The near-Earth asteroid (162173) Ryugu is a 900-m-diameter dark object expected to contain primordial material from the solar nebula. The Mobile Asteroid Surface Scout (MASCOT) landed on Ryugu’s surface on 3 October 2018. We present images from the MASCOT camera (MASCam) taken during the descent and while on the surface. The surface is covered by decimeter- to meter-sized rocks, with no deposits of fine-grained material. Rocks appear either bright, with smooth faces and sharp edges, or dark, with a cauliflower-like, crumbly surface. Close-up images of a rock of the latter type reveal a dark matrix with small, bright, spectrally different inclusions, implying that it did not experience extensive aqueous alteration. The inclusions appear similar to those in carbonaceous chondrite meteorites.

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Observations of the magnetization state of asteroids indicate diverse properties. Values between 1.9 × 10 −6 Am 2 /kg (Eros) and 10 −2 Am 2 /kg (Braille) have been reported. A more detailed understanding of asteroidal magnetic properties allows far-reaching conclusions of the magnetization mechanism as well as the strength of the magnetic field of the solar system regions the asteroid formed in. The Hayabusa2 mission with its lander Mobile Asteroid Surface Scout is equipped with a magnetometer experiment, MasMag. MasMag is a state-of-the-art three-axis fluxgate magnetometer, successfully operated also on Philae, the Rosetta mission lander. MasMag has enabled, after Eros for the second time ever, to determine the magnetic field of an asteroid during descent and on-surface operations. The new observations show that Ryugu, a low-albedo C-type asteroid, has no detectable global magnetization, and any local magnetization is either small (< 10 −6 Am 2 /kg) or on very small (subcentimeter) scales. This implies, for example, that energetic solar wind particles could reach and alter the surface unimpeded by strong asteroidal magnetic fields, such as minimagnetospheres in case of the Moon.Plain Language Summary Magnetic measurements in space near and on solar system bodies such as asteroids can provide important information about their formation history and their material properties. Hayabusa2, a Japanese mission, visited asteroid Ryugu, a type of asteroid (carbon rich) not visited before. Ryugu is a rubble pile, that is, an agglomeration of rocks and boulders. It is not expected to have any global magnetic field, but it can be magnetized on smaller scales (boulders or pebbles). Hayabusa2 carried therefore a small lander called Mobile Asteroid Surface Scout equipped also with an instrument for magnetic field measurement (magnetometer). In this study, we present observations from the magnetometer that were collected during Mobile Asteroid Surface Scout's descent and landing on the surface of Ryugu. The magnetic measurements show that Ryugu is not magnetized on boulder (greater than centimeter) scales. This gives us indication on its origin and evolution. In particular, it shows that Ryugu and the bodies it was created from did not possess any magnetic field generation mechanism and that they were not created in an environment with strong background magnetic field. The results are important inputs for theories about the solar system evolution that work with magnetic field as one of the drivers for dust accretion and planetary formation.

Now spanning a time frame of already 10 years, the plan to land a European Lander on an asteroid has finally been accomplished. The first idea was established around 2008 in the framework of the European Marco Polo Assessment, studying the possibility to collect a pristine sample of a Near-Earth Asteroid and returning it back to Earth. The lander named MASCOT (Mobile Asteroid Surface Scout) was proposed to be carried by the main spacecraft, to land on the surface and by the ability to relocate to investigate multiple surface locations in order to scout the best possible sampling site. After the discontinuation of the original study, MASCOT received an invitation from JAXA to join-in the Hayabusa2 mission, the direct follow-up of the first asteroid sampler Hayabusa. However, MASCOT was selected at a time (mid 2011) when its conceptual design and scientific payloads had not been fully defined; with the carrier spacecraft already in its critical design phase having most of its interfaces fixed; no heritage to use off-the-shelf bus equipment directly and only 3 years left until a proposed final delivery. The tight schedule, tightly defined envelope, and strict margins policy were challenges during its development at all levels. Nevertheless, Hayabusa2 and MASCOT were launched on December 3rd, 2014, and arrived at their destined target asteroid (162173) Ryugu on June 27, 2018. Finally, MASCOT was separated from its mother spacecraft and successfully landed on October 3rd, 2018, accomplishing the first ever landing of a European spacecraft on the surface of an asteroid. This paper provides a review of the performed MASCOT development process including its verification strategy from the first unit hardware test to the final check-out before launch. In addition, it also provides a historical comparison to former fast-paced programs in space.

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