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Posted by: ngunn Aug 25 2009, 10:47 AM

With thanks to Van Kane at futureplanets for links to the very interesting white papers for the next decadal survey, here is one that caught my eye:

http://www.google.com/url?sa=t&source=web&ct=res&cd=1&url=http%3A%2F%2Fwww8.nationalacademies.org%2Fssbsurvey%2FDetailFileDisplay.aspx%3Fid%3D14&ei=1bqTSqSfGeXRjAffs4j8DQ&usg=AFQjCNFLDpzeBXpmoGvWl6DEjXiZwrRDZg

Small but capable landers with enough power to talk directly to earth can be hurled into Titan's obliging atmosphere at interplanetary velocities and land safely without parachutes.

QUOTE:

Mission Architecture

A network mission could likely (depending on the ASRG/RTG provision) fit within the New Frontiers
budget envelope, in that 4 small landers could be delivered directly to Titan on a simple carrier from
interplanetary approach (somewhat reminiscent of the Pioneer Venus probes). The small semi-hard
landers would not need parachutes, legs nor sampling systems, and thus could be relatively simple and
inexpensive. In the Entry-Descent-and-Landing (EDL) context, a Titan network mission is considerably
easier than a corresponding Mars mission.

It will be recalled that the 1.3m diameter, 200kg Huygens probe, with a 1.3m parachute, hit the ground at
5 m/s and was unaffected by the 15g impact. A 10-50kg small station, with a Huygens-like 5cm foam
insulation layer, will encounter similar or lower impact speeds and loads without a parachute. The stations
might be simple DS-2 or Huygens-like capsules, or perhaps with simple self-righting petals like the
Russian Luna-9 or the Mars Pathfinder.

Communication, of course, would be direct-to-Earth. The key science goals can be met with modest
bandwidth (a Megabit per Titan day, or ~1 bps, is ample over year-long periods), compatible with low- or
medium-gain transmission to the DSN, especially if the bandwidth is leveraged by intelligent data
prioritization on the landers such as event-triggered sampling and data compression.

UNQUOTE

I have a queston regarding the range of arrival speed for which this could be done. Presumably when a very small payload is involved it is posible to send it on a much faster trajectory from Earth to its target than could be done for the likes of Cassini, should that be deemed worthwhile. I am wondering if Titan's atmosphere has an absolute maximum safe arrival speed?

Posted by: Holder of the Two Leashes Aug 25 2009, 02:34 PM

It all depends on how much of the probe weight you want to devote to heat shielding. The fastest entry speed to date has been the Galileo probe at Jupiter, which hit the atmosphere at slightly faster than 100,000 mph (160,000 kph). This was in an hydrogen/helium atmosphere rather than nitrogen, but due to Titan's lighter gravity the atmospheric scale is somewhat similar.

Going this fast, the Galileo probe paid for it with fully half its weight as heat shielding. Of course, with Jupiter you don't have much choice.

Nobody, I'm sure, is planning to send a probe to Titan that would arrive that fast.

Posted by: ngunn Aug 25 2009, 08:47 PM

I dunno. That's about three times faster than New Horizons, but the small Titan landers under discussion would be an order of magnitude less massive.

Posted by: vjkane Aug 25 2009, 10:42 PM

As I recall, Titan is one of the easiest atmospheres from a an entry and descent point of view. No deep gravity well, extended atmosphere (break early and slowly), and a thick atmosphere that makes retrorockets unnecessary. This is an ideal world to explore if it wasn't so damn far away.

However, Ralph's paper got me thinking, and you can see some thoughts I just posted at: http://futureplanets.blogspot.com/2009/08/titan-thought-experiment.html

Posted by: Holder of the Two Leashes Aug 26 2009, 02:03 PM

Doesn't matter.
Any chemical rocket in existance can't get anywhere near that velocity, even if it were only sending its last stage with zero payload. Besides, I would think a "New Horizons" velocity would get you to Titan plenty fast enough.

Posted by: ngunn Aug 26 2009, 02:12 PM

OK, I'll settle for that.

Posted by: Enceladus75 Aug 27 2009, 02:15 AM

This looks like a great and very workable proposal. But isn't Venus's atmosphere orders of magnitude thicker than Titan's, 90 atmpspheres a the surface? Would Titan's much thinner atmosphere really be able to slow down small unparachuted probes sufficiently enough to land intact?

If a network of probes is to be sent - I presume a major target would be the Northern Polar lake region and the possible cryovolcanic feature.

Posted by: stevesliva Aug 27 2009, 03:28 AM

And Titan's mass is an order of magnitude lower than Venus's. The acceleration due to gravity is probably a bigger deal than the deceleration provided by the atmosphere.

Posted by: rlorenz Aug 27 2009, 04:20 AM

Pressure (column mass) is what makes deceleration possible, but it is the scale height that makes
it easy. Titan comes 2nd place after Venus on the first, but is a clear winner on the 2nd factor.

P/g = column mass. On Titan this is 100,000 kg/m2
Whereas the column mass of a 50kg, 0.5 m2 probe would be only 10, so plenty of atmosphere to decelerate
small probes (think of sharing the momentum between the vehicle and the column of air it cookie-cutters out.)
For Mars at 6mb, the column mass of the atmosphere is only about the 10 kg/m2 so Mars is
marginal.

The scale height is what determines the size of the entry corridor, and for Titan that corridor is
very wide, so the guidance requirements for safe entry are very modest. (Or equivalently, lowers
the peak load - it spreads the deceleration over a longer distance/longer time)

So Venus could decelerate a more massive space vehicle than Titan could, but you'd need to deliver
it more accurately, and it would see much higher loads.

Arrival velocity for Titan is really determined by the interplanetary trajectory - acceleration by
Titan gravity doesnt add much ( Ventry^2 = Varrival^2 + Vescape^2) Vesc for TItan is only
2.5 km/s or so.

Posted by: nprev Aug 27 2009, 04:31 AM

Ralph, how much targeting accuracy could be practically achieved with this EDL architecture? It would be terrific if we could splash a gizmo into one of the large lakes with at least a conductivity sensor & an optical experiment (maybe to do a turbidity measurement or perhaps even a broadband spectrum?)

It's fun to think about the possible small instrument packages for this system.

Posted by: stevesliva Aug 27 2009, 05:36 AM

Bit of a facepalm on the Titan mass thing. At least it's probably related to column height.

Posted by: remcook Aug 27 2009, 07:07 AM

Don't worry, atmospheric scale height is inversely proportional to gravity

Posted by: Holder of the Two Leashes Aug 27 2009, 02:14 PM

Good discussion and useful data, but I think Enceladus75 and stevesliva were discussing the parachuting/landing phase rather than entry.