John,
Sorry to be late to the party, but I wanted to follow-up on Henry's
comments (which I mostly agree with).
At SmallSat conference this summer there was a workshop held by the Secure
World Foundation on cubesat post mission disposal best practices. I don't
think I have my notes on me since I'm on travel, but the consensus of the
meeting was pretty much what Henry said. Early cubesats often had to hich
rides to higher altitudes, and when combined with the early reliability
levels of cubesats, this made them look worse than they really are. But
lately, cubesats actually have a *better* rate of compliance with the
existing 25yr guidelines, precisely because almost all of them launch to
altitudes low enough to be naturally 25yr compliant. I remember several
reasons they mentioned (many of which Henry already pointed out):
1- University projects don't want missions that last too long, as it is
logistically hard to keep a long program staffed with the high turnover
rate you get from students graduating and such.
2- A few of the workshop members were saying that many US launch
providers/aggregators these days are actually acting as gatekeepers for
cubesats, and not allowing them to launch to orbits that won't naturally
decay within 25 yrs, even if they wanted to
3- Most of the big cubesat constellations are focused on missions that get
better resolution/performance at lower altitudes. While many of the bigger
cubesats are going for slightly higher altitudes to get a little more
lifespan, almost all are still lower than ~600ish km.
As for the rest of Henry's comments, I was actually just giving a
presentation today at a Spacecraft Anomalies/Failures workshop about why
historical constellation reliability data (from Iridium) suggests how
important having a "Plan B" for post mission disposal will be for the Big
LEO constellations. My only disagreement with Henry is that I think he may
be overly pessimistic on the inherent costs of deorbit services, or the
lack of a market for such deorbit services. At least, I think I've found a
path forward, and my startup is actively raising money to develop just such
a deorbit tug solution...
We'll see if I can earn myself an "I proved Henry was over-pessimistic"
t-shirt. :-)
~Jon
On Sat, Dec 9, 2017 at 12:28 PM, Henry Spencer <hspencer@xxxxxxxxxxxxx>
wrote:
On Sat, 9 Dec 2017, John Dom wrote:
Alarmist or serious threat ?
No. :-)
Not an entirely trivial issue, but not as bad as the scare stories tend to
suggest.
For the educational and experimental cubesats, there is a simple answer:
supply them with rides to quite low orbits, where their orbital life will
be short -- a year is plenty for most of their missions -- and they'll be
below most of the operational satellites. That would suit most of their
users fine. The reason they sometimes end up in higher orbits is not that
their users are malicious or uncaring, but that there simply aren't that
many rides to quite low orbit. Fix that, and firmly encourage them to use
such rides, and they largely stop being a problem.
(Deploying them from ISS, or from ISS supply runs, is a reasonably good
approach. That's low enough for a short life, and it guarantees that
they're below ISS -- which is something like half of the entire collision
cross-section of everything in LEO, and much more than half of the
political sensitivity.)
For the bigger nanosats, and the microsats on up, which are more likely to
be operational satellites of long-term value, the immediate answer is
firmer deorbiting requirements, and better funding for R&D on deorbit
methods. (Probably the most popular current approach is deployable drag
brakes, which are *not* a great choice because they bring the thing down
faster but don't reduce the volume swept out on the way down.) In the
longer term, unless deorbiting can be made very reliable -- tricky given
that the usual reason for deorbiting the thing is that it's broken! --
restrictions on large constellations may be needed.
Note, "deorbit methods" include non-on-board solutions like tugs. It's
not inconceivable that we'll eventually end up with a rule that you don't
get to launch a LEO constellation without showing a contract with a towing
service, committing you to pay for retrieval and disposal of any satellite
that isn't able to deorbit itself. (Longer term, this might apply to all
LEO satellites -- of any size -- not just constellations.) There is no
deep technological problem with providing towing services; it's just that
currently the costs are rather high and there's no market.
Henry
