The HTV-6 cargo spacecraft (named Kounotori) was successfully launched using the H-IIB rocket Friday, Dec. 9 from the Tanegashima Space Center in southern Japan, by the Japan Aerospace Exploration Agency (JAXA).
On Tuesday, Dec. 13, HTV-6 will dock with the International Space Station (ISS). Robotic ground controllers will install it on the Earth-facing side of the Harmony module, where it will spend more than five weeks, before returning to a fiery ending in Earth’s atmosphere.
But before its fiery end, HTV-6 will conduct an experiment with KITE – the Kounotori Integrated Tether Experiment – for one week after Kounotori undocks from the ISS, around Jan 20. It will release a 700-meter long Electrodynamic Tether to study the feasibility of the use of such tethers for in-space propulsion and space debris removal.
ISS Resupply Mission
The unpiloted cargo spacecraft, named “Kounotori” – the Japanese word for white stork, carries more than 4.5 tons of supplies, water, spare parts and experiment hardware for the six-person ISS crew.
Also aboard the resupply vehicle are six new lithium-ion batteries and adapter plates that will replace the (lower efficiency, heavier) nickel-hydrogen batteries currently used on the station to store electrical energy generated by the station’s solar arrays. These will be installed during a series of robotic operations and spacewalks between late December and mid-January.
Space Debris
Space debris is the collection of defunct man-made objects in space – old satellites, spent rocket stages, and fragments from disintegration, erosion, and collisions – including those caused by debris itself.
There are more than 20,000 pieces of debris larger than a softball orbiting the Earth. They travel at speeds up to 28,163 kmph, fast enough for a relatively small piece of orbital debris to damage a satellite or a spacecraft. There are 500,000 pieces of debris the size of a marble or larger. There are many millions of pieces of debris that are so small they can’t be tracked. Even tiny paint flecks can damage a spacecraft when traveling at high velocities.
Note that collisions with larger space debris objects results in large number of smaller fragments, which further increases the frequency of collisions.
The video below from NASA shows an animation of space debris around Earth. There is one distinct zone at geo-stationary orbit (GEO) around the equator at 35,680 km altitude, where all GEO satellites reside, a crowded set of zones in Low Earth Orbit (LEO) between 160 and 2,000 km and many other orbits, including some beyond GEO, in different orbital planes surrounding Earth.
See stuffin.space for a cool real-time interactive 3D map of objects in space including satellites and debris.
Satellite Population
ITEM |
NUMBER |
ORBIT |
Active Satellites |
1,569 |
About 50% are in LEO |
Inactive Defunct Satellites |
2,704 |
|
Proposed Boeing Constellation |
2,956 |
LEO |
Proposed SpaceX Constellation |
4,425 |
LEO |
Proposed OneWeb Constellation |
648 |
LEO |
Orbital Debris Catastrophes
In 1996, a French satellite was hit and damaged by debris from a French rocket that had exploded a decade earlier.
China's 2007 anti-satellite test used a missile with a kinetic kill vehicle to destroy an old 750-kg weather satellite at an altitude of 865 km, adding more than 3,000 pieces to the debris problem.
On Feb. 10, 2009, the U.S. Iridium 33 satellite collided, soon after launch, with the defunct Russian Cosmos 2251 satellite, at a speed of 42,120 km/h, at an altitude of 789 km. The collision added more than 2,000 pieces of trackable debris to the inventory of space junk.
In 2013, the space shuttle was destroyed by space debris with deadly consequences for its crew including Sandra Bullock and George Clooney — oh, that was fiction, in the movie Gravity. You can see the extreme dramatization of the event at www.youtube.com/...
Orbital Debris Consequences
The rising population of space debris increases the potential danger to all space vehicles, but especially to the International Space Station and crewed spacecraft.
The picture below shows a 7 mm-diameter circular chip in the fused-silica and borosilicate-glass window of the ISS gouged out by the impact from a tiny piece of space debris, possibly a paint flake or small metal fragment no bigger than a few thousandths of a millimetre across.
An object up to 1 cm in size could disable an instrument or a critical flight system on a satellite. Anything above 1 cm could penetrate the shields of the Station’s crew modules, and anything larger than 10 cm could shatter a satellite or spacecraft into pieces.
The Kessler syndrome, proposed by the NASA scientist Donald J. Kessler in 1978, is a scenario in which the density of objects in low Earth orbit (LEO) is high enough that collisions between objects could cause a cascade - each collision generating space debris that increases the likelihood of further collisions. One implication is that the distribution of debris in orbit could render space activities and the use of satellites in specific orbital ranges unfeasible for many generations.
The NASA Orbital Debris Program Office
The NASA Orbital Debris Program Office has taken the international lead in conducting measurements of the environment and in developing the technical consensus for adopting mitigation measures to protect users of the orbital environment.
However, it should be noted that, currently, no U.S. government entity has been assigned the task of removing existing on-orbit debris.
Tracking Debris
NASA and the DoD cooperate and share responsibilities for characterizing the satellite (including orbital debris) environment.
DoD’s Space Surveillance Network tracks discrete objects as small as 2 inches (5 centimeters) in diameter in low Earth orbit and about 1 yard (1 meter) in geosynchronous orbit. Using special ground-based sensors (radar and optical detectors such as Lidar (Light Detection and Ranging, a remote sensing method that uses light in the form of a pulsed laser to measure distances to objects)) and inspections of returned satellite surfaces, NASA statistically determines the extent of the population for objects less than 4 inches (10 centimeters) in diameter.
Mandatory Disposal of Defunct Satellites
The Iridium 33 collision and numerous near-misses have renewed calls for mandatory disposal of defunct satellites (typically by deorbiting them or at minimum sending them in graveyard orbit), but no such international law exists yet. Nevertheless, some countries have adopted such a law, such as France in December 2010. The United States Federal Communications Commission (FCC) requires all geostationary satellites launched after March 18, 2002, to commit to moving to a graveyard orbit at the end of their operational life.
Space Debris Removal Techniques
Several experimental techniques have been proposed for removal of space debris, including
- Spacecraft Tugs
- Space Nets and Collectors
- Laser Brooms
- Electrodynamic tethers
The laser broom concept uses a ground-based laser to ablate the front of the debris, producing a rocket-like thrust which slows the object causing it de-orbit and enter the atmosphere.
Other alternate proposals to the laser broom use a foamy ball of aerogel or a spray of water, inflatable balloons, boom electroadhesion, and dedicated anti-satellite weapons.
The Electrodynamic Tether (EDT)
An electrodynamic tether is a long conducting wire extended from a spacecraft that generates thrust through Lorentz-force interactions with a planetary magnetic field. This can be used to slow down and de-orbit a defunct satellite or large space debris object. The technology also has applications in spacecraft propulsion and momentum-transfers.
The gravity gradient field pulls the tether taut and tends to orient the tether along the vertical direction. As the tether orbits around the Earth, it crosses the Earth's magnetic field lines at orbital velocity (7-8 km/s in LEO orbits), which induces a voltage (a potential difference) along the length of the tether. This voltage, which is called the "motional EMF", can be up to several hundred volts per kilometer.
If the system has a means for collecting electrons from the ambient plasma (electrons, which inhabit space around Earth) at one end of the tether and expelling them back into the plasma at the other end of the tether, the motional EMF voltage will drive a current along the tether. This current will, in turn, interact with the Earth's magnetic field to cause a Lorentz force which will oppose the motion of the tether and whatever it is attached to. This "electrodynamic drag force" will slow down the tether and the spacecraft, thereby decreasing their orbital altitude and eventually drag them down into Earth’s atmosphere.
Essentially, the tether converts the orbital kinetic energy of the host spacecraft into electrical power, which is dissipated as ohmic heating in the tether.
Note that a current in the opposite direction would result in a force in the flight direction, the tether would thereby act as a propulsion system. That system would require electrical energy to drive the current.
Such EDTs can be built-in into future spacecraft for de-orbiting purposes.
They can also be used to de-orbit existing large pieces of space debris by attaching EDTs to them using special purpose space vehicles.
There are some obvious advantages to EDT — low cost, low weight, no propellant, no need for high electrical power. Other strong points of EDTs for debris removal include no requirement for thrust vectoring because of the substantial EDT thrust direction determined by the current–magnetic interaction and no restriction on the attachment point to the debris because of the lack of necessity of considering the center-of-mass of the object.
KITE Electrodynamic Tether
KITE (Kounotori Integrated Tether Experiment) is an experimental electrodynamic tether (EDT) on board the Kounotori-6 spacecraft. The EDT experiment will be conducted for a week following Kounotori-6's departure from the ISS around Jan 20.
The main objective of this experiment is the orbital demonstration of both extending an uncoated bare-tether, and driving electric currents through the EDT. These two technologies will contribute to gaining capabilities to remove space debris.
KITE will not perform any actual space debris removal — that will be left to future similar missions. Tethers 5 to 10 km long are envisioned for removal of large objects.
The orbit of HTV-6 will be 20 km (or more) below the ISS orbit at an altitude of 300–400 km and an orbital inclination of 52°.
An end-mass of about 20 kg will be released from the releasing mechanism onboard the HTV. The 700-m tether will be deployed using reels inside the end-mass. The end mass will be deployed in the “zenith” direction, pointing away from the spacecraft and Earth, since the electron emitter is located in the HTV, not the end-mass.
Installed on the exterior of HTV’s propulsion module is a Field Emission Cathode (FEC) to serve as a low-power and light-weight electron emitter. The FEC electron emitter will eject electrons collected by the tether into the space plasma, thereby generating a (positive) electrical current in the tether towards the end-mass.
The tether deployment motion will be observed using rendezvous sensors onboard the HTV (which are used to monitor docking with the ISS), and reflectors on the end-mass. Optical cameras are also installed on the HTV near the base of the tether, perhaps providing us with fascinating videos of the event. The HTV potential, current, ambient plasma, and geomagnetic field will be measured using a potential monitor with a plasma current probe and magnetic sensor.
After the experiment, the tether will be jettisoned from the spacecraft and both will proceed to fall and burn in Earth’s atmosphere.
Here is a video from JAXA depicting an animation of the EDT mission.
The Tether
The tether is not made up of a single conductor; instead it uses a mesh structure, similar to fishing nets, in order to avoid tether severing by impacts from small-sized debris. Each yarn comprises a thin aluminum and stainless-steel wire. The surface of the yarn is coated by a solid lubricant material possessing electrical conductivity.
Nitto Seimo, Japan’s largest maker of fishing nets, was asked by JAXA about a decade ago to develop the metal mesh line. Nitto Seimo started making minnow nets in 1910 and invented the knotless net machine in 1925.
“It was extremely difficult,” said Suzuki, whose employer also makes nets for trawling, fish farming and keeping animals out of agricultural areas. “At first, we could only make 20 or 30 centimeters. It took us until about 2010 until we could finally make several hundred meters.”
Concluding Remarks
Space debris is a clear and present danger to the future of space transportation and artificial satellites that serve humankind in the areas of communications, navigation, weather, remote sensing and space sciences. Minimizing future space debris and removal of the junk that has accumulated over decades has to be a top priority or else we run the risk of making several orbital ranges unusable for future spacecraft.
Recently announced satellite constellations will add a considerable number of new satellites into Low Earth Orbits (LEO), much more than the total number satellites in orbit today.
EDT is just one technology to address this problem, especially for large objects. Other technologies need to be developed and tested that can sweep up smaller sized bits and pieces of debris.
Many obstacles remain in the areas of technology, International co-operation or lack thereof and the expenses associated with this task. The United States needs to lead the world in this endeavor and lawmakers and Presidents need to provide the appropriate resources and leadership on this front.
References
- Electrodynamic tether — en.wikipedia.org/…
- NASA Orbital Debris Program Office — orbitaldebris.jsc.nasa.gov
- Space Debris wiki — en.wikipedia.org/…
- ESA Space Debris — www.esa.int/...
- HTV — en.wikipedia.org/...
- Preparation for On-Orbit Demonstration of Electrodynamic Tether on HTV — erps.spacegrant.org/…
- Tethers Unlimited — www.tethers.com/
- Understanding Space Debris. Causes, Mitigations, and Issues — www.aerospace.org/…
- 2009 satellite collision — en.wikipedia.org/…
Other diaries on space -
- A Star is Spaghettified — www.dailykos.com/...
- Asteroids and Planetary Defense — www.dailykos.com/…
- List of recent diaries — www.dailykos.com/...