Saturday 17 November 2018

Modelling the expected orbital lifespan of Orbital Reflector [UPDATED]

Update added 18 Nov 2018, 13:15 UT:
The launch of SSO-A with Orbital Reflector has been postponed, untill after Thanksgiving.

Update added 27 Nov 2018, 12:00 UT: 
New launch date of SSO-A with Orbital Reflector is on 28 Nov 2018 at 18:31:47 UT

Artist impression of Orbital Reflector. Image: Nevada Museum of Art

In just a few days from now, on 19 November 2018 at 18:32 UT, 28 November 2018 at 18:31:47 UT Spaceflight Industry's SSO-A SmallSat Express, a cubesat rideshare mission, will launch from Vandenberg SLC 4 on a SpaceX Falcon 9. SSO-A will release as much as 64 small spacecraft into space, over a 5-hour period, from two free-flying launch dispensers.

Onboard SSO-A is Orbital Reflector, a project by my artist friend Trevor Paglen. It is an interesting object, for several reasons. It is a cubesat that will inflate a large oblong balloon of about 30 by 1.4 meter, a bit shaped like an obelisk. The balloon is made of a very lightweight, Mylar-like foil that is highly reflective. Hence the name: Orbital Reflector. When reflecting sunlight, it should be easily visible from earth.

Orbital Reflector is Art. It is a sculpture in space, one that, in theory, you can see from everywhere in the world (but about reality: see later in this blog post). Trevor teamed up with the Nevada Museum of Art for this project, and it might be the first time a Museum has created an exhibit in Space.

Artist Trevor Paglen and an early spherical precursor prototype of the balloon (now at the Nevada Museum of Art)

Orbital Reflector will be released in a circular, 575 km altitude, sun-synchronous orbit with an orbital inclination of 97.6 degrees. The anticipated moment of release from the Lower Free Flying Dispenser (LoFF) is about 2h 18m (or about 1.5 revolutions) after launch, i.e. 20:50 UT, over Antarctica. At what moment the balloon will be inflated once Orbital Reflector has been released from the LoFF, is unknown to me.

My estimated initial orbit for the object:

1 70000U 18999A   18323.77222222  .00000000  00000-0  00000-0 0    09
2 70000 097.6000 032.4835 0001438 157.1159 325.9970 14.97378736    01

UPDATE (27 Nov 2018):

1 70000U 18999A   18332.77207176  .00000000  00000-0  00000-0 0    00
2 70000 097.6000 041.3000 0001438 157.1159 325.9970 14.97378736    04

... but once the balloon is inflated, the orbit will rapidly change.

The Falcon 9 Upper Stage is deorbited at the end of the first revolution (see map below), near Hawaii. The deorbit-burn might be visible from eastern Europe around 19:50 UT.

click map to enlarge

Orbital Reflector should initially have been launched in the spring, but launch delays pushed the date to 19 November. Unfortunately, due to this and due to the particularities of the orbital plane it is launched into, visibility of the satellite will initially be very bad, and will remain so for weeks.

The satellite will be making late evening passes (around 21:15 local time), remaining in the Earth's shadow and hence unilluminated by the sun in the northern hemisphere. New Zealand, southern Australia and South America in the southern hemisphere may have some spotting opportunity. But for Europe and the USA, initial spotting opportunities will be zero. It is the wrong season to see a satellite in this kind of orbital plane.

So the crucial question is: will Orbital Reflector survive long enough to carry over to spring and early summer, when viewing conditions are more positive? To answer this, I have done some modelling to get an indication of what orbital lifespan to expect.

SRP and modelling lifespans

Orbital Reflector in itself will be an interesting object to follow due to its highly unusual area-to-mass-ratio. Unlike typical satellites (which do experience SRP too but to a clearly lesser degree), this object will be under significant influence of Solar Radiation Pressure (SRP). And SRP will have a clear impact on its orbital lifetime, as we know from both theory and from data on the orbital evolution of earlier inflatable balloon satellites.

Earlier balloon satellites were Echo 1 (1960-009A), Echo 2 (1964-004A), and PAGEOS (1966-056A). Like Orbital Reflector, Echo 1 and PAGEOS were 30 meters wide. Echo 2 was slightly larger at 40 meters. They were spherical in shape, not oblong like Orbital Reflector. They also initially orbitted at much higher altitudes than Orbital Reflector will do: an initial altitude of 4225 km for PAGEOS, 1030 x 1315 km for Echo 2 and  1540 x 1670 km for Echo 1.

Echo 2 during development tests in 1961. Image NASA

The orbital evolution of all these three balloon satellites showed a strong influence of SRP on the evolution of apogee and perigee altitudes. SRP "pushes" and "pulls" on apogee and perigee of the orbit, with a quickly changing orbital eccentricity as a result. The effects can be well seen in the orbital history for Echo 1 and 2 and PAGEOS (source of orbital data used to make these diagrams is JSpOC):

click diagram to enlarge
click diagram to enlarge
click diagram to enlarge

A clear pattern is visible where the orbital eccentricity highly oscillates due to SRP. It initially is quickly pumped up, lowering perigee and raising apogee, then gets back to lower values again, and this cycle then repeats.

Something similar will happen to Orbital Reflector. SRP will quickly push perigee down and apogee up, pumping up the orbital eccentricity. The progressively lower perigee at the moments SRP pumps up the eccentricity, will speed up orbital decay.

I used GMAT 2018a to model the effects of SRP on the orbital evolution of Orbital Reflector. That is not something trivial to do, as there are a number of 'unknowns' involved for which I had to make educated guesses. The results below should be taken very cautiously for that reason.

For example, SRP depends on attitude of the spacecraft with regard to the direction of the sun. That attitude will change over time, and there is the question whether the oblong balloon will be (and stay) in stable attitude or start to tumble. SRP in itself creates a torque and might induce tumbling. Issues like these will strongly influence the amount of SRP, drag, and as a result the orbital lifespan. 

There are some uncertainties in the mass of Orbital Reflector as well: depending on whom you ask, it is either 2.2 or 3.2 kg. This is important, because SRP is highly dependent on the area-to-mass ratio (and so is the effect of drag on the object). If the balloon indeed settles in a least-drag orientation after deployment, as the designers expect, the drag surface is a constant 1.97 square meter.

Because Orbital Reflector is oblong, and because of the orientation of its orbital plane with respect to the sun, the SRP surface will vary between (almost) minimum and maximum values over one orbit. I have tried to accomodate this by running the model with an SRP surface that is 50% of the maximum value, i.e. 50% of 21 square meter = 10.5 square meter.

To show the non-triviality of SRP, I first ran the model without SRP, then with SRP, for comparison.

Below are the model results for Orbital Reflector (expressed as apogee and perigee altitude against date) if we ignore Solar Radiation Pressure, for two mass values: 2.2 and 3.2 kg.

model results for a mass of 2.2 kg and no SRP taken into account. Click diagram to enlarge

model results for a mass of 3.2 kg and no SRP taken into account. Click diagram to enlarge

Below are the results if we do implement modellation of Solar Radiation Pressure. The expected lifetime clearly shortens, by up to a third, and this is because of the progressively lower perigee as SRP pumps-up the eccentricity of the orbit. The grey lines in the diagrams are the data without SRP from the previous diagrams, as a reference:

model results for a mass of 2.2 kg with SRP taken into account. Click diagram to enlarge

model results for a mass of 3.2 kg with SRP taken into account. Click diagram to enlarge

These outcomes should be viewed with caution, as the modelling includes educated guesses and, to quote Monty Python, "it is only a model". It will be very interesting to see how the real orbital evolution compares to these model outputs.

What these model results do suggest, is that it is possible that Orbital Reflector, if it inflates and stays intact, might remain on-orbit long enough to carry it into the more favourable part of the year (late spring and/or summer of 2019) for visual sightings. So let's root that my models resemble reality, as I surely would like to see and image Orbital Reflector in the sky.

From the artist impressions, the balloon is flat-sided. This could mean that reflections might be specular, and bright only briefly in narrow zones on Earth, much like Iridium flares.

Sat for Art's sake?

Orbital Reflector is Art. It is distinctly non-utilitarian, in the common sense of that word: it just orbits. But it does have a deeper purpose than that. As Trevor recently put it himself:

"Orbital Reflector was designed as a provocation. An opportunity to think about outer space, the geopolitics of the heavens, and the militarization of earth orbits. It’s a project about public space, and a project about who gets to exercise power over our planetary commons, and on what terms"
In other words, Orbital Reflector is not just there to reflect light to people on Earth; it is also meant to make people reflect, pondering questions such as "who owns space?" and "what is happening out there?".

The question raised by Trevor is pertinent. Space is Public Space. At the same time, it is not public at all, but strongly the domain and playground of Nation States, and notably of the military of those Nation States.

Space is highly militarized. Not so surprising of course as the whole Space Age has its roots in the development of Ballistic Missiles. The role of the military in Space and Space innovation is often overlooked. While many people look at NASA as the big innovator in the US Space program, the real innovations in Space are often the product of another Space Agency, the NRO, which is NASA's shadier military cousin and generally unknown to the broader public, even though it sends billions worth of hardware per year into space. Hardware that plays a prominent role in geopolitics and modern warfare. They include highly detailed optical and radar imaging satellites, navigation satellites, communication satellites and giant listening radio "ears" in space. Long-time readers of this blog know what I am talking about.

Pondering Space and other things. Trevor Paglen (right) and the author of this blog (left), June 2018

It is very interesting that the only areas where it is internationally regulated what can and cannot be done in space, concern weapons in Space and national sovereignty in space. And this was done over 50 years ago already, as part of the 1967 “Treaty on Principles Governing the Activities of States in the Exploration and Use of Outer Space, including the Moon and Other Celestial Bodies” (or “Outer Space Treaty” in short).

The fact that after more than 60 years of Space exploration still only the military/national sovereignty aspect has been regulated, tells you how dominant that aspect of the use of space is. Nobody bothered to regulate other potential aspects of space (such as private enterprise).

We are however at a crossroads. Ideas for mining asteroids and for private crewed missions to Mars and the Moon (previously only in the realm of Nation States) have raised the topic of  private enterprise in space, and raised specific questions about regulating the exploitation of resources in Space and protection of historic sites in Space. We are standing at a decisive moment in the use of space too now that purely commercial, privatized space outfits have appeared on the launch market, taking over from companies closely alligned to what Eisenhower called the “Military-Industrial Complex”: new outfits like SpaceX, Rocket Lab and a number of other startups.

But this does not mean that the private sector takes over from the military. Some of these private firms (e.g. SpaceX) have been quickly drawn into the military sphere themselves, with lucrative launch contracts from the US military. Orbital ATK recently has been bough by Northrop Grumman, part of the "Military Industrial Complex" for decades. Meanwhile, three major spacefaring nations, the USA, China and Russia, have increased their military posturing in space, with ASAT tests and increased suggestions within the US military that the US should re-negotiate or even leave the Outer Space Treaty, as it is seen as restrictive to a more active, offensive use of space.

It is therefore a crucial time to bring up questions about who governs space, what is and what isn’t allowed there, who gets to put things up there, and to put to question the overarching role of the military in this all.

Paglen's Orbital Reflector encourages you to reflect upon these issues.

Note: I warmly thank Trevor Paglen, Amanda Horn, Zia Oboodiyat, Mark Caviezel and Ted Molczan for discussions and for providing viewpoints and data.

Edits of 27 Nov 2018: revised launch time, revised elset estimate, new map, and statement that the deorbit-burn might be visible from N-Europe