Rosetta-Why 10 Years?

[Nicholas Kang]

Recently, ESA space probe Rosetta had finally approach comet 67P. But tracing its history back, Rosetta have been in space since 2004-which means it was launched 10 years ago. My question is why it takes so long to trace just one comet, why not simply launch just years before the comet approach astronomers` desired orbit and then only launch this space probe?

 

Sorry for grammatical errors.

Edited August 13, 2014 by Nicholas Kang

[Greg H.]

Because that's how long it took to make get the probe going in the same direction at roughly the same speed as the comet would be going at the time the two intersected - close enough that it could grab hold of the comet.

 

I think it's amazing to be honest. They basically shot a bullet from a gun, and hit another bullet fired from another gun thousands of miles away, all while being blindfolded. The amount of math that goes into this is staggering, and if evenone calculation is off by a fraction, those errors compile over the long journey and you end up missing the target by thousands or millions of miles.

[Nicholas Kang]

You mean the comet is travelling very slow?

I think our space probe should be able to propel the probe at high speed, given the fact that progress in astronomical engineering has been astronomical. And Rosetta has been sleeping and only wake up early in the past few months ago, around New Year, I think it is January, iirc.

[Strange]

The comet is moving at up to 135,000 km/h and

 

Comet 67P/Churyumov-Gerasimenko loops around the Sun between the orbits of Jupiter and Earth, that is, between about 800 million and 185 million kilometres from the Sun. But rendezvousing with the comet required travelling a cumulative distance of over 6 billion kilometres. As no launcher was capable of directly injecting Rosetta into such an orbit, gravity assists were needed from four planetary flybys – one of Mars (2007) and three of Earth (2005, 2007 and 2009) – a long circuitous trip that took ten years to complete.

 

From: http://www.esa.int/Our_Activities/Space_Science/Rosetta/Frequently_asked_questions

[Nicholas Kang]

Great, the answer-gravity assists. Thanks, Strange, it is you who opened up my mind again. Once again, thanks Strange, that website answers everything. No more question.

[Enthalpy]

Our propulsion technology has not improved much since Von Braun or Korolev. They achieved 3km/s (kerosene) and 4.2km/s (hydrogen) ejection speed, we have 3.4km/s and 4.7km/s. This permits 500t rockets to send 1t probes with 10km/s relative to Earth.

 

This speed is insufficient to move easily in our solar system. Earth's speed around the Sun is 30km/s for instance. For every single travel, we have to find tricks and use about every help available, just to reach other bodies.

 

Presently we can send probes to our Moon, to Mars and Venus on a direct path and reach low circular orbits there, preferably by aerobraking there. That's all.

 

For Mercury we need assistance from Venus that takes years, for Jupiter and Saturn we better get help from Venus and can only reach very elliptic orbits at the target, for Uranus and Neptune humans have only achieved flybies up to now (though elliptic orbits around these planets would be accessible with patience). Ulysses' mission over the Solar poles was possible thanks to a Jupiter flyby only.

 

67P is not in the ecliptic plane. Reaching its orbit is very costly, much more that Mars, and needs help, typically through gravitational assistance. If you remember that one Earth orbit takes a year, one asteroid orbit for instance 4 years, it's clear that several planetary flybies bring you to 10 years.

 

Sometimes shorter paths would be feasible, but indirect ones permit much heavier hence capable probes, which scientists favour.

 

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Can propulsion improve?

 

Chemical rockets, no. We use already the best sensible propellants, which may improve by 0.2km/s - not sensible ones bring only 0.5km/s more anyway.

 

Then you have the solar thermal engine at 12km/s ejection speed, which opens many possibilities and eases many missions, but imposes strong constraints: small thrust, sunlight. I still check what it can do and how. It has never flown nor has been prototyped, to my knowledge.

 

Ion engines improve the ejection speed further but need much power, so their constraints are even worse. They exist, find wider use at satellites around Earth, and made one nice mission to asteroids.

 

Solar sails have not been built in the size and mass they need. These would bring the necessary performance, but only near the Sun (which permits some remote missions too). They should receive much more attention. For instance the Ulysses mission would have been faster, far better and lighter with a solar sail.

[Nicholas Kang]

Ion engines improve the ejection speed further but need much power, so their constraints are even worse. They exist, find wider use at satellites around Earth, and made one nice mission to asteroids.

 

I know that mission to asteroid, it is Hayabusa from JAXA Japan, isn`t it?

It is clear that an economical way of launching probes is a must now.

[Enthalpy]

 

I know that mission to asteroid, it is Hayabusa from JAXA Japan, isn`t it?

 

Hayabusa is one, Jpl made an other. Previously, many Earth-orbiting satellites have been using ion thrusters to keep their attitude and orbital position. The next trend is an ion engine to circularize the transfer orbit to geosynchronous - which makes rocket resigners, especially for Ariane 6, wonder whether the launcher shall target the Gso or Gto (as if a good design for the Esc-B couldn't do both!).

 

Ion engines are strongly limited by the electric power available on board, which they squander limitless. Solar panels limit them to a faint thrust hence lengthy operations when any possible, a nuclear reactor has other serious drawbacks - among others that it needs a huge cold sink. I consider the solar thermal engine a better compromise between thrust and efficiency when propulsion is needed.

[Nicholas Kang]

But then, you can't use them in outer solar system.

[Enthalpy]

http://www.scienceforums.net/topic/76627-solar-thermal-rocket/page-2#entry818683

missions around each equatorial moon of Jupiter or Saturn, heavy mission to Europa, elliptic mission in Uranus or Neptune orbit.

Proponents of solar sails claim they can do it as well.

 

I've read no proposal with an ion engine to orbit each moon of a giant planet, within a few years, ending with one ton.

[Nicholas Kang]

Jpl made an other

 

Which probe?

Dear Marc,

 

That link is really a place to set up your own online library. Good, I am feeling like moonguy, but why not many people did actually reply your posts. Your personal collections and exhibitions of data? Resource center in this forum?

 

http://www.scienceforums.net/topic/76627-solar-thermal-rocket/page-2#entry818683

missions around each equatorial moon of Jupiter or Saturn, heavy mission to Europa, elliptic mission in Uranus or Neptune orbit.

Proponents of solar sails claim they can do it as well.

 

I've read no proposal with an ion engine to orbit each moon of a giant planet, within a few years, ending with one ton.

 

Examples of missions in detail please, like from which space agency(ies), country(ies), date, etc.

[Carrock]

........Can propulsion improve?

 

Chemical rockets, no. We use already the best sensible propellants, which may improve by 0.2km/s - not sensible ones bring only 0.5km/s more anyway. .......

Spin polarized monatomic hydrogen has been the rocket fuel of the future since at least the 1950s.

 

I don't know the specific impulse but it is pretty high.

 

It's nowhere near as easy to handle as nitroglycerine and it requires a liquid helium plated fuel tank as it reacts explosively with anything else.

 

There are a few more similarly trivial problems to overcome before it comes into use.

[Enthalpy]

Jpl's probe with ion propulsion has orbited one asteroid and head to an other one

http://dawn.jpl.nasa.gov/mission/

 

The link here is to a thread where I describe how I imagine these missions are enabled by the sunheat engine, so none has been built up to now. Esa had conducted studies about this engine, but their design had difficulties (window, heat exchanger) which my design avoids. If you compare the figures (mass, duration...) of the missions I propose with the ones possible with chemical propulsion, it's a game changer. I'm sure companies or agencies will develop it; some questions here and elsewhere relate strongly with tecnologies I propose, so many people read my threads and invest some efforts in the development.

 

Few answers: that's not a worry to me. I describe my ideas in forum threads so people can jump in if they want. For instance on a chemical forum, they just advised that some ideas I proposed were already common, showing me where to search - fine! These threads receive few replies but many visits, so I'm pleased that they raise interest.

Spin polarized monatomic hydrogen has been the rocket fuel of the future since at least the 1950s.

 

Atomic hydrogen is efficient. Spin wouldn't change much, except maybe for a hope to store it... but how?

 

The sunheat engine I describe does dissociate a fraction of its hydrogen. That's why the chamber works at 30mbar instead of 1bar or 30bar. This partial dissociation raises the specific impulse from 800s to 1200s, and is obtained with a tungsten chamber.

[Carrock]

Sorry - I meant to say isospin monatomic hydrogen, where the atoms don't combine into dihydrogen unless a few atoms' spins are flipped.

The energy of reaction is enough to flip more spins for a runaway reaction.

I believe there is at least a 0.01% probabilty of this fuel being used in the next hundred years.

[Nicholas Kang]

Thanks for sharing.