Six probes still in flight

One of the strange features of robotic space exploration is that the dramatic moments — the launches, the landings, the headline-grabbing flybys — are bracketed by enormous quiet stretches of nothing happening. There are right now six science probes coasting somewhere between the Earth and their destinations. Most of them launched in the past two and a half years. Most of them will not arrive at the target for another five. One of them launched in 2016 and is still in flight. Here's what they are, where they're going, and what's coming.

Hera (ESA, Earth → Didymos system)

Launched: 7 October 2024 from Cape Canaveral, on a Falcon 9. Arrives: December 2026. Mission: measure exactly what NASA's DART probe did to a small asteroid two years ago.

Hera is the European half of the world's first planetary-defence demonstration. In September 2022 NASA flew DART into Dimorphos, a 160-metre moonlet orbiting the larger asteroid Didymos. Earth-based telescopes confirmed within weeks that the impact had measurably shortened Dimorphos's orbital period — the first time humanity had altered the trajectory of a celestial body. What no one yet knows is exactly what state the moonlet was left in. Did the impact compress it, fracture it, or rebuild it from the rubble? Hera, with two CubeSats riding alongside, will spend roughly six months in the Didymos system answering that question. It carries instruments to measure Dimorphos's mass, density, internal structure and post-impact crater geometry. The mission matters less for what it tells us about Dimorphos than for what it tells us about kinetic-impact deflection in general — the most plausible technique for steering a future Earth-bound asteroid out of harm's way.

ESCAPADE (NASA + UC Berkeley, Earth → Mars orbit)

Launched: 4 October 2024 from Cape Canaveral on the inaugural New Glenn flight. Arrives: September 2027 (Mars orbit insertion of both twin spacecraft, after a heliocentric loop). Mission: the first Mars mission to study the planet's magnetosphere with two spacecraft simultaneously.

ESCAPADE — Escape and Plasma Acceleration and Dynamics Explorers — consists of two identical 250-kilogram orbiters built by Rocket Lab, the first multi-spacecraft mission to Mars to launch on a single vehicle. The science problem is straightforward and important: Mars lost its global magnetic field early in its history, and as a result it lost most of its atmosphere to the solar wind. Single-orbiter missions like MAVEN have measured how that loss happens at a single point in time and space. ESCAPADE, with two craft in different orbits, can measure the cause and the response at once — what the solar wind does at one point and what the upper atmosphere does at another, simultaneously. The mission rode the maiden New Glenn launch for cost reasons (and got lucky that Blue Origin's first attempt reached orbit) and is now in cruise. The 2027 arrival is unusually late for a "fast" Mars mission because the mission was bumped from a 2024 trajectory and then had to take a long heliocentric path. Both spacecraft will operate for approximately a Martian year after orbit insertion.

Europa Clipper (NASA / JPL, Earth → Jupiter / Europa)

Launched: 14 October 2024 from Kennedy on a Falcon Heavy. Arrives: April 2030. Mission: the most thorough investigation ever attempted of an outer-solar-system ocean moon.

Europa Clipper is the largest planetary mission NASA has flown — a six-tonne spacecraft with thirty-metre solar arrays so large that the spacecraft only just fits inside the Falcon Heavy fairing in stowed configuration. It is also one of the agency's most genuinely high-stakes missions. Europa is, by most current estimates, the body in our solar system most likely to harbour extant life: an ice-covered ocean of liquid water, twice the volume of all of Earth's oceans, in continuous contact with a rocky seafloor that almost certainly hosts hydrothermal vents. Clipper will not look for life. It will fly past Europa roughly fifty times across four years, mapping the ice shell's thickness with radar, sampling plume material if any plumes are still venting, and characterising the chemistry of the surface and atmosphere. The mission's job is to find the science case strong enough to justify a follow-on lander, which is currently a paper study. Clipper itself spends nearly six years in cruise — a Mars gravity assist in 2025 (already complete), an Earth flyby in 2026, then a long quiet stretch out to Jupiter. A lot of the work between now and 2030 happens on the ground: the radar processing pipeline alone is a multi-year software development effort that's still ongoing.

Psyche (NASA / JPL, Earth → asteroid 16 Psyche)

Launched: 13 October 2023 from Kennedy on a Falcon Heavy. Arrives: August 2029. Mission: the first close investigation of a metallic asteroid.

Psyche is unusual because nobody is quite sure what it is. The asteroid 16 Psyche is roughly the size of Massachusetts, sits in the main belt, and reflects radar in a way that suggests it's largely made of metal — possibly the exposed iron-nickel core of a planetesimal that lost its rocky mantle in some long-ago collision. If that interpretation is right, Psyche offers something no other body in the solar system does: a direct view of the kind of differentiated planetary core that's normally buried thousands of kilometres beneath a planet's surface, including Earth's. The spacecraft uses solar-electric propulsion — Hall-effect thrusters running for thousands of hours at a time — and is currently most of the way through a Mars-gravity-assist trajectory that gets it to the asteroid belt in 2029. A 21-month survey at four progressively closer orbits will follow. The wider importance is operational as much as scientific: Psyche is by some distance the most ambitious deep-space mission ever flown on electric propulsion, and the lessons from its long thrust campaign will feed every NASA outer-planet mission that follows.

JUICE (ESA, Earth → Jupiter / Ganymede)

Launched: 14 April 2023 from Kourou on Ariane 5. Arrives: July 2031 in Jupiter orbit. Settles into Ganymede orbit December 2034. Mission: the first dedicated study of the largest moon in the solar system.

JUICE — Jupiter Icy Moons Explorer — is a contemporary of Europa Clipper but with a different target and a much longer cruise. After three Earth flybys, a Venus flyby and a fourth Earth flyby it arrives at Jupiter in 2031, spends three and a half years studying Ganymede, Callisto and Europa from Jovian orbit, and then in 2034 becomes the first spacecraft ever to enter orbit around a moon other than Earth's. Ganymede is the largest moon in the solar system — bigger than Mercury — and is the only moon with its own intrinsic magnetic field, which suggests a saltwater ocean larger than all of Earth's. The science overlap with Europa Clipper is intentional rather than redundant: Clipper does Europa thoroughly, JUICE does Ganymede thoroughly, and both contribute to a comparative picture of the icy-moon ocean worlds. The Ariane 5 that launched it was the second-to-last that ever flew, and the loss of that launcher means there is currently no European launch capability for a follow-on Jovian mission. The next ten years of European outer-solar-system science are riding on this one spacecraft.

OSIRIS-APEX (NASA, Earth → asteroid Apophis)

Launched: 8 September 2016 (as OSIRIS-REx). Arrives: 13 April 2029, the day Apophis flies past Earth. Mission: rendezvous with a 340-metre asteroid as it makes the closest pass of its orbit.

OSIRIS-APEX is the second life of the spacecraft that brought 120 grams of asteroid Bennu home to the Utah desert in September 2023. After dropping its sample-return capsule on Earth flyby, the mothership — by then nearly seven years out of warranty and short on fuel — was rebadged OSIRIS-APEX and given a new target: Apophis, which on 13 April 2029 will pass within 32,000 kilometres of Earth, closer than the geostationary belt and visible to the naked eye over Africa and Europe. APEX arrives at Apophis the same day, performing a rendezvous instead of a flyby, and will spend roughly eighteen months observing how the close encounter with Earth tidally distorts the asteroid's surface. Earth's gravity is expected to do measurable work on the asteroid's spin state and surface structure during the flyby; APEX will be there to record it. The mission is a good example of the kind of science that becomes possible when a long-running spacecraft has propellant left after its primary mission, and a new target opens up that wasn't on the original mission timeline.

What this list reveals

One thing worth noting from the table: every one of these missions launched on either an American Falcon 9 / Falcon Heavy / New Glenn, or the final Ariane 5 of the European launcher generation. The deep-space launch market is now almost entirely on either Falcon Heavy or New Glenn. Vulcan is in the queue for follow-on missions; Ariane 6 has the lift capacity but no qualified deep-space upper stage yet. China and Japan launch their own deep-space probes on national vehicles. The commercial-launch market for outer-solar-system payloads is, in practical terms, two American companies and Blue Origin trying to catch up.

Second: the average time-of-flight for these six missions is just over five years. That's a long stretch in which the team that launched the spacecraft slowly disperses, the subcontractors get bought, the operations software needs maintenance, and the ground stations need to be kept available. Every long-duration deep-space mission depends on the institutional ability to keep paying for that quiet middle, where nothing seems to happen and the headlines have moved on. NASA and ESA have learned the discipline. Anyone who follows the next generation of commercial deep-space ventures should watch closely how they manage the same quiet decades.

Third: between now and 2031 we will get arrivals at a metal asteroid, two ocean moons, a planetary-defence test target, a binary Mars-magnetosphere study, and a one-day rendezvous with the closest large asteroid to pass Earth in modern history. Five flagship-class scientific events, all already in the post. The next decade of planetary science is, in a literal sense, on its way.

Mission data is from the New Space Tracker missions tracker. Launch and arrival dates as of April 2026; ESA / NASA / JPL operating estimates.