Tracking Methods of Comets, Asteroids and Meteorites

Hollywood movies have depicted many times planetary disasters as a consequence of cosmic collisions. These grotesque situations seem to be the domain of the science fiction, but that is only because none of them has actually occurred during the history of mankind. But they have hit the Earth before humans, and not only once. It is just a matter of time before one such cosmic body will threaten the planet with extinction, and the only thing we know for sure is that it will happen sometime in the future. For humanity, tracking asteroids, meteorites, and comets is the first most important step in avoiding a hazardous collision. This paper provides an overview of the recent tracking history, as well as currently available and future tracking methods.

Despite their hazardous characteristic, tracking comets, asteroids and meteorites did not become urgency until the 1970s, when photometry, radiometry, and spectroscopy developed, contributing to the advances in the field, allowing better detection. [1] The Spacewatch program happened at the beginning of the 1980s, and marked the use of electronic techniques for asteroid tracking and observation, but their hazardous aspect was not to be acknowledged until the 1990s, especially after the impact of the comet Shoemaker-Levi into Jupiter in 1994. [2] The first program dedicated exclusively to the study of the flying rocks started as an idea in 1969 with the decision to build a telescope only for this purpose. Since very few scientists gave importance to this subject, it took years for the project to come into existence. A multi-mirror telescope was used until 1993 when the first Spacewatch 1.8 meter telescope was finally built. [3] The mission of the Spacewatch program was to continue the work done before by using photography, but implementing new technology, using faster detectors and charge-coupled scanning devices (CCD scanning), as well as computer processing. [4]

New technology helped the science of astronomy in tracking NEOs with impact possibility. Being able to observe and accurately calculate their trajectories is of crucial importance. Therefore, NASA has established the Near-Earth Object Program at the Jet Propulsion Laboratory. Its mission is to find asteroids that post the most serious threats to Earth. [5] Part of the Near-Earth Object program is NEAT – Near-Earth Asteroid Tracking. In cooperation with the US Air Force, NEAT uses a GEODSS telescope located in Hawaii to track NEOs. The telescope has a state of the art charge-coupled scanning (CCD) camera and computer system, and the data on any newly discovered Near-Earth object is immediately sent to JPL for analysis. A telescope equipped with three CCD cameras, and located at Palomar Mountain in California also joined the NEAT team, followed by the SkyMorph system allowing the search for moving objects, and in charge of archiving all images, so that no NEO gets lost. [6] Since the Near-Earth Object program started in 1998, more than 5500 asteroids and comets have been identified, of which about a thousand are potentially hazardous objects (PHA). PHAs are rocks that can come 7.5 million kilometers or closer to Earth in their trajectory around the sun, and are larger than 150 meters. [7]

Another method of tracking flying stones involves the use of the delay-Doppler radar, a very powerful tool that searches for asteroids and conveys information on their orbits, as well as and two-dimensional images of their physical characteristics. The precision of this method is invaluable in refining orbit prediction. [8] The first radar detection of an asteroid (1566 Icarus) occurred in 1968. Since then, the results of radar detection have grown exponentially. Until December 2011, the radar has detected 128 objects from the Asteroid Belt, 286 Near-Earth Asteroids, and 15 comets. One of the accomplishments of the delay-Doppler radar for the year of 2011 is the first detection of a comet since 2006, comet 45P/Honda-Mrkos-Pajdusakova. [9]

However, tracking can now be done by everybody. NASA has just announced the newest method of tracking meteoroids: the new iPhone and iPad Meteor Counter application. Using this free app, everyone interested can contribute to tracking the flying stones. The data collected from people using this app will be used to find new meteor showers, but also to keep track of these stones flying around Earth’s orbit. [10]

As far as future plans are concerned, the Large Synoptic Survey Telescope (LSST) intended to be functional by 2015 will take a picture of the sky every three nights in search for the faintest flying objects. LSST’s main mission will be to map small objects in the solar system, particularly near-Earth asteroids and Kuiper belt objects. LSST is planed to find about 80 percent of all the flying rocks of the solar system in one decade. [11] However, telescopes have blind spots, including the LSST, mainly when looking towards the sun, and can miss. Tracking comets, asteroids and meteorites can be faulty and incomplete. To avoid the blind spot issue, a suggestion was to place an infrared asteroid observatory in a Venus-like orbit that could make an inventory of all NEOs. Such a complete inventory would be valid for about a century. [12] This may be the technology of tomorrow in terms of detecting and tracking comets, asteroids and meteorites.

The science of comet, asteroid and meteorite observation and tracking has indeed developed tremendously in the past century, and has reached a quite trustworthy level. Earthlings can be confident that possible hazardous objects would be detected before catastrophic impacts. However, there is still work to do in order to detect almost all, if not all such hazardous objects. Time is precious in terms of detection, because the earlier a possible impact is discovered, the better and more precise its deflection will be. Keeping an eye on all the flying rocks is an important milestone in the survival of our species.

Asteroid Studies and Technological Developments

Human curiosity and eagerness for exploring the unknown has provided the species with an impulse towards always doing better, understanding more, and discovering new things. While from the very beginning people gazed at the sky and wondered about its mysteries, generation after generation humanity looked for new ways to support its thirst of knowledge. This is the reason behind all discoveries done my man, from fire and the wheel, to the telescope and ultimately spaceflight. New technology has not always been around to allow space exploration, but nevertheless, people have used whatever tools at hand to explore the heavens, and asteroids equally captured human attention. This paper provides an overview of the asteroid studies timeline, as well as of the latest technological developments that have influenced this field of study over the past century.

It all started in 1776 when Johann Titius of Wittenburg formulated the Titius-Bode planetary law, and concluded that, given the distances between planets, there must be an undiscovered one between Mars and Jupiter. This conclusion led to a passionate search for a missing celestial body. [1] In 1801 Giuseppe Piazzi discovered the asteroid Ceres, marking the beginning of asteroid studies. The discovery of Pallas in 1802, and of Juno in 1804 followed, and it became evident that there was not a single planet missing, but rather a whole bunch of small ones, later on acknowledged as the “asteroid belt”. At that time, the only means to conduct such studies were ground telescopes and spectroscopes. Discoveries were conducted visually and slowly, and resources were limited to observing the surface of an object, but not at all analyzing its composition. [2] 1807 marked the discovery of Vesta by Heinrich Olbers, but Vesta had to wait until 1994 when the Hubble Telescope had the means to look at its complete rotation. [3] In 1891 Max Wolf used photography for the first time to conduct asteroid observations, leading to the discovery of the first NEA (near-earth asteroid), Eros. However, the first proposals to focus on these potential hazardous objects and organize space missions to explore them did not come until the 1970s. In the seventies the development of photometry, radiometry, and spectroscopy contributed to the advances in the field, allowing the discovery of new structural characteristics in asteroids. [4] The beginning of the 1980s brought the Spacewatch program, and marked the use of electronic techniques for asteroid observation, but their hazardous aspect was not to be acknowledged until the 1990s, especially after the impact of the comet Shoemaker-Levi into Jupiter in 1994. [5] Later on, the space probe Dawn entered Vesta’s orbit in July of 2011, in order to answer questions about the origins of the solar system. After orbiting Vesta for a year, Dawn will move on to orbit Ceres for further studies. [6]

The first program dedicated exclusively to the study of the small objects in the solar system started as an idea in 1969 with the decision to build a telescope only for this purpose. Since very few scientists gave importance to this subject, it took years for the project to come into existence. A multi-mirror telescope was used until 1993 when the first Spacewatch 1.8 meter telescope was finally built. [7] The mission of the Spacewatch program was to continue the work done before by using photography, but implementing new technology, using faster detectors and charge-coupled scanning devices (CCD scanning), as well as computer processing. [8]

Obviously, the development of rocket and spacecraft technologies has further led to a revolution in asteroid studies. Missions were designed to have exploration of asteroids as primary goal (NEAR, MUSES-C, Dawn) or as secondary goal (Galileo, Deep Space 1, Stardust). [9] The next goal in space exploration is human landing on an asteroid by 2024. The Near-Earth Asteroid Rendezvous (NEAR) was the first NASA mission studying NEOs, orbiting and landing on an asteroid, providing the best ever taken images of Eros and Mathilde. [10] MUSES-C, a mission of the Japanese Institute of Space and Aeronautical Science, had as primary mission to obtain asteroid samples and return them to Earth. Launched in 2007, Dawn is in search for clues about the formation of the solar system, by visiting Vesta and Ceres. [11] Missions having asteroid research as secondary goal have also brought important contributions in the field. Galileo was the first NASA mission to come close to asteroids, by taking pictures of Gaspra and Ida, as well as discovering Dactyl. [12] Deep Space 1 conducted a flyby of Braille, coming as close as 26 kilometers away from the asteroid, while Stardust encountered Annefrank. [13] Such missions have tremendous contributions to expanding the knowledge and understanding of asteroids, generating huge advances in asteroid studies in the past couple of decades.

The new advances in technology also support the science of astronomy in detecting asteroids and NEOs with impact possibility. This is a crucial step for mitigation. The next step is precise calculation of asteroid orbits which again can be done with accuracy by computers. If deflection is required, the latest rocketry advances and missile encounters are well-founded and ready. In the same time, nuclear methods of deflection are considered to alter the orbit of a hazardous object. The Planetary Society is building its own spacecraft for testing solar sailing, a safer method of nudging an asteroid, and one day, a fleet of such laser spacecraft could orbit Earth and constantly protect the planet from NEOs. [14]

According to the newly released issue of Scientific American Magazine, nobody is actually in charge to save humanity if an impact should occur soon. [15] Prediction is crucial, but telescopes have blind spots, mainly when looking towards the sun, and can miss. To avoid the blind spot issue, a suggestion was to place an infrared asteroid observatory in a Venus-like orbit that could make an inventory of all NEOs. Such a complete inventory would be valid for about a century. [16] This may be the technology of tomorrow in terms of detection and prediction. Deflection however is another story.

The science of asteroid observation has indeed developed tremendously in the past century, and has reached a quite trustworthy level. Earthlings can be confident that possible hazardous objects would be detected before catastrophic impacts. However, there is still work to be done on deflection methods. It all depends on how much time is left to a possible impact. Either way, this field should be a priority of space exploration. After all, if such an impact occurs, there might be no more science or space exploration left to be done by the human species.

 

Notes

[1] Hobden, Heather. Asteroids - their history. http://www.cosmicelk.net/asteroids.htm (Accessed November 16, 2011), para. 1-3.

[2] Jet Propulsion Laboratory. Dawn – A Journey to the Beginning of the Solar System. Modern Era of Asteroid Study. http://dawn.jpl.nasa.gov/dawncommunity/flashbacks/fb_12.pdf (Accessed November 16, 2011), para. 1.

[3] Hobden, Heather. Asteroids - their history. http://www.cosmicelk.net/asteroids.htm (Accessed November 16, 2011), para. 14.

[4] Gehrels, Tom. History of Asteroid Research and Spacewatch. March 9, 1999. http://spacewatch.lpl.arizona.edu/arsw_tg.html (Accessed November 17, 2011), para 1.

[5] Ibid, para. 5.

[6] Hobden, Heather. Asteroids - their history. http://www.cosmicelk.net/asteroids.htm (Accessed November 16, 2011), para. 15.

[7] Gehrels, Tom. History of Asteroid Research and Spacewatch. March 9, 1999. http://spacewatch.lpl.arizona.edu/arsw_tg.html (Accessed November 17, 2011), para 18.

[8] Gehrels, Tom. History of Asteroid Research and Spacewatch. March 9, 1999. http://spacewatch.lpl.arizona.edu/arsw_tg.html (Accessed November 17, 2011), para 20.

[9] Jet Propulsion Laboratory. Dawn – A Journey to the Beginning of the Solar System. Modern Era of Asteroid Study. http://dawn.jpl.nasa.gov/dawncommunity/flashbacks/fb_12.pdf (Accessed November 16, 2011), para. 2.

[10] Ibid, para. 6.

[11] Ibid, para. 9.

[12] Ibid, para. 3.

[13] Ibid, para. 7.

[14] The Planetary Society. Organization’s Official Website. Projects. http://www.planetary.org/programs/list/ (Accessed November 18, 2011), para. 7.

[15] Lu, Edaward T. "Stop the Killer Rocks". (Forum). Scientific American 305, no. 6 (December 2011): 8.

[16] Ibid, 8.

 

Bibliography

Gehrels, Tom. History of Asteroid Research and Spacewatch. March 9, 1999. http://spacewatch.lpl.arizona.edu/arsw_tg.html (Accessed November 16, 2011).

Hobden, Heather. Asteroids - their history. http://www.cosmicelk.net/asteroids.htm (Accessed November 16, 2011).

Jet Propulsion Laboratory. Dawn – A Journey to the Beginning of the Solar System. Modern Era of Asteroid Study. http://dawn.jpl.nasa.gov/dawncommunity/flashbacks/fb_12.pdf (Accessed November 16, 2011).

Lu, Edaward T. "Stop the Killer Rocks". (Forum). Scientific American 305, no. 6 (December 2011).

The Planetary Society. Organization’s Official Website. Projects. http://www.planetary.org/programs/list/ (Accessed November 18, 2011).

Kepler and Search for Extrasolar Planets

Recently NASA announced an astounding discovery: The Kepler missions confirmed the first exoplanet orbiting in the habitable zone of a sun-like G-types star. The newly discovered planet Kepler-22b is a little more than double the size of Earth, and it is located 600 light years away. [1] That is a long way to go, but either way, this discovery is a milestone for the Kepler mission and the search for Earthlike planets.

Kepler space observatory launched in 2009 on a mission to scan an area of the Milky Way galaxy between Cygnus and Lyra in search for Earthlike life supporting planets located in habitable zones. [2] The telescope uses the transit method to detect if a planet passes in front of a star and blocks its light for a very short time. So far 2326 possible planet candidates circling over 1000 stars have been detected by Kepler. Of these, 207 have the size of Earth, 680 are much bigger than our planet, 1181 have the size of Neptune, 203 of Jupiter, and 55 are even larger than Jupiter. But more importantly, 48 of them are confirmed to orbit in habitable zones. [3]

Potential life habitats should be abundant in water and carbon, as well as other elements necessary in organic compounds, such as nitrogen. For the water to stay in liquid form, pressure should not be less than 610Pa. Temperature is also crucial: carbon based life can develop in environments with a temperature up to 160°C and down to several tens of degrees bellow 0°C. Habitats can exist either at the surface or below. [4] All these conditions create the so-called habitable zones. The habitable zone is the range of distances from a star within which water occupies large territories and remains in liquid form on the surface of a planet. [5] It is however not enough for a planet to be in the habitable zone to develop life. Volatiles are needed to form an atmosphere. The planet has to have sufficient mass so that the atmosphere could provide pressure for water to stay liquid; it has to be geologically active; its atmosphere should not leak into space. [6]

Some of the planets discovered by Kepler live in a binary system, orbiting two stars (Kepler-16b), some are much larger than Earth (Kepler-11), or much smaller and rocky (Kepler-10b); others live in complex solar systems with more planets (Kepler-18b, c, and d), yet others seem to be almost identical twins of our planet (Kepler-22b). [7] Kepler may be able to confirm that Earth is not that unique after all.

The latest discovery, Kepler-22b has a revolution period of 290 days, it is 2.4 times larger than Earth, and has a temperature of approximately 22°C. [8] It is yet to be discovered if the planet is gaseous or rocky, what is its mass, and if it has an atmosphere or magnetosphere. Since our technology is far from even studying in depth such a planet, let alone to visit it even by robotic missions, the promise of such discoveries is more in terms of revealing that Earthlike planets could have formed in other solar system and therefore could harbor life.

---

---

Notes:

[1] Phillips, Tony.  Kepler Confirms First Planet in Habitable Zone of Sun-like Star. NASA Science News. http://science.nasa.gov/science-news/science-at-nasa/2011/05dec_firstplanet/ (accessed December 12, 2011), para. 1

[2] Ames Research Center. Kepler: Importance of Planet Detection. http://kepler.nasa.gov/Mission/QuickGuide/ (accessed December 12, 2011), para. 1

[3] Phillips, Tony.  Kepler Confirms First Planet in Habitable Zone of Sun-like Star. NASA Science News. http://science.nasa.gov/science-news/science-at-nasa/2011/05dec_firstplanet/ (accessed December 12, 2011), para. 9

[4] Jones, Barrie William. Life in the Solar System and Beyond. Chichester: Praxis Publishing, 2004, 78.

[5] Ibid, 79.

[6] Ibid, 80.

[7] Ames Research Center. Kepler: Discoveries. http://kepler.nasa.gov/Mission/discoveries/ (accessed December 12, 2011), para. 1

[8] Phillips, Tony.  Kepler Confirms First Planet in Habitable Zone of Sun-like Star. NASA Science News. http://science.nasa.gov/science-news/science-at-nasa/2011/05dec_firstplanet/ (accessed December 12, 2011), para. 4.

---

References

Ames Research Center. Kepler: Discoveries. http://kepler.nasa.gov/Mission/discoveries/ (accessed December 12, 2011).

Ames Research Center. Kepler: Importance of Planet Detection. http://kepler.nasa.gov/Mission/QuickGuide/ (accessed December 12, 2011).

Jones, Barrie William. Life in the Solar System and Beyond. Chichester: Praxis Publishing, 2004

Phillips, Tony.  Kepler Confirms First Planet in Habitable Zone of Sun-like Star. NASA Science News. http://science.nasa.gov/science-news/science-at-nasa/2011/05dec_firstplanet/ (accessed December 12, 2011).

Image source: NASA/Ames/JPL-Caltech, http://www.nasa.gov/images/content/607691main_Kepler22bArtwork_946-710.jpg

Failures of Robotic Exploration to Mars

Mars is known has a reputation to be the “spacecraft cemetery” because lots of robotic missions to the Red Planet have been lost. Actually, an overwhelming 60 percent (26 of 43) of the probes launched by all space exploration nations to explore Mars have failed. NASA’s results are not far from these either: without taking into consideration the Mars Science Laboratory, which is on the way to Mars and hopefully will succeed, NASA launched 19 missions to the Red Planet, of which seven probes were lost: Mariner 3, Mariner 8, Mars Observer, Mars Climate Orbiter, Mars Polar Lander, and the two Deep Space 2 Probes. [1] What is going on?

Mariner 3, launched in 1964, did not make it to Mars because the shroud on top of the rocket where the probe was located did not open properly, and the solar panels did not open, so the spacecraft ended up orbiting the sun uselessly. [2] Mariner 8 failed during launch in 1971. [3] While the Mars Observer was successfully launched in 1992 and traveled all the way to Mars for 337 days, the contact with the probe was lost in 1993 just when it was about to enter orbit. [4] The Mars Climate Orbiter was launched in 1998 to function as a weather satellite in orbit around Mars, but it was also lost when it arrived to the Red Planet one year later, in 1999. [5] The Mars Polar Lander had as mission to land to the Martian South Pole. Launched in the beginning of 1999 together with the two Deep Space 2 probes, the entire missions with 3 probes on board was lost, again at their arrival to the Red Planet, at the end of 1999. [6]

Is there a Mars curse going on? One reason more probes are lost to Mars than any other celestial body is that more missions were launched to explore the Red Planet than any other body in the solar system. But there is also human error. Such an error occurred during inserting the Mars Climate Orbiter into orbit: because of a navigational error, the satellite occupied an orbit 100 kilometers lower than it was initially intended. The error was performed by NASA’s subcontractors, Lockheed Martin simply because they were not using the standard metric system, which in turn led to serious calculation errors. [7]

It is also important to notice that most failures have occurred in the early years of space exploration, and each of them helped improve the next mission. I would rather give some credit to the Red Planet than blame it for those missions that did not make it, by labeling such results as being a consequence of a Mars curse. It may also seem not to be the right approach to look into failures rather than successes, but I am a firm believer that we learn from the failed missions much more than we learn from the successful ones. Sending robotic explorers to Mars is a complex business, and the recently extremely successful missions to the Red Planet owe a lot to those that did not make it.

---

Notes:
[1] National Aeronautics and Space Administration. Mars Exploration Program: Historical Log. http://mars.jpl.nasa.gov/programmissions/missions/log/ (accessed December 5, 2011), p. 1.

[2] NASA. Mars: NASA Explores the Red Planet, Mariner 3 and 4. http://www.nasa.gov/mission_pages/mars/missions/mariner3-4-index.html (accessed December 5, 2011), para. 2.

[3] NASA. Mars: NASA Explores the Red Planet, Mariner 8 and 9. http://www.nasa.gov/mission_pages/mars/missions/mariner8-9-index.html (accessed December 5, 2011), para. 2.

[4] NASA. Mars: NASA Explores the Red Planet. http://www.nasa.gov/mission_pages/mars/missions/index-past.html (accessed December 5, 2011).

[5] Ibid.

[6] Ibid.

[7] O'Neill, Ian. The “Mars Curse”: Why Have So Many Missions Failed? Universe Today, March 22, 2008. http://www.universetoday.com/13267/the-mars-curse-why-have-so-many-missions-failed/ (accessed December 5, 2011), para. 9.

---

References

NASA. Mars: NASA Explores the Red Planet. http://www.nasa.gov/mission_pages/mars/missions/index-past.html (accessed December 5, 2011).

NASA. Mars: NASA Explores the Red Planet, Mariner 3 and 4. http://www.nasa.gov/mission_pages/mars/missions/mariner3-4-index.html (accessed December 5, 2011).

NASA. Mars: NASA Explores the Red Planet, Mariner 8 and 9. http://www.nasa.gov/mission_pages/mars/missions/mariner8-9-index.html (accessed December 5, 2011).

National Aeronautics and Space Administration. Mars Exploration Program: Historical Log. http://mars.jpl.nasa.gov/programmissions/missions/log/ (accessed December 5, 2011).

O'Neill, Ian. The “Mars Curse”: Why Have So Many Missions Failed? Universe Today, March 22, 2008. http://www.universetoday.com/13267/the-mars-curse-why-have-so-many-missions-failed/ (accessed December 5, 2011).

Science Fiction - Science Fact

I have never been a science fiction fan, and I am not familiar with many examples. Therefore the ones I have chosen may belong to common knowledge. For a science fiction thing that is already a science fact, I chose man’s trip to the moon, as illustrated by Jules Verne and transformed into a science fact by the Apollo program. For a science fiction thing that is still to become a science fact, but it may sometime soon, I chose extraterrestrial life.

In 1865 Jules Verne published From the Earth to the Moon. In this story, to reach the moon, the characters launch themselves from a canon in a projectile spaceship to land on the Moon. Since the novel does not conclude as to what happened to the characters afterwards, Jules Verne wrote a sequel entitled Around the Moon in which the author continues to follow the unfinished trip to the moon. I believe the idea that man could travel to the moon developed in this book has generated lots of confidence in people that such a thing could become a possibility. After all, Jules Verne’s literature brought some other ideas that later on became reality, such as the submarine in Twenty Thousand Leagues under the Sea, and the helicopter in Robur the Conqueror. A book like From the Earth to the Moon could have had an impact on the eventual development of space travel to the moon. Actually, during the return trip to Earth of Apollo 11, Neil Armstrong said:

"A hundred years ago, Jules Verne wrote a book about a voyage to the Moon. His spaceship, Columbia took off from Florida and landed in the Pacific Ocean after completing a trip to the Moon. It seems appropriate to us to share with you some of the reflections of the crew as the modern-day Columbia completes its rendezvous with the planet Earth and the same Pacific Ocean tomorrow." [1]

The Day the Earth Stood Still is probably one of the very few science fiction stories that captured my interest. At the beginning of this story, an astrobiologist is recruited by the government to participate in a highly classified operation that turns out to be the landing of an extraterrestrial spaceship in the Central Park of New York. I believe we are getting close to discover life elsewhere in the universe. While I am not convinced the reality will occur just like in The Day the Earth Stood Still, as intelligent life seems to be scarce, simple life forms have a good chance to thrive on various other celestial bodies. Earth’s extremophiles have proved that life can develop about anywhere, in any kind of environment. With this knowledge in mind, we will have to design future missions capable to reveal such a possibility. While Curiosity is on the way to forever change our views about the neighbor planet, missions like the Europa Jupiter System Mission seem realistic and possible in the next decade. Such a mission would search Europa’s surface for evidence of fossilized organisms that have been carried up through the cracks and deposited on the surface. This would be sufficient to reveal life on Europa, provided its already discovered liquid ocean has been harboring life for quite while. This seems to be a science fiction scenario about to become science fact, and I sure hope this will happen in my lifetime. 

---

Notes:

[1] NASA History. Apollo Flight Journal, Apollo 11, Day 8, part 2: More Television and Stowage for Re-entry. http://history.nasa.gov/ap11fj/25day8-reentry-stowage.htm (accessed November 28, 2011).

---

Reference

NASA History. Apollo Flight Journal, Apollo 11, Day 8, part 2: More Television and Stowage for Re-entry. http://history.nasa.gov/ap11fj/25day8-reentry-stowage.htm (accessed November 28, 2011).