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For now, the Planetary Defense Conference Exercise 2021's incoming fictitious(!) asteroid, 2021 PDC, seems headed for impact on October 20th, 2021, exactly 6 months after its discovery. Today (Monday, April 26th, 2021), the impact probability is 5%, in a steep rise from 1 in 2500 upon discovery six days ago. We all know how these things end. Or do we? Unless somebody wants to keep civil defense very busy very soon, the chance is 95% that it will not hit; instead fly by closely to Earth, swing by to a new orbit that takes it away essentially forever or back again sooner or later through a keyhole, for a re-play at different odds. This is where our story starts and the story sounds familiar: season's greetings from 2004 MN4, now better known as (99942) Apophis. One more thing is similar: the close fly-by is an easy launch opportunity to 'jump aboard' that potentially hazardous asteroid for planetary science and tracking of longterm Yarkovsky-shifted keyhole resonant return risks. Indeed, missions are currently being discussed to launch during the 2029 fly-by of Apophis to rendezvous and investigate it closely right after. Others strive for an earlier launch to rendezvous well before, to observe all of the close fly-by at Earth and what it might do to a likely delicate rubble pile asteroid. Presently, this is an unlikely if not impossible option for sudden encounters like 2021 PDC with a lead time of months. But when asteroid mining (...possibly the other ...-not-if of asteroids?) takes off in the same manner as low Earth orbit communications satellites, this option may become a reality. But for now, even if a suitable planetary mission were serendipitously ready atop a suitable launch vehicle, could you get it an asteroid lander within 6 months? Surprisingly, this option existed between late 2014 and late 2018 when the MASCOT Qualification Model turned Flight Spare was kept fully integrated and flight ready for on-ground testing to prepare for the Flight Model's brief but complete mission on Ryugu with JAXA's highly successful HAYABUSA2 probe. At the same time, the MASCOT2 detailed design study for ESA's former AIM mission within the common NASA-ESA AIDA mission to (65803) Didymos and its moonlet, Dimorphos (then affectionately known as 'Didymoon'), paved the way for long-life MASCOTs, many of which have been discussed and studied since. The thoughtful design of MASCOT’s hardware and software allowed for a very high degree of re-use and flexibility regarding scientific payloads. MASCOT2 was to investigate the interior of Didymoon by Low-Frequency Radar. Close encounters like Apophis' offer unique opportunities for Earth-based planetary radar assets to work with spacecraft near and landers on the passing asteroid. We present a range of options for radar- and composition-oriented long-life MASCOT variants - to be delivered to the surfaces of the respective asteroid bodies - for the presently most likely near miss of 2021 PDC and the most certain close fly-by of (99942) Apophis on Friday, April 13th, 2029.

The Hera mission has been approved for development and launch in the new ESA Space SafetyProgramme by the ESA Council at Ministerial Level, Space19+, in November 2019. Hera will contributeto the first deflection test of an asteroid, in the framework of the international NASA- and ESA supportedAsteroid Impact and Deflection Assessment (AIDA) collaboration.The impact of the NASA DART (Double Asteroid Redirection Test) spacecraft on the natural satelliteof the binary asteroid Didymos in late September 2022 will change its orbital period around Didymos.As Didymos is an eclipsing binary, and close to the Earth on this date, the change can be detected byEarth-based observers. ESA’s Hera spacecraft will rendezvous with Didymos four years after the impact.While performing the measurements necessary to understand the effect of the DART impact on Didymos’secondary, in particular its mass, its internal structure, the direct determination of the momentum transferand the detailed characterization of the crater left by DART, Hera will provide unique information onmany current issues in asteroid science.Hera will rendezvous for the first time with a binary asteroid, and in particular its secondary, of only160 m in diameter. So far, no mission has visited such a small rock in space. Moreover, for the firsttime, internal and subsurface properties will be directly measured. How do binaries form? What does a160 m-size rock in space look like? What are its internal properties? And what will be the size and themorphology of the crater left by DART, which will provide the first impact experiment at full asteroid scaleusing an impact speed close to the average speed between asteroids? What will be the exact momentumtransferred by DART, which needs the precise measurement of the mass of the target by Hera? Thesequestions and many others will be addressed by Hera as a natural outcome of its investigations focusedon planetary defense.The scientific legacy of the Hera mission will extend far beyond the core aims of planetary defense.Hera is thus the European contribution to the international asteroid exploration era.Acknowledgement: The authors acknowledge funding support from ESA and from the EuropeanUnion’s Horizon 2020 research and innovation programme under grant agreement No 870377 (projectNEO-MAPP).

The European Space Agency’s planetary defense Hera mission will launch to the Didymos binary asteroid system in 2023 (with bodies nicknamed Didymain and Didymoon). Once in vicinity of the asteroid, two 6U CubeSats will be deployed to contribute to the asteroid research and mitigation assessment objectives of the Hera mission. This paperwill describe the Juventas CubeSat, equipped with a low frequency radar payload to characterize the internal structure of Didymoon. Juventas is designed to be operated using the Hera mothercraft as a proxy. This mission architecture creates a new paradigm for CubeSats, requiring high levels of mission autonomy while operating in the challenging environment of a small-body binary asteroid. Juventas will utilize the inter-satellite link to Hera for performing radio science experiments, augmenting the characterization of the asteroid gravity field. Once the radar science and radio science observation objectives have been met, Juventas will perform an attempted landing on the surface of Didymoon to research its dynamical properties.