The Hera mission has been dwarfed in press coverage by the recent SpaceX Starship booster retrieval and the launch of Europa Clipper, both successful and significant. But let’s not ignore Hera. Its game plan is to check on the asteroid Dimorphos, which became the first body in the Solar System to have its orbit altered by human technologies when the DART spacecraft impacted it in 2022. Hera is all about assessing this double asteroid system to see first-hand the consequences of the impact, which shortened the smaller object’s orbit around asteroid Didymos by some 32 minutes.

That’s a pretty good result, some 25 times what NASA had defined as the minimum successful orbital period change, and we’re learning more about the ejecta, which involve tons of asteroidal rock. The collision occurred at 6.1 kilometers per second, to be more fully assessed by Hera’s twin CubeSat craft, which will make precise measurements of Dimorphos’ mass to analyze the efficiency of the impact. All this factors into planning for asteroid impact missions if an object ever threatens Earth.

Image: Astronomers using the NSF’s NOIRLab’s SOAR telescope in Chile captured the vast plume of dust and debris blasted from the surface of the asteroid Dimorphos by NASA’s DART spacecraft when it impacted on 26 September 2022. In this image, the more than 10,000 kilometer long dust trail — the ejecta that has been pushed away by the Sun’s radiation pressure, not unlike the tail of a comet — can be seen stretching from the center to the right-hand edge of the field of view. Credit: CTIO/NOIRLab/SOAR/NSF/AURA/T. Kareta (Lowell Observatory), M. Knight (US Naval Academy).

I want to draw your attention to those two CubeSats aboard Hera. My thinking is that these miniature spacecraft, built upon standardized 10-cm boxes, are a story as big as the results they’ll gather. There are two of them: Juventas is the product of GOMspace (Luxembourg), designed to make the first radar probe of the interior of an asteroid. Milani was produced by Tyvak International, an Italian operation. Its job is to perform multispectral mineral prospecting. Their recent activation for a check of on-board systems was in both cases completely successful. Says ESA engineer Franco Perez Lissi:

“Each CubeSat was activated for about an hour in turn, in live sessions with the ground to perform commissioning – what we call ‘are you alive?’ and ‘stowed checkout’ tests. The pair are currently stowed within their Deep Space Deployers, but we were able to activate every onboard system in turn, including their platform avionics, instruments and the inter-satellite links they will use to talk to Hera, as well as spinning up and down their reaction wheels which will be employed for attitude control.”

So this is good news for the Hera mission, but in the larger context we are seeing the continuing growth of miniaturized technologies that will improve the efficiency and capability of missions to much more distant targets. Juventas was activated on October 17 at a distance of 4 million kilometers from Earth; Milani’s turn came on October 24, with the craft 7.9 million kilometers out. Both will be deployed from their ‘mothership’ to make close approaches to Dimorphos upon arrival in 2026.

It was back in 2011 that NanoSail-D2 demonstrated successful sail deployment, to be followed by The Planetary Society’s LightSail-a in 2015. We were learning that a spacecraft as small as a CubeSat could carry a solar sail, leading to The Planetary Society’s subsequent LightSail missions. Sara Seager at MIT has investigated CubeSats as exoplanet research platforms, the notion being that a fleet of CubeSats could be deployed with each monitoring a single star. The first detection of an exoplanet by a CubeSat occurred in 2017 with the ASTERIA (Arcsecond Space Telescope Enabling Research In Astrophysics) 6U CubeSat space telescope.

NASA’s Mars Cube One (MarCO) CubeSats demonstrated multiple CubeSat operations beyond Earth orbit in 2018, and there have been a host of CubeSat projects in the hands of private companies and universities ranging from 2009’s AeroCube-3 to LunaH-Map in 2022, the latter aimed at mapping the distribution of hydrogen at the Moon’s south pole. QubeSat is a project out of UC-Berkeley to study quantum gyroscopes in low Earth orbit and explore precision control for small satellite navigation. CubeSats are cheap. Lose one at launch – and early failures abound, as witness Lunar Flashlight – and the impact on your budget is minimized.

Image: The first image captured by one of NASA’s Mars Cube One (MarCO) CubeSats. The image, which shows both the CubeSat’s unfolded high-gain antenna at right and the Earth and its moon in the center, was acquired by MarCO-B on May 9, 2018. Credit: NASA/JPL-Caltech.

Most CubeSat missions have been designed for low-Earth orbit but some scientists are aiming to go much farther. In a 2023 paper Slava Turyshev (JPL) and colleagues investigated small spacecraft (smallsats) with solar sails in missions to the outer system. In general, a ‘smallsat’ refers to a spacecraft that is both small and lightweight, usually less than 500 kilograms, and sometimes much less, as when we get into the realm of CubeSats.

Smallsats with solar sails are a combination of miniaturization and efficiency, for a sail requires no on-board propellant and can be sent on ‘sundiver’ trajectories that could achieve, in the view of the authors, velocities of 33 kilometers per second (7 AU per year). By comparison, Voyager 1’s pace is 17.1 kilometers per second. Using chemical propulsion, we would need 15 years to reach Uranus, so much faster travel times are welcome.

So let’s add to Sara Seager’s ideas on exoplanet constellations of CubeSats the idea of solar sail smallsats on fast trajectories for flyby missions, impactor missions like DART, and formation and swarm operations at targets like the ice giants, about which we know all too little. The challenge here will be the need for lightweight instrumentation and continuing miniaturization, both trends that seem to be accelerating. Two-years to Jupiter and three to Saturn at these velocities are tantalizing prospects as Turyshev and team continue to study sailcraft designs to reach the Sun’s gravity lens beginning at 550 AU.

How does a fleet of future smallsats hardened for deep space and using modularized components stack up against, say, a single enormous (by comparison) orbiter to Uranus or Neptune? The two could, of course, work together, but given the costs, flyby missions on the cheap in their tens or hundreds could offer priceless scientific return. We always return to the practicalities of prying money out of political entities, so finding ways to lighten the budget while accomplishing the mission will continue to be a priority. Will we see an ice giant orbiter off in the 2030s? Somehow I doubt it.

The exoplanet paper via CubeSat constellation mentioned above is “Demonstrating high-precision photometry with a CubeSat: ASTERIA observations of 55 Cancri e,” The Astronomical Journal Vol. 160, No. 1 (2020), 23 (full text). The Turyshev paper is “Science opportunities with solar sailing smallsats,” Planetary and Space Science Vol. 235 (1 October 2023). Full text. For more on all this, see Building Smallsat Capabilities for the Outer System, published last year in these pages.