An unfortunate result of 40 years of space activity is that there are now about 150,000 pieces of space junk orbiting the Earth in the 1 to 10 cm size range beneath 1500 km altitude. These items range from the explosive bolts and fairings we used to see in the Walter Cronkite coverage to 50,000 frozen marbles of sodium potassium reactor coolant, probably the result of kicking reactor cores into higher disposal orbits. The same spirit that fills New Mexico arroyos with junk cars and refrigerators has been operating in space! Just like those arroyos, space is a commons, but we are polluting it.
     The 1 to 10-cm range is important. Below 1 cm, Whipple shields can protect spacecraft, and above 10cm, the pieces are few, their orbits are known and a well-timed evasive maneuver can avoid them. In the middle range, debris are now sufficiently dense to threaten long-term space missions with large exposed cross-section.
     Because the sources of the junk were originally launched from Earth in many different directions, a satellite placed in orbit today can expect to see debris coming at it at with a relative speed of up to 27,000 miles per hour. Much faster than bullets out of the highest-performance research guns on Earth, a fleck of paint the size of a small sequin at this speed packs the energy of a 12-pound barbell dropped from a second-floor window, and a "BB" pellet has the same energy as a 100-pound anvil dropped off a seven-story building. Worse yet, debris in the very popular 800-1100 km-high orbit band stay around for several thousand years.
     However, up to now, it has been thought nothing could really be done about the problem except to stop making more debris.

A Solution called ORION
Photonic Associates of Santa Fe, NM has invented a laser debris sweeper. Called "ORION" by NASA, it is based on the fact that only a small change to the speed of an orbiting object can make it re-enter the atmosphere and burn up. For low-Earth-orbit debris, about a 3% change is enough. With hardware we can build today, it should take two years to clear out space.
     A study team convened by NASA verified that the concept would work [see bibliography].
     A high intensity pulsed laser can give this small velocity change to a debris object at great distance by turning its surface into a pulse jet, vaporizing a few-atoms-thick layer with each pulse and applying thrust to slow it down as effectively as if a tiny rocket motor were attached. This is a very efficient process, since only a small fraction of the object needs to be vaporized by the laser Ð its own speed provides most of the energy required to destroy it.
     There is no hazard from the re-entering objects, which are much too small to reach the ground before being completely vaporized.
     The laser station consists of four parts: a sensitive detector to see the debris at twilight and dawn, a high power repetitively pulsed laser, a large telescope mirror to focus the beam on the debris, plus a "rubber mirror" and "guidestar" laser to counteract the twinkle effect of the atmosphere and produce a good focus.
     Why do we want to struggle with the atmosphere when we could put the laser in space? The $10,000 per pound cost of putting anything in orbit, let alone maintaining it, is the main reason.

Finding the debris
One way of initially boresighting the debris is a very sensitive detector invented by Cheng Ho and Bill Priedhorsky at Los Alamos National Lab, which can see these small debris illuminated by sunlight at dawn or dusk at a range of by imaging and counting individual photons. Modifications will permit it to work at 1500km range. A small, short-pulse tracking laser can help compute an orbit for each object in 3 dimensions which is accurate enough for the ORION laser to later find and act on the object in the dark. This final process involves expanding the ORION beam footprint to match the track uncertainty, using its beam as target illuminator, then progressively narrowing the footprint with the aid of a quadrant detector and adaptive optics.

The ORION laser and beam director
A laser firing twice per second with an average power of just 30 kilowatts will do the trick. Each pulse of 15 kilojoules energy will have a duration of 10 billionths of a second. Even a six-meter-diameter mirror can only focus the laser beam to a spot which is larger than the debris Ð but still able to apply several hundred megawatts per square centimeter to the debris surface during the pulse to produce the flash-jet. Maximum range is about 1500 km.

Adaptive optics
A rubber mirror within the beam director corrects the beam several thousand times a second for twinkling due to turbulence in the air above.
     Information needed to correct for turbulence comes by comparing light from a star or any other very small object in the field of view, which is effectively a plane wave in space, with a flat reference surface. Since stars aren't always where you want them, an artificial star, made by exciting the sodium layer 90 km up with a separate orange "guidestar" laser beam, is used. Finally, a "windage" correction has to be applied to account for the fact that the debris object will have moved several meters ahead by the time the laser beam reaches it.

Footnotes
The ORION study was cosponsored by the USAF Space Command, directed by Ivan Bekey (then with NASA headquarters) and managed by Dr. Jon Campbell (NASA Marshall Spaceflight Center). Participants in the study were the author of the concept, Dr. Claude Phipps (Photonic Associates), Dr. Jim Reilly (Northeast Science and Technology, East Sandwich MA), Dr. Sid Sridharan (MIT Lincoln Labs), Dr. John Rather (then with NASA headquarters), Dr. Glen Zeiders (then with AmDyn Corporation) and Dr. David Spencer (USAF Phillips Laboratory).

To find out more...
1.I. Bekey, "Orion's laser: Hunting space debris", Aerospace America, May 1997, pp.38-44
2.C. R. Phipps and J. P. Reilly, "ORION: Clearing near-Earth space debris in two years using a 30-kW repetitively-pulsed laser", Proc. XI International Symposium on Gas Flow and Chemical Lasers and High Power Laser Conference, Edinburgh, 30 August, 1996, SPIE 3092, pp728-31 (1997)
3. J.W. Campbell, ed., Project ORION: Orbital Debris Removal Using Ground-Based Sensors and Lasers, NASA Marshall Spaceflight Center Technical Memorandum 108522 October 1996
4. C. R. Phipps, H. Friedman, D. Gavel, J. Murray, G. Albrecht, E. V. George, C. Ho, W. Priedhorsky, M. M. Michaelis and J. P. Reilly, "ORION: Clearing near-Earth space debris using a 20-kW, 530-nm, Earth-based, repetitively pulsed laser", Laser and Particle Beams, 14 (1) (1996) pp. 1-44
5. C. R. Phipps, Proceedings of the 11th International Workshop on Laser Interaction and Related Plasma Phenomena, Monterey, October 25-29, 1993, Laser and Particle Beams, 13(1) (1995) pp. 33-41

... or contact the author directly at CRPhipps@aol.com or crphipps@ni.net. He will be happy to send you a copy of the NASA TM or other papers, while they last.

Dr. Claude Phipps, SB MIT 61, SM MIT 63, received his Ph.D. from Stanford University in 1972. He developed the ORION concept while a senior R&D staff member at Los Alamos National Labs. He has numerous publications, including 31 invited presentations and a technical book chapter, on laser-surface interactions, laser system design and plasma physics. He presently heads a consulting firm, Photonic Associates of Santa Fe.

[Ed. Note: for more information about the Mars Pathfinder mission, and links to other online resources, visit the SpaceViews Mars Pathfinder page at http://www.seds.org/spaceviews/pathfinder/ ]

It will not land daintily on a set of landing legs with a dignified burst of retrorockets. It will land most undignified, swathed within a bundle of airbags that will protect it as it comes to a stop after bounding across the Martian terrain. But when you're trying to squeeze a Mars landing mission into the budget of a Discovery-class mission, you'll do what it takes to safely put your craft down on the Red Planet.
      Unlike the Viking landers in 1976, or the Surveyor landers on the Moon in the 1960s, Mars Pathfinder will use a landing procedure that eschews landing legs -- and the costs and complications that come with them -- with a simpler but untried system that, even if it works, may not be used again in future missions.
     The landing sequence beings about thirty minutes before Martian landfall. At that time the cruise stage of the spacecraft, which has protected and cooled Mars Pathfinder since launch, is jettisoned. An hour before the cooling fluid in the cruise stage was vented. The spacecraft and aeroshell now make their final approach to the planet.
     About five minutes before landing, the spacecraft encounters the Martian atmosphere at a speed of 26,500 kmph (16,400 mph). The heatshield dissipates the heat of entry into the tenuous Martian atmosphere as the spacecraft suddenly slows from nearly 7.5 km/s to only 0.4 km/s (900 mph). Deceleration reaches a peak of 20 g's, 20 times the force of Earth's gravity, during this time. Accelerometers on the spacecraft use this deceleration as a prompt to start the landing procedure.
     About 2 to 3 minutes after atmospheric entry, a parachute deploys. The 7.3-meter (24-foot) parachute, a modified version of the ones used in the Viking landings, will slow Pathfinder down to only 65 meters/second (145 mph).
     Twenty seconds after the parachute deploys, the heatshield is removed. The spacecraft itself starts to rappel down a cable attached to the back part of the aeroshell, to which the parachute is attached. This is designed to give the spacecraft room to inflate its airbags.
     Eight seconds before landing, when the spacecraft is about 300 meters (1,000 feet) above the surface, the airbags will inflate. The airbags are inflated in less than a third of a second to a pressure of 1 psi, only about 7 percent of normal atmospheric pressure on Earth but much higher than the pressure of the Martian atmosphere.
     When the spacecraft is just 100 meters (330 feet) above the surface (a radar altimeter on the spacecraft provides altitude information for this stage of the landing), a set of solid rocket motors above the spacecraft, on the back section of the aeroshell, fire. This effectively slows the spacecraft to a stop while still 10-20 meters (30-60 feet) above the surface.
     Then the airbags are put to use. A charge cuts the tether connecting the spacecraft to the parachute and aeroshell. The parachute and aeroshell fall away from the spacecraft, which, encased in airbags, drops to the surface. Pathfinder may hit the surface as fast as 25 meters/second (55 mph). The landing will take place at 1:07pm EDT July 4.
     Friction and gravity will bring Pathfinder to a stop, but not after the spacecraft bounces across the Martian terrain, going perhaps as high as 12 meters (40 feet) and making jumps of up to 200 meters (660 feet) between bounces. The bounce phase will take up to a few minutes to complete.
     Once the spacecraft comes to a stop, a charge will blow that will unlock the three pain petals that form the base of Mars Pathfinder, allowing them to open. As that happens, a set of internal lines in each of the airbags will start up, retracting the airbags. This process also deflates the bags as gas is vented out through special ports in the airbags. Mission planners have allowed three hours for this to happen.
     Throughout this time, the spacecraft is not in communication with the Earth. During the landing phase on a simple carrier wave is transmitted. Engineers on Earth will use the Doppler shift of the frequency to determine the speed of the spacecraft, and hence where in the landing phase it is at.
     If all goes well, and the spacecraft is on the ground and functioning well, the first images of the surface of Mars will be transmitted back at around 7-8pm EDT. If the situation looks good, mission controllers will tell Pathfinder to deploy the ramp for the rover. Sojourner could be moving across the Martian surface as early as midnight EDT.
     If there are problems with the landing, however, that keep Pathfinder from using its high-gain antenna, mission planners till tell the spacecraft to send back a series of highly-compressed black-and-white images using the low-gain antenna. Teams on Earth will use the images to diagnose the problem and determine the best solution.
     For more information on the entry, descent, and landing procedures for Mars Pathfinder, check out the Mars Pathfinder Web site, at http://mpfwww.jpl.nasa.gov/mpf/edl/edl1.html.

Their mutual love of auto racing brought them together thirty years ago. Now long-time friends Harry Dace and Jim Akkerman of Houston, Texas, are pushing the envelope even more. Their need for speed has made them business partners in Advent Launch Services. That's launch, as in space rockets. For less than $5,000 the two entrepreneurs will sell you a high-speed ride into space.
     There's a simple explanation for the transition from earthly car racing to sky-high rockets. Years ago when Jim was racing competitively as a hobby (at one time he held the Go-kartx record for the 2-1/2 mile track), he was in real life employed by NASA in its propulsion and power division. In other words, Jim was, and still is, a bona fide rocket scientist.
     Meanwhile Harry, a mere lad growing up in the '60s in the outskirts of Houston -- "Space City" -- read about Jim in a racing magazine and asked his dad to take him to meet his idol. The two became fast friends.
     Fast forward to the 1990s. Jim now was working for NASA's space shuttle program and Harry had become successful in the ventilation business. Around Harry's 40th birthday, he began to give serious thought to his life. "Half my life was over," he recalls. "I was getting bored and wanted to do something bigger. I asked myself, 'Who is the most intelligent person I can work with?'"
     The answer of course was Jim. Together the two friends worked on designs to improve the efficiency of air conditioning equipment. They patented, manufactured and sold their lucrative product, and again experienced the flush of business success. But Harry was bored still. He asked his rocket scientist friend what he wanted to do next. "I want to build rockets!" was Jim's immediate reply. So in the summer of 1995, Advent Launch Services was born.
     The design of the Advent rocket is not new. The seven-story reusable rocket will be machined from rugged, lightweight titanium and have the fewest possible moving components. The eight rocket engines will be fueled by inexpensive propellants liquid oxygen and natural gas. Jim had been working on the concept since the mid-'80s and at one time had tried to interest NASA in his design. Because the space agency was committed to building the shuttle, NASA formally declined and released to Jim most of the rights to the innovative rocket, freeing him and Harry to go forward with establishing Advent Launch Services.

Now what about that civilian rocket ride?

The Advent rocket will be launched from the sea. Six passengers and a pilot, possibly a former astronaut or military pilot, will ride comfortably in a spacious passenger cabin atop the rocket. Upon liftoff, riders will feel two g's, or twice the force of the Earth's gravity-similar to that experienced by astronauts during a space shuttle launch. The Advent rocket will accelerate to about 2,300 mph to an altitude close to 70 miles. There, passengers will enjoy a rare astronaut's-eye view of our beautiful planet Earth. After a weightless four-minute coast in space, the rocket will begin its glide down and passengers would re-experience gravity at about 3-1/2 g's, again much like the space shuttle. At 30,000 feet the rocket would level out and then make its cushioned landing in water.
     Recently Advent Launch Services formed the Civilian Astronauts Corps (CAC) to enable ordinary citizens to make the voyage into space and back. The CAC will offer the rocket rides through its membership and will support the final development and construction of the first Advent rocket, now called the CAC-1. The CAC-1 will be constructed in Houston, Texas, and be launched from international waters off the coast of Galveston, Texas. Persons from all over the United States, Canada, and even Europe have contacted Advent expressing their interest in the civilian ride to space. The Civilian Astronauts Corps, limited to 2,000 people, is expected to fill its ranks quickly.
     Safety issues have been carefully addressed. "The CAC-1 rocket will work," Jim states confidently. "The key is simplicity. What we have done is borrow from the technology that NASA has developed and then used only the simplest things that will do the job of getting people into space."
      Others have also expressed their confidence in the design and designer. The rocket is one of over a dozen proposals that have been officially entered to compete for the X Prize, a $10 million reward. The X Prize is based on the one given to Charles Lindbergh for being the first to fly solo across the Atlantic, thus proving the feasibility of transatlantic flight. The X Prize will be awarded to the first person or persons to demonstrate private flight into space.
     "We hope for nothing less than beginning the commercialization of space travel," explains Harry. It's a lofty goal but Harry and Jim are determined to do just that with Advent Launch Services. The two former car racing fans have entered a new space race. Along the way, they invite others to join them by buying their own tickets to ride.
     Advent Launch Services is located on the World Wide Web at http://www.advent-launch.com.