Five Years to Apophis; Can a New Mission be Ready?

Apophis is an asteroid that needs little introduction. Some asteroids may be familiar even to laypersons, such as Bennu, Ryugu, and Didymos; these asteroids have been visited by spacecraft relatively recently and are well-studied. Apophis however, is not well known because of a mission set to visit it, but because Apophis is set to visit us. On April 13th, 2029 Apophis will flyby the Earth and come within 20,000 miles of our home planet, far closer than our Moon and even closer than some satellites. However today the planetary science community has a strong interest in flipping the script; a desire to have humanity be the one to reach out first, by sending a spacecraft to Apophis before the asteroid’s encounter with Earth.

Apophis is a near-Earth asteroid (NEA) which was discovered back in 2004, and at the time of discovery showed a significant risk of impacting Earth in 2029. This potential impact has since been ruled out, as well as later potential impact dates in 2036 and 2068. These potential impact dates were ruled out by a radar observation campaign conducted in March of 2021 providing a better understanding of Apophis’ trajectory. While it originally sparked concern, Apophis is firmly understood to pose no threat to Earth. With concern arrested, what is left is a tremendous opportunity to investigate and explore how NEAs interact with our planet, and deepen our understanding of their physical properties in support of planetary defense.

One spacecraft is already en route to take advantage of this opportunity, OSIRIS-APEX, previously known as OSIRIS-REx, which completed its primary mission to return samples from asteroid Bennu in September of 2023. Under its new designation and mission extension, the spacecraft will follow behind Apophis during its 2029 encounter with Earth before finally catching up to the asteroid two weeks later. Once at Apophis, the OSIRIS-APEX spacecraft will begin proximity operations, orbiting the small asteroid and mapping its surface, identifying surface composition, and utilizing spare gas canisters meant for sample acquisition to disturb dust on Apophis’ surface to catch a glimpse at subsurface materials. This science operation is planned to last until 2031.

Apophis, being a small NEA, will be a familiar operational environment for the spacecraft, which spent nearly three years maneuvering around and studying Bennu in close proximity. The journey to Apophis will take OSIRIS-APEX closer to the Sun than it was originally designed for, spacecraft are built to very specific environmental tolerances, but the first of the mission extension’s closer approaches to the Sun has already been passed without incident.

The primary difference for the mission at Apophis will be Apophis’ properties as a target of scientific study and the overall mission goals. Bennu isa C-type “carbonaceous” asteroid, selected for study as C-types are more likely to hold organic compounds that could be building blocks for life. Apophis meanwhile is believed to be an Sq-type asteroid, a more common siliceous or “stony” asteroid with slight hints of olivine and pyroxene. Apophis, based on observations from the now-retired Arecibo observatory, is believed to be an elongated or perhaps bilobed-contact binary shape, another stark contrast to the “top-like” shape of Bennu and other asteroids. Apophis also rotates much slower than Bennu, taking over a day to spin once on its axis while Bennu only takes roughly four hours.

All of these features of Apophis could change by the time OSIRIS-APEX arrives, but not in the sense that this information will become outdated as more is learned; Apophis itself may literally change before OSIRIS-APEX catches up to it.

Where the mission at Bennu was investigating the ways in which asteroids are tied to the origin of life on Earth, the spacecraft’s new mission will be investigating how Earth’s influence affects nearby asteroids in return. Apophis’ encounter with Earth has the potential to impart significant changes upon the small asteroid. One aspect in which this interaction is well understood is in relation to Apophis’ orbit around the Sun. Near-Earth asteroids are sorted into different named groups based on their orbits and how they relate to Earth. Currently, Apophis spends most of its time closer to the Sun than Earth, and occasionally crosses Earth’s orbit; asteroids that behave this way are known as Atens. During the April 2029 encounter with Earth, Apophis will experience a change in velocity, boosting it into an orbit further away from the Sun but still occasionally crossing Earth’s path. Going from an Earth-crossing asteroid with a smaller orbit than Earth’s to one with a larger orbit means Apophis will be reclassified from an Aten type asteroid to an Apollo type.

The change to Apophis’ trajectory is the best understood of these changes as Apophis’ trajectory has been investigated for a long time; however, changes to Apophis’ spin-state, rate of rotation, potential resurfacing, or even reshaping, are far less understood. Predicting what features of Apophis might change and the extent to which they could would require data we do not currently have. Ground-based observatories on Earth will have limited ability to observe what changes occur during Apophis’ Earth-encounter, as will the OSIRIS-APEX spacecraft from a distance. Once OSIRIS-APEX arrives in the vicinity of Apophis, it will begin working backwards from data it collects in an attempt to understand how Apophis was changed by encountering Earth.

Letting Apophis flyby without having a purpose-built toolset to take advantage of the once-in-a-lifetime scientific opportunity it presents is considered by many to be suboptimal. For this reason it may be worth the investment to acquire data considered extremely vital by the scientific community, simply put: a before photo. Having data from a spacecraft in the proximity of Apophis ahead of its Earth encounter to compare with post-encounter data from OSIRIS-APEX would provide an unmatched glimpse into how planets influence nearby minor bodies. In times where money is tight and the pathways by which new missions can be approved are either delayed or uncertain, the question becomes how such a spacecraft could be rapidly readied to arrive at Apophis in just five years.

One concept for a new purpose-built mission to Apophis is the “Rapid Apophis Mission for SpacE Safety,” or “RAMSES” concept, which is being investigated by the European Space Agency. While the mission is not yet approved, RAMSES represents an ambitious undertaking. The mission would launch in April of 2028, and arrive at Apophis in February the following year, mere months ahead of its encounter with Earth. The spacecraft would slowly lower itself towards Apophis, transmitting data on the qualities of its surface, its exact rotation, provide clues on its internal structure and map Apophis down to 10 cm resolution.

RAMSES would also monitor Apophis through its Earth encounter, taking snapshots every minute or so from a safe distance, yet closer than the OSIRIS-APEX spacecraft. This would provide a near real-time feed of any changes Apophis undergoes during its encounter as they happen. RAMSES would spend some time with Apophis post-encounter before nominally ending operations in August of 2029, leaving OSIRIS-APEX to continue studying the asteroid on its own. In order to expedite the pre-launch process, the RAMSES mission uses a spacecraft nearly identical to the HERA spacecraft launching later this year. HERA is the European follow-up to NASA’s DART mission, and will travel to the binary near-Earth binary asteroid, Didymos, to observe DART’s impacts on the system. By using a platform already developed for exploring near-Earth asteroids, RAMSES skips many rigorous and lengthy development processes involved with creating a completely unique mission-specific spacecraft. RAMSES baselines launch on the heavily-delayed Ariane 6 rocket, expected to fly this year, but should the vehicle be unavailable a lower-energy backup trajectory using a Falcon 9 Block 5 has also been explored. It is unknown as of time of writing officially know if RAMSES will move forward until 2025; however, in a talk given to the 30th Meeting of the Small Bodies Assessment Group on January 31st, ESA’s Paolo Martino presented information on the RAMSES concept, and indicated that the feasibility of the mission’s approval may be communicated by ESA to planetary scientists in the coming months.

While RAMSES is tantalizing, its ambitious schedule does present risk, this is acknowledged by the concept team, and two backup dates where RAMSES could be launched to alternative asteroids have been determined. In the interests of time, there is an option even better than using an existing spacecraft design, and it’s using an existing spacecraft, and that mentality is becoming quite popular among scientists. This is, potentially, where Janus comes in, a pair of plucky spacecraft originally designed to perform flybys of two distinct binary asteroid systems. Being part of NASA’s SIMPLEx program for low-cost science missions, Janus was originally intended to ride along with the Psyche mission which launched in October of 2023 after heavy delays. Because Psyche’s launch timing and trajectory changed, the launch was no longer suitable for the Janus spacecraft to reach their asteroid targets, and the mission was canceled. The spacecraft have been partially disassembled and are to be shipped to NASA’s Langley Research Center for placement into long-term storage.

Presenting to the Small Bodies Assessment Group meeting on the 31st of January, Janus Principle Investigator Dr. Dan Scheeres elaborated on some potential new missions that could be undertaken by the Janus spacecraft, including opportunities to send the spacecraft to Apophis. Dr. Scheeres explained that many Near-Earth Asteroid targets are on the table, but it all depends on when Janus is given the go-ahead to pursue a new mission. Even though the spacecraft and their instrumentation are built and paid for, the process to reassemble the spacecraft and requalify some components still takes time, and until NASA says so that process cannot begin.

If Janus were to be given a green light and fast tracked, an opportunity to launch to Apophis exists as soon as next year, with the spacecraft riding along with NASA’s IMAP mission on February 1st 2025, with arrival at Apophis in 2028. Other opportunities for Janus to reach Apophis exist in the 2027 launch of NASA’s Near-Earth Object Surveyor Telescope, or even RAMSES if it is approved. For emphasis, Janus is not currently approved for anything other than entering storage in the hopes that it may be repurposed someday, but it remains to be seen if NASA will approve any revival.

As things stand, Apophis’ closeup with our planet is just over five years away, and if the world’s space agencies truly intend to be the one to reach out first, planning needs to be in full swing. In times of tight budgets and a full platter of planetary science commitments, undertaking a new endeavor to explore Apophis will require finding ways to reduce mission cost and development time. In the words of Planetary Science Professor Richard Binzel, “Apophis is coming, whether we are ready or not.”

Edited by Nik Alexander.