OSIRIS-APEX arrives at Apophis
The spacecraft that brought Bennu samples to Earth, now renamed OSIRIS-APEX, is heading for Apophis. It is due to arrive in June 2029 to see how the asteroid changed after its close pass by Earth.
The story
On 24 September 2023 the OSIRIS-REx spacecraft dropped a capsule holding about 121 grams of material from the asteroid Bennu, twice its 60-gram goal. Only the capsule came down to Earth, however. The spacecraft itself was still in good working order, so NASA decided not to retire it but to send it somewhere new.
Its new name is OSIRIS-APEX, and its new target is the asteroid Apophis. Apophis is about 340 metres across on average, and on 13 April 2029 it will pass about 32,000 km above Earth's surface. That is inside the ring where television and weather satellites orbit. An asteroid this size comes this close only about once every 800 years on average.
OSIRIS-APEX is due to arrive in June 2029, about two months after the close pass. The goal is to study the asteroid right after Earth's gravity has tugged on it. Once there, the spacecraft is planned to stay near Apophis for about 18 months, mapping its surface and measuring its shape and the way it spins.
One highlight of the plan is an engine experiment. The spacecraft will dip toward the surface and fire its thrusters, blowing away dust and small rocks. That will expose material underneath that has not been baked by sunlight and space radiation for long. On the way, the spacecraft flew past Earth in September 2025 and used Earth's gravity to adjust its path toward Apophis.
Mission goals
What Earth's gravity does to an asteroid has mostly been studied with calculations. As Apophis passes Earth, its spin rate or axis may change and rocks on its surface may slide. OSIRIS-APEX will measure the asteroid afterwards from close range, and scientists can compare those measurements with telescope observations made before the flyby.
There is a second spacecraft to compare with, too. ESA's Ramses mission is planned to reach Apophis in February 2029, before the flyby, and to watch the encounter from nearby. If both spacecraft keep to their plans, scientists will have a rare close-up record of one asteroid both before and after a major event.
Reusing a spacecraft brings benefits beyond saving money. Its cameras and instruments have already worked well at Bennu. Bennu is a dark asteroid rich in carbon, while Apophis is a stony asteroid made mostly of silicate rock. Measuring two very different asteroids with the same instruments makes their differences easier to compare fairly.
Mission chronology and background
OSIRIS-REx launched in September 2016 and reached Bennu in 2018. In October 2020 it touched the surface briefly and sucked up soil and pebbles, collecting so much that the lid had trouble closing. The capsule landed in the Utah desert in September 2023. Analysis found 14 of the 20 amino acids that living things use to build proteins, along with minerals that contain water.
Apophis was discovered in June 2004 at Kitt Peak Observatory in Arizona. At first its chance of hitting Earth in 2029 was calculated to be as high as 2.7 percent, which drew wide attention, but more observations removed that possibility. Radar observations in March 2021 confirmed there is no risk of impact for the next 100 years, and Apophis was taken off the risk list.
NASA's extra budget for this extended mission is reported to be about 200 million dollars, far less than building a new spacecraft from scratch. Because one spacecraft visits two asteroids in turn, the mission is seen as a good example of reusing hardware. Details such as the exact arrival date may still shift as the flight goes on.
Where to see it — sky and simulator
OSIRIS-APEX itself cannot be seen from the ground. Apophis, however, is expected to brighten to about third magnitude at its closest approach on 13 April 2029, and to be visible to the naked eye from Europe, Africa and western Asia, where it will be night. In East Asia and Australia it will be morning, and in North America daytime, so it will be hard to see there. Choose ‘Apophis flyby’ in the event list of NowSky's sky view to replay that night's sky.
In the orbit view, switch on asteroids and select Apophis to see its orbit weaving inside and outside Earth's. Move the time to 13 April 2029 to watch it come very close to Earth. After Earth's pull, the time Apophis takes to circle the Sun grows from about 0.89 years to about 1.16 years, and comparing the orbit before and after the flyby shows that change.
FAQ
Will Apophis hit Earth?
No. Radar observations in 2021 made the orbit precise enough to rule out any impact for the next 100 years. In 2029 it will pass very close, but it will miss.
How close does it come?
On 13 April 2029 it comes within about 32,000 km of Earth's surface. That is closer than the geostationary satellites, which orbit about 36,000 km up.
Why does it arrive after the flyby?
The available flight path and fuel set the arrival for around June 2029. That timing lets the spacecraft study the asteroid just after Earth's gravity has affected it. ESA's Ramses mission is planned to cover the time before the flyby.
Why reuse OSIRIS-REx?
After delivering its samples the spacecraft was still working well. Using instruments that have already been proven costs far less, and carries less risk, than building a new spacecraft.
Related
- Apophis flyby (2029)
- DART — humanity moves an asteroid
- Hayabusa2 reaches 1998 KY26
- Earth — object index
- Venus — object index
Times and figures are approximate — verify with official sources when planning travel. Simulation uses J2000 mean elements.