Was ESA Negligent in Continuing to Operate Envisat?

9 October 2012 by Michael Khan, posted in Uncategorized

A recent paper presented at the 63. International Astronautical Congress (IAC) in Naples levels grave allegations against the European Space Agency (ESA). The paper claims that ESA acted with negligence, or even gross negligence, when continuing to operate its large Earth observation satellite Envisat beyond the year 2010. Envisat turned defunct in April 2012. The paper further alleges that this negligence could expose ESA to liability claims in case Envisat should cause any damage to other satellites.

(Lesen Sie diesen Artikel hier auf Deutsch)

The paper is authored by Martha Mejía-Kaiser from the Universidad Autónoma Nacionál de México (UNAM). Mejía-Kaiser, a member of the International Institute of Space Law (IISL), is a doctor in political and social science (just the kind of person one would naturally turn to on technical matters related to satellite operations, right?). Her claims were eagerly picked up by the media, including Space News, a well-known publication in the space business. Strangely, the Space News article includes only quotes by Mejía-Kaiser. No attempt is made to verify her claims (though that would have easy), nor is there any comment by ESA representatives. In view of the gravity of the allegations, I would have thought it a matter of course to ask ESA officials to comment. But no.

The satellite in question is an 8 ton behemoth that was launched in 2002 and operated on a sun-synchronous orbit with a final altitude of 780 km, following a slight orbit lowering in 2010. On April 8, 2012, mission control suddenly and unexpectedly witnessed a total and permanent loss of communications from and to the satellite. All attempts of re-establishing contact failed and the spacecraft was later given up. The reason for the failure is as yet unknown.

Mejía-Kaiser's paper (number IAC-12.E7.5.11, title "ESA's Choice of Futures: Envisat Removal or First Liability Case") presents its case as follows:

  1. In 2010 ESA decided to lower Envisat's orbital altitude slightly to 780 km.
  2. Lowering the altitude further, to 750 km would have cost 60 kg of propellant.
  3. These 60 kg were still available in the spacecraft's tanks, so the further lowering would have been feasible.
  4. At an altitude of 750 km, the remaining orbital lifetime would have been reduced to 25 years, rather than around 150 years in the current 780 km orbit.
  5. A remaining orbital lifetime would have been in accordance with rules on space debris mitigation, which ESA imposed on itself, as a member of the Inter-Agency Space Debris Coordination Committee IADC.

Claims 1, 2 and 3 are correct but they don't mean much and certainly don't give rise to any allegations of negligence. The really major point is #4 but what Mejía-Kaiser is alleging simply is not true. It's not a matter of being debatable or dubious or anything like that. It just ain't so.

Mejía-Kaiser attempts to shore up her claim by citing a publicly available IADC document, namely "Support to the IADC Space Debris Mitigation Guidelines". However, her paper contains only an abbreviated quote from the IADC document, and it is obvious that the contents and implications of the quoted text were not understood. Page 21 of the IADC document, which anyone can freely download and check, is as follows:

[...] the removal of objects from LEO as soon as possible after the end of a mission is beneficial. Fortunately, natural forces, especially drag, work to clean debris from this region, although this is effective primarily for satellites below 700 km. It is recommended that orbital lifetime be reduced to less than 25 years at the end of mission (approximately 750 km circular orbit for A/m = 0.05 m2/kg, and approximately 600 km circular orbit for A/m=0.005 m2/kg, depending on solar activity to be more exact) [...]

Mejía-Kaiser explicitly refers to page 21 ofthe IADC document, but she quotes it thus:

“It is recommended that orbital lifetime be reduced to less than 25 years at the end of mission (approximately 750 km circular orbit for A/m = 0.05 m2/kg…)”

The part which Mejía-Kaiser has omitted and replaced by "..." is the crucial part. Therein lies the rub. The orbital lifetime depends on two factors: 1.) the exospheric density and 2.) the A/m-ratio, where A denotes the aerodynamic cross section and m the mass of the object.

The density is a function of the altitude and of numerous environmental parameters, the most important being the solar activity. At altitudes of 750 km and above a apacecraft will remain in orbit for decades, so short- and mid-term variations of the solar activity can be ignored. Instead, for a given altitude, the density to be assumed is averaged out over the range of solar activity.

The A/m-ratio is also fraught with a degree of uncertainty, due to the impossibility to determine the aerodynamic cross section accurately. This cross section is the projected area that is exposed to the flow of atmospheric particles and ths causes drag. Satellites are never evenly shaped bodies, they have appendages that stick out and it makes a big difference whether, e.g., a solar array faces the flow head-on, obliquely or  side-on. To predict the atmospheric drag accurately, one would have to know the satellite's attitude, its orientation in space, quite exactly.

However, not only is the attitude of a defunct body not exactly known - because in the absence of telemetry, there also is no data from the attitude sensors, it may also change as the object tumbles or turns in response to perturbing forces. If the effective aerodynamic cross section is not known, then the conservative approach would be to use the extreme case. I want to give Mejía-Kaiser the benefit of doubt and determine whether Envisat could possibly have had a lifetime of only 25 years at an altitude of 750 km, as she claims.

Envisat has a mass of 8000 kilograms. Its solar array has a surface area of 70 square meters,  viz. this ESA brochure, page 7. Let's assume that the main body (the "bus" in space engineers' parlance) and the SAR antenna have a combined cross section of 70 square meters too. In fact, they have rather less, but as I say, I'm giving Mejía-Kaiser the benefit of doubt. The combined cross section then adds up to 140 square meters. .

The A/m-ratio mentioned in the IADC document is then 140 sqm/8000 kg = 0.0175 sqm/kg. Again, in reality it would be quite a lot less, because I'm not only over-estimating the overall cross section, I'm also assuming that the defunct satellite will remain oriented such that the drag is maximized at all times - which is patently impossible. Look, I'm really trying hard to justify Mejía-Kaiser's claims of a 25 year lifetime.

The IADC document, from which I quoted the entire relevant paragraph above, recommends an maximum orbital altitude of 750 km for an A/m ration of 0.05  sqm/kg. That's about three times more than the figure of 0.0175 sqm/kg I obtain for Envisat even when making unrealistic assumptions on the cross section.

The lifetime depends linearly on the A/m-ratio. If an object with an A/m-ratio of 0.05 sqm/kg can be expected to have an orbital lifetime of 25 years at 750 km circular orbit altitude, then Envisat, with its A/m-ratio that is smaller by a factor of around three will also have thrice the lifetime. Three times 25 years is 75 years! This is the minimum lifetime Envisat could be expected to have even if the orbital altitude had been lowered to 750 km. Not 25 years, no, it's 75 years. In fact, it's significantly more than that if one makes realistic assumptions for the drag calculations. Decades more.

That's the way it is, based on clear and unambiguous data provided by the IADC - in fact, the very data that Mejía-Kaiser refers to in an attempt to justify her claims.

Still not convinced? Want a second opinion?

OK, so here goes: The authors of the IADC document used their orbital analysis to arrive at the figures they give. I have made similar calculations, completely independently of those made by the IADC document authors. I used different software from theirs, different models of the atmosphere, and I run it on different computers. Computations don't get more independent than that. Here come my results:

Orbital Lifetime of Spacecraft in Low Earth Orbit as Function of Mass/Area Ratio and Altitude, source: Michael Khan

Orbital Lifetime as Function of Orbital Altitude and m/A Ratio on Circular Earth Orbits, Source: Michael Khan

I use the m/A ratio rather than the A/m ratio used in the IADC document but that should not trouble you. It's just the inverse value. For a mass of 8000 kg and a cross section of 140 square meters the m/a ration turns out as m/A= 8000/140 kg/sqm = 57 kg/sqm. In the diagram I plot the lifetime for an m/A ratio of 25, 50 and 100 kg/sqm. Envisat lies between 50 und 100. Using those assumptions of mine, you'd find the expected lifetime for any given altitude just above the blue line. In reality they'd be somehat further up toward the green line.

Let's see what we get for 750 km. In the diagram, just a bit higher than the blue line (watch out, the vertical axis has a logarithmic scale!), for an altitude of 750 km, the resulting lifetime can be read off as around 70 years.  This is consistent with my rough calculation from earlier on based on the IADC data.

Let's face it, at 750 km of altitude, Envisat would not have stayed up for only 25 years, as Mejía-Kaiser claims. No way. The IADC data say so, my calculations say so, and I haven't seen any evidence to the contrary. That would have to be quite dramatic evidence, it would be contrary to decades of operational experience with spacecraft control. The least we can expect is around 75 years, and realistically, one can add several decades to that.

And what does that mean?

It simply means that Mejía-Kaiser's allegations are based on a claim that is verifiably incorrect:

  • The claim that ESA could have achieved a reduction of Envisat's orbital lifetime to 25 years by the simple expedient of lowering the spacecraft's orbital altitude to 750 km back in 2010 is patently and verifiably incorrect. This claim is simply not justified.
  • Because this claim is unjustified, the claim that ESA's decision not to lower the orbital altitude to 750 km constitutes an act of gross negligence that endangers other satellites is also unjustified.
  • Fact is: ESA never had any possibility of lowering Envisat's orbit to an altitude at which the remaining lifetime would have been reduced to 25 years. This would have required changing the design of the propulsion system and the capacity of the tanks in the design process, long before launch. However, back then there were no agreements on the mitigation of space debris. These came only in 2004.
  • Therefore, there is no justification for alleging that ESA disregarded agreements aiming at protecting near-Earth space from orbital debris.
  • If there is no justification for such an allegation, there can also be no justification for the claim that ESA is exposing itself to litigation in the event of damage to other spacecraft.

In summary, this means that there is no credible basis for Mejía-Kaiser's paper. End of the matter.

All references and data provided by me are public and can be verified by anyone. Nevertheless, I had no difficulty in disproving the numerous allegations which abound in Mejía-Kaiser's paper. Martha Mejía-Kaiser must have access to the same data available to me, as they are all on public record, so it would have been equally easy for her to check her claims before submitting and presenting her paper, th contents of which she will find very difficult to prove.

One may argue that it was not a good idea to launch a spacecraft without ensuring that it would not turn into a long-term debris problem. For any satellite launched today, this would no longer be acceptable. However, Envisat was not launched today, and at the time when the Envisat mission was planned, things were still done differently - not just by ESA, but by all space faring nations.  This is an example of a general lack of problem awareness that used to pervade the space business, and is changing now. It is however not a case of isolated, uncommon, unilateral or exceptional negligence.

Mejía-Kaiser's paper ends on a belligerent note, with a veritable broadside of insulting statements, e.g.: "... negligent conduct ...." ".... affects ESA's credibility ...." "...ESA's damaged credibility ....", ".... endangered astronauts ....", ".... ESA's actions and omissions harm the IADC's work of 20 years ...." ".... ESA's political decision to take risks ....", "... creates a non-measurable harm to the whole international community.". Whoever uses such language should be very certain that the perceived facts and the data used, and their interpretation are correct.

Mejía-Kaiser did not get her facts straight, and as a result, her paper is wide open to criticism and can quite easily be demolished, as I have demonstrated. Anyone with any knowledge of the topic at hand immediately sees that. However, the problem does not end there. Papers claiming things that just ain't so are written all the time, and in due course they are refuted by follow-up papers. That is the way science works. But part of the problem lies with the media - the onus is on them to dedicate time to the verification of their sources.

Good science, this paper wasn't.

4 Responses to “Was ESA Negligent in Continuing to Operate Envisat?”

  1. Markus Reply | Permalink

    Good analysis, Michael. I like the yoda-lingo at the end. However, again, one criticism to ESA remains - the lack of openness in response to the paper. Why didn't ESA post a press release with your analysis in it in response to the paper?

  2. Peter Reply | Permalink

    Time to rename Space News into Asterobloid (but we knew that).

  3. Michael Khan Reply | Permalink

    On 11. Oct. 2012 ESA issued a press release on this matter. This is of course formulated much more circumspectly and diplomatically than my above article but in the end says the same, namely that it has never been within the realm of the possible to lower Envisat to an orbit with a lifetime of at most 25 years.

    When reading that press release carefully you might be confused by the altitude values stated there, namely that the altitude after launch was 780 km and that this altitude was lowered too 768 km in 2010. In my article, I mention a current altitude of 780 km. I did not mention the original altitude, but if I had, I would have stated it as having been was some 17 km higher, around 797 km.

    Where does this discrepancy come from? There are different ways to compute the orbital altitude. The values in the current ESA press release (originally 780 km and 768 km now) are with respect to the Earth equator. The altitude figures I give are with respect to the mean radius of the geoid, a flattened ellipsoid that descibes the approximate shape of the Earth. In fact, earlier Envisat press releases also always give altitudes with respect to the mean radius of the geoid, not the Earth equattorial radius. cf. this one from Oct/2010, where the orbit lowering at the start ofthe mission phase 2010+ is described.

    To make things more complicated, the orbital elements vary considerably over one orbital revolution, especially for low, polar orbits. So it is not straightforward to state what exactly is meant when someone states an "altitude".

    On heavens-above.com you can find Two-Line-Elements for Envisat's current orbit. There, the number of revolutions per day is stated (with an absurdly high level of accuracy as 14.37548911. This leads to an orbital period of 6010.23 seconds and a semi-major axis of 7144.74 km. If you subtract the Earth equator radius of 6378.15 km from the semi-major axis, you get 766.6 km. But if you subtract the mean Earth radius, you end up at around 780 km.

  4. Ale Reply | Permalink

    Hi Micheal,
    Great analysis! I am currently carrying out a study aimed at finding a way to de-orbit Envisat and I would be grateful if you could provide me with the name of the software and atmospheric models you have used to model Envisat's lifetime?
    Many thanks.

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