OSIRIS-APEX arrives at Apophis

The Bennu sample courier, renamed, heads to Apophis to see how the Earth flyby reshaped it.

The story

OSIRIS-REx, which delivered 121 g of Bennu to Earth in September 2023, did not retire. With a quarter of its propellant left after dropping the sample capsule, NASA renamed it OSIRIS-APEX and sent it to a new target, Apophis, instead of building a new spacecraft — at a tenth of the cost of a new mission.

Apophis is a 340 m asteroid that on 13 April 2029 passes 32,000 km from Earth — inside the ring of geostationary satellites. An object this size coming this close happens once in thousands of years, and from Europe and Africa it is expected to cross the sky visible to the naked eye like a moving star.

OSIRIS-APEX is due to arrive in June 2029, just after the close approach — the first survey ever of an asteroid immediately after Earth's tides have shaken it: changes in spin, surface landslides, internal rearrangement. It plans to orbit Apophis for eighteen months, mapping it and firing its thrusters at the surface to blow away dust and expose the material beneath.

The first half of 2029, with the flyby in April and arrival in June, is set to be a festival of asteroid science. The conversion — several Venus flybys bending the orbit toward Apophis — stands as a model of spacecraft recycling. The orbit view shows Apophis's orbit.

Why it matters

The Apophis approach is a planetary-defence experiment provided by nature. At discovery in 2004 the 2029 impact probability was computed at 2.7 percent — Torino scale 4, the highest ever recorded — and later observations ruled out impacts in 2029, 2036 and 2068. It is a case of the near-Earth-object monitoring system working as intended.

What Earth's tides do to an asteroid is known only from theory. Apophis is predicted to change its spin state and shake its surface as it passes at 32,000 km, and OSIRIS-APEX is the only chance to compare before (from the ground) and after (on site).

Reusing the spacecraft is not only a budget matter. Instruments and navigation proven at Bennu carry less risk than a new mission, and comparing two asteroids — carbonaceous Bennu and silicate Apophis — with the same instruments is a scientific advantage.

How to watch

Apophis is now a 20th-magnitude target for large telescopes, but at the close approach on 13 April 2029 it brightens to 3rd magnitude, visible to the naked eye from Europe, Africa and western Asia. Select Apophis in the sky view for its position now, and choose "Apophis flyby" in the event list to replay that night's sky.

Switch on asteroids in the orbit view and select Apophis for its 0.9-year orbit weaving inside and outside Earth's; move the time to 13 April 2029 to see it come within 32,000 km. OSIRIS-APEX's track is drawn with its scheduled arrival.

After the flyby Earth's gravity changes Apophis's orbit substantially, moving it into the Apollo group outside Earth's orbit; comparing the orbits before and after in the orbit view shows the change.

Background

Apophis was discovered in June 2004 at Kitt Peak in Arizona and named for the Egyptian serpent of chaos. Radar observations in March 2021 refined the orbit enough to rule out any impact for the next hundred years, and it was removed from the risk list.

OSIRIS-REx launched in 2016, collected its sample from Bennu's surface in October 2020 and dropped the capsule in the Utah desert on 24 September 2023. The 121 g returned was twice the expected 60 g, and clays containing amino acids and water were confirmed.

ESA's Ramses mission is planned to launch in 2028 and reach Apophis in February 2029, before the flyby, to watch the approach on site, while OSIRIS-APEX covers the aftermath — two spacecraft dividing before and after at the same asteroid.

FAQ

Will Apophis hit Earth?

No. Radar observations in 2021 ruled out any impact for the next hundred years.

How close does it come?

32,000 km from Earth on 13 April 2029 — inside the ring of geostationary satellites.

Can it be seen with the naked eye?

At the flyby it brightens to 3rd magnitude, expected to look like a moving star from Europe, Africa and western Asia.

Why reuse OSIRIS-REx?

A quarter of its propellant remained, and proven instruments could be reused at a tenth of the cost of a new mission.

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Times and figures are approximate — verify with official sources when planning travel. Simulation uses J2000 mean elements.