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Philae landed on comet 67P this year. And it means a big breakthrough for the science and technology of human being.

However Hayabusa mission of Japan not only landed on an asteroid but also returned an asteroid sample to Earth in 2010.

Technologically ,which is more difficult?
And why is Philae more important?

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This is a very brief answer but I hope it helps.

Distance

Hayabusa collected samples from a near-earth asteroid while Philae landed on a comet after traveling a distance of over 6.4 billion kilometers.

The Type Of Mission

Hayabusa's did not literally land on the asteroid. It just touched down and collected samples. While, Philae had to land, steady itself, drill and study the composition. Plus, the asteroid on which Hayabusa landed was a near-earth asteroid and so it's nature could have been known easily. On the other hand, the Rosetta team came to know about the exact nature of 67P just weeks before the landing.

The Complexity

The Rosetta mission used about 6 gravity assists and performed 2 flybys. It was not an easy task to plan a journey of 10 years. Plus, when the probe reaches the comet, radio signals would take a lot of time to reach it and thus it would be very difficult to control the probe if something went wrong. Plus, the surface of an asteroid is vastly different from the surface of a comet. The Hayabusa Mission was not the first to touch down on an asteroid. It had been done once by NEAR Shoemaker and so we knew what to expect. See this for a detailed report.

I think this sums up how the Rosetta mission was a major breakthrough.

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  • $\begingroup$ How to collect samples without landing on it? Philae will take sample back? $\endgroup$
    – questionhang
    Jan 2, 2015 at 15:35
  • $\begingroup$ @questionhang I am not much aware of the technical diffrence between touchdown and landing but I can say thay Philae had to stabilise itself to remain on the comet while Hayabusa was more like go near, take samples and return. But even if you don't count this part, Rosetta's achievement was still greater. $\endgroup$
    – Yashbhatt
    Jan 2, 2015 at 17:20
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    $\begingroup$ I beg to differ (but not enough to downvote). The key difference between Japan's mission to 25143 Itokawa and ESA's mission to 67P/Churyumov–Gerasimenko is the European braggadocio of ESA (even in the face of failure, and let's face it, Philae was a failure) versus the subdued Japanese behavior of JAXA (even in the face of success, and let's face it, Hayabusa was a massive success). $\endgroup$
    – David Hammen
    Jan 3, 2015 at 10:05
  • $\begingroup$ @DavidHammen You are right. But Rosetta still gathered enough data to be popular. The diffrence was like landing on the moon and landing on Mars. $\endgroup$
    – Yashbhatt
    Jan 3, 2015 at 13:10
  • $\begingroup$ @questionhang: No, Rosetta/Philae is not a sample return mission. $\endgroup$
    – Keith Thompson
    Jan 4, 2015 at 2:32
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I find the manner in which the question is posed rather opinion-oriented. Nevertheless, I try to give a few objective add-ons.

Both missions have been great technical successes, having very different mission requirements, spacecraft designs and concepts of operation.

When looking at the scientific return, objectively Rosetta has blown it out (count it in number of Science/Nature/etc. publications and count the amount of new geophysical models for pristine solar system bodies. The comet has told us so much, that few weeks ago NASA has approved budget for supporting a mission concept to revisit the body.

Rosetta was, at the very origin of the project, conceived as an ESA-NASA cooperation, where JPL will build the lander. However, politics aside, Congress cut the flow and ESA had to turn to DLR to build a lander with limited budget, limited mass and limited volume. Therefore, Philae's platform had no steering (active AOCS) in contrast to Hayabusa, which was a fully-equipped platform.

The landing sequence for Philae was pre-planned. Philae didn't control its path, it was the comets inhomogeneous gravity at ~1m/s for several km. Propagate your uncertainties... Still, the landing was pin-point, damn the rebound on that hard rock.

Hayabusa performed an inertial hovering followed by a touch-and-go sequence. The operation was rehearsed twice before. The sample was taken with the equivalent of a vacum cleaner on a dusty body with such low density you could put your arm through and come out on the opposite hemisphere.

One important requirement in sample collection is to prevent contamination of the sample by the system and the environment. Specially during re-entry. For the small sample of Itokawian dust that was recovered, isolation conditions could not be proven. So most likely some material from spacecraft outgassing went in there. This could not be proven one way or the other. Nevetheless, technically speaking, this operation: sample capture, retrieval and re-entry was a breakthrough. And in fact, OSIRIS-REX is just going to try and repeat that on asteroid Bennu with a load of lessons learnt. I personally look forward to the science return and wish for a lot more data coming down this time.

So if you ask, why such a big boom for Rosetta, the key is, the few hours of experiments on the surface helped give context to large set of extended science observations by the orbiter. Something that had never been done before on a small body (be it a comet or an asteroid), actually only on Mars. This got the whole community very excited, for a good reason.

As an engineer, I understand the voices of "this wasn't a real landing", "this was luck", "it was a failure" etc. But you know, JPL redefined the landing gear of their next comet sample return proposal when Philae's harpoons didn't work. This is what erros are made for, to make it better the next chance. And this involves all agencies and all countries collaborating instead of competing to win a front page. It also involves the general public understanding the importance of each mission out there. Some prove technology, some bring a science storm. Let's leave politics to politians ...

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  • $\begingroup$ Technically, it is easier to land on an asteroid because of its larger gravity. In fact, I wanted to confirm comets are more important than asteroids, at least now. Comets carry a lot of information which asteroids do not have. Even Hayabusa was more successful, because of its target, it could not surpass Philae. Am I right? $\endgroup$
    – questionhang
    Jan 19, 2018 at 2:07
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    $\begingroup$ Gravitational potential is a function of body density and mass distribution, so we can't not state that asteroids have larger gravity than comets nor viceversa. Most comets are suspected to have an active nucleus, while asteroids are inert cold bodies. But this is an open research field, and in fact, the line that divides one group from the other is becoming more and more blurry. In space research, every lesson learnt counts as success. $\endgroup$
    – Ela
    Jan 23, 2018 at 10:22

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