The space tug is a general concept that many people in the space industry generally accept would be a tremendous asset to have available - they just want somebody else to fund its development: MDA Struggling to Find Customer for Satellite Servicing Business - SpaceNews.com. (See links here to articles about the MDA project.)

Dennis Wingo worked for several years on a venture in Europe called Orbital Recovery that intended to develop a vehicle that would attach itself to a geostationary communications satellite that was running out of fuel and extend its working life for several years. He spoke during the recent NewSpace 2010 meeting about one case where they offered to rendezvous with a satellite whose failed booster had left it in a useless orbit and move it to GEO. With an investment of about $5M, they could have made $150M within several months yet they still could not find any investors.

Besides saving commercial satellites, a system to keep derelict spacecraft from converting popular orbital bands into wasted spaces would clearly be useful:
/-- Envisat To Pose Big Orbital Debris Threat for 150 Years Experts Say - Space News - July.23.10
/-- Zombiesat has three more satellites in its crosshairs - spaceflightnow - July.25.10

Perhaps this DLR project in partnership with OHB Technology will be the one that finally results in the first practical tug.

The Canadian firm MDA Corp. seems quite serious about pursuing an in-space servicing capability: MDA Sees Business Case for In orbit Servicing - SpaceNews.com.

Here was an earlier article about the MDA project: MDA Planning In orbit Servicing Demo - SpaceNews.com - Mar.3.10.

If such a capability were available, perhaps it could have dealt with Galaxy 15: Shutdown Attempt on Wayward Intelsat Craft is Unsuccessful - Space News - May.4.10.

NASA has undertaken a Servicing Study (link via Life at the Frontier) that aims

to assess the feasibility, practicality, and cost of servicing satellites using elements of currently planned and future NASA human spaceflight systems and/or robotic technologies.

Goddard Space Flight Center is hosting a workshop on servicing this week. Many of the presentations have been posted already. They look very interesting. Examples I find of particular interest include:
/-- SpaceX Dragon as an In-Orbit Servicing Platform - shows examples of how Dragon could be used to service Hubble.
/-- LEO Depot Servicing Impact on Space Missions - Dallas Bienhoff of Boeing reviews propellant depots. Shows how a Falcon 9 could be used to build one.
/-- Commercial Human Spaceflight: Context and Capabiliteis - Bretton Alexander, CSF
/-- The Role of the Private Sector

Rob Coppinger points to plans by NASA to fund a study of a Lunar Cargo Tug using a VASIMR engine powered with solar panels: NASA to investigate VASIMIR propelled lunar tug concept - Hyperbola.

The Ad Astra Rocket Company page says,

An unmanned cargo capability based on VASIMR® propulsion offers significant cost savings to the proposed lunar exploration program. VASIMR® delivers the highest fraction of the initial mass in low Earth orbit (IMLEO) to the Moon, thereby reducing the cost per kg. In a 6 month lunar cargo mission, a VASIMR® with 5,000 s specific impulse can deliver approximately double the payload mass of a chemical rocket system.

The video shows a cargo delivery vehicle docking with the tug and transferring some of its fuel to the tug. Once in lunar orbit the CDV separates and lands at a Moon base. The video doesn't indicate if the tug is reused.

An update on the Common Extensible Cryogenic Engine (CECE), a deep-throttling, 15,000 pound thrust LOX/LH2 rocket engine : NASA Tests Engine Technology for Landing Astronauts on the Moon - NASA. It is being investigated by Pratt & Whitney Rocketdyne for NASA "to support a wide range of in-space applications such as the Crew Exploration Vehicle (CEV), Lunar Surface Access Module (LSAM) and in-space transfer systems".
===
Craig Covault reports for Spaceflight Now on a "top secret operation" (an oxymoronic phrase in the US) in which two spacecraft have carried out inspections of US military satellites in geostationary orbit over the past three years. This rendezvous and inspection capability in GEO is something quite new: Secret inspection satellites boost space intelligence ops - Spaceflight Now. This item also indicates the need for a space tug :

DSP 23, the last of a highly successful series flown since 1970, is only the second one to fail in orbit. But the DSP is now a ghost ship moving 69 miles per week east along the geosynchronous arc where many other spacecraft are parked.

The USAF is contacting operators who may need to move their spacecraft out of the way of the 5,000-pound hulk. If it were to collide with a Russian, Chinese or even European satellite, the crash would cause an international incident in space. It can not be shot down as was the failed USA 193 reconnaissance satellite, because the debris would stay in the geosynchronous arc.

Henry Spencer explains why low earth orbit is the place to build your transport for deep space, not on the ground: Future spaceships should be built in orbit - Short Sharp Science - New Scientist.

There is also a longer-term advantage: if you decide to launch everything on one big rocket, what happens when you outgrow that rocket? Even if your early expeditions stay within the rocket's capacity, presumably you'll want to do bigger and more complex ones later. What then? Develop a still-larger rocket?

Even people who don't want to depend on orbital assembly for the first expeditions to the Moon (or Mars, or wherever) often will concede that it will be necessary eventually. But then, where's the gain in delaying it?

If you're going to want to do orbital assembly anyway, you're better off starting it right away, so even early expeditions can benefit from it. The only reason to delay it is if you think there won't be any later expeditions - if you're planning a dead-end programme.

Rocketeers.co.uk points to a couple of interesting postings by Stephen Ashworth. I particularly like his focus on the need to develop an in-space infrastructure:
/-- Astronautical Evolution 38, 1 December 2008

... the asteroidal resource is completely invisible to the space agency paradigm of space exploration. That mode of planning excludes the possibility of systematic use of natural in-space materials, and it excludes the construction of permanent infrastructure on Earth-Mars cycler orbits. It will not contemplate anything that suggests permanent human activity in space.

/-- Astronautical Evolution 39, 6 December 2008

The problem is that space agencies apparently think of a space station as a mission, not as infrastructure. So, in their view, the station must inevitably come to the end of its mission and be trashed.

===
Via Ashworth I came across The Martian Trust, an effort by Charles Polk to create a viable business model to support Mars development following the example of the National Geographic Society. See some similar projects here.

Jon Goff outlines a lunar access architecture based on reusable lunar lander vehicles coupled with propellant depots in low earth orbit and lunar orbit : Lunar Depot-Enabled Multi-Sortie Missions - Selenian Boondocks

Following up on the orbital fuel depot discussion, Valtteri Maja points to his FLEX concept (August 2006), which focuses in particular on LOX depots : FLEX - Flexible Exploration Architecture - Gravity Loss - Nov.21.08
===
Thales Alenia might end up working on both American (COTS) and European (ATV) cargo delivery systems for the ISS depending on ESA and NASA decisions: Thales Alenia Space Forecasts Continued Growth Despite Global Downturn - Space News

Rob Coppinger doesn't believe in orbital propellant depots: The fantasy of orbital fuel depots - Hyperbola - Nov.19.08.

I expect that Jon Goff, a strong proponent of fuel depots, and other engineers who are more informed on this subject that I am will respond effectively to Rob's post.

[Update: Jon has now posted a response: The “Fantasy” of Propellant Depots? - Selenian Boondocks]

[Update Nov.21.08: Rand Simberg posted this item - Propellant Depot Dreams - Transterrestrial Musings, which included a comment from Tom Hill who linked to a article he wrote in 2004 about orbital supply depots Orbiting Suppy Depots by Tom Hill - AIAA SOS - Jan-Mar 2004.
]

I'll just make a few points here:

/-- Rob is correct that the currently planned commercial human spaceflight activities such as space tourism, Bigelow's habitats, etc. do not need fuel depots. It would be great if depots were available but they are not essential at the moment.

/-- Ares I/V/Orion program could easily eat up +$50B by 2020. The systems are all essentially throwaways and with only 3 or 4 missions per year at most, they will have extremely high operating costs. They are too expensive to use for any commercial operations and if the Moon/Mars program is canceled like Apollo, these systems will simply stop being used and their remnants will go to museums. All of that investment will be lost.

/-- Current proposals for fuel depots from engineers like Dallas Bienhoff of Boeing and Frank Zeglerof ULA (see the presentations from the SAS'08 conference) are to provide alternative architectures to Ares I/V that are significantly cheaper while also providing enhanced capabilities. E.g. Dallas shows that with a depot NASA could increase the mass placed on the Moon from 18 tons to 51 tons.

/-- Rob is correct that there are currently no near term markets for fuel depots. However, NASA could create one by operating its Constellation fuel depot in a manner similar to the COTS ISS cargo delivery program. That is, the agency would contract out to the cheapest contractor(s) for delivery of fuel to the depots. This benefits NASA in lowering costs for its missions and in addition acts to drive launch costs down for everyone as economies of scale kick in and as new entrants bring in improved vehicles.

/-- A fuel depot architecture for Constellation would initiate the creation of an in-space infrastructure (which would also include space tugs) that would benefit all space development ranging from commercial operations to other NASA deep space science missions. This infrastructure technology would continue to operate and develop regardless of whether the Moon/Mars mission funding was cut or eliminated. The billions spent by NASA on Constellation will have gone for something that represents a long term net gain in space capabilities rather than a net zero.

/-- Rob lists a lot of practical problems but those are all on the short term. What is key is to get the fuel depot development process going and Constellation could do that. Once a depot is in operation, other systems will gradually be adapted to using it.

/-- Finally, I'll state that the fuel depot is no more of a fantasy than Ares V. I don't "believe" the latter will ever be built. However, I do believe that fuel depots will be orbited eventually regardless of NASA programs.

SpaceX has posted the DragonLab DataSheet (pdf).

Jon Goff discusses a new paper on propellant depots from United Launch Alliance : ULA Propellant Depot Paper - Selenian Boondocks. He provides the file A Practical, Affordable Cryogenic Propellant Depot Based on ULA’s Flight Experience by Bernard F. Kutter, Frank Zegler, Gary O’Neil, Brian Pitchford (pdf).

As Jon has pointed out many times before, propellant depots are a tremendous enabling technology for much lower cost space travel. And as the paper emphasizes, it is a technology that is very nearly at hand. Only a modest "affordable" amount of development is needed to make it happen. For deep space missions, depots not only obviate the need for Ares V but allow for significantly more mass to be sent to the Moon or Mars. Furthermore, multiple commercial launch providers are available to deliver fuel to the depots economically. The paper concludes with:

A significant benefit associated with NASA’s use of commercial launch services is NASA’s potential to significantly reduce the cost of Exploration. This savings in turn would allow NASA to start the lunar exploration well before the current baseline of 2020. This savings would also allow NASA to fund other high priority elements, such as science and technology development. An added benefit of commercial launch services is that NASA would not be locked into a single launch solution as its needs and priorities change. For Exploration a major benefit of relying on-orbital fuel transfer is the flexibility to support evolving mission needs such as weight growth or Mars exploration without wholesale revamping of the Earth to orbit launch system.

Tom Hill says in a comment here that details of the TEMPO³ proposal, which just won the Mars Society's Mars Project Challenge competition, will be released soon. The general plan "is to use an inexpensive nanosatellite to conduct research into the feasibility of generating artificial gravity during the six-month journey to Mars".

He is the project manager and is looking for workers to join the project: Tethered Experiment for Mars interPlanetary Operations Cubed (TEMPO³) - The Mars Society.

Check Tom's Space: What Now? weblog for further updates.

A proposal to use the ISS for space travel: It's All Decked Out. Give It Somewhere to Go - washingtonpost.com. Regarding one point, though, I don't think that a slow cruise through the radiation belts with electric propulsion is a great idea.

If you could do such extensive mods to the station while in orbit and then fly it somewhere, then it seems like the station could just as easily be a place to assemble deep space transport systems. Dennis Wingo advocates using the ISS as such a base in a recent interview on the Space Show.

Following up on the propellant depot panel presentations at Space Access'08, Jon Goff discusses at length some of the system design and trade-off issues involved: Additional Thoughts on the SA'08 Propellant Depot Panel - Selenian Boondocks.

Rand Simberg comments here: More Propellant Depot Thoughts - Transterrestrial Musings

SEI and SpaceDev respond to the Planetary Society's Apophis Mission Design Competition: SpaceWorks Engineering, Inc. (SEI) and SpaceDev, Inc. Announce Radio Tagging Mission Design to Near-Earth Asteroid Apophis

Atlanta, September 28, 2007 — Engineering services and concept development firm SpaceWorks Engineering, Inc. (SEI) and small satellite developer SpaceDev, Inc. announce the submission of a joint proposal, the Foresight spacecraft, to the Planetary Society's Apophis mission design competition. Foresight is a low-cost, low-risk, minimal science approach to achieve the specific goal of obtaining accurate tracking to reduce the uncertainty in the orbit of potentially hazardous asteroid (PHA) Apophis.

continue...

Rand Simberg reports on a Boeing proposal presented at AIAA Space 2007 for fuel depots in low earth orbit that would provide NASA with a much more robust lunar program : Space Gas Station - Boeing Propellant Depot - Boeing has unveiled a radical redesign of NASA's plan to return to the lunar surface: save weight (and money) by saving gas for an orbital fill-'er-up, then shoot 15 times more material to the moon. Can the space agency jive with private space to get the new propellant depot off the ground? - Popular Mechanics - Sept.24.07. As mentioned earlier, such fuel depots would also become key building blocks for an in-space infrastructure on which a vastly expanded space presence can develop.

An innovative space tether experiment developed by the YES2 European student organization is scheduled for launch on Friday:
* Mission to Test 'Space Mail' Delivery System - Space.com
* YES2 (Young Engineers Satellite 2) blog
* YES2 at ESA
* YES Programme

If YES2 is successful it will be the first proof that 'space mail' can be sent using a relatively simple and cheap mechanism. Simple and cheap enough to send experiments from the International Space Station down to scientists on Earth.

Here are some animations and graphics for the mission.

DARPA starts a program that will seek "to produce a lightweight power and propulsion system capable of delivering persistent maneuverability on-orbit." : Fast Access Spacecraft Testbed - ISI Consulting - Sept.11.07.

More space tug news from Europe: European satellite servicing competitors announce new customers and new spacecraft - Flight Int. - Sept.10.07

Orbital Recovery has been reorganized as Orbital Satellite Services. The tug has also changed:

SMART-OLEV is 500kg (1,100lb) lighter than its 1,500kg predecessor Orbital Recovery's Conexpress-OLEV and will service one satellite by docking with its apogee kick motor.

They have "an undisclosed customer for its first mission".

The article also reports on the orbital refueling and services system from Kosmas Georing Services, which I mentioned the other day.