smith, intercom: "Uh oh..."

This was the last intercom statement captured by the crew cabin recorder. Smith may have been responding to indications on main engine performance or falling pressures in the external fuel tank. At about T+73.162 the vehicle breakup began.

Post-breakup flight controller dialog

At T+79.000 a TV tracking camera showed a cloud of smoke and flame where Challenger had been, with large burning debris pieces falling toward the ocean. 

T+89.000 seconds:
Flight: "FIDO, trajectories"
FIDO: "Go ahead."
Flight: "Trajectory, FIDO"
FIDO: "Flight, FIDO, filters (radar) got discreting sources. We're go."
FIDO: "Flight, FIDO, till we get stuff back he's on his cue card
   for abort modes"
Flight: "Procedures, any help?"
Unknown: "Negative, flight, no data."
GC: "Flight, GC, we've had negative contact, loss of downlink
     (of radio voice or data from Challenger)."
Flight: "OK, all operators, watch your data carefully."

At T+110.250 the Range safety officer sends radio signals that detonate the self-destruct packages on the solid rocket boosters (SRBs). He saw the breakup and judged the free-flying SRBs a possible threat to land or sea, so destroyed them, which is normal procedure for such a situation. 

T+1 min. 56 seconds
PAO: "Flight controllers here are looking very carefully at the situation.
Obviously a major malfunction."

T+2 min. 1 second
GC: "Flight, GC, negative downlink."
Flight: "Copy."

T+2 min. 8 seconds
PAO: "We have no downlink."

At T+2 min 20 seconds a TV tracking camera shows falling bits of debris and white contrails against the blue sky. Larger objects plummet toward the ocean, streaming thin vapor trails. 

T+2 min. 25 seconds
FIDO: "Flight, FIDO."
Flight: "Go ahead."
FIDO: "RSO (range safety officer) reports vehicle exploded."
Flight: (after a long pause): "Copy. FIDO, can we get any reports
   from recovery forces?"
FIDO: "Stand by."

T+2 min. 45 seconds
Flight: "GC, all operators, contingency procedures in effect."

Failure sequence

This sequence is from real time telemetry data and photographic analysis. Times are seconds after liftoff.

T+60.004: Internal pressure in the right-side SRB begins to drop because of the rapidly enlarging hole in the failed joint

T+60.238: Initial evidence of flame through the rupture impinging on the external tank

T+64.660: The plume suddenly changes shape, indicating a leak has begun in the liquid hydrogen tank, the aft portion of the external tank

T+64.937: Main engine nozzles pivot under computer control to compensate for the unbalanced thrust produced by the booster burn through

T+66.764: Pressure in the shuttle's external liquid hydrogen tank begins to drop, indicating a massive leak

T+72.284: The right SRB apparently pulls away from the aft strut attaching it to the external tank

T+73.124: The aft dome of the liquid hydrogen tank fails, producing a propulsive force pushing the tank into the liquid oxygen tank in the forward external tank. At the same time, the right SRB rotated about the forward attach strut and struck the intertank structure

With the external tank disintegrating, Challenger veered from its correct attitude with respect to the local air flow and was immediately torn apart by aerodynamic forces. The two SRBs, which can withstand greater aerodynamic loads, separated from the ET and began to fly independently.

No "explosion"

The Challenger begins to disintegrate.
File:Sts-33 tv1 01.jpg
Camera TV-1 shows vapor cloud

The Shuttle and External Tank did not actually "explode"; there was no detonation. Instead they rapidly disintegrated under tremendous aerodynamic forces, since the shuttle was near "Max Q", or maximum aerodynamic pressure. The more robustly constructed crew cabin and SRBs survived the breakup. As the detached cabin continued along its ballistic trajectory, the fuel and oxidizer stored in the ET and orbiter reaction control system burned in a few seconds, producing a massive fireball. Had there been a true explosion, the entire Shuttle would have been instantly destroyed, killing the crew at that moment.

The two separated SRBs continued to burn as they flew away from the fireball.

Cause and time of death

At least some of the astronauts were likely alive and briefly conscious after the breakup, because three of the four personal egress air packs (PEAPs) on the flight deck were found to have been activated. Investigators found their remaining unused air supply roughly consistent with the expected consumption during the 2 minute 45 second post-breakup trajectory. PEAP switch design makes activation from the vehicle breakup or water impact very unlikely.

At vehicle breakup, the robustly constructed crew cabin detached in one piece and slowly tumbled. NASA estimated separation forces at about 12 to 20 times the force of gravity (g) very briefly; within two seconds forces were below four g, within ten seconds the cabin was in free fall. These forces were likely insufficient to cause major injury.

Whether the astronauts remained conscious long after the breakup is unknown, and largely depends on whether the detached crew cabin maintained pressure integrity. If it did not, time of useful consciousness at that altitude is just a few seconds. The PEAPs supplied only unpressurized air, not oxygen, hence would not have helped much.

The crew cabin impacted the ocean surface at roughly 207 miles per hour (333 km/hour), causing acceleration of over 200 g, far beyond the structural limits of the crew compartment or crew survivability levels.

On July 28, 1986, Rear Admiral Richard H. Truly, NASA's Associate Administrator for Space Flight and a former astronaut, released a report from Joseph P. Kerwin, biomedical specialist from the Johnson Space Center in Houston, Texas, relating to the deaths of the astronauts in the accident. Dr. Kerwin, a veteran of the Skylab 2 mission had been commissioned to undertake the study soon after the accident.

According to the Kerwin Report: "The findings are inconclusive. The impact of the crew compartment with the ocean surface was so violent that evidence of damage occurring in the seconds which followed the explosion was masked. Our final conclusions are:

  • the cause of death of the Challenger astronauts cannot be positively determined;
  • the forces to which the crew were exposed during Orbiter breakup were probably not sufficient to cause death or serious injury; and
  • the crew possibly, but not certainly, lost consciousness in the seconds following Orbiter breakup due to in-flight loss of crew module pressure."

His report is available from NASA's History Office.

Solid rocket boosters destroyed

After the orbiter and external tank disintegrated at about T+73.213 seconds, the two SRBs (Solid Rocket Booster) continued in uncontrolled powered flight for another 37 seconds. The SRB casings are one-half-inch thick steel and much stronger than the orbiter and ET. The RSO (Range Safety Officer) then detonated the self-destruct packages on board both boosters to prevent damage to objects on earth.

The same destruct signal would have destroyed the External Tank had it not already disintegrated.

Crew escape was not possible

Crew escape was not possible during powered flight.

Modified SR-71 ejection seats and full pressure suits were used on the first four shuttle orbital missions, considered test flights. They were removed for the operational missions that followed, during which the astronauts wore only cloth flight suits.

NASA reasoned that retaining ejection seats was possible for the commander and pilot, but they were impractical for the rest of the crew, especially the three below deck. Unlike a fighter pilot under a thin canopy, the crew below deck were in the center of the forward fuselage, surrounded by vehicle structure on all sides. Additionally, ejection seats might have problems at Max Q (period of maximum aerodynamic pressure), or because of the SRB exhaust plume. The seats were primarily intended for an escape during landing, since the shuttle is unpowered when landing and has only one chance to make the runway. It would be possible to design the crew cabin as a self-contained escape vessel, but this would have been prohibitively expensive, complex, and have added excessive weight to the vehicle. For more details see shuttle ejection escape systems.

While launch escape systems were often considered during shuttle development, NASA eventually decided the shuttle was sufficiently reliable to not need one. Also launch escape systems entail significant weight and space penalties, and the associated pyrotechnics incur safety issues. Several fighter pilots have been killed due to uncommanded ejections. After the Challenger loss, a bail-out system was designed to give the crew the option to leave the shuttle under certain conditions, not including the Challenger scenario. For more details see Post-Challenger abort enhancements.

Aftermath

Investigation

File:ChallengerCrewRemains.jpg
The Challenger crewmember remains being transferred from 7 hearse vehicles to a MAC C-141 transport plane at the SLF for transport to Dover Air Force Base, Delaware.

Initial criticism was focused on the space shuttle external tank, manufactured by Martin Marietta at the Michoud Assembly Facility in New Orleans, by NASA officials who immediately assumed that the fuel tank had failed and exploded. (New Orleans Times-Picayune, Page One, January 29, 1986). The pressure of the initial investigation caused one of the engineers to suffer a nervous breakdown and he never worked again. The focus of the investigation soon shifted however, to the O-rings on the solid rocket booster, manufactured by Morton Thiokol.

The Presidential Commission on the Space Shuttle Challenger Accident, also known as the Rogers Commission (after its chairman), was formed to investigate the disaster. The Commission members were Chairman William P. Rogers, Vice Chairman Neil Armstrong, David Acheson, Eugene Covert, Richard Feynman, Robert Hotz, Donald Kutyna, Sally Ride, Robert Rummel, Joseph Sutter, Arthur Walker, Albert Wheelon, and Chuck Yeager.

Feynman famously demonstrated during a televised hearing how the O-rings became less resilient and subject to seal failures at ice-cold temperatures by immersing a sample of the material in a glass of ice water. He was so critical of flaws in NASA's "safety culture" that he threatened to not sign off on the report unless it included his assessment, which appeared as Appendix F. He pointed to the discrepancy between management claiming a 1 in 100,000 chance of serious failure and the engineers claiming 1 in only 100, a risk one thousand times greater. The commission worked for several months and published a report of their findings.

Cause of failure

For various reasons the SRBs are fabricated in four separate segments, not as a single unit. The four SRB segments are then shipped from the manufacturer to Cape Kennedy where they are assembled "in the field". A joint exists between each SRB segment called a "field joint". The joint exists in both the SRB solid propellant and the 1/2 inch (12 mm) thick outer steel casing. Each field joint is sealed with two O-rings between the steel casing segments.

The entire length of the SRB is internally hollow, like a lead pencil without the lead. During operation, the entire internal length of the SRB is internally burning and under pressure. The Challenger accident happened because the aft field joint of the right SRB failed, allowing the pressurized hot gasses and eventually flame to "blow by" the O-ring and impact the adjacent external tank, causing structural failure.

In hindsight the field joint was inadequately designed, but likely would not have caused a fatal problem if launched under normal Florida temperatures (over 50 °F or 10 °C). The abnormally low temperatures of the last Challenger mission, combined with the inadequate joint design produced a seal failure. Engineers at SRB manufacturer Morton Thiokol were aware of the problem and warned against launching, but the warning did not reach NASA management.

Tributes

The Space Shuttle Challenger Memorial, where some remains were buried together.

On the night of the disaster, President Ronald Reagan had been scheduled to give his annual State of the Union address. He initially announced that the address would go on as scheduled, but under mounting pressure he postponed the State of the Union address for a week and gave a national address on the Challenger disaster from the Oval Office of the White House. The speech was written by Peggy Noonan. At its end, he made the following statement, quoting from the poem "High Flight" by John Gillespie Magee, Jr.: "We will never forget them, nor the last time we saw them, this morning, as they prepared for their journey and waved goodbye and 'slipped the surly bonds of earth' to 'touch the face of God.'" Three days later, he and his wife Nancy traveled to the Johnson Space Center for a memorial service to honor the astronauts.

Funeral ceremonies

The remains of the crew that were identifiable were returned to their families on April 29, 1986. Two of the crewmembers, Dick Scobee and Michael Smith, were buried by their families at Arlington National Cemetery at individual grave sites. The rest (the unassignable human remains), was buried at the Space Shuttle Challenger Memorial in Arlington on May 20, 1986.

Recovery of debris

On board the Challenger was an American flag that was sponsored by Boy Scout Troop 514 of Monument, Colorado. It was recovered intact, still sealed in its cargo bag.

Debris from Challenger washed up on Florida beaches for years after the incident. On December 17, 1996, more than ten years after the incident, two large pieces of the shuttle washed up on Cocoa Beach.

References in popular culture

Plaque of the Challenger and Columbia crews at KSC
         The cast and crew of Star Trek
    wish to dedicate this film to the men and
    women of the spaceship Challenger whose
    courageous spirit shall live into the 23rd Century
    and beyond....
  • There was a television movie, Challenger made in 1990 about the disaster.
  • French electronic music artist Jean-Michel Jarre originally wanted Ronald McNair to play saxophone in outer space for his album Rendez-Vous. After the disaster, he dedicated the song to McNair's memory. The music video for Fourth Rendez-Vous features footage of the crew as they board the spaceship, and later, transparent silhouettes of astronauts in space, perhaps a homage that the spirits of the deceased will stay with us forever.
  • The episode of the NBC television series Punky Brewster entitled "Accidents Happen," dealt with the title character's reaction to the disaster as she watched it on TV at school. It first aired March 9, 1986.
  • In the popularistic scientific work The Golem at Large: What you should know about technology by Harry Collins and Trevor Pinch the chapter The naked launch: assigning blame for the Challenger explosion is a re-examination of the shuttle launch.
  • Michael Longcor has a tribute to the crew of the Challenger on his album Lovers, Heroes, and Rogues. The song is called "Pillar of Hell."
  • John Denver also wrote a tribute to the crew of the Challenger on his album Take Me Home: The John Denver Story The song is called Flying For Me.
  • The Character Stan Smith from the popular cartoon American Dad quotes President Reagans National Address " slipped the surly bonds of earth' to 'touch the face of God." after a particularly impressive swing in a game of baseball

See also

References

  1. Rogers Commission report (1986). "Report of the Presidential Commission on the Space Shuttle Challenger Accident, Volume I, chapter 3". Retrieved January 4. {{cite web}}: Check date values in: |accessdate= (help); Unknown parameter |accessyear= ignored (|access-date= suggested) (help)
  2. Joseph P. Kerwin (1986). "Challenger crew cause and time of death". Retrieved February 9. {{cite web}}: Check date values in: |accessdate= (help); Unknown parameter |accessyear= ignored (|access-date= suggested) (help)
  3. Rogers Commission report (1986). "Rogers Commission report, Volume I, chapter 9, Range Safety Activities, January 28, 1986". Retrieved January 4. {{cite web}}: Check date values in: |accessdate= (help); Unknown parameter |accessyear= ignored (|access-date= suggested) (help)
  4. Richard Feynman. What Do You Care What Other People Think? ISBN 0586218556. Describes the inner workings of the Rogers Commission, the confusion and misjudgement that plagued NASA and the moment when the cause of the Challenger disaster was revealed.
  5. Vaughan, D. (1996) The Challenger Launch Decision: Risky Technology, Culture and Deviance at NASA ISBN 0226851761
  6. Edward Tufte. (1997) Visual Explanations, ISBN 0961392126, Chapter 2.

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